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                                    The gut microbiota — masters of host
                                    development and physiology
                                    Felix Sommer1,2 and Fredrik Bäckhed1,2,3
                                    Abstract | Establishing and maintaining beneficial interactions between the host and its
                                    associated microbiota are key requirements for host health. Although the gut microbiota has
                                    previously been studied in the context of inflammatory diseases, it has recently become clear
                                    that this microbial community has a beneficial role during normal homeostasis, modulating
                                    the host’s immune system as well as influencing host development and physiology, including
                                    organ development and morphogenesis, and host metabolism. The underlying molecular
                                    mechanisms of host–microorganism interactions remain largely unknown, but recent studies
                                    have begun to identify the key signalling pathways of the cross-species homeostatic
                                    regulation between the gut microbiota and its host.

Microbiota                         All higher animals are associated with a diverse micro-             gram, the ileum 107 cells per gram and the colon up to
The sum of all microorganisms      bial community that is composed mainly of bacteria                  1012 cells per gram) and along the tissue–lumen axis
(including bacteria, archaea,      but also includes archea, viruses, fungi and protozoa.              (with few bacteria adhering to the tissue or mucus but
eukaryotes and viruses) that       Microorganisms cover essentially all host mucosal sur-              a large number being present in the lumen)4. Second,
reside in and/or on a host or
a specified part of a host (such
                                   faces, but most reside within the gastrointestinal tract.           bacterial diversity increases in the same axes and manner
as the gastrointestinal tract).    Studies had traditionally focused on examining the                  as microbial density 4. Many bacterial species are present
                                   role of the microbiota during human disease, for exam-              in the lumen, whereas fewer, but well-adapted species,
1
 Wallenberg Laboratory             ple in inflammatory diseases such as colitis. However,              including several proteobacteria and Akkermansia
for Cardiovascular and             in the past decade, the field of microbiota research has            muciniphila, adhere and reside within the mucus layer
Metabolic Research,                exploded, resulting in the publication of a plethora of             close to the tissue5,6. Colonization of the host begins dur-
Sahlgrenska University             reports that describe both the individual members of our            ing birth, and the composition of the microbiota changes
Hospital, Department of
Molecular and Clinical
                                   intestinal microbiota and their wide-ranging impact on              throughout host development (BOX 1).
Medicine, University               host physiology. Thus, the traditional anthropocentric                  In the adult intestine, a total of about 10 14 bacte-
of Gothenburg.                     view of the gut microbiota as pathogenic and solely                 rial cells are present, which is ten times the number of
2
 Sahlgrenska Center for            an immunological threat has been substituted with                   human cells in the body 7. Their combined genomes
Cardiovascular and Metabolic
                                   an appreciation of its mainly beneficial influence on               (known as the microbiome) contain more than 5 mil-
Research, Department of
Molecular and Clinical             human health.                                                       lion genes, thus outnumbering the host’s genetic poten-
Medicine, University of                The ‘normal’ gut microbiota is dominated by anaer-              tial by two orders of magnitude2,8. This large arsenal of
Gothenburg, SE‑413 45              obic bacteria, which outnumber aerobic and faculta-                 gene products provides a diverse range of biochemical
Gothenburg, Sweden.                tive anaerobic bacteria by 100- to 1,000‑fold1. In total,           and metabolic activities to complement host physiol-
3
 Novo Nordisk Foundation
Center for Basic Metabolic
                                   the intestinal microbiota consists of approximately                 ogy. In fact, the metabolic capacity of the gut microbiota
Research, Section for              500–1,000 species that, interestingly, belong to only               equals that of the liver, and the intestinal microbiota can
Metabolic Receptology and          a few of the known bacterial phyla2,3. By far the most              therefore be considered as an additional organ9. These
Enteroendocrinology,               abundant phyla in the human gut are Firmicutes and                  bacteria are essential for several aspects of host biology.
Faculty of Health Sciences,
                                   Bacteriodetes, but other species present are mem-                   For example, they facilitate the metabolism of otherwise
University of Copenhagen,
Copenhagen DK‑2200,                bers of the phyla Proteobacteria, Verrumicrobia,                    indigestible polysaccharides and produce essential vita-
Denmark.                           Actinobacteria, Fusobacteria and Cyanobacteria2,3. Two              mins; they are required for the development and differ­
Correspondence to F.B.             gradients of microbial distribution can be found in the             entiation of the host’s intestinal epithelium and immune
e‑mail:                            gastrointestinal tract. First, microbial density increases          system; they confer protection against invasion by
Fredrik.Backhed@wlab.gu.se
doi:10.1038/nrmicro2974
                                   both from the proximal to the distal gut (the stomach               opportunistic pathogens10; and they have a key role in
Published online                   contains 101 microbial cells per gram of content, the               maintaining tissue homeostasis. Recent studies have
25 February 2013                   duodenum 103 cells per gram, the jejunum 104 cells per              also revealed that the human microbiota influences


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REVIEWS

 Box 1 | Colonization of the host
                                                                                                       are pathogens and thus the cause of infectious diseases.
                                                                                                       The realization that we live in a microbially dominated
 Human babies are colonized during passage through the birth canal by environmental                    world and in fact benefit greatly from our microbiota
 microorganisms (for example, from the mother’s vagina or skin) and during breast                      has led to a paradigm shift in immunology. Thus, the
 feeding by microorganisms present in the milk137. Owing to the highly oxidative                       definition of self in the superorganism theory has been
 environment in the gastrointestinal tract of the newborn, primary colonizers are
                                                                                                       extended to incorporate the constituents of both our
 facultative anaerobic bacteria such as proteobacteria, which are thought to adjust the
 environmental conditions by decreasing the oxygen concentration to allow successive
                                                                                                       own body and our microbiota12. It is also now widely
 colonization by anaerobic microorganisms such as members of the genus Bacteroides                     accepted that the host’s mucosal immune system is char-
 and members of the phyla Actinobacteria and Firmicutes. During the first year of life,                acterized by tolerance to microorganisms rather than
 the intestinal microbiota composition is simple and fluctuates widely between                         responsiveness13. Furthermore, it has even been specu-
 individuals and over time. Microbial signatures stabilize and start to resemble the ‘adult            lated that the highly sophisticated adaptive immune
 state’ when the infant reaches 1–2 years of age4.                                                     system of jawed vertebrates evolved to keep control of
   Interestingly, conflicting evidence has been published concerning the driving force                 the mutualistic or beneficial symbiosis with our complex
 for microbial transmission. In early studies of twins, the faecal microbial compositions in           microbial ecosystem14.
 the mother and her children were similar, indicating a mainly maternal transmission108,111.               The intestine, one essential organ in which the
 However, in a more recent and extensive study, the same research group found that the
                                                                                                       mucosal immune system operates, has to accomplish
 faecal microbiota of children was no more similar to that of their mothers than to that of
 their biological fathers, and genetically unrelated but co‑habiting mothers and fathers
                                                                                                       two apparently confounding tasks. First, it needs to
 were significantly more microbially similar to one another than to members of different               facilitate nutrient absorption; thus, the total surface area
 families138. This indicates that, as well as genetics and kinship, environmental factors              of the gastrointestinal tract amounts to about 200 m2 in
 have a considerable effect on the microbial composition of the infant.                                humans15. Second, it needs to be resistant to infection
                                                                                                       and inhibit microbial translocation across the tissue bar-
                                                                                                       rier. Bacterial densities in the gut are the highest known
                                  the development and homeostasis of other host tissues,               in any habitat to date and reach up to 1012 cells per
                                  including the bone11.                                                gram in the lower intestine16. This highly dense micro-
                                      The microbiota also benefits from this mutualistic               bial community and the host intestinal epithelial cell
                                  association, as the mammalian intestine is a nutrient-rich           (IEC) lining are separated by only a few micro­ etres of
                                                                                                                                                         m
                                  environment that is maintained at a constant tempera-                mucus in the small intestine and up to several hundred
                                  ture. However, it is also a dynamic habitat that undergoes           micrometers in the colon, depending on the location17.
                                  constant and rapid changes in its physiological para­                Because of this unique nature of the intestinal tract, its
Mutualistic                       meters owing to variations in, for example, host diet,               mucosal immune system needs to fulfil several special
Pertaining to a relationship      lifestyle, hygiene or use of antibiotics, all of which affect        requirements. It has to be non-responsive to or tolerant
between two organisms:
beneficial to both organisms.
                                  gut microbial composition (FIG. 1). Thus, unlike the host            towards the huge number of mutualistic micro­organisms
The term originates from the      genome, the microbiome can change rapidly as a result of             that reside in the intestinal lumen. At the same time, it is
Latin word mutuus (lent,          modifications in either the composition of the microbial             thought that the mucosal immune system has to assure
borrowed or mutual).              community or individual microbial genomes, resulting                 a beneficial microbiota composition by keeping patho-
                                  in modified transcriptomic, proteomic and metabolic                  bionts in check, restricting microbial overgrowth and
Superorganism
A term that extends the           profiles. Accordingly, the establishment and preserva-               reacting to penetrating microorganisms that breach the
classical biological definition   tion of beneficial interactions between the host and its             intestinal chemical and physical barriers (such as secreted
of an organism (a living system   associated intestinal microbiota are key requirements                soluble immunoglobulin A (IgA), antimicrobial peptides
capable of autonomous             for health.                                                          (AMPs), the mucus layer and the tightly interconnected
metabolism and reproduction)
by including the many
                                      The dynamic fluctuations in the microbiota com-                  IEC lining). In turn, the intestinal microbiota has a key
microorganisms that live in and   bined with the vast numbers of bacterial cells and their             role in directing several aspects in the development and
on that host organism, thus       close proximity to the epithelial tissue represent a mas-            regulation of the host’s immune tissues, immune cell
yielding a superior degree of     sive challenge to host immunity, as microbial growth                 populations and immune mediators.
complexity. The term originates
                                  has to be restricted to ensure a beneficial homeostasis.
from the Latin supra (above)
and the Greek organon (organ,     Furthermore, activation of the host immune system                    Mucus layer properties depend on intestinal bacteria.
instrument, tool).                has to be controlled to circumvent the detrimental                   The intestinal mucus layer covers the epithelial cell
                                  effects of chronic inflammation, so the interaction of               lining and functions as a lubricant, facilitating gastro-
Symbiosis                         the gut microbiota with the host has to be tightly regu-             intestinal transport, and as a protective layer against
Any close physical association
between two organisms,
                                  lated. In this Review, we discuss recent insights into the           bacterial invasion, owing to its physical properties 18.
usually from different species.   impact of the normal microbiota on the development                   The colonic mucus layer is in fact composed of two
This includes mutualism,          and homeostasis of the immune system and other tis-                  layers17. Both the inner and outer mucus layers are
commensalism and parasitism.      sues and organs, as well as on host physiology. We also              secreted by goblet cells and are mainly made up of gel-
The term originates from the
                                  highlight recent advances in deciphering the under­                  forming highly glyco­ ylated proteins termed mucins18.
                                                                                                                              s
Greek words syn (together) and
bio (life).                       lying molecular mechanisms of host–microorganism                     Mucin 2 (MUC2) is the main mucin in the small and
                                  interactions.                                                        large intestines of both mice and humans18. The entire
Pathobionts                                                                                            mucus layer represents a selective microbial habitat
Normally harmless                 Tailoring immune development                                         owing to microbial adhesion via lectins and glycosidases
microorganism that can
become pathogens under
                                  Immunology was originally based on the concept of                    that are expressed by only specific bacteria, and it also
certain environmental             ‘self ’ versus ‘non-self ’ discrimination, with the assump-          serves as nutrient source19,20. However, bacteria are found
conditions.                       tion that, because they are non-self, all micro­organisms            only in the outer layer 17, probably owing to the specific


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                                 Antibiotics                          Lifestyle                             Diet                      Hygiene


                           Hyperimmunity                                                                                         Immunodeficiency
                           IL-6                                            Altered intestinal microbiota                         NOD2
                           IL-12                                                                                                 IL-10
                           TNF



                                                                     Chronic                              Metabolic
                                                                   inflammation                            dysfunction




                       Figure 1 | Factors shaping intestinal microbial composition and effects of dysbiosis on host health.  The composition
                       of the gut microbiota is influenced by various environmental factors, including the use of antibiotics, lifestyle, diet and
                       hygiene preferences. The host’s genetic disposition also has a role: hyperimmunity (owing Nature Reviews | Microbiology
                                                                                                                      to over-representation of
                       pro-inflammatory mediators such as interleukin‑6 (IL‑6), IL‑12 or tumour necrosis factor (TNF)) or immunodeficiency
                       (owing to mutations in regulatory immune proteins such as NOD2 (nucleotide-binding oligomerization domain protein 2)
                       or IL‑10) can influence the gut microbiota composition. In turn, dysbiosis affects levels of immune mediators and induces
                       both chronic inflammation and metabolic dysfunction.


                       structure of the mucus layer as a whole, which is formed               mislocalization of the same microbiota or merely on
                       of interconnected sheets that create pores smaller than a              increased bacterial load.
                       bacterial cell and thus inhibit penetration21.
                           Comparisons of germ-free and conventionally raised                 Microorganisms induce the development of lymphoid
                       animals revealed that microorganisms have major effects                structures. The lymphatic system consists of a network
                       on mucus thickness and composition; compared with                      of lymphatic vessels connecting the primary and second-
                       conventionally raised animals, germ-free animals have                  ary lymphoid organs. The main functions of this sys-
                       fewer goblet cells, a thinner mucus layer and also a                   tem are the recirculation of interstitial fluid and blood
                       higher percentage of neutral mucins in the colon 22.                   as well as the transport of lymphocytes (such as B cells
                       Stimulation with bacterial products such as lipopolysac-               and T cells) (BOX 2) and antigen-presenting cells to lymph
                       charide (LPS) and peptidoglycan is sufficient to estab-                nodes. Lymphoid tissue is classified as primary (thy-
                       lish conventional mucus properties in germ-free mice23,                mus and bone marrow) and secondary (lymph nodes,
                       but the underlying mechanisms for how the gut micro­                   Peyer’s patches, tonsils, spleen and lymphoid follicles).
                       biota modulates goblet cells and mucus layer properties                Lymphocytes are generated in primary lymphoid tissues
                       remain largely elusive.                                                and are then transported to secondary lymphoid tissues,
                           Notably, Muc2‑deficient mice or those with aberrant                where the mature lymphocytes are exposed to antigens
                       mucin glycosylation profiles (owing to a lack of specific              by antigen-presenting cells and are thus activated to
                       glycosyl transferases) show bacterial overgrowth and                   initiate an adaptive immune response. The cellular
                       either develop spontaneous colitis or are more suscep-                 interactions that occur during lymphoid tissue develop-
                       tible to chemical induction of colitis, an effect that can             ment and maturation are similar for both primary and
                       be ameliorated by treatment with antibiotics23–25. This                secondary lymphoid organs, although the molecular
                       demonstrates the importance of the mucus layer for                     frameworks differ a little (for details see REF. 26).
                       homeostasis in the gut and also highlights the recip-                      In addition to host genetics, several environmen-
                       rocal interaction between the mucus layer and the gut                  tal factors, including contact with microorganisms,
                       microbiota. It remains to be clarified whether disease                 influence both the development and maturation of the
                       onset in these mouse strains depends on a selectively                  immune system. The development of secondary gut-
                       altered and thus more colitogenic microbiota, on                       associated lymphoid tissue (GALT), such as Peyer’s


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 Box 2 | Lymphocyte subtypes
                                                                                                      induce the differentiation of stromal organizer cells to
                                                                                                      express several cytokines and adhesion molecules that
 All lymphocytes differentiate and mature in primary lymphoid organs (the thymus and                  attract further immune cells, causing GALT forma-
 bone marrow). Mature naive lymphocytes migrate to secondary lymphoid tissues,                        tion26. Maturation of these tissues, including an increase
 where they become activated by antigen-presenting cells such as dendritic cells.                     in tissue size and the development of germinal centres
 Gut-associated secondary lymphoid tissues include Peyer’s patches, mesenteric lymph
                                                                                                      (sites of B cell proliferation, differentiation and somatic
 nodes and lymphoid follicles139. Here, we list and describe the lymphocytes that are
                                                                                                      hypermutation in lymph nodes), depends on postnatal
 known to be modulated by the gut microbiota43,140,141.
                                                                                                      microbial colonization28 (FIG. 2). Consequently, Peyer’s
 Lymphoid tissue inducer cells
                                                                                                      patches, mesenteric lymph nodes and splenic white pulp
 (LTi cells). A unique T cell subpopulation that is characterized by the expression of
 RORγt, CD4 and interleukin‑7 receptor‑α and the absence of CD3, B220 (an isoform                     are underdeveloped in germ-free mice29.
 of CD45) and CD11c (also known as integrin αX). Their function is to recruit B cells and                 Furthermore, in parallel with microbial coloniza-
 T cells and thereby promote the formation of secondary lymphoid tissues.                             tion, clusters of LTi-like cells termed cryptopatches
 Natural killer cells                                                                                 form at birth in the connective tissue between intestinal
 (NK cells). Lymphocytes that recognize the abnormal antigen signatures of infected or                crypts, known as the lamina propria30. Cryptopatches
 tumour cells, which NK cells kill by lysis or apoptosis. NK cells resemble cytotoxic T cells         recruit B cells and develop into isolated lymphoid fol-
 in function but belong to the innate immune system. They express various NK cell                     licles (ILFs), a type of lymphoid tissue that is structurally
 receptors, including NKp46 (in mice; NKp44 in humans) and NKG2D. They can activate                   similar to Peyer’s patches and serves as an inductive site
 B cells and T cells and thereby stimulate an adaptive immune response.                               for intestinal immune reactions31,32. This process also
 Natural killer T cells                                                                               depends on the gut microbiota, as ILFs fail to develop
 (NKT cells). These cells have properties of both T cells and NK cells, as they co-express            fully in germ-free mice33. ILF formation can be induced
 NK cell markers with a T cell receptor. NKT cells mainly recognize lipids and glycolipids            by exposing germ-free mice to purified peptidoglycan
 presented by antigen-presenting cells via CD1d. Following activation, NKT cells                      from Gram-negative bacteria, indicating that this pro-
 produce pro-inflammatory cytokines such as tumour necrosis factor (TNF) and                          cess is driven solely by a specific microbial pattern34.
 interleukin‑17 (IL‑17). Invariant NKT (iNKT) cells are a specific subpopulation expressing
                                                                                                      Stromal and epithelial cells recognize the peptido­
 an invariant T cell receptor.
                                                                                                      glycan of resident microorganisms mainly via signalling
 T helper 1 cells                                                                                     through the pattern recognition receptor (PRR) NOD1
 (TH1 cells). A subset of TH lymphocytes that is characterized by the expression of
                                                                                                      (nucleotide-binding oligomerization domain contain-
 interferon‑γ and transforming growth factor-β (TGFβ). TH1 cell differentiation is
 induced by contact with activated macrophages or NK cells.
                                                                                                      ing 1) but also partially through another family of PRRs,
                                                                                                      the Toll-like receptors (TLRs). Activation of NOD1
 T helper 2 cells
                                                                                                      by the gut microbiota causes increased expression of
 (TH2 cells). A subset of TH lymphocytes that is characterized by the expression of the
 cytokines IL‑4, IL‑5 and IL‑13. TH2 cell differentiation is induced in response to, for
                                                                                                      CC-chemokine ligand 20 (CCL20) and presumably also
 example, allergens and extracellular microorganisms.                                                 of β‑defensin 3, both of which activate ILF formation
                                                                                                      by binding to CC-chemokine receptor 6 (CCR6) on LTi
 T helper 17 cells
 (TH17 cells). A subset of TH cells that is characterized by the expression of IL‑17, which
                                                                                                      cells34.
 stimulates stromal cells to express the pro-inflammatory cytokines IL‑6 and IL‑8,
 thereby attracting neutrophils and promoting inflammation to clear out invading                      The gut microbiota modulates immune cell differentia-
 microorganisms.                                                                                      tion. In addition to regulating the development of lym-
 Regulatory T cells                                                                                   phoid structures, the gut microbiota has been shown
 (TReg cells). A T cell subpopulation that is characterized by the expression of CD4, CD25            to modulate the differentiation of immune cell subsets
 and FOXP3 and the production of the anti-inflammatory cytokines TGFβ and IL‑10.                      (BOX 2) and, therefore, maintain homeostatic interactions
 These cells can be subdivided into natural TReg cells, which differentiate from CD4+                 between the host and the gut microbiota.
 T cells in the thymus, and inducible TReg cells, which arise from naive T cells in secondary             After birth, LTi-like cells that express nuclear RORγt
 lymphoid tissues. Both cell types function to suppress immune activation and prevent                 but lack NKp46 (in mice; also known as NCR1) or
 self-reactivity, thereby reducing the risk of autoimmune disease.                                    NKp44 (in humans; also known as NCR2) markers accu-
 Type 1 regulatory T cells                                                                            mulate in both the mouse and human GALT and lamina
 (TR1 cells). These CD4+CD25+FOXP3− T cells are functionally equivalent to the                        propria35–37. Interestingly, the RORγt+NKp46− LTi-like
 IL‑10‑producing TReg cells. They respond to microorganisms and regulate intestinal                   cells can differentiate into RORγt+NKp46+ natural killer
 tolerance through the secretion of IL‑10.
                                                                                                      (NK)‑like cells, which differ from regular NK cells (BOX 2)
 B cells                                                                                              in that they have intermediate expression of NK1.1 (also
 Lymphocytes that are activated when the unique B cell receptor binds its specific                    known as KLRB1C) and do not produce interleukin‑1β
 antigen and that then mediate humoral immunity through the production of antibodies.
                                                                                                      (IL‑1β) or kill tumour cells36,37. This differentiation
 B cells are also involved in lymphoid tissue organization.
                                                                                                      requires both IL‑23, which is produced by activated
                                                                                                      myeloid cells and epithelial or endothelial cells, and the
                                                                                                      presence of the intestinal microbiota, as germ-free mice
                                  patches and mesenteric lymph nodes, is initiated pre-               have fewer RORγt+NKp46+ NK‑like cells than conven-
                                  natally in the sterile environment of the fetus through             tionally raised mice37. These cells produce IL‑22, which in
Somatic hypermutation             induction by lymphoid tissue inducer (LTi) cells 27.                mice promotes the integrity of the intestinal barrier and
A programmed process of           Briefly, mesenchymal cells are induced by retinoic acid             reduces bacterial infiltration by inducing epithelial repair
mutation affecting the variable
regions of immunoglobulin
                                  to produce CXC-chemokine ligand 13 (CXCL13), which                  via signal transducer and activator of transcription 3
genes during affinity             recruits LTi precursor cells and stimulates their cluster-          (STAT3) signalling and the production of antimicrobial
maturation of B cell receptors.   ing, leading to their maturation into LTi cells. These then         proteins38. Thus, the normal gut microbiota promotes


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a Germ-free mice                                                                           Conventionally raised mice

                     ↓ Mucus thickness
                       Altered mucus properties                                                                     Microbiota


                     Mucus


                                                   Intestinal
                                                   epithelial cell

                                                   Goblet cell


                          Villus




                                                          IgA


         ↓ Vessel
         density
                                                 AMP             Intestinal
                                                                 crypt
    Blood
    vessel




b                                                                                                                   c




                                                                                                                                    PSA
                                                                                                                                                       TReg cell
                                                                                                                     B. fragilis
                                                                                       Peyer’s patch
                                 Immature
                                 Peyer’s patch


                                 Immature                                                                               SFB                           TH17 cell
                                                                                             MLN
    ↓ AMP and                    MLN
    IgA production                       T cell
                        B cell
                                                                                      IgA-producing
                      Dendritic cell                                                  plasma cell


                                   Figure 2 | Microbiota-induced maturation of the gastrointestinal tract.  The microbiota promotes substantial
                                   changes in gut morphology, including villus architecture, crypt depth, stem cell proliferation, bloodReviews | Microbiology
                                                                                                                                   Nature vessel density, mucus
                                   layer properties and maturation of mucosa-associated lymphoid tissues. a | In germ-free mice, the villi in the distal small
                                   intestine are longer and thinner and have a less complex vascular network than the villi of conventionally raised animals. In
                                   the absence of bacteria, intestinal crypts are less deep and contain fewer proliferating stem cells. Furthermore, germ-free
                                   animals show reduced mucus thickness and altered mucus properties. b | Moreover, very few isolated lymphoid follicles,
                                   immature Peyer’s patches and immature mesenteric lymph nodes (MLNs) are present under germ-free conditions, and
                                   levels of both immunoglobulin A (IgA) and antimicrobial peptides (AMPs) are lower than in conventionally raised animals.
                                   c | In conventionally raised mice, polysaccharide A (PSA) of Bacteroides fragilis is known to induce the expansion of
                                   CD4+CD25+FOXP3+ regulatory T (TReg) cells, which have an anti-inflammatory effect and dampen immune responses. By
                                   contrast, segmented filamentous bacteria (SFB) have been shown to induce the expansion of T helper 17 (TH17) cells,
                                   which are pro-inflammatory.


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                                    intestinal barrier function by modulating mucosal                    increased levels of the immunosuppressive cytokine
                                    homeostasis, in part by promoting the differentiation of             IL‑10 (REF. 47). Notably, compared with non-colonized
                                    RORγt+NKp46+ NK‑like cells.                                          germ-free mice, the Clostridia species-colonized mice
                                        The intestinal microbiota also modulates the abun-               were more resistant to chemically induced disruption
                                    dance of invariant NK T cells (iNKT cells), a unique                 of the gut epithelium and displayed attenuated levels of
                                    T cell subset that expresses an invariant T cell receptor            antigen-induced serum IgE. Furthermore, a similar TReg
                                    α-chain. These cells promote inflammation, as follow-                cell response was observed when germ-free mice were
                                    ing activation they secrete pro-inflammatory T helper 1              colonized with altered Schaedler flora, a cocktail of eight
                                    (TH1)- and TH2‑type chemokines and cytokines, including              defined bacteria including three Clostridia species48.
                                    interferon-γ, IL‑2, IL‑4, IL‑13, IL‑17A, IL‑21 and tumour            However, the specific species in this community that are
                                    necrosis factor (TNF)39,40. In contrast to the NK‑like cells         functionally responsible and the underlying molecular
                                    described above, there are more iNKT cells in the colon              mechanisms of this effect are so far unknown.
                                    of germ-free mice than the colon of conventionally raised                In contrast to the bacteria mentioned above,
                                    mice41, which suggests that the gut microbiota promotes              segmented filamentous bacteria (SFB) elicit a pro-
                                    homeostasis by decreasing the number of these pro-                   inflammatory immune response by promoting the dif-
                                    inflammatory cells. Importantly, a recent study elegantly            ferentiation of TH17 cells and, to a lesser extent, TH1
                                    demonstrated an age dependency of the microbial effects              cells49,50. SFB reside in the small intestine of mice and
                                    on the iNKT population and revealed that colonization of             are in direct contact with epithelial cells, which these
                                    neonatal but not adult germ-free mice with conventional              bacteria seem to readily invade. Invasion leads to local
                                    gut microbiota normalized iNKT cell numbers and pro-                 actin polymerization in the epithelium at the interac-
                                    tected against oxazolone-induced colitis as well as against          tion site, and this presumably initiates a signalling event
                                    ovalbumin-induced allergic lung inflammation41.                      that activates the differentiation of TH17 cells. Notably,
                                        It has become evident that the gut microbiota shapes             mono-association of germ-free mice with SFB is suffi-
                                    the T cell landscape not only in the lamina propria but              cient to restore susceptibility to TH17 cell-mediated arthri-
                                    also systemically and therefore modulates the homeo-                 tis and experimental autoimmune encephalomyelitis51,52. So
                                    stasis of the superorganism42 (FIG. 2). Intestinal mucosal           far, however, it is not known whether TH17 cell differ-
                                    T cells are important ‘legislators’ of intestinal homeo-             entiation is induced by IEC-produced mediators, by
                                    stasis because they not only defend against intestinal               direct interaction with antigen-presenting cells in the
                                    pathogens, but also promote wound healing, barrier                   lamina propria (dendritic cells or macrophages) or by
                                    repair and regeneration as they rapidly accumulate at                bacterially secreted signalling molecules (for example,
                                    sites of injury and infection43. T cells can be assigned             metabolites)15.
                                    to subpopulations that drive either a pro-inflammatory
                                    immune response (TH1, TH2 and TH17 cells) or an anti-                Gut microorganisms tweak the production of immune
                                    inflammatory immune response (CD4+CD25+FOXP3+                        mediators. It is clear that the gut microbiota regulates
                                    regulatory T (TReg) cells or CD4+CD25+FOXP3− type 1                  the production of cytokines and chemokines to influ-
                                    regulatory T (TR1) cells), depending on the cytokines                ence the T cell repertoire of the intestine and surround-
                                    that they produce44 (BOX 2). The balance between both                ing tissue, but there is evidence that these bacteria also
                                    pro- and anti-inflammatory T cell subpopulations deter-              modulate the production of other soluble immune
                                    mines the overall immune equilibrium.                                mediators (FIG. 2). IgA is produced by plasma cells (dif-
                                        Interestingly, individual members of the gut micro-              ferentiated B cells) in the lamina propria and is trans­
                                    biota have been found to drive specific T cell responses             cytosed through the intestinal epithelium into the
                                    (FIG. 2c). The Gram-negative bacterium Bacteroides fragilis          lumen, where it binds microbial antigens and thereby
                                    elicits an anti-inflammatory response by inducing the                prevents bacterial translocation and infection53. The dif-
                                    differentiation of CD4+ T cells into TReg cells locally in           ferentiation of B cells into IgA-producing plasma cells is
                                    the intestinal lamina propria but also in the circula-               induced by sensing of gut microbiota-derived flagellin
Experimental autoimmune
encephalomyelitis                   tion45. TReg cells produce IL‑10 and thereby suppress the            via TLR5 on lamina propria dendritic cells54. IgA has a
An animal model of                  pro-inflammatory TH17 response46. This skewing event is              key role in barrier homeostasis, as IgA-deficient mice
T cell-mediated autoimmune          mediated by polysaccharide A (PSA) on the outer mem-                 produce gut microbiota-specific serum IgG antibodies,
disease in general and in           brane of the bacterium, which is recognized by TLR2 on               indicating that there is a breach in the mucosal barrier
particular of demyelinating
diseases of the central nervous
                                    CD4+ T cells and activates a signalling cascade involving            of these mice and subsequent induction of the systemic
system, such as multiple            myeloid differentiation 88 (MYD88) to induce TReg cell               immune system55. Furthermore, a recent study showed
sclerosis.                          differentiation45. Indeed, a mutant strain of B. fragilis            that microbial modulation of IgA homeostasis is in part
                                    lacking PSA fails to initiate differentiation of TReg cells,         dependent on the host protein programmed cell death 1
T follicular helper cells
                                    whereas purified PSA has the same effect as the wild-type            (PD1), which is expressed by T follicular helper cells in
A T cell subtype that resides in
the B cell follicles of secondary   bacterium29.                                                         the germinal centre56. PD1‑deficient mice harbour an
lymphoid organs and                     Bacteria from the Gram-positive class Clostridia have            altered IgA repertoire owing to changes in B cell matura-
expresses the B cell homing         similar effects on the host immune system. A mixture of              tion, leading to modified IgA specificity. This altered IgA
receptor CXC-chemokine              46 Clostridia spp. belonging to clusters IV and XIVa was             repertoire then shifts the normal gut microbiota com-
receptor 5. These T cells
mediate B cell activation and
                                    isolated from mouse faeces, and colonization of germ-                position by reducing the numbers of bacteria from the
trigger the formation of the        free mice with this mixture induced the expansion of                 genera Bifidobacterium and Bacteroides and increasing
germinal centre.                    TReg cells in the mucosal lamina propria and thereby                 the number from the family Enterobacteriaceae56.


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                                        In addition to IgA, the gut microbiota regulates the             D. melano­gaster insulin signalling involving phospho-
                                    production of AMPs. These molecules are produced by                  inositide 3‑kinase and the forkhead transcription factor
                                    IECs as a consequence of their tight contact with a dense            FOXO by an as-yet-unknown mechanism and thereby
                                    and highly diverse microbial community and include                   tunes the homeostatic programmes in the fly. The squid
                                    defensins, C‑type lectins (such as REG3β and REG3γ),                 Euprymna scolopes has developed a close symbiosis with
                                    ribonucleases (for example, angiopoietin 4 (ANG4))                   the bacterium Vibrio fischeri, in which the bacterium
                                    and S100 proteins (for example, psoriasin (also known                releases a tetrapeptide peptidoglycan monomer that,
                                    as S100A7)), which rapidly kill or inactivate microorgan-            together with the lipid A component of LPS, is sufficient
                                    isms (see REF. 57 for detailed information). Some AMPs,              to drive the development of a light-emitting ‘organ’ in the
                                    such as α‑defensins and β‑defensin 1, are expressed                  squid68. Peptidoglycan signalling through a nuclear pep-
                                    constitutively 58, whereas others, such as ANG4 and                  tidoglycan recogniton protein induces apoptosis, which
                                    REG3γ, are induced following a microbial encounter 59,60,            is an integral part of light organ morphogenesis69. This
                                    either via PRR signalling (through TLRs and NOD-like                 organ camouflages the squid at night, as it resembles a
                                    receptors (NLRs)) or in a PRR-independent manner                     star to predators below; remarkably, the organ is reas-
                                    (for example, by microbially fermented butyrate)61,62.               sembled every night, as the bacteria are expelled every
                                    Furthermore, intestinal lymphocyte-derived IL‑17 and                 morning 70.
                                    IL‑22, which are bacterially modulated (see above),                      In humans and other mammals, studies have shown
                                    induce the production of AMPs by IECs and Paneth                     that the intestinal microbiota has a considerable effect
                                    cells59,63. Induction of AMPs in epithelial cells is likely to       on the development of the gastrointestinal tract (FIG. 3).
Crypts of Lieberkühn                be one important mechanism for preventing breaches                   In newborns, the gastrointestinal tract is structurally
Tubular invaginations of the        of the mucosal barrier and, in particular, protecting the            and functionally immature, and maturation is induced
intestinal epithelium around        stem cell niche in the crypts of Lieberkühn. Furthermore,            by many factors, one them being the gut microbiota71
the villi. The crypt base
contains Paneth cells, which
                                    AMPs not only help to sustain host–microorganism                     (BOX 1). Notably, the changes in microbiota composi-
secrete mainly antimicrobial        segregation, but also affect the microbiota composition.             tion during weaning in both mice and humans coincide
peptides as well as other           Mice that are deficient in MYD88, NOD2 or matrilysin                 with gut maturation, indicating that specific bacteria in
immune factors, and                 (MMP7; a protease involved in the regulation of defen-               the pre- and post-weaning microbiota have differen-
continually dividing stem
                                    sin activity), as well as mice that are transgenic for               tial effects72. The most prominent feature of germ-free
cells that are the source of
all intestinal epithelial cells.    α‑defensin 5, have an altered microbiota owing to shifted            animals is a greatly enlarged caecum73. Furthermore,
                                    AMP production64–66.                                                 the overall intestinal surface area in germ-free mice is
Xenobiotic metabolism                                                                                    reduced compared with that of conventional mice74.
The metabolism of foreign           Regulation of host physiology                                        Germ-free mice are impaired in brush border differ-
compounds that are neither
produced by nor naturally
                                    As mentioned above, the microbiome contains >5 million               entiation75 and have reduced villus thickness76 owing to
found in the host, such as          genes, many of which encode biosynthetic enzymes, pro-               reduced cell regeneration77 and a longer cell cycle time78.
drugs.                              teases and glycosidases, thereby greatly expanding the               The number of serotonin-producing enterochromaffin
                                    host’s own biochemical and metabolic capability 2,3. The             cells is also higher in the gut of germ-free compared
Enterochromaffin cells
                                    effects of the gut microbiota on host metabolism — for               with conventional mice16, but interestingly, germ-free
A subtype of enteroendocrine
cells in the intestinal or          example, through the microbiota metabolizing other-                  mice have lower levels of serotonin79, and this is thought
respiratory epithelium.             wise indigestible polysaccharides, producing essential               to correlate with decreased intestinal peristaltic activ-
Enterochromaffin cells are the      vitamins and carrying out xenobiotic metabolism — have               ity and a prolonged gastrointestinal transit time80,81.
main source of serotonin in the     long been appreciated10. However, the effects on host                Last, microorganisms modulate epithelial permeability
body and are thereby involved
in the regulation of intestinal
                                    physiology exceed these purely biochemical properties.               in the gastrointestinal tract. For example, in mice the
peristalsis and nausea.             In fact, the microbiota also influences a wide range of host         Gram-negative bacterium Bacteroides thetaiotaomicron
                                    processes and characteristics that were thought to depend            increases the resistance of the gut to injury by inducing
Desmosome                           solely on the genetic programme of the host, including               the expression of SPRR2A, which is involved in desmo-
A type of junctional complex
                                    organ development and morphogenesis, cell proliferation,             some maintenance82. Moreover, several Lactobacillus
that is mainly found in epithelia
(specifically, in the lateral       bone mass, adiposity and even behaviour (FIG. 3). Below,             strains rigidify tight junctions between epithelial cells,
plasma membrane of the              we discuss recent advances in our understanding of the               resulting in reduced epithelial permeability 83. PRR sig-
epithelial cell) and mediates       microbial modulation of these physiological properties               nalling was found to be important in this process, as pep-
cell‑to‑cell adhesion to allow      of the superorganism.                                                tidoglycan-induced TLR2 signalling in epithelial cells
cells to withstand shearing
forces.
                                                                                                         improves tight junction function and reduces apoptosis
                                    Development and morphogenesis. Microorganisms                        rates, thus enhancing barrier integrity and facilitating
Tight junctions                     affect not only the development of immune tissues,                   wound repair after injury 84,85.
Junctional complexes that are       but also the development and morphogenesis of other                      Interestingly, recent work has shown that, in addition
present only in vertebrates (the
                                    organs and body structures in a range of species. For                to promoting gastrointestinal tract morphogenesis, the
invertebrate equivalents are
the septate junctions) and are      example, the symbiotic interaction between the fruit                 gut microbiota influences the remodelling of the vascu-
located at the transition of the    fly Drosophila melanogaster and one of its gut bacteria,             lar system76,86 (FIG. 3). Colonization of the gut in germ-
apical and lateral membrane,        Acetobacter pomorum, affects several host physiologi-                free mice causes restructuring of intestinal villi, which
closely connecting two              cal properties, including developmental rate, body size,             shorten and widen to prevent microbial infiltration. This
epithelial cells and thereby
making the epithelium
                                    wing area, metabolism and stem cell activity 67. Acetic              restructuring increases the demand for oxygen in the gut
impermeable to water and            acid produced by the pyrroloquinoline quinone-depend-                epithelium and leads to increased amounts of sprout-
solutes.                            ent alcohol dehydrogenase of A. pomorum triggers                     ing endothelial cells and, consequently, angiogenesis76.


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                                                                   Behaviour                       microbiota are required for regaining tissue homeostasis
                                                                   Synaptic connectivity ↓         following injury in the intestine in mice85. Importantly,
                                                                   Anxiety               ↑         the gut microbiota also has direct effects on tissue
                                                                   Pain perception       ↑         homeo­ tasis, as germ-free mice have reduced epithelial
                                                                                                           s
                                                                                                   cell turnover in the small intestine owing to reduced
                                                                                                   IEC proliferation, reduced crypt-to-tip cellular migra-
Metabolism                                                                                         tion and reduced apoptosis75,88,89 (FIG. 3). As the crypt
Energy expenditure ↓                                                                               contains proliferative IECs and the villus contains differ-
Nutrient accessibility ↑                                           Intestinal function             entiated IECs that are in contact with the gut microbiota,
Short-chain fatty acids ↑                                          GALT maturation           ↑     these observations suggest that epithelial cells along the
Adiposity               ↑                                          Tissue regeneration       ↑     crypt–tip axis differ in their responses to microbial
                                                                   Gut motility              ↑
                                                                                                   contact.
                                                                   Permeability              ↓
                                                                                                       Imbalanced cellular homeostasis can result in
Intestinal vessel                                                                                  the development of cancer, and inflammation has
formation                                                                                          a crucial role in cancer initiation and progression90.
TF glycosylation                                                   Bone homeostasis
                                                                                                   Microorganisms modulate inflammation and thus could
  Thrombin cleavage                                                TH17 cells                ↑
                                                                   TNF in colon and bone     ↑     influence carcinogenesis91. Indeed, an increased bacterial
    PAR1 activation
                                                                   Osteoclastogenesis        ↑     load was detected in colonic biopsies from patients with
     TF phosphorylation
       ANG1 expression                                             Bone mass                 ↓     colorectal cancer or colonic adenomas92. In contrast to
        Vascularization                                                                            the decreased microbial diversity that is associated with
                                                                                                   obesity and inflammatory bowel disease3,93, microbial
                                                                                                   diversity is increased in patients with colorectal adeno-
                                                                                                   mas94. In two mouse models of carcinogenesis, germ-
                                                                                                   free animals were protected from or showed reduced
                                                                                                   cancer development compared with conventionally
Figure 3 | Microbial impact on host physiology.  The gut microbiota has been shown
                                                              Nature Reviews | Microbiology        raised mice95,96. Notably, bacteria are also required for
to affect several aspects of host physiology; arrows represent either stimulatory or
inhibitory effects of the gut microbiota on host physiological processes. The microbiota           the production of secondary bile acids, which have
has been shown to influence intestinal function in the host, promoting gut-associated              carcinogenic effects97. Some microbial species, such as
lymphoid tissue (GALT) maturation, tissue regeneration (in particular of the villi) and gut        B. fragilis, Streptococcus gallolyticus or Fusobacterium
motility, and reducing the permeability of epithelial cells lining the gut, thus promoting         nucleatum, have been associated with cancer develop-
barrier integrity. Similarly, the gut microbiota influences the morphogenesis of the               ment 98–100. This suggests that certain groups of bacteria
vascular system surrounding the gut. This is associated with increased glycosylation of            promote, whereas others protect against, colon cancer.
tissue factor (TF), which leads to cleavage of thrombin, in turn activating proteinase-            Therefore, selective manipulation of the gut microbiota
activated receptor 1 (PAR1). This then phosphorylates TF to promote epithelial                     might provide new avenues to prevent carcinogenesis90.
expression of angiopoietin 1 (ANG1), which promotes increased vascularization.                         In addition to its effect on immune system and gut
Changes in the microbiota composition or a complete lack of a gut microbiota has been
                                                                                                   homeostasis, the gut microbiota affects homeostasis in
shown to affect metabolism, behaviour and tissue homeostasis, suggesting that the
microbiota also regulates these processes. Specifically, the gut microbiota can influence          other tissues, for example by altering bone mineral den-
the host’s nervous system, decreasing synaptic connectivity and promoting anxiety-like             sity in mice (FIG. 3). Bone remodelling occurs through
behaviour and pain perception. In the case of host metabolism, the gut microbiota has              the antagonistic activity of bone-forming osteoblasts
been shown to facilitate energy harvest from the diet, to modulate host metabolism (for            and bone-resorbing osteoclasts101. Bone cells express
example, by decreasing energy expenditure) and to promote host adiposity. Finally, the             receptors for serotonin, which, as mentioned above, is
gut microbiota can influence tissue homeostasis, for example decreasing bone mass by               reduced in germ-free mice, and serotonin signalling
promoting the function of osteoclasts (which cause bone resorption) and increasing                 inhibits bone formation102,103. Furthermore, bone loss is
the numbers of pro-inflammatory T helper 17 (TH17) cells.                                          associated with inflammation, and T cells are responsible
                                                                                                   for bone resorption in autoinflammatory diseases104,105.
                                                                                                   TH17 cells and the pro-inflammatory cytokines TNF and
                              This process is associated with increased glycosylation              IL‑1β all promote bone resorption by inducing osteo-
                              and surface translocation of tissue factor (TF), leading             clastogenesis105–107. Consistent with this, germ-free mice
                              to increased activation of thrombin. In turn, thrombin               have been shown to have a higher bone mineral den-
                              activates proteinase-activated receptor 1 (PAR1), which              sity than conventional mice, highlighting the fact that
                              phosphorylates TF and promotes epithelial expression of              microbial modulation of T cell function (see above),
                              angiopoietin 1, a protein that is required for increased             serotonin levels and cytokine profiles might contribute
                              vascularization76.                                                   to microbial modulation of bone homeostasis11. Taken
                                                                                                   together, these findings suggest that the gut microbiota
                              Tissue and organ homeostasis. Tissue homeostasis                     can be considered an environmental factor that might
                              requires a balance between cell renewal and death, and               contribute to osteoporosis.
                              thus a tightly regulated cell cycle, which is also modified
                              by microorganisms. For example, in D. melanogaster,                  Metabolism and adiposity. The microbiome encodes
                              infection with the pathogenic bacterium Erwinia caroto­              a more versatile metabolome than the host 9. Although
                              vora induces stem cell proliferation and epithelial cell             gut microbial composition differs significantly between
                              renewal87. Similarly, TLR signals derived from the gut               individuals, a core microbiome can be identified,


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                                                    © 2013 Macmillan Publishers Limited. All rights reserved
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                                  indicating the requirement for stable functional meta-               modulates mate choice in D. melanogaster 123; larval
                                  bolic interactions with the host 108. Studies have sug-              settle­ment of the marine tubeworm Hydroides elegans is
                                  gested that there are differences in the gut microbiota              regulated by the biofilm bacterium Pseudoalteromonas
                                  composition between obese and non-obese individuals,                 luteoviolacea 124; and the composition of the human
                                  although results about the gut microbiota composition                skin microbiota influences attraction for mosquitoes125,
                                  in obese individuals have been conflicting 109. Compared             with potential consequences for disease spread. In mice,
                                  with the gut microbiome of non-obese mice, that of                   the gut microbiota modulates the levels of several
                                  obese mice is enriched in genes encoding carbohydrate                signalling molecules, such as brain-derived neuro-
                                  metabolism enzymes and was demonstrated to have                      trophic factor and noradrenaline, in different areas of
                                  a greater capacity to extract energy from the diet and               the brain126. Germ-free mice display an altered stress
                                  to generate short-chain fatty acids110 (FIG. 3). Moreover,           response, dysregulation of the hypothalamus–pituitary–­
                                  obese humans harbour an altered microbiota with                      adrenal gland axis and decreased inflammatory pain
                                  reduced diversity 93,108,111, but the functional impact of           perception127,128.
                                  this reduced diversity on the development of obesity is                  To date, the best studied microbial effects are the
                                  not yet clear. A recent report demonstrated that the gut             effects on anxiety-like behaviour. Dysbiosis, as a result
                                  microbiome is altered in Chinese individuals with type 2             of either pathogenic infection or antibiotic treatment,
                                  diabetes112. Strikingly, the composition of the gut micro-           increases anxiety-like behaviour in conventionally
                                  biota was able to predict type 2 diabetes in a second,               raised mice129,130, whereas germ-free mice show little
                                  smaller cohort.                                                      anxiety-like behaviour 131,132. The neurological defects in
                                      A link between the gut microbiota and metabolism                 germ-free mice can be resolved only by colonization of
                                  has also been demonstrated in studies using germ-free                neonates, indicating that there is a critical time window
                                  mice. These mice have reduced adiposity and require                  in which microbially induced maturation of the nervous
                                  a higher caloric intake to achieve the same weight as                system occurs128,131. Notably, compared with the striatum
                                  conventionally raised mice113. This has in part been attrib-         of conventionally raised mice, the striatum of germ-free
                                  uted to reduced energy extraction from a carbohydrate-               mice has higher levels of the synaptic proteins synap-
                                  rich diet in germ-free mice113. However, these mice are              tophysin and PSD95 (also known as DLG4), which are
                                  also resistant to diet-induced obesity when fed a fat- and           both involved in synaptogenesis, indicating that the gut
                                  sucrose-rich ‘Western’ diet containing almost no com-                microbiota might affect synaptic connectivity 131,133.
                                  plex carbohydrates114. Thus, the gut microbiota is likely to             For further discussion of microbial effects on the
                                  directly modulate host metabolism (FIG. 3). For example,             development of the nervous system and behaviour,
                                  compared with in conventionally raised mice, the small               readers are referred to a recent review 134.
                                  intestine of germ-free mice has a higher expression of
                                  angiopoietin-like protein 4 (ANGPTL4; also known as                  Conclusion
                                  fasting-induced adipose factor), which promotes fatty                Research over the past decade has accumulated a large
                                  acid oxidation in skeletal muscle114. Furthermore, the               body of evidence linking alterations in the gut microbial
                                  gut microbiota might also contribute to increased adi-               composition to several diseases, such as inflammatory
                                  posity and impaired glucose metabolism by stimulating                bowel disease, asthma, arthritis, obesity and cardiovascu-
                                  inflammation and macrophage accumulation in adipose                  lar disease. Furthermore, it is now clear that the normal
                                  tissue115. Indeed, LPS from Gram-negative bacteria                   intestinal microbiota also influences numerous physio-
                                  promotes hepatic insulin resistance116.                              logical aspects in the healthy host, including organ mor-
                                      In addition to obesity, an altered gut microbiome                phogenesis, immune system and gastrointestinal tract
                                  was recently associated with symptomatic atheroscle-                 development and maturation, intestinal vascularization,
                                  rosis117. The microbiomes of patients who have had a                 tissue regeneration, carcinogenesis, bone homeostasis,
                                  stroke were shown to have lower levels of carotene-                  metabolism and behaviour.
                                  and lycopene-producing enzymes and higher levels of                      An important insight that has come from these stud-
                                  peptidoglycan-producing enzymes than the microbiomes                 ies it that the timing of colonization of germ-free mice
                                  of individuals who have not had a stroke, suggesting                 seems to be crucial if these mice are to recapitulate the
                                  that patients who have had a stroke have a more inflam-              phenotypes of conventionally raised mice. Whereas col-
                                  matory gut milieu. Furthermore, cardiovascular disease               onization of adult germ-free mice restores adiposity to
                                  in humans is associated with altered microbial metabo-               normal levels113, colonization before weaning is required
                                  lism of dietary choline118. For further details about how            to normalize behaviour and protect against the iNKT cell
                                  host–microorganism interactions can programme host                   accumulation that is associated with asthma and inflam-
                                  metabolism, readers are referred to a recent review 119.             matory bowel disease41,131. Such early microbial coloniza-
                                                                                                       tion might have an epigenetic effect on the host through
                                  Effects on the brain and behaviour. Animal behaviour and             early-life imprinting, but it remains to be demonstrated
                                  social context have been shown to shape the microbiota               how the gut microbiota achieves this. It is possible that
Dysbiosis                         composition in several species, including bumble­bees120,            in Tlr2‑deficient mice, the altered microbiota contrib-
An imbalance in the structural    the squid E. scolopes 121 and chimpanzees122. In turn, the           utes to altered DNA methylation patterns in cells of the
and/or functional configuration
of the microbiota, leading to a
                                  impact of the microbiota reaches far outside the gastro­             colonic mucosa135. The impact of the gut microbiota as
disruption of host–               intestinal tract, also affecting behaviour (FIG. 3). For             a modulator of methylation in other organs remains to
microorganism homeostasis.        example, the gut bacterium Lactobacillus plantarum                   be identified.


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REVIEWS

Gnotobiotic                              Analyses of biopsy samples obtained from gnoto­                              challenge when developing diagnostic markers based
Pertaining to an organism:            biotic mouse models or from mice treated with antibiot-                         on the gut microbiota. Although comparisons of gnoto­
associated with a defined             ics have been extensively used to elucidate how the gut                         biotic and conventionally raised animals are useful for
microbiota. For example,              microbiota modulates metabolic interactions and gene                            identifying important physiological functions that are
laboratory mice can be reared
under sterile (germ-free)
                                      expression in different tissues. However, further studies                       modulated by the gut microbiota, such comparisons
conditions or colonized with a        are required to expand our currently limited knowledge                          cannot be automatically extrapolated to humans, and it
specific collection of                and to establish how the gut microbiota regulates the                           remains unclear whether an altered microbiota associ-
microorganisms. From the              functions of distinct cell populations in the gut.                              ated with a disease in humans is causing, contributing
Greek gnosis (known or
                                          It is important to stress that findings obtained from                       to or merely a consequence of the disease state.
knowledge) and bios (life),
                                      the study of animal models remain to be translated to                              Immense progress has been made not only in identi-
                                      diagnostic, prophylactic or therapeutic treatments for                          fying, isolating and culturing members of the gut micro-
                                      humans. One potential caveat is that microbiota mem-                            biota, but also in the development of genetic tools, such
                                      bers differ not only among host species but also between                        as whole-genome sequencing, and in the availability of
                                      individual host organisms136. For example, the potent                           novel genetic models to dissect the interplay between
                                      immune system-modulating SFB are found only in mice                             the microbiome, host genetics and host physiology.
                                      and have not yet been detected in humans. Diet is also                          Combining these tools for further studies in the upcom-
                                      one important factor modulating the composition of the                          ing years will greatly deepen our understanding of the
                                      gut microbial ecosystem136. Thus, variation in dietary                          molecular targets in the homeostatic interaction between
                                      habits among humans might contribute to the large                               the gut microbiota and the host, and thereby promises to
                                      inter-individual differences in the relative abundances                         reveal new ways to treat chronic inflammatory diseases
                                      of given microorganisms. This might constitute a major                          and maintain health.

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236 | APRIL 2013 | VOLUME 11	                                                                                                                           www.nature.com/reviews/micro

                                                                  © 2013 Macmillan Publishers Limited. All rights reserved
The gut microbiota — masters of host development and physiology
The gut microbiota — masters of host development and physiology

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The gut microbiota — masters of host development and physiology

  • 1. REVIEWS The gut microbiota — masters of host development and physiology Felix Sommer1,2 and Fredrik Bäckhed1,2,3 Abstract | Establishing and maintaining beneficial interactions between the host and its associated microbiota are key requirements for host health. Although the gut microbiota has previously been studied in the context of inflammatory diseases, it has recently become clear that this microbial community has a beneficial role during normal homeostasis, modulating the host’s immune system as well as influencing host development and physiology, including organ development and morphogenesis, and host metabolism. The underlying molecular mechanisms of host–microorganism interactions remain largely unknown, but recent studies have begun to identify the key signalling pathways of the cross-species homeostatic regulation between the gut microbiota and its host. Microbiota All higher animals are associated with a diverse micro- gram, the ileum 107 cells per gram and the colon up to The sum of all microorganisms bial community that is composed mainly of bacteria 1012 cells per gram) and along the tissue–lumen axis (including bacteria, archaea, but also includes archea, viruses, fungi and protozoa. (with few bacteria adhering to the tissue or mucus but eukaryotes and viruses) that Microorganisms cover essentially all host mucosal sur- a large number being present in the lumen)4. Second, reside in and/or on a host or a specified part of a host (such faces, but most reside within the gastrointestinal tract. bacterial diversity increases in the same axes and manner as the gastrointestinal tract). Studies had traditionally focused on examining the as microbial density 4. Many bacterial species are present role of the microbiota during human disease, for exam- in the lumen, whereas fewer, but well-adapted species, 1 Wallenberg Laboratory ple in inflammatory diseases such as colitis. However, including several proteobacteria and Akkermansia for Cardiovascular and in the past decade, the field of microbiota research has muciniphila, adhere and reside within the mucus layer Metabolic Research, exploded, resulting in the publication of a plethora of close to the tissue5,6. Colonization of the host begins dur- Sahlgrenska University reports that describe both the individual members of our ing birth, and the composition of the microbiota changes Hospital, Department of Molecular and Clinical intestinal microbiota and their wide-ranging impact on throughout host development (BOX 1). Medicine, University host physiology. Thus, the traditional anthropocentric In the adult intestine, a total of about 10 14 bacte- of Gothenburg. view of the gut microbiota as pathogenic and solely rial cells are present, which is ten times the number of 2 Sahlgrenska Center for an immunological threat has been substituted with human cells in the body 7. Their combined genomes Cardiovascular and Metabolic an appreciation of its mainly beneficial influence on (known as the microbiome) contain more than 5 mil- Research, Department of Molecular and Clinical human health. lion genes, thus outnumbering the host’s genetic poten- Medicine, University of The ‘normal’ gut microbiota is dominated by anaer- tial by two orders of magnitude2,8. This large arsenal of Gothenburg, SE‑413 45 obic bacteria, which outnumber aerobic and faculta- gene products provides a diverse range of biochemical Gothenburg, Sweden. tive anaerobic bacteria by 100- to 1,000‑fold1. In total, and metabolic activities to complement host physiol- 3 Novo Nordisk Foundation Center for Basic Metabolic the intestinal microbiota consists of approximately ogy. In fact, the metabolic capacity of the gut microbiota Research, Section for 500–1,000 species that, interestingly, belong to only equals that of the liver, and the intestinal microbiota can Metabolic Receptology and a few of the known bacterial phyla2,3. By far the most therefore be considered as an additional organ9. These Enteroendocrinology, abundant phyla in the human gut are Firmicutes and bacteria are essential for several aspects of host biology. Faculty of Health Sciences, Bacteriodetes, but other species present are mem- For example, they facilitate the metabolism of otherwise University of Copenhagen, Copenhagen DK‑2200, bers of the phyla Proteobacteria, Verrumicrobia, indigestible polysaccharides and produce essential vita- Denmark. Actinobacteria, Fusobacteria and Cyanobacteria2,3. Two mins; they are required for the development and differ­ Correspondence to F.B.  gradients of microbial distribution can be found in the entiation of the host’s intestinal epithelium and immune e‑mail: gastrointestinal tract. First, microbial density increases system; they confer protection against invasion by Fredrik.Backhed@wlab.gu.se doi:10.1038/nrmicro2974 both from the proximal to the distal gut (the stomach opportunistic pathogens10; and they have a key role in Published online contains 101 microbial cells per gram of content, the maintaining tissue homeostasis. Recent studies have 25 February 2013 duodenum 103 cells per gram, the jejunum 104 cells per also revealed that the human microbiota influences NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | APRIL 2013 | 227 © 2013 Macmillan Publishers Limited. All rights reserved
  • 2. REVIEWS Box 1 | Colonization of the host are pathogens and thus the cause of infectious diseases. The realization that we live in a microbially dominated Human babies are colonized during passage through the birth canal by environmental world and in fact benefit greatly from our microbiota microorganisms (for example, from the mother’s vagina or skin) and during breast has led to a paradigm shift in immunology. Thus, the feeding by microorganisms present in the milk137. Owing to the highly oxidative definition of self in the superorganism theory has been environment in the gastrointestinal tract of the newborn, primary colonizers are extended to incorporate the constituents of both our facultative anaerobic bacteria such as proteobacteria, which are thought to adjust the environmental conditions by decreasing the oxygen concentration to allow successive own body and our microbiota12. It is also now widely colonization by anaerobic microorganisms such as members of the genus Bacteroides accepted that the host’s mucosal immune system is char- and members of the phyla Actinobacteria and Firmicutes. During the first year of life, acterized by tolerance to microorganisms rather than the intestinal microbiota composition is simple and fluctuates widely between responsiveness13. Furthermore, it has even been specu- individuals and over time. Microbial signatures stabilize and start to resemble the ‘adult lated that the highly sophisticated adaptive immune state’ when the infant reaches 1–2 years of age4. system of jawed vertebrates evolved to keep control of Interestingly, conflicting evidence has been published concerning the driving force the mutualistic or beneficial symbiosis with our complex for microbial transmission. In early studies of twins, the faecal microbial compositions in microbial ecosystem14. the mother and her children were similar, indicating a mainly maternal transmission108,111. The intestine, one essential organ in which the However, in a more recent and extensive study, the same research group found that the mucosal immune system operates, has to accomplish faecal microbiota of children was no more similar to that of their mothers than to that of their biological fathers, and genetically unrelated but co‑habiting mothers and fathers two apparently confounding tasks. First, it needs to were significantly more microbially similar to one another than to members of different facilitate nutrient absorption; thus, the total surface area families138. This indicates that, as well as genetics and kinship, environmental factors of the gastrointestinal tract amounts to about 200 m2 in have a considerable effect on the microbial composition of the infant. humans15. Second, it needs to be resistant to infection and inhibit microbial translocation across the tissue bar- rier. Bacterial densities in the gut are the highest known the development and homeostasis of other host tissues, in any habitat to date and reach up to 1012 cells per including the bone11. gram in the lower intestine16. This highly dense micro- The microbiota also benefits from this mutualistic bial community and the host intestinal epithelial cell association, as the mammalian intestine is a nutrient-rich (IEC) lining are separated by only a few micro­ etres of m environment that is maintained at a constant tempera- mucus in the small intestine and up to several hundred ture. However, it is also a dynamic habitat that undergoes micrometers in the colon, depending on the location17. constant and rapid changes in its physiological para­ Because of this unique nature of the intestinal tract, its Mutualistic meters owing to variations in, for example, host diet, mucosal immune system needs to fulfil several special Pertaining to a relationship lifestyle, hygiene or use of antibiotics, all of which affect requirements. It has to be non-responsive to or tolerant between two organisms: beneficial to both organisms. gut microbial composition (FIG. 1). Thus, unlike the host towards the huge number of mutualistic micro­organisms The term originates from the genome, the microbiome can change rapidly as a result of that reside in the intestinal lumen. At the same time, it is Latin word mutuus (lent, modifications in either the composition of the microbial thought that the mucosal immune system has to assure borrowed or mutual). community or individual microbial genomes, resulting a beneficial microbiota composition by keeping patho- in modified transcriptomic, proteomic and metabolic bionts in check, restricting microbial overgrowth and Superorganism A term that extends the profiles. Accordingly, the establishment and preserva- reacting to penetrating microorganisms that breach the classical biological definition tion of beneficial interactions between the host and its intestinal chemical and physical barriers (such as secreted of an organism (a living system associated intestinal microbiota are key requirements soluble immunoglobulin A (IgA), antimicrobial peptides capable of autonomous for health. (AMPs), the mucus layer and the tightly interconnected metabolism and reproduction) by including the many The dynamic fluctuations in the microbiota com- IEC lining). In turn, the intestinal microbiota has a key microorganisms that live in and bined with the vast numbers of bacterial cells and their role in directing several aspects in the development and on that host organism, thus close proximity to the epithelial tissue represent a mas- regulation of the host’s immune tissues, immune cell yielding a superior degree of sive challenge to host immunity, as microbial growth populations and immune mediators. complexity. The term originates has to be restricted to ensure a beneficial homeostasis. from the Latin supra (above) and the Greek organon (organ, Furthermore, activation of the host immune system Mucus layer properties depend on intestinal bacteria. instrument, tool). has to be controlled to circumvent the detrimental The intestinal mucus layer covers the epithelial cell effects of chronic inflammation, so the interaction of lining and functions as a lubricant, facilitating gastro- Symbiosis the gut microbiota with the host has to be tightly regu- intestinal transport, and as a protective layer against Any close physical association between two organisms, lated. In this Review, we discuss recent insights into the bacterial invasion, owing to its physical properties 18. usually from different species. impact of the normal microbiota on the development The colonic mucus layer is in fact composed of two This includes mutualism, and homeostasis of the immune system and other tis- layers17. Both the inner and outer mucus layers are commensalism and parasitism. sues and organs, as well as on host physiology. We also secreted by goblet cells and are mainly made up of gel- The term originates from the highlight recent advances in deciphering the under­ forming highly glyco­ ylated proteins termed mucins18. s Greek words syn (together) and bio (life). lying molecular mechanisms of host–microorganism Mucin 2 (MUC2) is the main mucin in the small and interactions. large intestines of both mice and humans18. The entire Pathobionts mucus layer represents a selective microbial habitat Normally harmless Tailoring immune development owing to microbial adhesion via lectins and glycosidases microorganism that can become pathogens under Immunology was originally based on the concept of that are expressed by only specific bacteria, and it also certain environmental ‘self ’ versus ‘non-self ’ discrimination, with the assump- serves as nutrient source19,20. However, bacteria are found conditions. tion that, because they are non-self, all micro­organisms only in the outer layer 17, probably owing to the specific 228 | APRIL 2013 | VOLUME 11 www.nature.com/reviews/micro © 2013 Macmillan Publishers Limited. All rights reserved
  • 3. REVIEWS Antibiotics Lifestyle Diet Hygiene Hyperimmunity Immunodeficiency IL-6 Altered intestinal microbiota NOD2 IL-12 IL-10 TNF Chronic Metabolic inflammation dysfunction Figure 1 | Factors shaping intestinal microbial composition and effects of dysbiosis on host health.  The composition of the gut microbiota is influenced by various environmental factors, including the use of antibiotics, lifestyle, diet and hygiene preferences. The host’s genetic disposition also has a role: hyperimmunity (owing Nature Reviews | Microbiology to over-representation of pro-inflammatory mediators such as interleukin‑6 (IL‑6), IL‑12 or tumour necrosis factor (TNF)) or immunodeficiency (owing to mutations in regulatory immune proteins such as NOD2 (nucleotide-binding oligomerization domain protein 2) or IL‑10) can influence the gut microbiota composition. In turn, dysbiosis affects levels of immune mediators and induces both chronic inflammation and metabolic dysfunction. structure of the mucus layer as a whole, which is formed mislocalization of the same microbiota or merely on of interconnected sheets that create pores smaller than a increased bacterial load. bacterial cell and thus inhibit penetration21. Comparisons of germ-free and conventionally raised Microorganisms induce the development of lymphoid animals revealed that microorganisms have major effects structures. The lymphatic system consists of a network on mucus thickness and composition; compared with of lymphatic vessels connecting the primary and second- conventionally raised animals, germ-free animals have ary lymphoid organs. The main functions of this sys- fewer goblet cells, a thinner mucus layer and also a tem are the recirculation of interstitial fluid and blood higher percentage of neutral mucins in the colon 22. as well as the transport of lymphocytes (such as B cells Stimulation with bacterial products such as lipopolysac- and T cells) (BOX 2) and antigen-presenting cells to lymph charide (LPS) and peptidoglycan is sufficient to estab- nodes. Lymphoid tissue is classified as primary (thy- lish conventional mucus properties in germ-free mice23, mus and bone marrow) and secondary (lymph nodes, but the underlying mechanisms for how the gut micro­ Peyer’s patches, tonsils, spleen and lymphoid follicles). biota modulates goblet cells and mucus layer properties Lymphocytes are generated in primary lymphoid tissues remain largely elusive. and are then transported to secondary lymphoid tissues, Notably, Muc2‑deficient mice or those with aberrant where the mature lymphocytes are exposed to antigens mucin glycosylation profiles (owing to a lack of specific by antigen-presenting cells and are thus activated to glycosyl transferases) show bacterial overgrowth and initiate an adaptive immune response. The cellular either develop spontaneous colitis or are more suscep- interactions that occur during lymphoid tissue develop- tible to chemical induction of colitis, an effect that can ment and maturation are similar for both primary and be ameliorated by treatment with antibiotics23–25. This secondary lymphoid organs, although the molecular demonstrates the importance of the mucus layer for frameworks differ a little (for details see REF. 26). homeostasis in the gut and also highlights the recip- In addition to host genetics, several environmen- rocal interaction between the mucus layer and the gut tal factors, including contact with microorganisms, microbiota. It remains to be clarified whether disease influence both the development and maturation of the onset in these mouse strains depends on a selectively immune system. The development of secondary gut- altered and thus more colitogenic microbiota, on associated lymphoid tissue (GALT), such as Peyer’s NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | APRIL 2013 | 229 © 2013 Macmillan Publishers Limited. All rights reserved
  • 4. REVIEWS Box 2 | Lymphocyte subtypes induce the differentiation of stromal organizer cells to express several cytokines and adhesion molecules that All lymphocytes differentiate and mature in primary lymphoid organs (the thymus and attract further immune cells, causing GALT forma- bone marrow). Mature naive lymphocytes migrate to secondary lymphoid tissues, tion26. Maturation of these tissues, including an increase where they become activated by antigen-presenting cells such as dendritic cells. in tissue size and the development of germinal centres Gut-associated secondary lymphoid tissues include Peyer’s patches, mesenteric lymph (sites of B cell proliferation, differentiation and somatic nodes and lymphoid follicles139. Here, we list and describe the lymphocytes that are hypermutation in lymph nodes), depends on postnatal known to be modulated by the gut microbiota43,140,141. microbial colonization28 (FIG. 2). Consequently, Peyer’s Lymphoid tissue inducer cells patches, mesenteric lymph nodes and splenic white pulp (LTi cells). A unique T cell subpopulation that is characterized by the expression of RORγt, CD4 and interleukin‑7 receptor‑α and the absence of CD3, B220 (an isoform are underdeveloped in germ-free mice29. of CD45) and CD11c (also known as integrin αX). Their function is to recruit B cells and Furthermore, in parallel with microbial coloniza- T cells and thereby promote the formation of secondary lymphoid tissues. tion, clusters of LTi-like cells termed cryptopatches Natural killer cells form at birth in the connective tissue between intestinal (NK cells). Lymphocytes that recognize the abnormal antigen signatures of infected or crypts, known as the lamina propria30. Cryptopatches tumour cells, which NK cells kill by lysis or apoptosis. NK cells resemble cytotoxic T cells recruit B cells and develop into isolated lymphoid fol- in function but belong to the innate immune system. They express various NK cell licles (ILFs), a type of lymphoid tissue that is structurally receptors, including NKp46 (in mice; NKp44 in humans) and NKG2D. They can activate similar to Peyer’s patches and serves as an inductive site B cells and T cells and thereby stimulate an adaptive immune response. for intestinal immune reactions31,32. This process also Natural killer T cells depends on the gut microbiota, as ILFs fail to develop (NKT cells). These cells have properties of both T cells and NK cells, as they co-express fully in germ-free mice33. ILF formation can be induced NK cell markers with a T cell receptor. NKT cells mainly recognize lipids and glycolipids by exposing germ-free mice to purified peptidoglycan presented by antigen-presenting cells via CD1d. Following activation, NKT cells from Gram-negative bacteria, indicating that this pro- produce pro-inflammatory cytokines such as tumour necrosis factor (TNF) and cess is driven solely by a specific microbial pattern34. interleukin‑17 (IL‑17). Invariant NKT (iNKT) cells are a specific subpopulation expressing Stromal and epithelial cells recognize the peptido­ an invariant T cell receptor. glycan of resident microorganisms mainly via signalling T helper 1 cells through the pattern recognition receptor (PRR) NOD1 (TH1 cells). A subset of TH lymphocytes that is characterized by the expression of (nucleotide-binding oligomerization domain contain- interferon‑γ and transforming growth factor-β (TGFβ). TH1 cell differentiation is induced by contact with activated macrophages or NK cells. ing 1) but also partially through another family of PRRs, the Toll-like receptors (TLRs). Activation of NOD1 T helper 2 cells by the gut microbiota causes increased expression of (TH2 cells). A subset of TH lymphocytes that is characterized by the expression of the cytokines IL‑4, IL‑5 and IL‑13. TH2 cell differentiation is induced in response to, for CC-chemokine ligand 20 (CCL20) and presumably also example, allergens and extracellular microorganisms. of β‑defensin 3, both of which activate ILF formation by binding to CC-chemokine receptor 6 (CCR6) on LTi T helper 17 cells (TH17 cells). A subset of TH cells that is characterized by the expression of IL‑17, which cells34. stimulates stromal cells to express the pro-inflammatory cytokines IL‑6 and IL‑8, thereby attracting neutrophils and promoting inflammation to clear out invading The gut microbiota modulates immune cell differentia- microorganisms. tion. In addition to regulating the development of lym- Regulatory T cells phoid structures, the gut microbiota has been shown (TReg cells). A T cell subpopulation that is characterized by the expression of CD4, CD25 to modulate the differentiation of immune cell subsets and FOXP3 and the production of the anti-inflammatory cytokines TGFβ and IL‑10. (BOX 2) and, therefore, maintain homeostatic interactions These cells can be subdivided into natural TReg cells, which differentiate from CD4+ between the host and the gut microbiota. T cells in the thymus, and inducible TReg cells, which arise from naive T cells in secondary After birth, LTi-like cells that express nuclear RORγt lymphoid tissues. Both cell types function to suppress immune activation and prevent but lack NKp46 (in mice; also known as NCR1) or self-reactivity, thereby reducing the risk of autoimmune disease. NKp44 (in humans; also known as NCR2) markers accu- Type 1 regulatory T cells mulate in both the mouse and human GALT and lamina (TR1 cells). These CD4+CD25+FOXP3− T cells are functionally equivalent to the propria35–37. Interestingly, the RORγt+NKp46− LTi-like IL‑10‑producing TReg cells. They respond to microorganisms and regulate intestinal cells can differentiate into RORγt+NKp46+ natural killer tolerance through the secretion of IL‑10. (NK)‑like cells, which differ from regular NK cells (BOX 2) B cells in that they have intermediate expression of NK1.1 (also Lymphocytes that are activated when the unique B cell receptor binds its specific known as KLRB1C) and do not produce interleukin‑1β antigen and that then mediate humoral immunity through the production of antibodies. (IL‑1β) or kill tumour cells36,37. This differentiation B cells are also involved in lymphoid tissue organization. requires both IL‑23, which is produced by activated myeloid cells and epithelial or endothelial cells, and the presence of the intestinal microbiota, as germ-free mice patches and mesenteric lymph nodes, is initiated pre- have fewer RORγt+NKp46+ NK‑like cells than conven- natally in the sterile environment of the fetus through tionally raised mice37. These cells produce IL‑22, which in Somatic hypermutation induction by lymphoid tissue inducer (LTi) cells 27. mice promotes the integrity of the intestinal barrier and A programmed process of Briefly, mesenchymal cells are induced by retinoic acid reduces bacterial infiltration by inducing epithelial repair mutation affecting the variable regions of immunoglobulin to produce CXC-chemokine ligand 13 (CXCL13), which via signal transducer and activator of transcription 3 genes during affinity recruits LTi precursor cells and stimulates their cluster- (STAT3) signalling and the production of antimicrobial maturation of B cell receptors. ing, leading to their maturation into LTi cells. These then proteins38. Thus, the normal gut microbiota promotes 230 | APRIL 2013 | VOLUME 11 www.nature.com/reviews/micro © 2013 Macmillan Publishers Limited. All rights reserved
  • 5. REVIEWS a Germ-free mice Conventionally raised mice ↓ Mucus thickness Altered mucus properties Microbiota Mucus Intestinal epithelial cell Goblet cell Villus IgA ↓ Vessel density AMP Intestinal crypt Blood vessel b c PSA TReg cell B. fragilis Peyer’s patch Immature Peyer’s patch Immature SFB TH17 cell MLN ↓ AMP and MLN IgA production T cell B cell IgA-producing Dendritic cell plasma cell Figure 2 | Microbiota-induced maturation of the gastrointestinal tract.  The microbiota promotes substantial changes in gut morphology, including villus architecture, crypt depth, stem cell proliferation, bloodReviews | Microbiology Nature vessel density, mucus layer properties and maturation of mucosa-associated lymphoid tissues. a | In germ-free mice, the villi in the distal small intestine are longer and thinner and have a less complex vascular network than the villi of conventionally raised animals. In the absence of bacteria, intestinal crypts are less deep and contain fewer proliferating stem cells. Furthermore, germ-free animals show reduced mucus thickness and altered mucus properties. b | Moreover, very few isolated lymphoid follicles, immature Peyer’s patches and immature mesenteric lymph nodes (MLNs) are present under germ-free conditions, and levels of both immunoglobulin A (IgA) and antimicrobial peptides (AMPs) are lower than in conventionally raised animals. c | In conventionally raised mice, polysaccharide A (PSA) of Bacteroides fragilis is known to induce the expansion of CD4+CD25+FOXP3+ regulatory T (TReg) cells, which have an anti-inflammatory effect and dampen immune responses. By contrast, segmented filamentous bacteria (SFB) have been shown to induce the expansion of T helper 17 (TH17) cells, which are pro-inflammatory. NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | APRIL 2013 | 231 © 2013 Macmillan Publishers Limited. All rights reserved
  • 6. REVIEWS intestinal barrier function by modulating mucosal increased levels of the immunosuppressive cytokine homeostasis, in part by promoting the differentiation of IL‑10 (REF. 47). Notably, compared with non-colonized RORγt+NKp46+ NK‑like cells. germ-free mice, the Clostridia species-colonized mice The intestinal microbiota also modulates the abun- were more resistant to chemically induced disruption dance of invariant NK T cells (iNKT cells), a unique of the gut epithelium and displayed attenuated levels of T cell subset that expresses an invariant T cell receptor antigen-induced serum IgE. Furthermore, a similar TReg α-chain. These cells promote inflammation, as follow- cell response was observed when germ-free mice were ing activation they secrete pro-inflammatory T helper 1 colonized with altered Schaedler flora, a cocktail of eight (TH1)- and TH2‑type chemokines and cytokines, including defined bacteria including three Clostridia species48. interferon-γ, IL‑2, IL‑4, IL‑13, IL‑17A, IL‑21 and tumour However, the specific species in this community that are necrosis factor (TNF)39,40. In contrast to the NK‑like cells functionally responsible and the underlying molecular described above, there are more iNKT cells in the colon mechanisms of this effect are so far unknown. of germ-free mice than the colon of conventionally raised In contrast to the bacteria mentioned above, mice41, which suggests that the gut microbiota promotes segmented filamentous bacteria (SFB) elicit a pro- homeostasis by decreasing the number of these pro- inflammatory immune response by promoting the dif- inflammatory cells. Importantly, a recent study elegantly ferentiation of TH17 cells and, to a lesser extent, TH1 demonstrated an age dependency of the microbial effects cells49,50. SFB reside in the small intestine of mice and on the iNKT population and revealed that colonization of are in direct contact with epithelial cells, which these neonatal but not adult germ-free mice with conventional bacteria seem to readily invade. Invasion leads to local gut microbiota normalized iNKT cell numbers and pro- actin polymerization in the epithelium at the interac- tected against oxazolone-induced colitis as well as against tion site, and this presumably initiates a signalling event ovalbumin-induced allergic lung inflammation41. that activates the differentiation of TH17 cells. Notably, It has become evident that the gut microbiota shapes mono-association of germ-free mice with SFB is suffi- the T cell landscape not only in the lamina propria but cient to restore susceptibility to TH17 cell-mediated arthri- also systemically and therefore modulates the homeo- tis and experimental autoimmune encephalomyelitis51,52. So stasis of the superorganism42 (FIG. 2). Intestinal mucosal far, however, it is not known whether TH17 cell differ- T cells are important ‘legislators’ of intestinal homeo- entiation is induced by IEC-produced mediators, by stasis because they not only defend against intestinal direct interaction with antigen-presenting cells in the pathogens, but also promote wound healing, barrier lamina propria (dendritic cells or macrophages) or by repair and regeneration as they rapidly accumulate at bacterially secreted signalling molecules (for example, sites of injury and infection43. T cells can be assigned metabolites)15. to subpopulations that drive either a pro-inflammatory immune response (TH1, TH2 and TH17 cells) or an anti- Gut microorganisms tweak the production of immune inflammatory immune response (CD4+CD25+FOXP3+ mediators. It is clear that the gut microbiota regulates regulatory T (TReg) cells or CD4+CD25+FOXP3− type 1 the production of cytokines and chemokines to influ- regulatory T (TR1) cells), depending on the cytokines ence the T cell repertoire of the intestine and surround- that they produce44 (BOX 2). The balance between both ing tissue, but there is evidence that these bacteria also pro- and anti-inflammatory T cell subpopulations deter- modulate the production of other soluble immune mines the overall immune equilibrium. mediators (FIG. 2). IgA is produced by plasma cells (dif- Interestingly, individual members of the gut micro- ferentiated B cells) in the lamina propria and is trans­ biota have been found to drive specific T cell responses cytosed through the intestinal epithelium into the (FIG. 2c). The Gram-negative bacterium Bacteroides fragilis lumen, where it binds microbial antigens and thereby elicits an anti-inflammatory response by inducing the prevents bacterial translocation and infection53. The dif- differentiation of CD4+ T cells into TReg cells locally in ferentiation of B cells into IgA-producing plasma cells is the intestinal lamina propria but also in the circula- induced by sensing of gut microbiota-derived flagellin Experimental autoimmune encephalomyelitis tion45. TReg cells produce IL‑10 and thereby suppress the via TLR5 on lamina propria dendritic cells54. IgA has a An animal model of pro-inflammatory TH17 response46. This skewing event is key role in barrier homeostasis, as IgA-deficient mice T cell-mediated autoimmune mediated by polysaccharide A (PSA) on the outer mem- produce gut microbiota-specific serum IgG antibodies, disease in general and in brane of the bacterium, which is recognized by TLR2 on indicating that there is a breach in the mucosal barrier particular of demyelinating diseases of the central nervous CD4+ T cells and activates a signalling cascade involving of these mice and subsequent induction of the systemic system, such as multiple myeloid differentiation 88 (MYD88) to induce TReg cell immune system55. Furthermore, a recent study showed sclerosis. differentiation45. Indeed, a mutant strain of B. fragilis that microbial modulation of IgA homeostasis is in part lacking PSA fails to initiate differentiation of TReg cells, dependent on the host protein programmed cell death 1 T follicular helper cells whereas purified PSA has the same effect as the wild-type (PD1), which is expressed by T follicular helper cells in A T cell subtype that resides in the B cell follicles of secondary bacterium29. the germinal centre56. PD1‑deficient mice harbour an lymphoid organs and Bacteria from the Gram-positive class Clostridia have altered IgA repertoire owing to changes in B cell matura- expresses the B cell homing similar effects on the host immune system. A mixture of tion, leading to modified IgA specificity. This altered IgA receptor CXC-chemokine 46 Clostridia spp. belonging to clusters IV and XIVa was repertoire then shifts the normal gut microbiota com- receptor 5. These T cells mediate B cell activation and isolated from mouse faeces, and colonization of germ- position by reducing the numbers of bacteria from the trigger the formation of the free mice with this mixture induced the expansion of genera Bifidobacterium and Bacteroides and increasing germinal centre. TReg cells in the mucosal lamina propria and thereby the number from the family Enterobacteriaceae56. 232 | APRIL 2013 | VOLUME 11 www.nature.com/reviews/micro © 2013 Macmillan Publishers Limited. All rights reserved
  • 7. REVIEWS In addition to IgA, the gut microbiota regulates the D. melano­gaster insulin signalling involving phospho- production of AMPs. These molecules are produced by inositide 3‑kinase and the forkhead transcription factor IECs as a consequence of their tight contact with a dense FOXO by an as-yet-unknown mechanism and thereby and highly diverse microbial community and include tunes the homeostatic programmes in the fly. The squid defensins, C‑type lectins (such as REG3β and REG3γ), Euprymna scolopes has developed a close symbiosis with ribonucleases (for example, angiopoietin 4 (ANG4)) the bacterium Vibrio fischeri, in which the bacterium and S100 proteins (for example, psoriasin (also known releases a tetrapeptide peptidoglycan monomer that, as S100A7)), which rapidly kill or inactivate microorgan- together with the lipid A component of LPS, is sufficient isms (see REF. 57 for detailed information). Some AMPs, to drive the development of a light-emitting ‘organ’ in the such as α‑defensins and β‑defensin 1, are expressed squid68. Peptidoglycan signalling through a nuclear pep- constitutively 58, whereas others, such as ANG4 and tidoglycan recogniton protein induces apoptosis, which REG3γ, are induced following a microbial encounter 59,60, is an integral part of light organ morphogenesis69. This either via PRR signalling (through TLRs and NOD-like organ camouflages the squid at night, as it resembles a receptors (NLRs)) or in a PRR-independent manner star to predators below; remarkably, the organ is reas- (for example, by microbially fermented butyrate)61,62. sembled every night, as the bacteria are expelled every Furthermore, intestinal lymphocyte-derived IL‑17 and morning 70. IL‑22, which are bacterially modulated (see above), In humans and other mammals, studies have shown induce the production of AMPs by IECs and Paneth that the intestinal microbiota has a considerable effect cells59,63. Induction of AMPs in epithelial cells is likely to on the development of the gastrointestinal tract (FIG. 3). Crypts of Lieberkühn be one important mechanism for preventing breaches In newborns, the gastrointestinal tract is structurally Tubular invaginations of the of the mucosal barrier and, in particular, protecting the and functionally immature, and maturation is induced intestinal epithelium around stem cell niche in the crypts of Lieberkühn. Furthermore, by many factors, one them being the gut microbiota71 the villi. The crypt base contains Paneth cells, which AMPs not only help to sustain host–microorganism (BOX 1). Notably, the changes in microbiota composi- secrete mainly antimicrobial segregation, but also affect the microbiota composition. tion during weaning in both mice and humans coincide peptides as well as other Mice that are deficient in MYD88, NOD2 or matrilysin with gut maturation, indicating that specific bacteria in immune factors, and (MMP7; a protease involved in the regulation of defen- the pre- and post-weaning microbiota have differen- continually dividing stem sin activity), as well as mice that are transgenic for tial effects72. The most prominent feature of germ-free cells that are the source of all intestinal epithelial cells. α‑defensin 5, have an altered microbiota owing to shifted animals is a greatly enlarged caecum73. Furthermore, AMP production64–66. the overall intestinal surface area in germ-free mice is Xenobiotic metabolism reduced compared with that of conventional mice74. The metabolism of foreign Regulation of host physiology Germ-free mice are impaired in brush border differ- compounds that are neither produced by nor naturally As mentioned above, the microbiome contains >5 million entiation75 and have reduced villus thickness76 owing to found in the host, such as genes, many of which encode biosynthetic enzymes, pro- reduced cell regeneration77 and a longer cell cycle time78. drugs. teases and glycosidases, thereby greatly expanding the The number of serotonin-producing enterochromaffin host’s own biochemical and metabolic capability 2,3. The cells is also higher in the gut of germ-free compared Enterochromaffin cells effects of the gut microbiota on host metabolism — for with conventional mice16, but interestingly, germ-free A subtype of enteroendocrine cells in the intestinal or example, through the microbiota metabolizing other- mice have lower levels of serotonin79, and this is thought respiratory epithelium. wise indigestible polysaccharides, producing essential to correlate with decreased intestinal peristaltic activ- Enterochromaffin cells are the vitamins and carrying out xenobiotic metabolism — have ity and a prolonged gastrointestinal transit time80,81. main source of serotonin in the long been appreciated10. However, the effects on host Last, microorganisms modulate epithelial permeability body and are thereby involved in the regulation of intestinal physiology exceed these purely biochemical properties. in the gastrointestinal tract. For example, in mice the peristalsis and nausea. In fact, the microbiota also influences a wide range of host Gram-negative bacterium Bacteroides thetaiotaomicron processes and characteristics that were thought to depend increases the resistance of the gut to injury by inducing Desmosome solely on the genetic programme of the host, including the expression of SPRR2A, which is involved in desmo- A type of junctional complex organ development and morphogenesis, cell proliferation, some maintenance82. Moreover, several Lactobacillus that is mainly found in epithelia (specifically, in the lateral bone mass, adiposity and even behaviour (FIG. 3). Below, strains rigidify tight junctions between epithelial cells, plasma membrane of the we discuss recent advances in our understanding of the resulting in reduced epithelial permeability 83. PRR sig- epithelial cell) and mediates microbial modulation of these physiological properties nalling was found to be important in this process, as pep- cell‑to‑cell adhesion to allow of the superorganism. tidoglycan-induced TLR2 signalling in epithelial cells cells to withstand shearing forces. improves tight junction function and reduces apoptosis Development and morphogenesis. Microorganisms rates, thus enhancing barrier integrity and facilitating Tight junctions affect not only the development of immune tissues, wound repair after injury 84,85. Junctional complexes that are but also the development and morphogenesis of other Interestingly, recent work has shown that, in addition present only in vertebrates (the organs and body structures in a range of species. For to promoting gastrointestinal tract morphogenesis, the invertebrate equivalents are the septate junctions) and are example, the symbiotic interaction between the fruit gut microbiota influences the remodelling of the vascu- located at the transition of the fly Drosophila melanogaster and one of its gut bacteria, lar system76,86 (FIG. 3). Colonization of the gut in germ- apical and lateral membrane, Acetobacter pomorum, affects several host physiologi- free mice causes restructuring of intestinal villi, which closely connecting two cal properties, including developmental rate, body size, shorten and widen to prevent microbial infiltration. This epithelial cells and thereby making the epithelium wing area, metabolism and stem cell activity 67. Acetic restructuring increases the demand for oxygen in the gut impermeable to water and acid produced by the pyrroloquinoline quinone-depend- epithelium and leads to increased amounts of sprout- solutes. ent alcohol dehydrogenase of A. pomorum triggers ing endothelial cells and, consequently, angiogenesis76. NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | APRIL 2013 | 233 © 2013 Macmillan Publishers Limited. All rights reserved
  • 8. REVIEWS Behaviour microbiota are required for regaining tissue homeostasis Synaptic connectivity ↓ following injury in the intestine in mice85. Importantly, Anxiety ↑ the gut microbiota also has direct effects on tissue Pain perception ↑ homeo­ tasis, as germ-free mice have reduced epithelial s cell turnover in the small intestine owing to reduced IEC proliferation, reduced crypt-to-tip cellular migra- Metabolism tion and reduced apoptosis75,88,89 (FIG. 3). As the crypt Energy expenditure ↓ contains proliferative IECs and the villus contains differ- Nutrient accessibility ↑ Intestinal function entiated IECs that are in contact with the gut microbiota, Short-chain fatty acids ↑ GALT maturation ↑ these observations suggest that epithelial cells along the Adiposity ↑ Tissue regeneration ↑ crypt–tip axis differ in their responses to microbial Gut motility ↑ contact. Permeability ↓ Imbalanced cellular homeostasis can result in Intestinal vessel the development of cancer, and inflammation has formation a crucial role in cancer initiation and progression90. TF glycosylation Bone homeostasis Microorganisms modulate inflammation and thus could Thrombin cleavage TH17 cells ↑ TNF in colon and bone ↑ influence carcinogenesis91. Indeed, an increased bacterial PAR1 activation Osteoclastogenesis ↑ load was detected in colonic biopsies from patients with TF phosphorylation ANG1 expression Bone mass ↓ colorectal cancer or colonic adenomas92. In contrast to Vascularization the decreased microbial diversity that is associated with obesity and inflammatory bowel disease3,93, microbial diversity is increased in patients with colorectal adeno- mas94. In two mouse models of carcinogenesis, germ- free animals were protected from or showed reduced cancer development compared with conventionally Figure 3 | Microbial impact on host physiology.  The gut microbiota has been shown Nature Reviews | Microbiology raised mice95,96. Notably, bacteria are also required for to affect several aspects of host physiology; arrows represent either stimulatory or inhibitory effects of the gut microbiota on host physiological processes. The microbiota the production of secondary bile acids, which have has been shown to influence intestinal function in the host, promoting gut-associated carcinogenic effects97. Some microbial species, such as lymphoid tissue (GALT) maturation, tissue regeneration (in particular of the villi) and gut B. fragilis, Streptococcus gallolyticus or Fusobacterium motility, and reducing the permeability of epithelial cells lining the gut, thus promoting nucleatum, have been associated with cancer develop- barrier integrity. Similarly, the gut microbiota influences the morphogenesis of the ment 98–100. This suggests that certain groups of bacteria vascular system surrounding the gut. This is associated with increased glycosylation of promote, whereas others protect against, colon cancer. tissue factor (TF), which leads to cleavage of thrombin, in turn activating proteinase- Therefore, selective manipulation of the gut microbiota activated receptor 1 (PAR1). This then phosphorylates TF to promote epithelial might provide new avenues to prevent carcinogenesis90. expression of angiopoietin 1 (ANG1), which promotes increased vascularization. In addition to its effect on immune system and gut Changes in the microbiota composition or a complete lack of a gut microbiota has been homeostasis, the gut microbiota affects homeostasis in shown to affect metabolism, behaviour and tissue homeostasis, suggesting that the microbiota also regulates these processes. Specifically, the gut microbiota can influence other tissues, for example by altering bone mineral den- the host’s nervous system, decreasing synaptic connectivity and promoting anxiety-like sity in mice (FIG. 3). Bone remodelling occurs through behaviour and pain perception. In the case of host metabolism, the gut microbiota has the antagonistic activity of bone-forming osteoblasts been shown to facilitate energy harvest from the diet, to modulate host metabolism (for and bone-resorbing osteoclasts101. Bone cells express example, by decreasing energy expenditure) and to promote host adiposity. Finally, the receptors for serotonin, which, as mentioned above, is gut microbiota can influence tissue homeostasis, for example decreasing bone mass by reduced in germ-free mice, and serotonin signalling promoting the function of osteoclasts (which cause bone resorption) and increasing inhibits bone formation102,103. Furthermore, bone loss is the numbers of pro-inflammatory T helper 17 (TH17) cells. associated with inflammation, and T cells are responsible for bone resorption in autoinflammatory diseases104,105. TH17 cells and the pro-inflammatory cytokines TNF and This process is associated with increased glycosylation IL‑1β all promote bone resorption by inducing osteo- and surface translocation of tissue factor (TF), leading clastogenesis105–107. Consistent with this, germ-free mice to increased activation of thrombin. In turn, thrombin have been shown to have a higher bone mineral den- activates proteinase-activated receptor 1 (PAR1), which sity than conventional mice, highlighting the fact that phosphorylates TF and promotes epithelial expression of microbial modulation of T cell function (see above), angiopoietin 1, a protein that is required for increased serotonin levels and cytokine profiles might contribute vascularization76. to microbial modulation of bone homeostasis11. Taken together, these findings suggest that the gut microbiota Tissue and organ homeostasis. Tissue homeostasis can be considered an environmental factor that might requires a balance between cell renewal and death, and contribute to osteoporosis. thus a tightly regulated cell cycle, which is also modified by microorganisms. For example, in D. melanogaster, Metabolism and adiposity. The microbiome encodes infection with the pathogenic bacterium Erwinia caroto­ a more versatile metabolome than the host 9. Although vora induces stem cell proliferation and epithelial cell gut microbial composition differs significantly between renewal87. Similarly, TLR signals derived from the gut individuals, a core microbiome can be identified, 234 | APRIL 2013 | VOLUME 11 www.nature.com/reviews/micro © 2013 Macmillan Publishers Limited. All rights reserved
  • 9. REVIEWS indicating the requirement for stable functional meta- modulates mate choice in D. melanogaster 123; larval bolic interactions with the host 108. Studies have sug- settle­ment of the marine tubeworm Hydroides elegans is gested that there are differences in the gut microbiota regulated by the biofilm bacterium Pseudoalteromonas composition between obese and non-obese individuals, luteoviolacea 124; and the composition of the human although results about the gut microbiota composition skin microbiota influences attraction for mosquitoes125, in obese individuals have been conflicting 109. Compared with potential consequences for disease spread. In mice, with the gut microbiome of non-obese mice, that of the gut microbiota modulates the levels of several obese mice is enriched in genes encoding carbohydrate signalling molecules, such as brain-derived neuro- metabolism enzymes and was demonstrated to have trophic factor and noradrenaline, in different areas of a greater capacity to extract energy from the diet and the brain126. Germ-free mice display an altered stress to generate short-chain fatty acids110 (FIG. 3). Moreover, response, dysregulation of the hypothalamus–pituitary–­ obese humans harbour an altered microbiota with adrenal gland axis and decreased inflammatory pain reduced diversity 93,108,111, but the functional impact of perception127,128. this reduced diversity on the development of obesity is To date, the best studied microbial effects are the not yet clear. A recent report demonstrated that the gut effects on anxiety-like behaviour. Dysbiosis, as a result microbiome is altered in Chinese individuals with type 2 of either pathogenic infection or antibiotic treatment, diabetes112. Strikingly, the composition of the gut micro- increases anxiety-like behaviour in conventionally biota was able to predict type 2 diabetes in a second, raised mice129,130, whereas germ-free mice show little smaller cohort. anxiety-like behaviour 131,132. The neurological defects in A link between the gut microbiota and metabolism germ-free mice can be resolved only by colonization of has also been demonstrated in studies using germ-free neonates, indicating that there is a critical time window mice. These mice have reduced adiposity and require in which microbially induced maturation of the nervous a higher caloric intake to achieve the same weight as system occurs128,131. Notably, compared with the striatum conventionally raised mice113. This has in part been attrib- of conventionally raised mice, the striatum of germ-free uted to reduced energy extraction from a carbohydrate- mice has higher levels of the synaptic proteins synap- rich diet in germ-free mice113. However, these mice are tophysin and PSD95 (also known as DLG4), which are also resistant to diet-induced obesity when fed a fat- and both involved in synaptogenesis, indicating that the gut sucrose-rich ‘Western’ diet containing almost no com- microbiota might affect synaptic connectivity 131,133. plex carbohydrates114. Thus, the gut microbiota is likely to For further discussion of microbial effects on the directly modulate host metabolism (FIG. 3). For example, development of the nervous system and behaviour, compared with in conventionally raised mice, the small readers are referred to a recent review 134. intestine of germ-free mice has a higher expression of angiopoietin-like protein 4 (ANGPTL4; also known as Conclusion fasting-induced adipose factor), which promotes fatty Research over the past decade has accumulated a large acid oxidation in skeletal muscle114. Furthermore, the body of evidence linking alterations in the gut microbial gut microbiota might also contribute to increased adi- composition to several diseases, such as inflammatory posity and impaired glucose metabolism by stimulating bowel disease, asthma, arthritis, obesity and cardiovascu- inflammation and macrophage accumulation in adipose lar disease. Furthermore, it is now clear that the normal tissue115. Indeed, LPS from Gram-negative bacteria intestinal microbiota also influences numerous physio- promotes hepatic insulin resistance116. logical aspects in the healthy host, including organ mor- In addition to obesity, an altered gut microbiome phogenesis, immune system and gastrointestinal tract was recently associated with symptomatic atheroscle- development and maturation, intestinal vascularization, rosis117. The microbiomes of patients who have had a tissue regeneration, carcinogenesis, bone homeostasis, stroke were shown to have lower levels of carotene- metabolism and behaviour. and lycopene-producing enzymes and higher levels of An important insight that has come from these stud- peptidoglycan-producing enzymes than the microbiomes ies it that the timing of colonization of germ-free mice of individuals who have not had a stroke, suggesting seems to be crucial if these mice are to recapitulate the that patients who have had a stroke have a more inflam- phenotypes of conventionally raised mice. Whereas col- matory gut milieu. Furthermore, cardiovascular disease onization of adult germ-free mice restores adiposity to in humans is associated with altered microbial metabo- normal levels113, colonization before weaning is required lism of dietary choline118. For further details about how to normalize behaviour and protect against the iNKT cell host–microorganism interactions can programme host accumulation that is associated with asthma and inflam- metabolism, readers are referred to a recent review 119. matory bowel disease41,131. Such early microbial coloniza- tion might have an epigenetic effect on the host through Effects on the brain and behaviour. Animal behaviour and early-life imprinting, but it remains to be demonstrated social context have been shown to shape the microbiota how the gut microbiota achieves this. It is possible that Dysbiosis composition in several species, including bumble­bees120, in Tlr2‑deficient mice, the altered microbiota contrib- An imbalance in the structural the squid E. scolopes 121 and chimpanzees122. In turn, the utes to altered DNA methylation patterns in cells of the and/or functional configuration of the microbiota, leading to a impact of the microbiota reaches far outside the gastro­ colonic mucosa135. The impact of the gut microbiota as disruption of host– intestinal tract, also affecting behaviour (FIG. 3). For a modulator of methylation in other organs remains to microorganism homeostasis. example, the gut bacterium Lactobacillus plantarum be identified. NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | APRIL 2013 | 235 © 2013 Macmillan Publishers Limited. All rights reserved
  • 10. REVIEWS Gnotobiotic Analyses of biopsy samples obtained from gnoto­ challenge when developing diagnostic markers based Pertaining to an organism: biotic mouse models or from mice treated with antibiot- on the gut microbiota. Although comparisons of gnoto­ associated with a defined ics have been extensively used to elucidate how the gut biotic and conventionally raised animals are useful for microbiota. For example, microbiota modulates metabolic interactions and gene identifying important physiological functions that are laboratory mice can be reared under sterile (germ-free) expression in different tissues. However, further studies modulated by the gut microbiota, such comparisons conditions or colonized with a are required to expand our currently limited knowledge cannot be automatically extrapolated to humans, and it specific collection of and to establish how the gut microbiota regulates the remains unclear whether an altered microbiota associ- microorganisms. From the functions of distinct cell populations in the gut. ated with a disease in humans is causing, contributing Greek gnosis (known or It is important to stress that findings obtained from to or merely a consequence of the disease state. knowledge) and bios (life), the study of animal models remain to be translated to Immense progress has been made not only in identi- diagnostic, prophylactic or therapeutic treatments for fying, isolating and culturing members of the gut micro- humans. One potential caveat is that microbiota mem- biota, but also in the development of genetic tools, such bers differ not only among host species but also between as whole-genome sequencing, and in the availability of individual host organisms136. For example, the potent novel genetic models to dissect the interplay between immune system-modulating SFB are found only in mice the microbiome, host genetics and host physiology. and have not yet been detected in humans. 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