Normal flora or resident microbiota refers to the diverse microbial populations that inhabit human bodies. These microbes outnumber human cells 10 to 1 and play important roles in nutrition, immune system development, and preventing pathogen colonization. Disruptions to the normal microbiota are associated with diseases like antibiotic-associated diarrhea, autoimmune disorders, and metabolic syndrome. Probiotics, prebiotics, and fecal microbiota transplantation can be used to treat diseases by modifying the gut microbiota.

1. NORMAL COMMENSAL
FLORA AND THEIR
APPLIED IMPORTANCE
Presented by: Dr. Devyashree Medhi
Post Graduate Trainee (Third Year)
Dept. of Microbiology
Gauhati Medical College and Hospital
Moderator: Dr. Lahari Saikia
Professor and Head
Dept. of Microbiology
Gauhati Medical College and Hospital

2. CONTENTS
• INTRODUCTION
• SITES AND DISTRIBUTION
• INTRODUCTION OF NORMAL FLORA INTO HUMAN BODY
• FACTORS THAT CONTRIBUTE TO BACTERIAL GUT COMMPOSITION
• APPLIED IMPORTANCE OF NORMAL MICROBIAL FLORA
• MICROBIOTA IN DISEASE

3. INTRODUCTION
• Normal flora or resident microbiota is
the diverse group of microbial
population that every human harbors
on every surface of the body exposed
to the external environment.
• Bacterial cells outnumber human cells
by 10:1 ratio.
• They typically fall into one of the
following two categories:
i) Resident flora: They are permanent
members of the community and
when disturbed they again re-
establish themselves. They are
beneficial to the host.
ii) Transient flora: They colonize for a
short duration and most of them
are potential pathogens.

4. • Most of the normal flora consist of bacteria and to a lesser extent fungi
• Overall, anaerobic flora predominates over aerobes
• The presence of viruses and parasites are doubtful.
• Although life is possible without normal residential flora, they are
indispensable in maintaining the health and normal function of the host.

5. SITES
• Skin
• Conjunctiva
• External ear
• Nose, nasopharynx and sinuses
• Oral cavity
• Upper respiratory tract
• Gastrointestinal tract
• Genitourinary tract

6. Distribution of the human
microbiome in the most
representative body sites
Vaginal
Bacteroides
Mobiluncus
Group B streptococci
Peptostreptococci
CONS

7. INTRODUCTION OF NORMAL FLORA
INTO THE HUMAN BODY

8. .
The presence of the normal
microbial flora in a given anatomical
site depend on the following factors:
 Local temperature
 Moisture
 pH
 Presence of certain nutrients and
inhibitory substances
 Environment (hospital/community)
 Immune status of the individual
• The gut composition of infants and
toddlers varies significantly with
time.
• Flora resembling that of adults is
achieved by approximately 3 years
of age.

9. FACTORS THAT CONTRIBUTE TO BACTERIAL
GUT COMPOSITION

10. APPLIED
IMPORTANCE

11. Normal flora is indispensable in maintaining
proper nutritional status in the host
• They synthesize and excrete vitamins in excess of their own needs, which can
be absorbed as nutrients by their host.
• They extract additional calories from otherwise indigestible oligosaccharides.
• They aid in the proper utilization of nutrients by modulation of absorptive
capacity of the intestinal epithelium.

12. • In humans, enteric bacteria secrete Vitamin K and Vitamin B12 and
certain other vitamins of the B-complex group.
• Gut Bifidobacterium strains conjugate dietary linoleic acid also known
as Vitamin F. It has a wide variety of biological effects like formation of
lipids in the cell membrane, prostaglandins, leukotrienes e.t.c.
• Oral microbiota is required for reduction of dietary nitrate to
biologically active nitrite which, among other functions, is essential for
protecting the cardiovascular system and the gastric mucosa.

13. Prevent colonization by pathogen
• Normal microbiota provides its host with a physical barrier to incoming pathogens by
competitive exclusion, such as occupation of attachment sites, consumption of nutrient
sources, maintaining an acidic environment and production of antimicrobial substances.
• They also stimulates the host to produce various antimicrobial compounds the secretion
of which is enhanced in acidic conditions.
• This has been demonstrated that germ-free animals can be infected by
10 Salmonella bacilli, while the infectious dose for conventional animals is near
106 cells.

14. Stimulate the development of certain tissue
• Bacterial colonization of the intestine plays a major role in the post-natal development and
maturation of the immune and endocrine systems and, therefore, the central nervous system.
• Experimental data suggest a role of enteric bacteria in the gut–brain axis.
• There is a substantial body of evidence to link the pathogenesis of irritable bowel syndrome with
gut bacteria dysbiosis.
• The symptoms of autism typically become apparent before a child is 3 years old. Some evidence
supports the alterations of the fecal microbiota in patients with autism, with an increase in
several subtypes of Clostridium. In some cases, treatment with non-absorbed antibiotics can
improve symptoms.

15. Development of host’s defense mechanisms
• The importance of the gut microbiota in the development of both the intestinal mucosal and
systemic immune systems can be readily appreciated from studies of germ free animals.
• Germ free animals contain abnormal numbers of several immune cell types and immune cell
products. They have deficits in local and systemic lymphoid structures. Spleens and lymph nodes
are poorly formed. Peyer’s patches are hypoplastic and the number of mature isolated lymphoid
follicles are decreased. The number of their IgA-producing plasma cells is reduced, as are the
levels of secreted immunoglobulins. They also exhibit irregularities in cytokine levels.

16. Probiotics
• Probiotics are live micro-organisms which, when administered in adequate amounts as
part of food, confer a health benefit on the host.
• Probiotics may play a beneficial role in several medical conditions, including antibiotic
induced diarrhea, gastroenteritis, irritable bowel syndrome, inflammatory bowel disease,
cancer, depressed immune function, infant allergies, failure-to-thrive, hyperlipidemia,
hepatic diseases, Helicobacter pylori infections, and others, all of which were suggested
by certain research studies to improve with the use of probiotics

17. Probiotic microbes exert beneficial effect via a wide array of actions. These
include
 resistance to colonization
 production of antimicrobial substances
 inhibition of pathogen adhesion
 degradation of toxins
 stimulation of local and peripheral immunity
 stimulation of brush border enzyme activity
 stimulation of secretory-IgA
 prevention of microbial translocation.

18. • The major source of cholesterol in the human body
includes biosynthesis by the liver and absorption by
the intestines. These two factors determine the
overall cholesterol level.
• The hypo-cholesterolemic effect of the probiotics
has also been attributed to their ability to bind
cholesterol in the small intestines.
• Recent research has found that several pathways
may be involved, but these mechanisms are still not
clearly understood. The overall cholesterol
reduction mechanism of probiotic microorganisms
are represented in Figure.

19. Prebiotics
• Prebiotics are nondigestible dietary ingredient that beneficially affects the host by
selectively stimulating the growth or activity of a limited number of bacteria in the
colon.
• They are not digestible by human enzymes. Rather, they are fermented and digested by
the microbiota of the intestine. Fructooligosaccharides, inulin, oligofructose, lactulose,
and galactooligosaccharides have been identified as prebiotics.
• The combination of probiotics and prebiotics is termed ‘synbiotic’, and is an exciting
concept aimed at optimizing the impact of probiotics on the gut microbial ecosystem.

20. Fecal microbiota transplantation
• Fecal microbiota transplantation is the administration of of fecal matter from a donor
into the intestinal tract of a recipient in order to directly change the recipient’s gut
microbial composition and confer health benefits.
• FMT has been used to successfully treat recurrent Clostridium difficile infection.
• There are preliminary indications to suggest that it may also carry therapeutic potential
for other conditions such as inflammatory bowel disease, obesity, metabolic syndrome,
and functional gastrointestinal disorders.
• The major concern about this approach is the potential risk of transmission of infectious
diseases.

21. Preventative and therapeutic purposes
• Oxalobacter formigenes has the ability to degrade dietary oxalates,
reducing urinary oxalate excretion, which prompted its successful use in
clinical trials as a therapeutic and prophylactic option in calcium oxalate
nephrolithiasis and associated renal failure.
• Furthermore, gut inhabitants can prove invaluable in preventing adverse
outcomes following inadvertent environmental exposure to toxic
compounds: the toxicity of hydrazine, a highly toxic compound used in a
variety of industrial processes, is greatly reduced by the gut microbiota.

22. Dietary modifications for preventative and
therapeutic purposes
• It has been observed that phenolic compounds have the ability
to reduce or reverse the development of colitogenic changes at
the intestinal mucosa, thus offering a prophylactic and
sometimes a therapeutic means against colorectal carcinomas.
• The individual’s microbiota composition and its ability to bio-
transform nutritional compounds with potential medicinal
significance should be considered when recommending dietary
interventions.

23. MICROBIOTA IN DISEASE
Mechanisms of fine balance

24. Antibiotic-associated diarrhea
• Antibiotic-associated diarrhea is caused by an altered balance in gut microbial
communities, resulting in both decreased fermentation of indigestible carbohydrates and
rapid overgrowth of opportunistic micro-organisms with potential pathogenicity.
• It ranges from mild episodes that resolve when antibiotics are stopped to serious
complications such as toxic megacolon, bowel perforation and death.
• Risk is increased with different clinical conditions such as extremes of age, co-morbidity,
oral broad spectrum antibiotics, prolonged antibiotic duration, previous episodes of
antibiotic-associated diarrhea and hospitalization.

25. Autoimmunity and related disorders
• A number of studies using molecular techniques confirm that the gut microbial
ecosystem differs between children with and without atopic eczema.
• In fact, children born by means of caesarean section are colonized by bacteria
originating from the hospital environment and not from maternal skin. A cohort study
by Penders et al. suggests that early colonization by Escherichia coli increases the risk of
developing eczema and colonization with Clostridium difficile is associated with a higher
risk of eczema, recurrent wheeze and allergic sensitization.
• Undoubtedly, there are other environmental conditions, such as feeding regimen,
antibiotic intake or hygiene, which are well-recognized risk factors

26. Pseudomonas aeruginosa
• Pseudomonas aeruginosa is an example of a mutualistic bacterium that can turn into a pathogen
and cause disease if it gains entry into the circulatory system.
• It can result in infections in skin, bone, joint, gastrointestinal, and respiratory systems.
• However, it produces antimicrobial substances such as pseudomonic acid that is exploited
commercially as Mupirocin. This works against staphylococcal and streptococcal infections.
• Pseudomonas aeruginosa also produces substances that inhibit the growth of fungus species
such as Candida krusei, Candida albicans, Torulopsis glabrata, Saccharomyces
cerevisiae and Aspergillus fumigatus. It can also inhibit the growth of Helicobacter pylori.

27. Bacterial translocation
• Intestinal bacteria play a role in the pathogenesis of sepsis by
bacterial translocation, defined as the passage of viable bacteria from
the gastrointestinal tract through the epithelial mucosa.
• Extensive work on bacterial translocation has been performed in
animal models and occurs notably in hemorrhagic shock, burn injury,
trauma, intestinal ischemia, intestinal obstruction, severe
pancreatitis, acute liver failure and cirrhosis.

28. Metabolic syndrome
• In animal models, it seems that transplantation of gut microbiota from obese mice to non-obese, germ-
free mice resulted in transfer of metabolic syndrome-associated features from the donor to the recipient.
• The mechanisms advocated are the provision of additional energy by the conversion of dietary fibre to
SCFA, effects on gut-hormone production and increased intestinal permeability, causing elevated systemic
levels of lipopolysaccharides.
• The contact with these antigens seems to contribute to low-grade inflammation, a characteristic trait of
obesity and metabolic syndrome. Presumably, obesity affects the diversity of the gut microbiota and,
probably, the way individuals harvest energy from nutrients.
• However diet or other undetermined variables may also account for the observed microbiome changes.
• Further research on human subjects must be conducted.

29. Gut microbiota and cancer therapy
• The relationship between gut resident microbiota and their host is
complex.
• Some bacterial subpopulations are able to rise during gut dysbiosis
and, in turn, to trigger the formation of an inflammatory and pro-
cancerogenic environment.
• On the other hand, many gut derived probiotics like lactobacillus
rhamnosus are able to protect the host, re-establishing the conditions
of a healthy intestinal microbiota within dysbiotic patients, including
cancer patients.

31. • Three parallel studies identified specific gut species populating the gastro-
intestinal tract of cancer immunotherapy responders, are able to improve the
efficacy of immunotherapy treatments.
• That questions the usage of both probiotics and FMT in cancer therapy, either
as tools to repopulate cancer patients’ damaged intestine or even as proper
adjuvants in immunotherapy and other kinds of anti-cancer therapies.
• Correspondingly, care needs to be pursued as patients are often
immunocompromised, therefore it is important to evaluate the specific side
effects of administering selected bacterial species to such sensitive
individuals.

32. Reference
• Linking the gut microbiota to human health. Alonso VR, Guarner. British Journal
of Nutrition (2013), 109, S21–S26 doi:10.1017/S0007114512005235
• Review Article Effects of Probiotics, Prebiotics, and Synbiotics on
Hypercholesterolemia: A Review. Anandharaj M, Shivashankari B, Rani RP.
Chinese Journal of Biology .Volume 2014, Article ID 572754.
http://dx.doi.org/10.1155/2014/572754
• Gupta S, Allen-Vercoe E, Petrof EO. Fecal microbiota transplantation: in
perspective. Therap Adv Gastroenterol. 2016;9(2):229–239.
doi:10.1177/1756283X15607414
• Ma L, Hu L, Feng X, Wang S. Nitrate and Nitrite in Health and Disease. Aging Dis.
2018 Oct 1;9(5):938-945. doi: 10.14336/AD.2017.1207. PMID: 30271668; PMCID:
PMC6147587.
• Gut Microbiota in Health and Disease.Sekirov I, Russell SL SHANNON L, Antunes
LCM, Finlay BB. Physiol Rev 90: 859–904, 2010; doi:10.1152/physrev.00045.2009.
• Gut microbiota and cancer: From pathogenesis to therapy. Vivarelli S, Salemi R et
al. Cancers (Basel).2019 jan;11(1) 38.