3. CELL WALL
The cell wall is the outer most layer of the cell.
In many cases the cell wall comes in direct contact
with the environment.
• Protection of the cell.
• Maintains the shapes of the cell.
• Maintains the osmotic integrity of the cell.
• Prevents expulsion of ions, molecules and water.
4. • Assist some cells in attaching to other cells or in eluding
• Not present in animal cells, so can target cell wall of
bacteria with antibiotics.
• Providing attachment sites for bacteriophages.
• Play an essential role in cell division.
• Providing a rigid platform for surface appendages-
flagella, fimbriae and pili.
• Peptidoglycan, also known as murein, is
a polymer consisting of sugars and amino acids that
forms a mesh-like layer outside the cell membrane of
most bacteria forming the cell wall.
• The sugar component consists of alternating residues
of β-(1,4) linked N-acetylglucosamine and N-
• These subunits which are related to glucose in their
structure are covalently joined to one another to
form glycan chains.
7. • Attached to the N-acetylmuramic acid is a peptide
chain of four amino acids. The peptide chain can be
cross-linked to the peptide chain of another strand
forming the peptidoglycan.
• Tetra peptide
• Meso-diaminopimilic acid
• D-Glutamic acid
10. Gram Positive Cell wall
• Usually thick, homogenous, composed mainly
• It accounts for 50-90% of the dry weight of the
• Contain large amount of teichoic acids
(polymers of glycerol or ribitol joined by
13. Teichoic acid
• Teichoic acids are connected to either peptidoglycan or to
plasma membrane lipids.
• Absent in gram negative bacteria.
Function of Teichoic Acid:
. Antigenic determinant
-Receptor molecule for bacteriophages.
. Participate in the supply of Mg to the cell by binding Mg++
. Regulate normal cell division.
For most part, protein is not found as a constituent of the G+ cell
wall except M protein on group streptococci.
14. Gram Negative Cell Wall
• Multi layered and more complex than Gram
positive cell walls.
• Peptidoglycan of gram negative bacteria is
thin and comprises only 10% or less of cell
• Outer membrane lies outside the thin
• Most abundant protein is Braun’s lipoprotein.
• The region between the cytoplasmic membrane and
the outer membrane is filled with a gel-like fluid
• In gram negative bacteria, all secreted proteins are
contained within the periplasm, unless they are
specifically translocated across the outer membrane.
• Periplasm is filled with the proteins that are involved
in various cellular activities, including nutrient
degradation and transport.
17. Outer membrane
• Peptidoglycan layer is surrounded by outer
membrane in the gram negative bacteria.
• Its outside leaflet is made up of lipopolysaccharides,
rather than phospholipids.
• For this reason, the outer membrane is also called
the lipopolysaccharide layer or LPS.
• The outer membrane functions as a protective
barrier and excludes many toxic compounds.
18. • Lipopolysaccharide molecule is extremely important
from a medical stand point.
• It consists of three parts, two of them are medically
1. Lipid A…..embedded in membrane.
2. Core polysaccharide…..located on the surface of
3. O antigens….which are short polysaccharides
extended out from core.
19. • Lipid A: The chemical makeup of lipid A molecule
plays significant role in our body’s ability to recognize
the presence of invading bacteria.
• Contains two glucosamine sugar derevetives.
• It is toxic in nature, as a result the LPS can act as an
endotoxin, causing symptoms like fever, diarrhea
• O-antigen: It is composed of carbohydrates,
including glucose, galactose, mannose and some
other sugars in varying combinations.
• The O-antigens can resist react with their specific
antibodies by changing nature of their O side chains
to avoid detection
20. • Porin proteins: Three
porin molecules cluster
together and span the
outer membrane to
form a narrow channel
molecules smaller than
about 600 to 700 Da
1. Number of flagella
Three possible locations
Polar, Lateral and
• Sheathed flagella
• Periplasmic flagella
26. -Flagella can switch
Physical factors- Toque, temperature,
pH, salt concentration.
Genetic factors- Point mutation
• Polymorphism of flagella-
-some mutant flagella, such as straight
flagella, are too stiff to transform into
• Helical transformation is necessary for
untangling a jammed bundle of tangled
27. 3. Flagellin
Component protein filament is c/a Flagellin
Many bacteria have one kind few have two kind of Flagllin
Mol. Wt. 20-60 kDa.
• Amino Acid Sequences
Terminal Regions -Conserved,
Central Region - Highly Variable
Eg. Salmonella serotype variation
• In the filament, the terminal regions are located at
the innermost radius of a cylindrical structures,
whereas the central region is exposed to the outside.
28. 4. Cap protein
• flagellin can polymerize into flagella-invitro
• flagellin assembly requires Cap protein- invivo
• without Cap protein or Flid the flagellin is
secreted into the medium as monomers.
• located at the tip- pentamer, forming a
shorter., more sharply curved
(almost in a right angle)
Length -55 nm(+ 6nm)
A polyhook -indefinite length,
seen in mutants.
2. Hook protein
• Hook- polymer, hook protein or
• Mol. size m 29 kDa (Bacillus
subtilis) to 76 kDa (Helicobacter
• 42 kDa for most species.
30. 3. Scaffolding protein
FlgD-polymerize the hook
protein (Sits at tip)
FlgD is c/a Scaffolding protein
bcz of temporary existence
4. Hook-associated protein
• HAPs- two minor proteins
between the hook and
31. C. Basal structure
• The basal body typically consists of four rings and one rod
1. Basal body
• The basal body contains rings and a rod penetrating them.
• four rings –gram negatives,
• two rings - gram-positives,
• The structure of the basal body of S. typhimurium has been
32. 2.LP-ring complex
• L ring, - LPS layer of the outer
• P ring - peptidoglycan layer.
• The component proteins,
• FlgH for the L ring
• FlgI for the P ring, have signal
• LP-ring complex, resistant to
extremes of pH or temperature.
• Role- Ambiguous, bcz mutants
lacking the complex still can swim,
and LP complex is not found in
33. 1. MS-ring complex
• single type of protein, FliF, self-assembles into a
complex consisting of the M and S rings and part of the
• FliF is 65 kDa, the largest of the flagellar proteins
• Is the structural center of the basal structure and plays
an important role in flagellar assembly
34. 4. Rod
• The rod is not as simple as its name
suggests; it consists of at least four
• No intermediate rod structure
-- a whole rod or no rod at all.
The C ring is a fragile component of the basal
• The C ring consists of the switch proteins
(FliG, FliM, and FliN) and so is
sometimes called the switch
• 20–40 copies of FliG, 20–40 copies of FliM,
and several 100 copies of FliN.
• Role in flagellar formation, torque
generation, and the switching of
There is no correlation between bacterial flagella and eukaryotic
The rotational force (torque) of the flagellar motor is
difficult to measure directly, but can be estimated
from the rotational speed of flagella
1. Rotational direction
• 70% - by CCW rotation, (Enterobacteriaceae)
• majority CW-Rhodobacter sphaeroides,
2. Rotational speed
• torque of the flagellar motor cannot be directly measured
1. highest speed-200 Hz for S. typhimurium
2. High viscocity slows down the speed. ion
36. B. Energy source
• The energy source of torque generation in the
flagellar motor is not ATP but proton-motive force
• PMF is the electrochemical potential of the
proton,and results in the flow of protons from
outside toinside the cell.
C. Switching of rotational direct
• Switching the rotational direction of flagella is the
primary basis of chemotaxis
• an effector binds to the switch complex in the
flagellar motor. The effector is the phosphorylated
form of CheY, a signalling protein in the sensory
A. Flagellar genes
• There are more than 50 flagellar genes, which are
divided into three types
1. The fla genes: flg, flh, fli, and flj;
one for each of the clusters of genes scattered in several regions
around the chromosome.
2. The mot genes.
• Mutants that produce paralyzed flagella are called
motility deficient (Mot) mutants.
• There are only two mot genes (motA and motB) in S. typhimurium,
3. The che genes.
• Mutants that can produce functional flagella but that cannot show a
normal chemotactic behavior are called chemotaxis deficient (Che)
• two types, general chemotaxis mutants and specific chemotaxis
38. B. Gene clusters in four regions
• Flagellar genes are found in gene clusters on
the chromosome, They are in four regions
• Region I -the flg genes
• Region II- the flh genes and mot and che genes
• Regions IIIa and IIIb- fli genes
39. The Kinetics of Morphogenesis
• In order to achieve coherent cell activities,
flagellar construction has to be synchronized with cell
1. Filament growth
• A defined number of flagella have to be supplied at
each cell division.
• The number of flagella must be genetically controlled.
• On the other hand, filament growth seems free from
genetic control, because it continues over generations.
• The elongation rate of filaments is estimated to vary
inversely to the length
41. • Chemotaxis in microbiology
refers to the migration of cells
toward attractant chemicals or
away from repellents.
• Motility involves one or several
flagella, or whether it occurs by a
mechanism such as gliding
motility that does not involve
• Attractants: amino acids,
peptides, and sugars
• Repellents: phenol and acid
42. RESPONSE STRATEGY
A. Biased random walk
• In a constant environment,
motile bacteria generally move
in a random walk of straight
runs punctuated by brief
periods of reversal that serve to
the direction of the next run.
• Individual cells never have to
determine in which direction
they want to move. Instead, they
simply determine whether they
want to continue on course or
44. B. Temporal sensing and
• 1970s, through the work of
Macnab, Koshland, Berg, and
• chemotaxis depends on a
temporal rather than a spatial
• As the cell moves if the
comparison is favorable, the
cell tends to keep going; if not,
it tends to change direction.
45. • C. Excitation and adaptation
• Bacteria must have a way of comparing the past with
the present—they must have memory.
• Bacteria do not respond to absolute concentrations
of attractant and repellent chemicals. They respond
only to changes.
• There is a close relationship between memory and
• The sense and degree of excitation and adaptation
in response to a new place in time are only
determined in relation to the memory of the old one
• Pili, also known as fimbriae, are
polymeric organelles expressed on
the surface of bacteria.
• 5-20ϻm × 2 to 11 nm
• Pili are composed of single or
multiple types of protein subunits,
called pilins or fimbrins, which are
typically arranged in a helical
• Pilus architecture varies from thin,
twisting threadlike fibers to thick,
rigid rods with small axial holes.
47. • Pili with diameters of 2–3 nm,
are often referred to as
“fibrillae”. Eg. K88 and K99 pili,
• Pili which tend to coil up into a
fuzzy adhesive mass on the
bacterial surface, are referred
as thin aggregative pili or curli.
• Pili are expressed peritrichously
• pili, can be localized to one
pole- eg. type 4
48. • Pili expressed by gram-negative bacteria have
been extensively characterized, and the
expression of pili by gram-positive bacteria has
also been reported.
• Primary function – adhesion , adhesins
-spread of both pathogenic and commensal
• - act as receptors for bacteriophage, facilitate
DNA uptake and transfer (conjugation),
• Pili were first noted in early electron microscopic
investigations as nonflagellar, filamentous appendages
• In 1955, Duguid -“fimbriae” (plural, from Latin for
thread or fiber) and correlated their presence with the
ability of E. coli to gglutinate red blood cells.
• In 1965 Brinton introduced the term “pilus” (singular,
from Latin for hair) to describe the fibrous structures
(the F pilus) associated with the conjugative transfer of
genetic material between bacteria
• Duguid and co-workers, pili expressed by
different E. coli strains were distinguished on the
basis of their ability to bind to and agglutinate
red blood cells (hemagglutination) in a mannose
sensitive (MS) (Type 1 pili)as opposed to a
mannose resistant (MR) fashion.
• Other bases for classification
• Adhesive and antigenic traits
• Distribution among bacterial strains
• Microscopic charecterization
• Assembly mechanism-Gram negateive 6 types
51. • These pili are very diverse and possess a
myriad of architectures and different receptor
binding specificities and functions
• Pili are now known to be encoded by virtually
all gram-negative organisms and are some of
the best-characterized colonization and
virulence factors in bacteria.
52. Molecular structure
Type 1 pili
• The P pilus tip is a 2-
composed of a distally
located adhesin PapG,
a tip pilin PapE, and
adaptor pilins PapF and
• type 1 pilus has a short,
3-nm-wide fibrillar tip
made up of the
adhesin, FimH, and two
additional pilins, FimG
55. • 50% glycerol can cause
the pilus rod to
reversibly unwind into a
2-nm-thick linear fiber
similar in appearance to
the tip fibrillum.
• Bullitt and Makowski
(1995) have proposed
that unwinding will help
to withstand better the
stress, such as shearing
forces from the bulk flow
of fluid through the
urinary tract, without
56. Charecters of Pili
• P pili are major virulence factors associated
with pyelonephritis caused by uropathogenic
E. coli. UPEC
• type 1 pili appear to be more rigid and prone
to breaking than P pili.
• K88 and K99 pili are significant virulence
factors expressed by enterotoxigenic E. coli
(ETEC) strains. They are relatively rigid and rod
57. REGULATION OF PILUS
• Pilus biogenesis, in general, is a tightly
• Ideally, the costs in energy and other resources
required for pilus ssembly must be balanced
with any potential benefits.
• Pathogenic and other bacteria must also
control pilus expression, in some cases,
to avoid attachment to unfavorable sites
58. Biogenesis depends on
• Oxygen tension,
• Carbon source, and
• Nutrient availability
59. ROLE OF PILI IN DISEASE PROCESSES
• Adherance- colonization-ETEC,UPEC
• Virulance K88, K99
• Uptake of DNA-Conjugation(Resistance,
• Biofilm fomation- Antibiotic Resistance
• To be continued…