Bacterial Structure in Relationship to Pathogenicity (page 2)
(This chapter has 2 pages)
© Kenneth Todar, PhD
are proteins in the
outermost cell envelope of a
broad range of bacteria. S-layers are composed of a
protein or glycoprotein species (mw 40-200 kDa) and exhibit either
oblique, square or hexagonal lattice symmetry with unit cell dimensions
in the range of 3 to 30 nm. S-layers are generally 5 to 10 nm thick and
show pores of identical size (diameter, 2 - 8 nm) and morphology.
Gram-negative and Gram-positive bacteria, as well a many archaea,
possess a regularly structured layer called an S-layer attached to the
outermost portion of their cell wall. It is composed of protein or
glycoprotein and in electron micrographs, has a pattern resembling a
tiled surface. Transmission electron micrograph of a freeze-etched,
metal shadowed preparation of a bacterial cell with an S-layer with
symmetry. Bar = 100nm.
S-layers have been associated with a number of possible functions that
relate to pathogenicity. S-layers can function as adhesins, enabling
to adhere to host cell membranes and tissue surfaces in order to
colonize. Many of the cell-associated protein adhesins used by
pathogens are components of the S-layer. The
S-layer may protect bacteria from harmful enzymes or changes in pH.
Like many other surface components, S-layers contribute to virulence by
bacterium against complement and attack by phagocytes.
The cell wall of a bacterium is
an essential structure that protects the delicate cell protoplast from
osmotic lysis. The cell wall of Bacteria consists of a polymer of
cross-linked by short chains of amino acids (peptides). This molecule
a type of peptidoglycan called murein. Murein is unique
the Domain, Bacteria. In the Gram-positive bacteria,
the cell wall is thick
(15-80 nanometers), consisting of several layers of peptidoglycan
with molecules called teichoic acids. In the Gram-negative
the cell wall is relatively thin (10 nanometers) and is composed of a
layer of peptidoglycan surrounded by a membranous structure called the
The structure of the muramic
acid subunit in the peptidoglycan Escherichia. coli. The
consists of N-acetyl glucosamine (NAG) attached (via a beta 1,4 link)
to N-acetyl-muramic acid (NAM). Attached to the NAM is a peptide chain,
which (in the case of E. coli, as illustrated) consists of
D-glutamate, diaminopimelic acid and D-alanine. Some antibiotics,
bacitracin, act by blocking the synthesis of the muramic acid subunit.
Penicillin and related antibiotics (beta lactams), as well as
block the assembly of the muropeptide subunits into the peptidoglycan
The cell wall, more properly the cell
envelope, is a complicated structure, fundamentally different in
Gram-positive and Gram-negative bacteria. Cell wall components are
determinants of virulence in both groups of bacteria. Endotoxin,
inherent to all Gram-negative bacteria, is toxic to animals in a
of ways. Peptidoglycan and LPS, as well as some teichoic acids, induce
the alternate complement pathway leading to inflammation. Teichoic
and O-specific polysaccharides may be used as adhesins by
and Gram-negative bacteria, respectively. Some cell wall components
against phagocytic engulfment or digestion. Variations in the
structure of cell wall components may be at the basis of antigenic
as well as specific host resistance to pathogens.
E. coli 0157.
electron micrograph (CDC). O157 refers to the antigenic type of E.
which, in this case, is based on the precise molecular structure of the
O-specific polysaccharide in the cell wall LPS.
The essential outer membrane of Gram-negative bacteria is the target
for attack by complement, hydrophobic agents and certain antibiotics.
(peptidoglycan) is dismantled by a host enzyme, lysozyme, found in most
body fluids. Several antibiotics, mainly the beta lactams, exert their
antimicrobial effect by blocking the synthesis and assembly of
Schematic drawing the outer
membrane of a Gram-negative bacterium.
The membranes of bacteria are
similar to the cell membranes of eucaryotes, except that bacterial
consist of saturated or monounsaturated fatty acids (rarely
fatty acids) and do not normally contain sterols. The plasma membrane
an exceptionally dynamic structure in bacteria which mediates
transport, secretion and energy generation. In terms of pathogenesis of
a bacterium, it is often dependent upon the integrity and function
of its plasma membrane. The membrane might be responsible for secretion
of toxins, resistance to antimicrobial agents, tactic responses or
other environmental signals to turn on genes for virulence.
Endospores are bacterial
(resting cells) formed by a few groups of bacteria as intracellular
but ultimately they are released as free endospores. Biologically,
are a fascinating type of cell. Endospores exhibit no signs of life,
described as cryptobiotic. They are highly resistant to environmental
such as high temperature (some endospores can be boiled for hours and
their viability), irradiation, strong acids, disinfectants, etc. They
are thought to be the most durable cell produced in nature. Although
they retain viability indefinitely, such that under appropriate
conditions, they germinate back into vegetative cells.
Endospores are formed mainly by two genera of Gram-positive
the aerobic sporeformers, and Clostridium, the anaerobic
Both genera contain pathogens, and the endospores produced by these
invariably play some role in the toxicity, transmission or survival of
Spore stain of a Bacillus
species. (CDC). Mature spores stain green whether free or still
the vegetative sporangium. Vegetative cells and sporangia stain red.
Schaeffer-Fulton stain technique was applied. The primary stain,
green, is forced into the spores by heating the prepared slide to
for 4-5 minutes. After washing, the vegetative cells are counterstained
END OF CHAPTER
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