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Bacterial Structure in Relationship to Pathogenicity (page 1)
(This chapter has 2 pages)
© Kenneth Todar, PhD
The Importance of the Bacterial Surface
All of the various surface components of a bacterial cell are
in its ecology since they mediate the contact of the bacterium with its
environment. The only "senses" that a bacterium possesses result from
contact with its environment. It must use its surface components to
the environment and respond in a way that supports its own existence
survival in that environment. The surface properties of a bacterium are
determined by the exact molecular composition of its membrane and cell
envelope, including capsules, glycocalyx, S layers, peptidoglycan and
LPS, and the other surface structures, such as flagella and pili or
The surface of Streptococcus
pyogenes. High magnification electron micrograph of an ultra-thin
by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission.
Laboratory of Bacterial Pathogenesis and Immunology ,
Rockefeller University. At this magnification, especially in the cell
the left, the cell wall and cell surface fibrils, consisting mainly of
M protein, are well defined. The interdigitaion of these fibrils
neighboring cells of different chains can also be seen.
Bacterial surface components may have a primary biological
that has nothing to do with pathogenicity. Thus, the function of the
in the outer membrane of Gram-negative bacteria has to do with its
characteristics, rather than its toxicity for animals. However, there
endless examples wherein a bacterial surface component plays an
role in the pathogenesis of infectious disease. Bacterial surface
may act as (1) permeability barriers that allow selective
of nutrients and exclusion of harmful substances (e.g. antimicrobial
agents); (3) adhesins used to attach or adhere to
specific surfaces or
(3) enzymes to mediate specific reactions on the cell surface
in the survival of the organism; (4) protective structures against
engulfment or killing; (5) antigenic disguises to bypass
activation of host immune defenses; (6) endotoxins,
generally cell wall components, that cause an inflammatory response in
the host; (7) "sensing
proteins" that can respond to temperature, osmolarity, salinity,
oxygen, nutrients, etc., resulting in a molecular signal to
genome of the cell that will cause expression of some determinant of
(e.g. an exotoxin).
In medical situations, the surface components of bacterial cells are
major determinants of virulence for many pathogens. In animals, they
may be used to
tissues, resist phagocytosis and immune responses, and to induce
complement activation and harmful immune responses.
The surface of Bacillus
From Mesnage, et al. Journal of Bacteriology (1998) 180, 52-58. http://www.pasteur.fr/recherche/unites/scme/Biblio/capsulea.htm.
The bacterial membrane is evident as the innermost layer surrounding
denotes the peptidoglycan cell wall.S
refers to the S-layer which consists of two proteins including the
denotes the poly-D-glutamic acid capsule that is exterior to and
covers the S-layer proteins.
The Structure of the Bacterial Surface
Structurally, a bacterial cell has three architectural regions: appendages
(proteins attached to the cell surface) in the form of flagella and
a cell envelope consisting of a
cell wall and plasma membrane; and a cytoplasmic
region that contains the cell genome (DNA) and ribosomes and
various sorts of inclusions. The surface components of a bacterium are
the constituents of its cell envelope and appendages.
Flagella are filamentous protein
structures attached to the cell surface that provide swimming movement
for most motile bacterial cells. The diameter of a bacterial flagellum
is about 20 nanometers, well-below the resolving power of the light
The flagellar filament is rotated by a motor apparatus in the plasma
allowing the cell to swim in fluid environments. Bacterial flagella are
powered by proton motive force (chemiosmotic potential) established on
the bacterial membrane.
Bacteria are known to exhibit a variety of types of tactic
i.e., the ability to move (swim) in response to environmental stimuli.
For example, during chemotaxis a bacterium can sense the
and quantity of certain chemicals in its environment and swim towards
(if they are useful nutrients) or away from them (if they are harmful
During aerotaxis, bacteria swim toward or away from O2.
For a few pathogens motility is known to be a determinant of
In the case of Vibrio cholerae, the vibrios apparently swim
into the intestinal mucosa to avoid being flushed out by the
action of the gut. Flagella are antigenic, and therefore, vulnerable to
attack by host antibody molecules. Antibody molecules directed against
flagellar antigens can agglutinate and/or immobilize bacterial cells,
possibly opsonize them from phagocytosis, which presumably would aid in
flagellar stain (CDC). Bacterial flagella are below the resolving power
of the light microscope. In order to be visualized, the bacteria must
reacted with a stain that precipitates along the flagellar filaments,
increases their effective diameter to the point of resolution. Vibrio
cholerae is motile by means of a single polar flagellum
into one pole of the cell.
Fimbriae and Pili
are interchangeable terms used to designate short, hair-like structures
on the surfaces of bacterial cells. Fimbriae are shorter and stiffer
flagella, and slightly smaller in diameter. Like flagella, they are
of protein. A specialized type of pilus (always called a pilus), the F
or sex pilus, mediates the transfer of DNA between mating
but the function of the smaller, more numerous common pili is quite
Inasmuch as many bacteria are able to exchange genes for virulence by
of conjugation, the sex pilus which confers the ability to conjugate,
well play a role in the their assembly of virulence determinants.
Common pili or
are often involved in adherence (attachment) of bacterial cells to
in nature. In medical situations, they are major determinants of
virulence because they allow pathogens to attach to (colonize) tissues
and, sometimes, to resist attack by phagocytic white blood cells. As
on the bacterial cell, the functions of fimbriae overlap with those of
capsules discussed below. Fimbriae are also antigenic and secretory
(IgA) will often block bacterial colonization, while circulating
(IgG or IgM) will opsonize bacterial cells for phagocytosis.
Electron micrograph by David M. Phillips,
Unlimited, with permission. This pathogen utilizes its fimbriae in
order to initially colonize the urethral or cervical epithelium.
Most bacteria contain some sort of a polysaccharide layer outside of
the cell wall or outer membrane. In a general sense, this layer is
a capsule. A true capsule is a
detectable layer of polysaccharides deposited outside the cell wall. A
less discrete structure or matrix which embeds the cells is a called a
layer. Slime layers are equivalent to biofilms (below) A type of
capsule found in bacteria called a glycocalyx
is a thin layer of tangled polysaccharide fibers which is a
observed on the surface of cells growing in nature. Some
microbiologists consider all types of exopolysaccharides to be
Capsules, slime layers, and glycocalyx are known to mediate specific or
non specific adherence of bacteria to particular surfaces. The also
protect bacteria from engulfment by predatory phagocytes and
attack by antimicrobial agents.
In nature, and in many medical situations, colonies of bacteria
and live in a biofilm, made up principally of capsule material.
A biofilm usually consists of a consortium (mixture) of bacteria living
in a matrix of slime which is secreted by one of the bacterial members.
Dental plaque is an example of a natural biofilm, as is a slimy mass of
bacteria attached to a rock in a mountain stream. In medical
bacteria in a biofilm may have certain advantages over planktonic
For example, biofilm bacteria may be less susceptible to phagocytosis,
or neutralizing antibodies.
Many polysaccharide capsules possess an antigenic epitope so they
induce and react with host antibodies. Where the capsule is a main
of virulence of a pathogen (e.g. Streptococcus pneumoniae)
against the bacterium neutralize its virulence.
Bacterial capsules visualized
by various techniques. Left. Streptococcus pneumoniae -India
capsule outline (K.Todar); Middle. Bacillus anthracis
antibody (CDC); Right.
Streptococcus pyogenes -transmission electron
micrograph by Maria Fazio and Vincent A. Fischetti, Ph.D. with
Laboratory of Bacterial Pathogenesis and Immunology ,
Rockefeller University. S. pneumoniae capsular material is
of polysaccharide. The capsule is the pathogen's most important
of virulence because it allows the bacterial cells to escape phagocytes
in the lung. The B.anthracis capsule is composed of
acid. Its capsule is antiphagocytic, and it protects the bacteria from
complement- mediated lysis in serum or blood. The capsule of S.
is composed of hyaluronic acid, the same polymer as found in human
tissue. The capsule is an antigenic disguise that prevents recognition
of the streptococci by phagocytes
the immune system.