3. BACTERIAL ADHESION
● Adherence is an essential step in bacterial pathogenesis or
infection, required for colonizing a new host.
● Bacterial adhesins are also a potential target for prophylaxis
or treatment of bacterial infections.
● Bacteria are typically found attached to and living in close
association with surfaces.
● During the bacterial lifespan, a bacterium is subjected to
frequent shear-forces.
● In the crudest sense, bacterial adhesins serve as anchors
allowing bacteria to overcome these environmental shear
forces, thus remaining in their desired environment.
● However, bacterial adhesins do not serve as a sort of
universal bacterial velcro.
● Rather, they act as specific surface recognition molecules,
allowing the targeting of a particular bacterium to a
particular surface.
4. Bacterial adherence to mucosal surfaces
In its simplest form, bacterial adherence or attachment to a
eukaryotic cell or tissue surface requires the participation of two
factors:
1. Receptor
The receptors so far defined are usually specific carbohydrate or
peptide residues on the eukaryotic cell surface.
1. Ligand
The bacterial ligand, called an adhesin, are cell-surface
components or appendages of bacteria that facilitate adhesion
or adherence to other cells or to surfaces, usually in the host they
are infecting or living in. Adhesins are a type of virulence factor.
Adhesins and receptors usually interact in a complementary and
specific fashion with specificity comparable to enzyme-substrate
relationships and antigen-antibody reactions.
5. Specific adherence of bacteria to
cell and tissue surfaces
Several types of observations have provided indirect
evidence for specificity of adherence of bacteria to host
cells or tissues:
● Tissue tropism.
● Species specificity
● Genetic specificity within a species
6. Tissue tropism.
Particular bacteria are known to have an apparent
preference for certain tissues over others,
● e.g. S. mutans is abundant in dental plaque but does not
occur on epithelial surfaces of the tongue;
● the reverse is true for S. salivarius which is attached in
high numbers to epithelial cells of the tongue but is
absent in dental plaque.
● Corynebacterium diphtheriae colonizes exclusively in
the throat.
7. Species specificity
Certain pathogenic bacteria infect only certain species of
animals, e.g.
● N. gonorrhoeae and Bordetella pertussis infections are
limited to humans;
● enteropathogenic E. coli k-88 infections are limited to
pigs;
● E. coli cfa i and cfa ii infect humans;
● E. coli k-99 strains infect calves;
● group a streptococcal infections occur only in humans.
In addition, certain indigenous species and symbionts are
quite specific in their associations with specific animal hosts.
8. Genetic specificity within a
species
Certain strains or races within a species may be genetically
immune to a pathogen, e.g.
● Certain pigs are not susceptible to E. coli k-88 infections
● males are not susceptible to mastitis( inflammation of breast
tissue);
● females are not susceptible to orchitis (infection of testicles);
● A percentage of females are not susceptible to urinary tract
infection caused by E. coli
Although other explanations are possible, the above
observations might be explained by the existence of specific
interactions between microorganisms and eukaryotic tissue
surfaces which allow microorganisms to become established on
the surface.
9. Mechanisms of adherence to cell or tissue
surfaces
The mechanisms for adherence may involve two steps:
Nonspecific adherence:
● Reversible attachment of the bacterium to the eucaryotic surface
(sometimes called "docking") nonspecific adherence involves
nonspecific attractive forces which allow approach of the bacterium
to the eucaryotic cell surface.
● Possible interactions and forces involved are:
■ Hydrophobic interactions
■ Electrostatic attractions
■ Atomic and molecular vibrations resulting from fluctuating
dipoles of similar frequencies
■ Brownian movement
■ Recruitment and trapping by biofilm polymers interacting with
the bacterial glycocalyx (capsule)
10. Specific adherence:
● Irreversible permanent attachment of the microorganism to the
surface (sometimes called "anchoring").
● Specific adherence involves permanent formation of many specific
lock-and-key bonds between complementary molecules on each
cell surface.
● Complementary receptor and adhesin molecules must be accessible
and arranged in such a way that many bonds form over the area of
contact between the two cells.
● Once the bonds are formed, attachment under physiological
conditions becomes virtually irreversible.
● Several types of experiments provide direct evidence that receptor
and/or adhesin molecules mediate specificity of adherence of
bacteria to host cells or tissues. These include:
■ The bacteria will bind isolated receptors or receptor analogs.
■ The isolated adhesins or adhesin analogs will bind to the
eucaryotic cell surface.
11. ● Adhesion (of the bacterium to the eucaryotic cell surface)
is inhibited by:
■ Isolated adhesin or receptor molecules
■ Adhesin or receptor analogs
■ Enzymes and chemicals that specifically destroy
adhesins or receptors
■ Antibodies specific to surface components (i.e.,
adhesins or receptors)
12. Structure for adherence
Through the mechanisms of evolution, different species of bacteria have
developed different solutions to the problem of attaching receptor
specific proteins to the bacteria surface. Bacteria possess several
structures which help in adhesion of cells
Capsules
● Capsule components of certain bacteria (e.g. Streptococcus,
staphylococcus, klebsiella, neisseria, haemophilus) mediate
adhesion to host cell surface. Capsule may also act indirectly to
block adhesion, by masking adhesins and blocking their access to
cellular receptors by steric hindrance. The capsule also acts as
chelator of cations, some of which may also be required for proper
function of other adhesins.
S layer
● It consists of glycoprotein and self-assembling units (external to cell
wall) which also help in adhesion
13. Fimbriae
Fimbriae are present on cell surface and cause
bacterial adhesion. Adhesin is located at the tip
or along the whole length of fimbriae. Fimbriae
have been classified into the five types:
● Type 1
● Type 2
● Type 3
● Type 4
● Type 5
14. Type 1
● They are rigid fimbriae that exhibit mannose-sensitive
haemagglutination e.g. E. Coli).
● Type-1 fimbriae enable E. coli to bind to d-mannose residues on
eucaryotic cell surfaces. They are said to be "mannose-sensitive"
since exogenous mannose blocks binding to receptors on red blood
cells.
● Although the primary 17kda fimbrial subunit is the major protein
component of type-1 fimbriae, the mannose-binding site is not
located here, but resides in a minor protein (28-31kda) located at
the tips or inserted along the length of the fimbriae.
By genetically varying the minor "tip protein" adhesin, the organisms can
gain ability to adhere to different receptors. For example,
● Tip proteins on pyelonephritis-associated (pap) pili recognize
a galactose-galactose disaccharide,
● While tip proteins on S-fimbriae recognize sialic acid. S fimbriae
are able to recognize receptor molecules containing sialic acid
and are produced by pathogenic E. coli strains causing urinary
tract infection.
15. Type 2
● Similar to type 1 but not induce haemagglutination
e.g. Actinomyces naeslundii.
Type 3
● Flexible and mannose-resistant fimbriae. They are
common among the enterobacteriaceae e.g.
Klebsiella pneumoniae.
Type 4
● They consist of n-methyl-phenylalanine in the amino
terminus region of the major subunits e.g.
Pseudomonas aeruginosa
Type 5
● Thinner than type 1, mannose-sensitive and a few in
number.
