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Bacteria may cause illness by exploiting the following:
- production of structures that enable the microorganism
to attach to the surface at which they cause disease;
- pili are hair-like structures that enable bacteria
to stick to surfaces through a specific mechanism; fimbriae
work in a similar fashion;
- Streptococcus mutans produces large
amount of extracellular slime that allow it, and other microorganisms,
to stick firmly to the surface of teeth. The acid produced
my metabolism of sugars in the diet then etches the surface of the
tooth to which these bacteria are stuck, initiating dental caries.
- production of one or more toxins;
- There are two classes of bacterial toxins: exotoxins
and endotoxins;
- exotoxins are enzymes that
are secreted by bacteria;
"Lockjaw" is an alternative name for tetanus. It is
so called because the tetanus toxin, produced by Clostridium
tetani, interferes with the transmission of nerve impulses
leading to a paralysis in which muscles go into spasm. The face
muscles are among the first to be affected, leading to the classical
"risus sardonicus" or sardonic smile, characteristic
of the disease.
Clostridium botulinum is a close relative of Clostridium
tetani. It, too, produces a neurotoxin but this causes
a flaccid paralysis where muscles go limp.
Another clostridium, Clostridium difficile causes pseudomembranous
colitis, associated with the use of particular antibiotics.
The disease results from the activity of two toxins that damage
the gut lining, causing a copious diarrhoea.
Vibrio cholerae causes cholera in which the victim
suffers such a profuse diarrhoea that stools take on the appearance
of water in which rice has been boiled. This is due to the activity
of cholera toxin, activating the production of cyclic AMP in
gut epithelial cells.
The genes that code for many exotoxins are located on "plasmids"
-extrachromosomal circles of DNA that carry genetic information
that is not essential for the survival of the bacterium. The
gene that codes for diphtheria toxin is unusual - it is carried
on a bacteriophage, a bacterial virus, that integrates its DNA
into the chromosome of the host bacterium.
- Endotoxin is a component of the outer
membrane of Gram-negative bacteria.
This structure lies beyond the cell wall. It does not have the
characteristic phospholipid bilayer seen in most membranes.
Rather, the inner surface comprises mostly phospholipid while
the outer layer comprises a complex known as lipopolysacchride.
The sugar complexes that make up the polysaccharide component
provide the bacterium with its antigenic structure. The lipid
in the outer layer of the Gram-negative outer membrane is a
complex lipid known as Lipid A. When purified
and injected into experimental subjects, this produces the generalised
symptoms associated with endotoxin - low blood pressure and
raised temperature. If sufficient endotoxin is present, this
may cause endotoxic shock, generalised organ failure, intravascular
coagulation and death.
- production of "aggressins";
- Aggressins are bacterial enzymes that, in contrast to exotoxins,
typically have localised effects. Examples include coagulase,
produced by Staphylococcus aureus. This
catalyses the formation of fibrin from fibrinogen, causing plasma
to clot. Staphylococcus aureus tends to produce defined
lesions - this is because the activity of coagulase is to "wall
off" the bacteria from the host defences. Another aggressin
is streptokinase, produced by Streptococcus
pyogenes. This aggressin digests fibrin, allowing
the infecting bacteria to spread through affected tissues. Streptokinase
has found a significant medical application as a clot-buster, used
to treat people with a thrombosis. Other aggressins include collagenase
and hyaluronidase, produced by a range of bacteria
that use these enzymes to facilitate spread of infection through
tissues.
- initiating undesirable consequences of the host defences.
- Certain pathogenic bacteria produce soluble antigens. These may
then combine with antibodies to produce circulating immune
complexes. Under certain conditions these can become trapped
in blood vessels compromising their function. A good example of
the problems that may result from circulating immune complexes is
glomerulonephritis - kidney damage following an
infection with Streptococcus pyogenes.
- Rheumatic fever results from immunological cross-reactions
between human tissue antigens and antigens on Streptococcus
pyogenes. Following a streptococcal infection, antibodies raised
against the bacterial antigens cross-react with the antigens produced
by human tissue, leading to autoimmune disease.
- The tubercle, the characteristic lesion of tuberculosis,
has as a major component of the lesion mixtures of giant cells formed
by the fusion of several macrophages as well as activated macrophages
and lymphocytes, cells important in immune defences. Mycobacterium
tuberculosis thus produces disease by over-stimulation
of our immune response to infection.
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