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Back to Bacteria Index
Clostridium tetani
Member of genus Clostridium
Epidemiology
There are about one million cases of tetanus reported worldwide and
about 70 in the United States annually. Most cases in the US are in
the elderly who have allowed their innoculations to lapse, whereas
most cases in developing countries are in newborns.
History
Tetanus, the disease, was known to the ancients, who recognized the
relationship between wounds and fatal muscle spasms. In 1884, the
strychnine-like toxin of tetanus was isolated from free-living,
anaerobic soil bacteria. In 1890, tetanus toxoid was developed,
providing active immunization against the illness. This is our modern
tetanus vaccine.
Morphology
Clostridium tetani is a gram positive, spore-forming, obligate
anaerobic, rod-shaped bacterium of the phylum firmicutes. C. tetani's
appearance on gram stain is said to resemble tennis rackets or
drumsticks. Found in nature as spores in soil or parasitising the
gastrointestinal tracts of animals, these bacteria cause serious
toxicity in humans. See excellent article on tetanus.
Cultural characteristics
C. tetani is an obligately anaerobic bacillus that stains gram
positive in fresh cultures, but may stain gram negative in older
cultures.
Pathogenesis
During growth, the germ possesses many flagellae and have
limited mobility. Two toxins are elaborated, tetanospasmin and
tetanolysin, the latter of which is of uncertain toxicity and
significance. As the organism matures, it develops a terminal spore,
giving it the characteristic tennis racket appearance. Spores are
exteremely hardy but can be rendered harmless by iodine, hydrogen
peroxide or heat and pressure treatment (autoclaving). From a clinical
viewpoint, growing the organism from a wound is of no significance.
Toxicity
Tetanospasmin is similar in structure to botulinum toxin, but very
different in effect. It is a zinc-dependent metalloproteinase. There
is a heavy protein chain and a light chain connected by a disulfide
bridge. The heavy chain attaches to the cellular receptor, and the
light chain produces the toxic effects. It enters the central nervous
system by binding to the ends of axons and traveling up the axon to
the cell bodies. There, it blocks the release of neurotransmitter from
inhibitory neurons to motor cells. The motor neurons then fire because
of unopposed action of excitatory neurons, leading to diffuse muscle
spasm.
Diseases caused by clostridium tetani
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