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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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