Neurotoxigenic Clostridia
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Neurotoxigenic Clostridia CESARE MONTECUCCO, ORNELLA ROSSETTO AND MICHEL R. POPOFF
Introduction Certain bacterial species of the genus Clostridium are characterized by their ability to produce extremely potent neurotoxins: tetanus neurotoxin (TeNT) and botulinum neurotoxin (BoNT). TeNT inhibits neurotransmitter release of synapses of the central nervous system (CNS) causing the spastic paralysis of tetanus; BoNT inhibits the release of acethylcholine at peripheral cholinergic nerve terminals causing the flaccid paralysis of botulism. To date, one TeNT and seven (A–G) serologically distinct BoNTs are known (Schiavo et al., 2000).
Clostridium Tetani Morphological and Cultural Characteristics Tetanus neurotoxin (TeNT) is produced by a uniform group of bacteria belonging to the Clostridium tetani species. These bacteria are usually 0.3– 0.6 µm in width and may vary considerably in length between 3 and 12 µm. They are Gram positive in young cultures, but they lose the Gram coloration upon prolonged incubations. Clostridium tetani is usually highly motile by peritrichous flagella, a property responsible for the swarming growth on agar medium, though some strains have no flagella. Clostridium tetani sporulates by forming translucid terminal enlargements, which give the typical drumstick appearance (Clostridium in Latin). The sporulation rate varies with the nature of the culture medium, strains, pH and temperature; sporulation does not take place above 41°C and at pH < 6 (Bytchenko, 1981). Spores are inactivated within one h at 100°C. Germination of C. tetani spores occurs both under anaerobic and aerobic conditions, but the outgrowth of C. tetani is strictly dependent upon a low oxidation-reduction potential (Smith and Williams, 1984). Clostridium tetani is strictly anaerobic, and motile strains swarm over the entire surface of the agar leading to a transparent film. Discrete colonies (2–5 mm) can be obtained with media containing 3–4% agar. On blood agar, colonies are slightly raised, semitransl-
ucent, gray, with an irregular margin and surrounded by a narrow zone of hemolysis. Clostridium tetani grows fairly well on the usual media containing peptones or tissue extracts. Most of the usual biochemical tests used for Clostridium identification are negative, as no carbohydrates are acidified, and there is no proteolysis, and no lipase or lecithinase production. Gelatin is liquefied slowly (2–7 days) with production of H2S and indole (Smith and Williams, 1984).
Genetic Characteristics The Clostridium genus encompasses more than 100 species that display a wide range of phenotypes and genotypes (Hippe et al., 1992). Clostridium tetani has been classified in group II on the basis of 23S rRNA homology (Johnson and Francis, 1975). The complete genome sequence of a toxigenic C. tetani strain consists of a 2,799,250-bp chromosome containing 2372 putative genes and a 74,082-bp plasmid, containing 61 genes (Brüggemann et al., 2003). Clostridium tetani possesses many genes for peptida
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