Microbiology And Toxins

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C. botulinum is a gram-positive spore-forming obligate anaerobe, present in the soil worldwide, and has been identified in up to 18.5% of the U.S. soil surveyed (3).

C. botulinum is made of four groups of Clostridia (groups I-IV), linked by their ability to produce potent neurotoxins which have identical pharmacologic modes of action. C. botulinum produces seven closely related serological toxins (A-G). Human illness is usually caused by type A, B, or E toxin, and rarely by type Ci, C2, D, F, or G (4-8). Types A and B toxins are highly poisonous proteins resistant to digestion by gastrointestinal enzymes. Unexpressed toxin genes can be found in other clostridial species (and more than one toxin type in a single botulinal strain), confounding molecular diagnostics (9).

Each of the four groups of C. botulinum are distinguished by its characteristic biochemical activities. The production of each toxin appears to depend on the presence of a plasmid that encodes the toxin gene. All of the toxins are large, single polypeptides of similar structure. Elimination of the plasmid renders the bacteria nontoxigenic. The molecular weights of the toxins, which now are believed to be cellular proteins released during lysis, vary within the range of 130 to 150 kDa. The active moiety of the protein may be as small as 10 kDa. The toxin exerts its action through affecting the transmission at all peripheral cholinergic junctions. It interferes with the normal release of acetylcholine from nerve terminals in response to depolarization (10). The toxin binds irreversibly, and recovery of function depends on ultraterminal sprouting of the nerve to form new motor end plates. Following absorption or dissemination, the toxins give rise to neurologic symptoms by interfering with the release of acetylcholine from the terminal endings of cholinergic nerve fibers (10). Type A toxin may cause more severe disease than types B and E because of differences in amount of ingested toxin, absorption, or receptor affinity (11).

Botulinum toxins are the most potent toxins known. The estimated lethal doses for purified crystalline botulinum toxin type A for a 70-kg man are 0.09 to 0.15 |ig when introduced intravenously, 0.80 to 0.90 |ig when introduced inhalationally, and 70 |ig when introduced orally (12). Botulinal toxin type A has therapeutic value in the treatment of several neurologic and ophthalmologic disorders through chemical denervation (13). It is used as a therapeutic agent through local instillations in strabismus, blepharospasm, and other facial nerve disorders.

C. botulinum sporulates under stress and survives standard cooking and food processing but not industrial canning. The spores are highly heat-resistant; they may survive several hours at 100°C; however, exposure to moist heat at 120°C for 30 minutes will kill the spores. Botulinum toxins are temperature sensitive, and are inactivated by heating to 85°C for five minutes (14). Germination can occur in anaerobic milieu, nonacidic pH, and low salt and sugar content (15). The conditions in the normal human intestine do not support germination, vegetation, and toxin production by C. botulinum. However, this can occur in a small number of infants who develop infant botulism or adults who develop adult toxemic infectious botulism (16).

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