Clostridium Botulinum - Neurotoxin Types

Neurotoxin Types

Neurotoxin production is the unifying feature of the species C. botulinum. Seven types of toxins have been identified and allocated a letter (A-G). Most strains produce one type of neurotoxin but strains producing multiple toxins have been described. Clostridium botulinum producing B and F toxin types have been isolated from human botulism cases in New Mexico and California. The toxin type has been designated Bf as the type B toxin was found in excess to the type F. Similarly, strains producing Ab and Af toxins have been reported. There is evidence that the neurotoxin genes have been the subject of horizontal gene transfer, possibly from a viral source. This theory is supported by the presence of integration sites flanking the toxin in some strains of C. botulinum. However, these integrations sites are degraded indicating that the C. botulinum acquired the toxin genes quite far into the evolutionary past.

Only types A, B, E, and F cause disease in humans while types C and D cause disease in cows, birds, and other animals but not in humans. The "gold standard" for determining toxin type is a mouse bioassay, but the genes for types A, B, E, and F can now be readily differentiated using Real-time polymerase chain reaction (PCR).

A few strains from organisms genetically identified as other Clostridium species have caused human botulism: Clostridium butyricum has produced type E toxin and Clostridium baratii had produced type F toxin. The ability of C. botulinum to naturally transfer neurotoxin genes to other clostridia is concerning, especially in the food industry where preservation systems are designed to destroy or inhibit only C. botulinum but not other Clostridium species.