C. botulinum is a large Gram-positive rod much like the rest of the clostridia. Its sporesresist boiling for long periods, and moist heat at 121°C is required for certain destruction. Germination of spores and growth of C. botulinum can occur in a variety of alkaline or neutral foodstuffs when conditions are sufficiently anaerobic.
The major characteristic of medical importance is that when C. botulinum grows un-der these anaerobic conditions, it elaborates a family of neurotoxins of extraordinary toxi-city. Botulinum toxin is the most potent toxin known in nature, with an estimated lethal dose for humans of less than 1 μg. Like tetanospasmin, botulinum toxin is a metallopro-teinase that acts on the presynaptic membranes at neuromuscular junctions. Once bound, it cleaves proteins involved in the release of acetylcholine at the synapse. The major ef-fect of this blockage of acetylcholine release is paralysis of the motor system, but it also causes dysfunction of the autonomic nervous system.
C. botulinum is classified into multiple types (A to G) based on the antigenic speci-ficity of the neurotoxins. All of the toxins are heat labile and destroyed rapidly at 100°C but are resistant to the enzymes of the gastrointestinal tract. If unheated toxin is ingested, it is readily absorbed and distributed in the bloodstream.