Chemical overview

There are seven serologically distinct toxin types, designated A through G; 3 subtypes of A have been described. The toxin is a two-chain polypeptide with a 100-kDa heavy chain joined by a disulfide bond to a 50-kDa light chain. This light chain is an enzyme (a protease) that attacks one of the fusion proteins (SNAP-25, syntaxin or synaptobrevin) at a neuromuscular junction, preventing vesicles from anchoring to the membrane to release acetylcholine. By inhibiting acetylcholine release, the toxin interferes with nerve impulses and causes flaccid (sagging) paralysis of muscles in botulism as opposite to the spastic paralysis seen in tetanus.
It is possibly the most acutely toxic substance known, with a median lethal dose of about 1 ng/kg, meaning that a few hundred grams could theoretically kill every human on earth (for perspective, the rat poison strychnine, often described as highly toxic, has an LD50 of 1,000,000 ng/kg, and it would take four hundred tons to kill every human).
It is also remarkably easy to come by: Clostridium spores are found in soil practically all over the earth.
Food-borne botulism usually results from ingestion of food that has become contaminated with spores (such as a perforated can) in an anaerobic environment, allowing the spores to germinate and grow. The growing (vegetative) bacteria produce toxin. It is the ingestion of preformed toxin that causes botulism, not ingestion of the spores or vegetative organism.
Infant (intestinal) and wound botulism both result from infection with spores which subsequently germinate, resulting in production of toxin and the symptoms of botulism.
The toxin itself is rapidly destroyed by heat, such as in thorough cooking. However, the spores which produce the toxin are heat-tolerant and will survive boiling at 100 degrees Celsius for an extended period of time. 3-8