Chemical overview and lethality
Botulinum toxin, or botulin, 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 as seen in botulism, as opposite to the spastic paralysis seen in tetanus.
There are seven serologically distinct toxin types, designated A through G. Three subtypes of A have been described. Botulinum toxin type A (BTA-A) and botulinium toxin type B (BTX-B) are use medicinally and cosmetically under various trade names.
Botulinum toxin is possibly the most acutely toxic substance known, with a median lethal dose of about 1 nanogram per kilogram (ng/kg) (Arnon et al. 2001), 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 would thus take about six metric tons to kill every human.)
Botulinum toxin 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 that 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 (Licciardello et al. 1967). However, the spores that produce the toxin are heat-tolerant and will survive boiling at 100 degrees Celsius for an extended period of time (Setlowa 2007).