Clostridium botulinum is Gram positive, spore-forming anaerobic bacteria, which can produce botulinum neurotoxins (BoNTs). The toxins block the release of neurotransmitter, acetylcholine, at peripheral cholinergic nerve terminal and cause flaccid paralysis of muscle in human and animals, a condition known as botulism. BoNTs are classified into seven different serotypes (designated as BoNT/ A-BoNT/G), in which serotype A, B, E, and F cause botulism in human. BoNTs are comprised of one domain of light chain (L-chain) at N-terminus and two domains of heavy chains (H-chain) at C-terminus. The function of L-chain is to cleave SNARE (soluble N-ethylmaleimide-sensitive factor attachmentprotein receptors) proteins that involve in the exocytosis of neurotransmitter whereas H-chain is responsible for binding of toxin with nerve terminal and translocating of L-chain into cytosol from synaptic vesicle. The BoNTs are usually produced as complexes called progenitor toxin complex (PTC). They bind together with neurotoxin-associated proteins (NAPs), which are haemagglutinin (HA) and non-toxin non-haemagglutinin (NTNH). The NAPs can protect BoNTs from gastrointestinal environment and facilitate the absorption andtranslocation of neurotoxin into main circulation. The genes encoding BoNTs and NAPs are arranged as gene cluster, which are organized in two operons: ha and orfX operons. Mostly, they are located on the chromosome, large plasmid, or bacteriophage at the specific location and can be transferred horizontallyto other clostridia strains.
