Genetic diversity within the botulinum neurotoxin-producing bacteria and their neurotoxins

• Genetic diversity of botulinum neurotoxin-producing clostridia is discussed.
• Toxin subtypes are compared and delineated.
• Mechanisms for the movement of toxin genes are explained.
• Toxin cluster types are illustrated.
• Toxin locations within genomes are highlighted.

The recent availability of multiple Clostridium botulinum genomic sequences has initiated a new genomics era that strengthens our understanding of the bacterial species that produce botulinum neurotoxins (BoNTs). Analysis of the genomes has reinforced the historical Group I–VI designations and provided evidence that the bont genes can be located within the chromosome, phage or plasmids. The sequences provide the opportunity to examine closely the variation among the toxin genes, the composition and organization of the toxin complex, the regions flanking the toxin complex and the location of the toxin within different bacterial strains. These comparisons provide evidence of horizontal gene transfer and site-specific insertion and recombination events that have contributed to the variation observed among the neurotoxins. Here, examples that have contributed to the variation observed in serotypes A-H strains are presented to illustrate the mechanisms that have contributed to their variation.