Clostridium botulinum is a species of spore-forming anaerobic bacteria defined 
by the expression of any one or two of seven serologically distinct botulinum 
neurotoxins (BoNTs) designated BoNT/A-G. This Gram-positive bacterium was first 
identified in 1897 and since then the paralyzing and lethal effects of its toxin 
have resulted in the recognition of different forms of the intoxication known as 
food-borne, infant, or wound botulism. Early microbiological and biochemical 
characterization of C. botulinum isolates revealed that the bacteria within the 
species had different characteristics and expressed different toxin types. To 
organize the variable bacterial traits within the species, Group I-IV 
designations were created. Interestingly, it was observed that isolates within 
different Groups could express the same toxin type and conversely a single Group 
could express different toxin types. This discordant phylogeny between the toxin 
and the host bacteria indicated that horizontal gene transfer of the toxin was 
responsible for the variation observed within the species. The recent 
availability of multiple C. botulinum genomic sequences has offered the ability 
to bioinformatically analyze the locations of the bont genes, the composition of 
their toxin gene clusters, and the genes flanking these regions to understand 
their variation. Comparison of the genomic sequences representing multiple 
serotypes indicates that the bont genes are not in random locations. Instead the 
analyses revealed specific regions where the toxin genes occur within the 
genomes representing serotype A, B, C, E, and F C. botulinum strains and C. 
butyricum type E strains. The genomic analyses have provided evidence of 
horizontal gene transfer, site-specific insertion, and recombination events. 
These events have contributed to the variation observed among the neurotoxins, 
the toxin gene clusters and the bacteria that contain them, and has supported 
the historical microbiological, and biochemical characterization of the Group 
classification within the species.