Figure 4. Model of horizontal BoNT gene transfer of the hybrid botulinum toxin complex.

Here, the ancestral BoNT gene cluster is transferred into the clostridial bacterium producing an HA complex with vacuolating toxicity [HA (VT⁺)], yielding an ancestor of the C. botulinum serotype C and D strains. Meanwhile, the serotype A and B strains also produce the toxin complex with HA components, but these strains do not exhibit vacuolating cytotoxicity. Therefore, the ancestral BoNT gene cluster transfers into the HA complex-producing bacterium [HA (VT⁻)], whose vacuolating toxicity is lost during its molecular evolution. The BoNT gene cluster transfer into a clostridial bacterium with no HA gene may yield ancestors of the serotype E and F strain.