New Insights On Botulism Botulinum Neurotoxins And Botulinum Toxin Producing Clostridia PDF Download

Classical clostridial neurotoxins including tetanus neurotoxin and seven antigenically distinct serotypes of botulinum neurotoxin are potent proteinaceous toxins composed of three functional domains. Responsible for the paralytic diseases tetanus and botulism, the clostridial neurotoxin family is produced by multiple species of Clostridium and localizes to plasmids, bacteriophage, and the bacterial chromosome suggestive of a deep history of horizontal gene transfer. In Group I Clostridium botulinum, the species responsible for most human foodborne botulism cases, three serotypes of botulinum neurotoxins occur both within a family of large conjugative plasmids and at discrete sites within the chromosome. To further understand the transfer and localization of these toxins in Group I C. botulinum, a bioinformatic survey of CRISPR-Cas systems was conducted to test whether these endogenous genomic defense complexes play a potential role regulating toxin transfer at the species level. No evidence was found for direct targeting of botulinum neurotoxin associated genes by CRISPR-Cas systems, but CRISPR-Cas systems were predicted to frequently target botulinum neurotoxin producing, conjugative plasmids. Further, these plasmids themselves were found to possess a unique nuclease deficient variant of the type I-B CRISPR-Cas system, which could play roles in plasmid incompatibility, maintenance, and survival. Coupled with the high frequency of chromosomally localized type I and III CRISPR-Cas systems, these results indicate that CRISPR-Cas systems are tolerant of botulinum toxins as an accessory gene product but may nonetheless play a significant role in the regulation of their trafficking throughout Group I C. botulinum. Since 2015, a fast-growing outgroup of botulinum-like toxins have been identified in diverse bacterial species within and beyond genus Clostridium. These homologous toxins possess comparable domain structures to clostridial neurotoxins and are frequently conserved at key functional motifs capable of enzymatically cleaving known botulinum substrates. These attributes make them excellent tools for exploring the evolution of clostridial neurotoxins and may additionally confer unique and pharmacologically desirable activities. However, their capacity to cause disease akin to tetanus or botulism at similar potency to the classical toxin family remains largely unknown. Recombinant expression of a botulinum-like toxin from Enterococcus demonstrated a capacity to cause botulism in mice, though this required a significantly larger quantity of toxin relative to the classical neurotoxin serotypes. Chimeric constructs composed of the enzymatic and translocative domains of the botulinum-like toxin and binding domain of botulinum serotype A was intermediately toxic between the two. These findings coupled with mixed toxicity results from additional botulinum-like toxins indicates botulinum-like toxins vary significantly in mammalian potency and function, requiring investigation on a case-by-case basis.