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

Abstract

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.

Impact of Clostridium botulinum genomic diversity on food safety

“…subtype A2 is a hybrid of subtypes A1 and A3 [37], subtype F6 is a hybrid of subtypes F1 and F2 [38]. Interestingly, subtype F6 neurotoxin is uniquely formed by Group II strains, while subtypes F1 and F2 are only formed by Group I strains [23, 39•]. This seems to be a rare example of neurotoxin genes crossing between Groups I and II, as no neurotoxin subtypes are known to be formed by both Group I and Group II strains (e.g.…”

“…Genes encoding the neurotoxin and accessory proteins (e.g. non-toxic-non-haemagglutinin (NTNH)) are co-located in one of two conserved neurotoxin complex clusters ( ha cluster or orf-X cluster) at one of several specific insertion sites on the chromosome or a plasmid [7•, 26•, 37, 38, 39•]. All type B and some type A neurotoxin genes are located in the ha cluster, that comprises genes encoding the neurotoxin, NTNH, three haemagglutinins, and a positive regulator ( botR ).…”

“…This cluster includes genes encoding the neurotoxin, NTNH, and four open reading frames of unknown function ( p47 , orf-X1 , orf-X2 , orf-X3 ). The gene ( botR ) encoding the positive regulatory protein is present in the orf-X gene cluster of Group I, but not Group II, strains [6•, 37, 39•]. …”

“…plasmid, phage), and various recombination/insertion events (e.g. simple recombination, insertions facilitated by transposases/insertion sequence elements, insertion of genetic material following targeting of homologous genes [21, 37, 38, 39•]). For example, it is postulated that a single recombination event has given rise to strains that possess an ha gene cluster and form subtype A1 neurotoxin, a very successful lineage that is frequently associated with foodborne botulism [21, 39•].…”

Diversity of the Germination Apparatus in Clostridium botulinum Groups I, II, III, and IV

“…There are seven confirmed botulinum neurotoxins (types A to G), and approximately forty different subtypes (Carter and Peck, 2015; Hill et al, 2015; Williamson et al, 2016). The botulinum neurotoxin is the most powerful toxin known, with as little as 30–100 ng sufficient to cause human botulism.…”