Stability of toxigenicity in proteolytic Clostridium botulinum type B upon serial passage

Botulinum neurotoxin X lacks potency in mice and in human neurons

Botulinum neurotoxins (BoNTs) are a class of toxins produced by Clostridium botulinum (C. botulinum) and other species of Clostridia. BoNT/X is a putative novel botulinum neurotoxin identified through genome sequencing and capable of SNARE cleavage, but its neurotoxic potential in humans and vertebrates remained unclear. The C. botulinum strain producing BoNT/X, Strain 111, encodes both a plasmid-borne bont/b2 as well as the chromosomal putative bont/x. This study utilized C. botulinum Strain 111 from Japan as well as recombinantly produced full-length BoNT/X to more fully analyze this putative pathogenic toxin. We confirmed production of full-length, catalytically active native BoNT/X by C. botulinum Strain 111, produced as a disulfide-bonded dichain polypeptide similar to other BoNTs. Both the purified native and the recombinant BoNT/X had high enzymatic activity in vitro but displayed very low potency in human-induced pluripotent stem cell-derived neuronal cells and in mice. Intraperitoneal injection of up to 50 µg of native BoNT/X in mice did not result in botulism; however, mild local paralysis was observed after injection of 2 μg into the gastrocnemius muscle. We further demonstrate that the lack of toxicity by BoNT/X is due to inefficient neuronal cell association and entry, which can be rescued by replacing the receptor binding domain of BoNT/X with that of BoNT/A. These data demonstrate that BoNT/X is not a potent vertebrate neurotoxin like the classical seven serotypes of BoNTs.

IMPORTANCE

The family of botulinum neurotoxins comprises the most potent toxins known to humankind. New members of this family of protein toxins as well as more distantly related homologs are being identified. The discovery of BoNT/X via bioinformatic screen in 2017 as a putative new BoNT serotype raised concern about its potential as a pathogenic agent with no available countermeasures. This study for the first time assessed both recombinantly produced and native purified BoNT/X for its vertebrate neurotoxicity.

Clostridial Neurotoxins: Structure, Function and Implications to Other Bacterial Toxins

Gram-positive bacteria are ancient organisms. Many bacteria, including Gram-positive bacteria, produce toxins to manipulate the host, leading to various diseases. While the targets of Gram-positive bacterial toxins are diverse, many of those toxins use a similar mechanism to invade host cells and exert their functions. Clostridial neurotoxins produced by Clostridial tetani and Clostridial botulinum provide a classical example to illustrate the structure-function relationship of bacterial toxins. Here, we critically review the recent progress of the structure-function relationship of clostridial neurotoxins, including the diversity of the clostridial neurotoxins, the mode of actions, and the flexible structures required for the activation of toxins. The mechanism clostridial neurotoxins use for triggering their activity is shared with many other Gram-positive bacterial toxins, especially molten globule-type structures. This review also summarizes the implications of the molten globule-type flexible structures to other Gram-positive bacterial toxins. Understanding these highly dynamic flexible structures in solution and their role in the function of bacterial toxins not only fills in the missing link of the high-resolution structures from X-ray crystallography but also provides vital information for better designing antidotes against those toxins.

Virulence Plasmids of the Pathogenic Clostridia

The clostridia cause a spectrum of diseases in humans and animals ranging from life-threatening tetanus and botulism, uterine infections, histotoxic infections and enteric diseases, including antibiotic-associated diarrhea, and food poisoning. The symptoms of all these diseases are the result of potent protein toxins produced by these organisms. These toxins are diverse, ranging from a multitude of pore-forming toxins to phospholipases, metalloproteases, ADP-ribosyltransferases and large glycosyltransferases. The location of the toxin genes is the unifying theme of this review because with one or two exceptions they are all located on plasmids or on bacteriophage that replicate using a plasmid-like intermediate. Some of these plasmids are distantly related whilst others share little or no similarity. Many of these toxin plasmids have been shown to be conjugative. The mobile nature of these toxin genes gives a ready explanation of how clostridial toxin genes have been so widely disseminated both within the clostridial genera as well as in the wider bacterial community.

Novel Botulinum Neurotoxins: Exploring Underneath the Iceberg Tip

Botulinum neurotoxins (BoNTs), the etiological agents of botulism, are the deadliest toxins known to humans. Yet, thanks to their biological and toxicological features, BoNTs have become sophisticated tools to study neuronal physiology and valuable therapeutics for an increasing number of human disorders. BoNTs are produced by multiple bacteria of the genus Clostridium and, on the basis of their different immunological properties, were classified as seven distinct types of toxin. BoNT classification remained stagnant for the last 50 years until, via bioinformatics and high-throughput sequencing techniques, dozens of BoNT variants, novel serotypes as well as BoNT-like toxins within non-clostridial species have been discovered. Here, we discuss how the now “booming field” of botulinum neurotoxin may shed light on their evolutionary origin and open exciting avenues for future therapeutic applications.

Impact of Clostridium botulinum genomic diversity on food safety

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C. botulinum Groups I and II form botulinum neurotoxin and cause foodborne botulism.

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Increased knowledge of C. botulinum Group I and II genomes and neurotoxin diversity.

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Impact on food safety via improved surveillance and tracing/tracking during outbreaks.

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New insights into C. botulinum biology, food chain transmission, evolution.

The deadly botulinum neurotoxin formed by Clostridium botulinum is the causative agent of foodborne botulism. The increasing availability of C. botulinum genome sequences is starting to allow the genomic diversity of C. botulinum Groups I and II and their neurotoxins to be characterised. This information will impact on microbiological food safety through improved surveillance and tracing/tracking during outbreaks, and a better characterisation of C. botulinum Groups I and II, including the risk presented, and new insights into their biology, food chain transmission, and evolution.

Implications of Genome-Based Discrimination between Clostridium botulinum Group I and Clostridium sporogenes Strains for Bacterial Taxonomy

Taxonomic classification of Clostridium botulinum is based on the production of botulinum neurotoxin (BoNT), while closely related, nontoxic organisms are classified as Clostridium sporogenes. However, this taxonomic organization does not accurately mirror phylogenetic relationships between these species. A phylogenetic reconstruction using 2,016 orthologous genes shared among strains of C. botulinum group I and C. sporogenes clearly separated these two species into discrete clades which showed ∼93% average nucleotide identity (ANI) between them. Clustering of strains based on the presence of variable orthologs revealed 143 C. sporogenes clade-specific genetic signatures, a subset of which were further evaluated for their ability to correctly classify a panel of presumptive C. sporogenes strains by PCR. Genome sequencing of several C. sporogenes strains lacking these signatures confirmed that they clustered with C. botulinum strains in a core genome phylogenetic tree. Our analysis also identified C. botulinum strains that contained C. sporogenes clade-specific signatures and phylogenetically clustered with C. sporogenes strains. The genome sequences of two bont/B2-containing strains belonging to the C. sporogenes clade contained regions with similarity to a bont-bearing plasmid (pCLD), while two different strains belonging to the C. botulinum clade carried bont/B2 on the chromosome. These results indicate that bont/B2 was likely acquired by C. sporogenes strains through horizontal gene transfer. The genome-based classification of these species used to identify candidate genes for the development of rapid assays for molecular identification may be applicable to additional bacterial species that are challenging with respect to their classification.

Historical and current perspectives on Clostridium botulinum diversity

For nearly one hundred years, researchers have attempted to categorize botulinum neurotoxin-producing clostridia and the toxins that they produce according to biochemical characterizations, serological comparisons, and genetic analyses. Throughout this period the bacteria and their toxins have defied such attempts at categorization. Below is a description of both historic and current Clostridium botulinum strain and neurotoxin information that illustrates how each new finding has significantly added to the knowledge of the botulinum neurotoxin-containing clostridia and their diversity.

Complete nucleotide sequence of a plasmid containing the botulinum neurotoxin gene in Clostridium botulinum type B strain 111 isolated from an infant patient in Japan

Botulinum neurotoxins (BoNTs) are highly potent toxins that are produced by Clostridium botulinum. We determined the complete nucleotide sequence of a plasmid containing the botulinum neurotoxin gene in C. botulinum type B strain 111 in order to obtain an insight into the toxigenicity and evolution of the bont gene in C. botulinum. Group I C. botulinum type B strain 111 was isolated from the first case of infant botulism in Japan in 1995. In previous studies, botulinum neurotoxin subtype B2 (BoNT/B2) produced by strain 111 exhibited different antigenic properties from those of authentic BoNT/B1 produced by strain Okra. We have recently shown that the isolates of strain 111 that lost toxigenicity were cured of the plasmid containing the bont/B2 gene. In the present study, the plasmid (named pCB111) was circular 265,575 bp double-stranded DNA and contained 332 predicted open reading frames (ORFs). 85 gene products of these ORFs could be functionally assigned on the basis of sequence homology to known proteins. The bont/B2 complex genes were located on pCB111 and some gene products may be involved in the conjugative plasmid transfer and horizontal transfer of bont genes. pCB111 was similar to previously identified plasmids containing bont/B1, /B5, or/A3 complex genes in other group I C. botulinum strains. It was suggested that these plasmids had been derived from a common ancestor and had played important roles for the bont gene transfer between C. botulinum.

Multi-locus variable number tandem repeat analysis for Clostridium botulinum type B isolates in Japan: comparison with other isolates and genotyping methods

Clostridium botulinum produces botulinum neurotoxin (BoNT) and causes botulism in humans and animals. Recently, 15-loci multi-locus variable number tandem repeat analysis (MLVA) for C. botulinum was developed for high-resolution and inter-lab comparative genotyping. This study examines the relation between MLVA and other genotyping methods such as pulsed-field gel electrophoresis (PFGE), multi-locus sequence typing (MLST), BoNT/B subtyping and bont/b gene location to evaluate MLVA as a method applicable to the genetic markers for C. botulinum type B. Japanese isolates were genotyped using MLVA and were compared with strains from other sources reported previously. Results show that the discriminatory power of MLVA was comparable to that of PFGE and higher than that of MLST. The topology of the minimum spanning tree (MST) constructed using MLVA data was very consistent with the phylogenetic classifications of PFGE and MLST. The MST topology also represented genetic diversity between the strains possessing bont/b gene on chromosomes and plasmids. Some Japanese isolates including those associated with infant botulism were inferred to be related to isolates of Europe origin from MLVA genotyping results. The MLVA scheme used for this study is apparently useful not only for high-resolution molecular typing, but also for phylogenetic characterization of C. botulinum type B.