Genetic diversity among Botulinum Neurotoxin-producing clostridial strains

Effects of botulinum neurotoxin-associated proteins on cellular organelle dynamics in NRK-52E cells

Clostridium botulinum produces seven distinct serotypes (A-G) of botulinum neurotoxin (BoNT), a potent biological toxin that affects the nervous system. In its natural state, BoNT forms a progenitor toxin complex with neurotoxin-associated proteins (NAPs), substantially enhancing its stability and oral toxicity. Recent studies have shown that NAPs in certain serotypes induce vacuolation and death in various epithelial cell types. In this study, we examined the effects of the serotype D NAPs complex (NAPs/D) on intracellular organelles in NRK-52E cells, a rat kidney cell line. NRK-52E cells were selected because they exhibit a unique response, showing vacuolation without undergoing cell death upon exposure to NAPs/D. Our findings indicated that NAPs/D-induced vacuole formation was associated with the reduced size of the Golgi apparatus. Additionally, vacuoles were associated with endocytic vesicles, implying that NAPs/D modulate endocytic pathways and affect intracellular transport. Furthermore, NAPs/D exposure activated early autophagic signaling, as evidenced by increased LC3 expression, although vacuole formation appeared to occur independently of the complete autophagy process. These findings provide valuable insights into the cellular mechanisms underlying BoNT toxicity and highlight the complex interplay between NAPs/D-induced vacuolation, endocytosis, and autophagy. This study highlights potential therapeutic approaches to mitigate the effects of botulism by targeting these pathways.

Prevalence of toxigenic Clostridium botulinum in food products sold in Indian retail markets

INTRODUCTION

Clostridium botulinum and botulinum neurotoxins have been recognized as an important food safety hazard. The objective of this study was to screen the prevalence of C. botulinum and botulinum toxin in various food products sold in the retail markets of India.

MATERIAL AND METHODS

A total of 236 food products collected from Indian retail markets were screened for preformed botulinum toxin using a mouse bioassay. This was followed by enrichment and isolation and further testing of isolates for botulinum toxin production using an additional mouse bioassay. A toxin neutralization test with type specific antitoxins, 16S rRNA gene sequencing and BoNT (botulinum neurotoxin) gene amplification confirmed the presence of C. botulinum and its toxin production potential.

RESULTS AND DISCUSSION

Preformed botulinum toxin was not detected in any of the products. The overall prevalence of C. botulinum in the products was 11 %, but pure cultures of C. botulinum could be isolated only from 10 samples. The isolates were identified as C. botulinum type A, type B and type E based on toxin typing with type specific antitoxins and amplification of the botulinum neurotoxin (BoNT) gene. One of the isolates was designated as C. botulinum subtype A1B. This is the first report of isolation of C. botulinum type E and subtype A1B in seafood from India. The presence of C. botulinum in the products tested can be a significant public health hazard since the organism can grow when exposed to favourable conditions and can produce neurotoxin in the food.

An Effective Prophylactic and Therapeutic Protection Against Botulinum Type A Intoxication in Mice and Rabbits Using a Humanized Monoclonal Antibody

Botulinum neurotoxins (BoNTs) are the most potent toxins on Earth and are classified as Category A biological agents. BoNTs lead to paralysis in humans and cause botulism. Antibody therapeutics can effectively treat toxin-mediated infectious diseases. In this study, we generated a pharmaceutical humanized monoclonal antibody (HZ45 mAb) to prevent or treat botulism. HZ45 binds to the heavy chain receptor (HCR) domain of the toxin, preventing the toxin from entering the cell. The mAb was produced using hybridoma technology and phage display. We evaluated HZ45 mAb for the neutralization of BoNT serotype A (BoNT/A) in mice and rabbits. The survival results showed that pretreatment with HZ45 mAb provided 100% protection at a dose of 0.1 mg per mouse against a maximum of 100 LD50 of BoNT/A. To assess the therapeutic efficacy of HZ45 mAb in New Zealand white rabbits (NZWs), a 5 mg dose was administered 4 or 8 h after challenge with 10 LD50. The results indicated that 5 mg of HZ45 could treat the NZWs within 8 h after exposure to 10 LD50 botulinum. Consequently, in an in vivo context, including mice and rabbits, HZ45 mAb could protect against botulinum type A intoxication.

SiO2/Si interferometers designed for on-site botulinum neurotoxin serotypes B and C quantification and biological activity assessment

Botulinum neurotoxins (BoNT), the agent causing botulism, exhibit the highest potency among bacterial toxins and pose a significant threat to both humans and animals. The current in vivo method (mouse lethality assay, MLA) is inappropriate for real-time and pen-side assessment of the occurring outbreak or case. Herein, we describe a reflective-based biosensor capable of detecting the toxin's type and activity state by competitive immunoassay and endopeptidase activity, respectively. Two serotypes (BoNT-B and BoNT-C) were assessed independently by SiO2/Si interferometers using simulating antigens or specific peptides (i.e., synaptosomal-associated protein and vesicle-associated membrane protein). The reflective responses of both platforms were biochemically amplified using 4-chloro-1-naphthol oxidation into insoluble products. The resulting refractive index variations were observed in real-time using reflective interferometric Fourier transform spectroscopy. The portable bioassay depicted high sensitivity, selectivity, and near real-time rapid assessment (0.14 and 0.47 pg mL-1 for BoNT-C and BoNT-B, negligible interferences with interfering serotypes, respectively). Additionally, the applicability of the bioassay for monitoring extracted toxins was tested against the in vivo method with satisfactory performance values. Overall, the presented approach can improve field assessment of new botulism incidences by acquiring epidemiological data in near real-time, thus eliminating the need for the MLA approach.

Recombinant Detoxified Holotoxin as a Potent Candidate Vaccine Against Botulism

Vaccination may represent a suitable strategy for preventing botulism. The recombinant expression of toxin functional domains can induce effective immune responses against botulism. This study aims to develop a safe and effective recombinant detoxified full-length BoNTA vaccine. In this study, we engineered and mutated the toxin activity-related sites on the basis of the full-length BoNTA protein and constructed three detoxified full-length toxin mutants. They were recombinant expressed and purified in Escherichia coli, and the BoNTA/M4 mutant was determined to have the highest safety, with a murine lethal dose of 50% (MLD50). The M4 protein was used as the antigen for three immunizations, and the serum titers, neutralizing activity, and BoNTA protective effects of immunized mice were evaluated. The results show that, in comparison to the receptor-binding domain BoNTA/Hc protein, the full-length detoxified mutant M4 protein exhibited superior immunogenicity and could induce higher levels of specific antibodies, and the resulting immune serum could effectively protect mice against higher doses of BoNTA challenge. This study laid the foundation for research on a novel recombinant detoxified full-length botulinum toxin vaccine.

Expression of Recombinant Clostridial Neurotoxin by C. tetani

Tetanus neurotoxins (TeNT) and botulinum neurotoxins (BoNTs) are closely related ~150 kDa protein toxins that together comprise the group of clostridial neurotoxins (CNTs) expressed by various species of Clostridia. While TeNT is expressed as a single polypeptide, BoNTs are always produced alongside multiple non-toxic proteins that form a stabilizing complex with BoNT and are encoded in a conserved toxin gene cluster. It is unknown how tent evolved without a similar gene cluster and why complex-free TeNT is secreted as a stable and soluble protein by C. tetani, whereas complexing proteins appear to be essential for BoNT stability in culture supernatants of C. botulinum. To assess whether the stability of TeNT is due to an innate property of the toxin or is a result of C. tetani's intra- and extra-cellular environment, both TeNT and complex-free BoNT/A1ERY were expressed recombinantly in atoxic C. tetani and analyzed for expression and stability. The strong clostridial ferredoxin (fdx) promotor resulted in the expression of recombinant TeNT at greater levels and earlier time points than endogenously produced TeNT. Recombinant BoNT/A1ERY was similarly expressed by atoxic C. tetani, although partial degradation was observed. The rBoNT/A1ERY produced in C. tetani was also partially proteolytically processed to the dichain form. Investigations of bacterial growth media and pH conditions found that the stability of rTeNT and rBoNT/A1ERY in spent media of C. tetani or C. botulinum was affected by growth media but not by pH. These data indicate that the distinct metabolism of C. tetani or C. botulinum under various growth conditions is a primary factor in creating a more or less favorable environment for complex-free CNT stability.

Comparative Phylogenetic Analysis and Protein Prediction Reveal the Taxonomy and Diverse Distribution of Virulence Factors in Foodborne Clostridium Strains

Background

Clostridium botulinum and Clostridium perfringens, 2 major foodborne pathogenic fusobacteria, have a variety of virulent protein types with nervous and enterotoxic pathogenic potential, respectively.

Objective

The relationship between the molecular evolution of the 2 Clostridium genomes and virulence proteins was studied via a bioinformatics prediction method. The genetic stability, main features of gene coding and structural characteristics of virulence proteins were compared and analyzed to reveal the phylogenetic characteristics, diversity, and distribution of virulence factors of foodborne Clostridium strains.

Methods

The phylogenetic analysis was performed via composition vector and average nucleotide identity based methods. Evolutionary distances of virulence genes relative to those of housekeeping genes were calculated via multilocus sequence analysis. Bioinformatics software and tools were used to predict and compare the main functional features of genes encoding virulence proteins, and the structures of virulence proteins were predicted and analyzed through homology modeling and a deep learning algorithm.

Results

According to the diversity of toxins, genome evolution tended to cluster based on the protein-coding virulence genes. The evolutionary transfer distances of virulence genes relative to those of housekeeping genes in C. botulinum strains were greater than those in C. perfringens strains, and BoNTs and alpha toxin proteins were located extracellularly. The BoNTs have highly similar structures, but BoNT/A/B and BoNT/E/F have significantly different conformations. The beta2 toxin monomer structure is similar to but simpler than the alpha toxin monomer structure, which has 2 mobile loops in the N-terminal domain. The C-terminal domain of the CPE trimer forms a "claudin-binding pocket" shape, which suggests biological relevance, such as in pore formation.

Conclusions

According to the genotype of protein-coding virulence genes, the evolution of Clostridium showed a clustering trend. The genetic stability, functional and structural characteristics of foodborne Clostridium virulence proteins reveal the taxonomy and diverse distribution of virulence factors.

Real-time PCR assays that detect genes for botulinum neurotoxin A-G subtypes

The role of Real-Time PCR assays for surveillance and rapid screening for pathogens is garnering more and more attention because of its versatility and ease of adoption. The goal of this study was to design, test, and evaluate Real-Time TaqMan PCR assays for the detection of botulinum neurotoxin (bont/A-G) genes from currently recognized BoNT subtypes. Assays were computationally designed and then laboratory tested for sensitivity and specificity using DNA preparations containing bont genes from 82 target toxin subtypes, including nine bivalent toxin types; 31 strains representing other clostridial species; and an extensive panel that consisted of DNA from a diverse set of prokaryotic (bacterial) and eukaryotic (fungal, protozoan, plant, and animal) species. In addition to laboratory testing, the assays were computationally evaluated using in silico analysis for their ability to detect bont gene sequences from recently identified toxin subtypes. Seventeen specific assays (two for each of the bont/C, bont/D, bont/E, and bont/G subtypes and three for each of the bont/A, bont/B, and bont/F subtypes) were designed and evaluated for their ability to detect bont genes encoding multiple subtypes from all seven serotypes. These assays could provide an additional tool for the detection of botulinum neurotoxins in clinical, environmental and food samples that can complement other existing methods used in clinical diagnostics, regulatory, public health, and research laboratories.

The impact of orphan histidine kinases and phosphotransfer proteins on the regulation of clostridial sporulation initiation

Sporulation is an important feature of the clostridial life cycle, facilitating survival of these bacteria in harsh environments, contributing to disease transmission for pathogenic species, and sharing common early steps that are also involved in regulating industrially important solvent production by some non-pathogenic species. Initial genomics studies suggested that Clostridia lack the classical phosphorelay that phosphorylates Spo0A and initiates sporulation in Bacillus, leading to the hypothesis that sporulation in Clostridia universally begins when Spo0A is phosphorylated by orphan histidine kinases (OHKs). However, components of the classical Bacillus phosphorelay were recently identified in some Clostridia. Similar Bacillus phosphorelay components have not yet been found in the pathogenic Clostridia or the solventogenic Clostridia of industrial importance. For some of those Clostridia lacking a classical phosphorelay, the involvement of OHKs in sporulation initiation has received support from genetic studies demonstrating the involvement of several apparent OHKs in their sporulation. In addition, several clostridial OHKs directly phosphorylate Spo0A in vitro. Interestingly, there is considerable protein domain diversity among the sporulation-associated OHKs in Clostridia. Further adding to the emergent complexity of sporulation initiation in Clostridia, several candidate OHK phosphotransfer proteins that were OHK candidates were shown to function as phosphatases that reduce sporulation in some Clostridia. The mounting evidence indicates that no single pathway explains sporulation initiation in all Clostridia and supports the need for further study to fully understand the unexpected and biologically fascinating mechanistic diversity of this important process among these medically and industrially important bacteria.

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.

Pathogenicity and virulence of Clostridium botulinum

Clostridium botulinum, a polyphyletic Gram-positive taxon of bacteria, is classified purely by their ability to produce botulinum neurotoxin (BoNT). BoNT is the primary virulence factor and the causative agent of botulism. A potentially fatal disease, botulism is classically characterized by a symmetrical descending flaccid paralysis, which is left untreated can lead to respiratory failure and death. Botulism cases are classified into three main forms dependent on the nature of intoxication; foodborne, wound and infant. The BoNT, regarded as the most potent biological substance known, is a zinc metalloprotease that specifically cleaves SNARE proteins at neuromuscular junctions, preventing exocytosis of neurotransmitters, leading to muscle paralysis. The BoNT is now used to treat numerous medical conditions caused by overactive or spastic muscles and is extensively used in the cosmetic industry due to its high specificity and the exceedingly small doses needed to exert long-lasting pharmacological effects. Additionally, the ability to form endospores is critical to the pathogenicity of the bacteria. Disease transmission is often facilitated via the metabolically dormant spores that are highly resistant to environment stresses, allowing persistence in the environment in unfavourable conditions. Infant and wound botulism infections are initiated upon germination of the spores into neurotoxin producing vegetative cells, whereas foodborne botulism is attributed to ingestion of preformed BoNT. C. botulinum is a saprophytic bacterium, thought to have evolved its potent neurotoxin to establish a source of nutrients by killing its host.

Exploring the genetic background of the botulism neurotoxin BoNT/B2 in Spain

To determine whether the neurotoxin BoNT/B2 causing botulism in Spain is clonal, the genetic diversity and phylogenetic relationships of Clostridium botulinum from food-borne episodes and infant cases of the condition were explored. The botulinum toxin gene (bont) subtype, the variable region of the flagellin gene (flaVR), and a seven-gene multi-locus sequence type were examined by sequencing 37 BoNT-positive cultures obtained over the period 2010 to 2022. Out of 37 botulism events, 16 food-borne episodes and 16 infant cases were associated with bont/b2. Eight bont/b2 alleles were detected [nucleotide distance range 0.0259-0.415%, Hunter and Gaston discrimination index (HGDI) 0.71]. The most common bont/b2 allele corresponded to that of strain Prevot 25 NCASE and its single and double locus variations (87.5%). Four known flaVR types were identified (HGDI 0.79), along with one previously unknown (flaVR-15). Sixteen sequence types (STs) (HGDI 0.89) were recorded including seven new STs (ST164-ST170; 10 new alleles) and five new STs (ST171-ST175; with new allele combinations) were also noted. Correlations among some STs and flaVR types were seen. Overall, the present results show that the combined analysis of bont/b2-flaVR-ST at the nucleotide level could be used to track botulism events in Spain. The neurotoxin BoNT/B2 has largely been responsible for human botulism in Spain. The polymorphism analysis of bont/b2, flaVR typing, and sequence type determinations, revealed a wide variety of clones to be responsible for human botulism, ruling out a common source of acquisition. IMPORTANCE Botulism, a potentially fatal disease, is classically characterized by a symmetrical descending flaccid paralysis, which if left untreated can lead to respiratory failure and death. Botulinum neurotoxin (BoNT), produced by certain species of Clostridium, is the most potent biological toxin known, and the direct cause of botulism. This study characterizes the acquisition in Spain of two forms of botulism, i.e., food-borne and infant botulism, which are largely caused by the main neurotoxin BoNT/B2. Polymorphism analysis of the bont/b2 gene, typing of the flagellin variable region sequence (flaVR), and multilocus sequence typing, were used to explore the genetic background of Clostridium botulinum group I. To our knowledge, this is the first phylogenetic and typing study of botulism undertaken in Spain.

Rapid SERS Detection of Botulinum Neurotoxin Type A

Surface-enhanced Raman scattering (SERS) is a powerful technique for decoding of 2-5-component mixes of analytes. Low concentrations of analytes and complex biological media are usually non-decodable with SERS. Recognition molecules, such as antibodies and aptamers, provide an opportunity for a specific binding of ultra-low contents of analyte dissolved in complex biological media. Different approaches have been proposed to provide changes in SERS intensity of an external label upon binding of ultra-low contents of the analytes. In this paper, we propose a SERS-based sensor for the rapid and sensitive detection of botulinum toxin type A. The silver nanoisland SERS substrate was functionalized using an aptamer conjugated with a Raman label. The binding of the target affects the orientation of the label, providing changes in an analytical signal. This trick allowed detecting botulinum toxin type A in a one-stage manner without additional staining with a monotonous dose dependence and a limit of detection of 2.4 ng/mL. The proposed sensor architecture is consistent with the multiarray detection systems for multiplex analyses.

Recent Developments in Vaccine Design: From Live Vaccines to Recombinant Toxin Vaccines

Vaccines are one of the most effective strategies to prevent pathogen-induced illness in humans. The earliest vaccines were based on live inoculations with low doses of live or related pathogens, which carried a relatively high risk of developing the disease they were meant to prevent. The introduction of attenuated and killed pathogens as vaccines dramatically reduced these risks; however, attenuated live vaccines still carry a risk of reversion to a pathogenic strain capable of causing disease. This risk is completely eliminated with recombinant protein or subunit vaccines, which are atoxic and non-infectious. However, these vaccines require adjuvants and often significant optimization to induce robust T-cell responses and long-lasting immune memory. Some pathogens produce protein toxins that cause or contribute to disease. To protect against the effects of such toxins, chemically inactivated toxoid vaccines have been found to be effective. Toxoid vaccines are successfully used today at a global scale to protect against tetanus and diphtheria. Recent developments for toxoid vaccines are investigating the possibilities of utilizing recombinant protein toxins mutated to eliminate biologic activity instead of chemically inactivated toxins. Finally, one of the most contemporary approaches toward vaccine design utilizes messenger RNA (mRNA) as a vaccine candidate. This approach was used globally to protect against coronavirus disease during the COVID-19 pandemic that began in 2019, due to its advantages of quick production and scale-up, and effectiveness in eliciting a neutralizing antibody response. Nonetheless, mRNA vaccines require specialized storage and transport conditions, posing challenges for low- and middle-income countries. Among multiple available technologies for vaccine design and formulation, which technology is most appropriate? This review focuses on the considerable developments that have been made in utilizing diverse vaccine technologies with a focus on vaccines targeting bacterial toxins. We describe how advancements in vaccine technology, combined with a deeper understanding of pathogen-host interactions, offer exciting and promising avenues for the development of new and improved vaccines.

Navigating the Complexities Involving the Identification of Botulinum Neurotoxins (BoNTs) and the Taxonomy of BoNT-Producing Clostridia

Botulinum neurotoxins are a varied group of protein toxins that share similar structures and modes of activity. They include at least seven serotypes and over forty subtypes that are produced by seven different clostridial species. These bacterial species are not limited strictly to BoNT-producers as neuro-toxigenic and non-neuro-toxigenic members have been identified within each species. The nomenclature surrounding these toxins and associated bacteria has been evolving as new isolations and discoveries have arisen, resulting in challenges in diagnostic reporting, epidemiology and food safety studies, and in the application of therapeutic products. An understanding of the intricacies regarding the nomenclature of BoNTs and BoNT-producing clostridia is crucial for communication that allows for accurate reporting of information that is pertinent to each situation.

Characterization of a novel tetravalent botulism antitoxin based on receptor-binding domain of BoNTs

Botulinum neurotoxin (BoNTs; serotypes A, B, E, and F) cause botulism disease in humans, which could be effectively treated using antitoxins. Herein, we established a novel receptor-binding domain (RBD)-based antitoxin using recombinant C terminal heavy chain (Hc) domains of BoNTs as immunogens. Immunization of horses with these recombinant Hc domains allowed the purification and digestion of IgGs from hyper-immune sera to produce high-quality and high-efficiency monovalent botulism antitoxin F(ab')₂ against each BoNT (M-BATs). However, these M-BATs could not bind or neutralize other serotypes of BoNTs, and that there were no cross-protective effects among these M-BATs. This suggested the need to prepare tetravalent antitoxins to neutralize the four BoNTs simultaneously. Thus, these M-BATs were formulated into a novel tetravalent botulism antitoxin (T-BAT), in which a 10-ml volume contained 10000 IU of BoNT/A and 5000 IU of BoNT/B, BoNT/E, and BoNT/F antitoxins. The novel antitoxin preparation could prevent and treat the four mixed botulinum neurotoxins simultaneously in vivo, representing strong efficacy in an animal poisoning model. Moreover, these antibodies in T-BAT could bind the RBD, whereas conventional antitoxins based on inactivated toxins mainly bind the light chain or heavy chain translocation domain (HN) and weakly bind the important RBD in current experimental conditions. The high levels of RBD-specific novel antitoxins can efficiently bind the RBD and neutralize natural or recombinant toxins containing this RBD. The findings of the present study experimentally support the use of RBD-specific antitoxins to treat BoNT serotype A, B, E, and F-mediated botulism. This study demonstrated the concept of developing potent novel multivalent antitoxins against all BoNTs or other toxins, using the RBD of these toxins as an alternative antigen to inactivated toxins. KEY POINTS: • Antitoxins based on the receptor-binding domains of botulinum neurotoxins were made. • Novel antitoxin binds RBD; traditional antitoxin mainly binds light chain or HN domain. • A tetravalent antitoxin could prevent and treat the four mixed neurotoxins in vivo.

Molecular Diversity of BoNT-Producing Clostridia—A Still-Emerging and Challenging Problem

The diversity of BoNT-producing Clostridia is still a worrying problem for specialists who explore the evolutionary and taxonomic diversity of C. botulinum. It is also a problem for epidemiologists and laboratory staff conducting investigations into foodborne botulism in humans and animals, because their genetic and phenotypic heterogeneity cause complications in choosing the proper analytical tools and in reliably interpreting results. Botulinum neurotoxins (BoNTs) are produced by several bacterial groups that meet all the criteria of distinct species. Despite this, the historical designation of C. botulinum as the one species that produces botulinum toxins is still exploited. New genetic tools such as whole-genome sequencing (WGS) indicate horizontal gene transfer and the occurrence of botulinum gene clusters that are not limited only to Clostridium spp., but also to Gram-negative aerobic species. The literature data regarding the mentioned heterogeneity of BoNT-producing Clostridia indicate the requirement to reclassify C. botulinum species and other microorganisms able to produce BoNTs or possessing botulinum-like gene clusters. The aim of this study was to present the problem of the diversity of BoNT-producing Clostridia over time and new trends toward obtaining a reliable classification of these microorganisms, based on a complex review of the literature.

Tracing Foodborne Botulism Events Caused by Clostridium botulinum in Xinjiang Province, China, Using a Core Genome Sequence Typing Scheme

Foodborne botulism is a rare but life-threatening illness resulting from the action of a potent toxin mainly produced by Clostridium botulinum. It grows in an oxygen-deficient environment and is extremely viable in meat and soy products, making it one of the most virulent bacteria. How to track foodborne botulism events quickly and accurately has become a key issue. Here, we investigated two foodborne botulism events that occurred in Xinjiang in 2019 based on whole-genome sequencing and also successfully traced the relationship between clinical and food C. botulinum isolates using whole-genome core gene markers. All 59 isolates were classified as group I strains. Of the strains isolated in this study, 44 were found to be botulinum toxin A(B), and 15 isolates contained only the toxin B locus. Both the toxin A and B gene segments were located on the chromosome and organized in an ha cluster. Antibiotic resistance and virulence factors were also investigated. A set of 329 universal core gene markers were established using C. botulinum strains from a public database. These core gene markers were applied to the published C. botulinum genomes, and three outbreaks were identified. This work demonstrates that universal core gene markers can be used to trace foodborne botulism events, and we hope that our work will facilitate this effort in future. IMPORTANCE In this study, we analyzed 59 foodborne botulism (FB)-related strains isolated in Xinjiang Province, China. Our findings not only reveal the group classification, neurotoxin locus organization, antibiotic resistance and virulence factors of these strains but also establish a set of core gene markers for tracing foodborne botulism events, which was verified using published genomes. These findings indicate that these gene markers might be used as a potential tracing tool for FB events caused by C. botulinum group I strains, which have relatively stable genomic components.

Development and evaluation of a tetravalent botulinum vaccine

Botulinum neurotoxins (BoNTs) are the most toxic known proteins. Naturally occurring botulism in humans is caused by botulinum serotypes A, B, E, and F. Vaccination is an effective strategy to prevent botulism. In this study, a tetravalent botulinum vaccine (TBV) that can prevent serotypes A, B, E, and F was developed using the C-terminal receptor-binding domain of BoNT (Hc) as an antigen. To develop a suitable vaccine formulation, in vitro binding experiments of antigens and aluminum adjuvant in different buffers, and in vivo experiments of TBV at different antigen concentrations, were conducted. Our results showed that the optimal vaccine formulation buffer was a pH 6.0 phosphate buffer, and the suitable antigen concentration was 40 or 80 µg/ml of each antigen. A pilot-scale TBV was then prepared and evaluated for immunogenicity and stability. The results showed that TBV could elicit strong protective efficacy against each BoNT in mice, and remain effective after two years of storage at 4ºC, indicating that the preparation was stable and highly effective. Adsorption experiments also showed that the antigens could be well adsorbed by the aluminum adjuvant after 2 years of storage. Our results provide valuable experimental data supporting the development of a tetravalent botulinum vaccine, which is a promising candidate for the prevention of botulinum serotypes A, B, E, and F.

Multicolor fluorescence activated cell sorting to generate humanized monoclonal antibody binding seven subtypes of BoNT/F

Generating specific monoclonal antibodies (mAbs) that neutralize multiple antigen variants is challenging. Here, we present a strategy to generate mAbs that bind seven subtypes of botulinum neurotoxin serotype F (BoNT/F) that differ from each other in amino acid sequence by up to 36%. Previously, we identified 28H4, a mouse mAb with poor cross-reactivity to BoNT/F1, F3, F4, and F6 and with no detectable binding to BoNT/F2, F5, or F7. Using multicolor labeling of the different BoNT/F subtypes and fluorescence-activated cell sorting (FACS) of yeast displayed single-chain Fv (scFv) mutant libraries, 28H4 was evolved to a humanized mAb hu6F15.4 that bound each of seven BoNT/F subtypes with high affinity (KD 5.81 pM to 659.78 pM). In contrast, using single antigen FACS sorting, affinity was increased to the subtype used for sorting but with a decrease in affinity for other subtypes. None of the mAb variants showed any binding to other BoNT serotypes or to HEK293 or CHO cell lysates by flow cytometry, thus demonstrating stringent BoNT/F specificity. Multicolor FACS-mediated antibody library screening is thus proposed as a general method to generate multi-specific antibodies to protein subtypes such as toxins or species variants.

Draft Genome Sequences of Two Clostridium botulinum Group II Strains Carrying Phage-Like Plasmids

Clostridium botulinum is responsible for botulism, a potentially lethal foodborne intoxication. Here, we report the draft genome sequences of C. botulinum group II strains 202F (serotype F) and Hazen (serotype E). The genomes share many similarities, including multiple mobile genetic elements.

Crystal Structures of Botulinum Neurotoxin Subtypes A4 and A5 Cell Binding Domains in Complex with Receptor Ganglioside

Botulinum neurotoxins (BoNT) cause the potentially fatal neuroparalytic disease botulism that arises due to proteolysis of a SNARE protein. Each BoNT is comprised of three domains: a cell binding domain (H_C), a translocation domain (H_N), and a catalytic (Zn²⁺ endopeptidase) domain (LC). The H_C is responsible for neuronal specificity by targeting both a protein and ganglioside receptor at the neuromuscular junction. Although highly toxic, some BoNTs are commercially available as therapeutics for the treatment of a range of neuromuscular conditions. Here we present the crystal structures of two BoNT cell binding domains, H_C/A4 and H_C/A5, in a complex with the oligosaccharide of ganglioside, GD1a and GM1b, respectively. These structures, along with a detailed comparison with the previously reported apo-structures, reveal the conformational changes that occur upon ganglioside binding and the interactions involved.

Extensive Genome Exploration of Clostridium botulinum Group III Field Strains

In animals, botulism is commonly sustained by botulinum neurotoxin C, D or their mosaic variants, which are produced by anaerobic bacteria included in Clostridium botulinum group III. In this study, a WGS has been applied to a large collection of C. botulinum group III field strains in order to expand the knowledge on these BoNT-producing Clostridia and to evaluate the potentiality of this method for epidemiological investigations. Sixty field strains were submitted to WGS, and the results were analyzed with respect to epidemiological information and compared to published sequences. The strains were isolated from biological or environmental samples collected in animal botulism outbreaks which occurred in Italy from 2007 to 2016. The new sequenced strains belonged to subspecific groups, some of which were already defined, while others were newly characterized, peculiar to Italian strains and contained genomic features not yet observed. This included, in particular, two new flicC types (VI and VII) and new plasmids which widen the known plasmidome of the species. The extensive genome exploration shown in this study improves the C. botulinum and related species classification scheme, enriching it with new strains of rare genotypes and permitting the highest grade of discrimination among strains for forensic and epidemiological applications.

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.

A Four-Monoclonal Antibody Combination Potently Neutralizes Multiple Botulinum Neurotoxin Serotypes C and D

Human botulism can be caused by botulinum neurotoxin (BoNT) serotypes A to G. Here, we present an antibody-based antitoxin composed of four human monoclonal antibodies (mAbs) against BoNT/C, BoNT/D, and their mosaic toxins. This work built on our success in generating protective mAbs to BoNT /A, B and E serotypes. We generated mAbs from human immune single-chain Fv (scFv) yeast-display libraries and isolated scFvs with high affinity for BoNT/C, BoNT/CD, BoNT/DC and BoNT/D serotypes. We identified four mAbs that bound non-overlapping epitopes on multiple serotypes and mosaic BoNTs. Three of the mAbs underwent molecular evolution to increase affinity. A four-mAb combination provided high-affinity binding and BoNT neutralization of both serotypes and their mosaic toxins. The mAbs have potential utility as therapeutics and as diagnostics capable of recognizing and neutralizing BoNT/C and BoNT/D serotypes and their mosaic toxins. A derivative of the four-antibody combination (NTM-1634) completed a Phase 1 clinical trial (Snow et al., Antimicrobial Agents and Chemotherapy, 2019) with no drug-related serious adverse events.

Genetic Characterization of Clostridium botulinum Isolated from the First Case of Infant Botulism in Korea

Botulism is a neuroparalytic disease caused by a neurotoxin produced by Clostridium botulinum. This study aimed to genetically characterize C. botulinum strain isolated from the first case of infant botulism in Korea reported on June 17, 2019. We isolated C. botulinum strain CB-27 from a stool sample of the patient and analyzed the toxin types and toxin gene cluster compositions of the strain using a mouse bioassay, real-time PCR, and genome sequencing. Toxin gene cluster analysis showed that strain CB-27 possesses a C. botulinum neurotoxin type A harboring an unexpressed B gene. Although the nucleotide and amino acid sequences of toxin genes as well as the toxin gene cluster arrangements in strain CB-27 were identical to those of the known strain CDC_69094, the total nucleotide sequences of the toxin gene clusters of CB-27 differed from those of CDC_69094 by 0.47%, indicating genetic diversity of toxin gene clusters of CB-27 among other previously reported C. botulinum strains. To our knowledge, this is the first description of a C. botulinum strain with two separate toxin gene clusters in Korea.

Clostridium botulinum Type B Isolated From a Wound Botulism Case Due to Injection Drug Use Resembles Other Local Strains Originating From Hawaii

Clostridium botulinum produces botulinum neurotoxin (BoNT), which can lead to death if untreated. In the United States, over 90% of wound botulism cases are associated with injection drug use of black tar heroin. We sought to determine the phylogenetic relatedness of C. botulinum isolated from an injection drug use wound botulism case and isolates from endogenous infant botulism cases in Hawaii. Nineteen C. botulinum type B isolates from Hawaii and one type B isolate from California were analyzed by whole-genome sequencing. The botulinum toxin gene (bont) subtype was determined using CLC Genomics Workbench, and the seven-gene multi-locus sequence type (MLST) was identified by querying PubMLST. Mashtree and pairwise average nucleotide identity were used to find nearest neighbors, and Lyve-SET approximated a phylogeny. Eighteen of the isolates harbored the bont/B5 gene: of those, 17 were classified as sequence type ST36 and one was classified as ST104. A single isolate from Hawaii harbored bont/B1 and was determined to belong to ST110, and the isolate from California harbored bont/B1 and belonged to ST30. A tree constructed with Lyve-SET showed a high degree of homology among all the Hawaiian C. botulinum isolates that harbor the bont/B5 gene. Our results indicate that the bont/B-expressing isolates recovered from Hawaii are closely related to each other, suggesting local contamination of the drug paraphernalia or the wound itself with spores rather than contamination of the drug at manufacture or during transport. These findings may assist in identifying interventions to decrease wound botulism among persons who inject drugs.

Genomic Characterization of Strains From a Cluster of Infant Botulism Type A in a Small Town in Colorado, United States

Three cases of infant botulism were reported in a small Colorado town between 1981 and 1984. The first two cases occurred in 1981, 6 months apart, and the third case occurred in 1984. Clostridium botulinum type A was isolated from stool of all three case patients and from environmental samples of the patient's homes. An epidemiological investigation and follow-up study were conducted from 1981 to 1986 and concluded the cases were likely related. In this study, we sought to determine whether the C. botulinum type A clinical isolates were related to each other and to isolates obtained from environmental samples. We performed whole genome sequencing (WGS) for 17 isolates associated with this potential cluster of infant botulism. Fifteen isolates were confirmed to be C. botulinum type A(B) and contained botulinum toxin gene subtypes A1 and B5 by WGS; these strains formed a monophyletic cluster in a phylogeny and were considered closely related to each other (0-18 high-quality single-nucleotide polymorphisms), but distinct from other C. botulinum type A(B) in Colorado and elsewhere in the United States. Results of our study suggest that the three infant botulism cases could have represented a cluster due to a C. botulinum type A(B) strain present in the environment.

Rapid Detection of Clostridium botulinum in Food Using Loop-Mediated Isothermal Amplification (LAMP)

Botulinum neurotoxins are considered as one of the most potent toxins and are produced by Clostridium botulinum. It is crucial to have a rapid and sensitive method to detect the bacterium Clostridium botulinum in food. In this study, a rapid detection assay of C. botulinum in food using loop-mediated isothermal amplification (LAMP) technology was developed. The optimal primers were identified among three sets of primers designed specifically based on the partial ntnh gene encoding nontoxic-nonhaemagglutinin (NTNH) for rapid detection of the target DNA in plasmids. The optimal temperature and reaction time of the LAMP assay were determined to be 64 °C and 60 min, respectively. The chemical kit could be assembled based on these optimized reaction conditions for quick, initial high-throughput screening of C. botulinum in food samples. The established LAMP assay showed high specificity and sensitivity in detecting the target DNA with a limit of 0.0001 pg/ul (i.e., ten times more sensitive than that of the PCR method) and an accuracy rate of 100%. This study demonstrated a potentially rapid, cost-effective, and easy-operating method to detect C. botulinum in food and clinical samples based on LAMP technology.

The Distinctive Evolution of orfX Clostridium parabotulinum Strains and Their Botulinum Neurotoxin Type A and F Gene Clusters Is Influenced by Environmental Factors and Gene Interactions via Mobile Genetic Elements

Of the seven currently known botulinum neurotoxin-producing species of Clostridium, C. parabotulinum, or C. botulinum Group I, is the species associated with the majority of human botulism cases worldwide. Phylogenetic analysis of these bacteria reveals a diverse species with multiple genomic clades. The neurotoxins they produce are also diverse, with over 20 subtypes currently represented. The existence of different bont genes within very similar genomes and of the same bont genes/gene clusters within different bacterial variants/species indicates that they have evolved independently. The neurotoxin genes are associated with one of two toxin gene cluster types containing either hemagglutinin (ha) genes or orfX genes. These genes may be located within the chromosome or extrachromosomal elements such as large plasmids. Although BoNT-producing C parabotulinum bacteria are distributed globally, they are more ubiquitous in certain specific geographic regions. Notably, northern hemisphere strains primarily contain ha gene clusters while southern hemisphere strains have a preponderance of orfX gene clusters. OrfX C. parabotulinum strains constitute a subset of this species that contain highly conserved bont gene clusters having a diverse range of bont genes. While much has been written about strains with ha gene clusters, less attention has been devoted to those with orfX gene clusters. The recent sequencing of 28 orfX C. parabotulinum strains and the availability of an additional 91 strains for analysis provides an opportunity to compare genomic relationships and identify unique toxin gene cluster characteristics and locations within this species subset in depth. The mechanisms behind the independent processes of bacteria evolution and generation of toxin diversity are explored through the examination of bacterial relationships relating to source locations and evidence of horizontal transfer of genetic material among different bacterial variants, particularly concerning bont gene clusters. Analysis of the content and locations of the bont gene clusters offers insights into common mechanisms of genetic transfer, chromosomal integration, and development of diversity among these genes.

Genomic Characterization of Newly Completed Genomes of Botulinum Neurotoxin-Producing Species from Argentina, Australia, and Africa

Botulinum neurotoxin-producing clostridia are diverse in the types of toxins they produce as well as in their overall genomic composition. They are globally distributed, with prevalent species and toxin types found within distinct geographic regions, but related strains containing the same toxin types may also be located on distinct continents. The mechanisms behind the spread of these bacteria and the independent movements of their bont genes may be understood through examination of their genetic backgrounds. The generation of 15 complete genomic sequences from bacteria isolated in Argentina, Australia, and Africa allows for a thorough examination of genome features, including overall relationships, bont gene cluster locations and arrangements, and plasmid comparisons, in bacteria isolated from various areas in the southern hemisphere. Insights gained from these examinations provide an understanding of the mechanisms behind the independent movements of these elements among distinct species.

Structural and Biochemical Characterization of Botulinum Neurotoxin Subtype B2 Binding to Its Receptors

Botulinum neurotoxins (BoNTs) can be used therapeutically to treat a wide range of neuromuscular and neurological conditions. A collection of natural BoNT variants exists which can be classified into serologically distinct serotypes (BoNT/B), and further divided into subtypes (BoNT/B1, B2, …). BoNT subtypes share a high degree of sequence identity within the same serotype yet can display large variation in toxicity. One such example is BoNT/B2, which was isolated from Clostridium botulinum strain 111 in a clinical case of botulism, and presents a 10-fold lower toxicity than BoNT/B1. In an effort to understand the molecular mechanisms behind this difference in potency, we here present the crystal structures of BoNT/B2 in complex with the ganglioside receptor GD1a, and with the human synaptotagmin I protein receptor. We show, using receptor-binding assays, that BoNT/B2 has a slightly higher affinity for GD1a than BoNT/B1, and confirm its considerably weaker affinity for its protein receptors. Although the overall receptor-binding mechanism is conserved for both receptors, structural analysis suggests the lower affinity of BoNT/B2 is the result of key substitutions, where hydrophobic interactions important for synaptotagmin-binding are replaced by polar residues. This study provides a template to drive the development of future BoNT therapeutic molecules centered on assessing the natural subtype variations in receptor-binding that appears to be one of the principal stages driving toxicity.

Diversity of the Genomes and Neurotoxins of Strains of Clostridium botulinum Group I and Clostridium sporogenes Associated with Foodborne, Infant and Wound Botulism

Clostridium botulinum Group I and Clostridium sporogenes are closely related bacteria responsible for foodborne, infant and wound botulism. A comparative genomic study with 556 highly diverse strains of C. botulinum Group I and C. sporogenes (including 417 newly sequenced strains) has been carried out to characterise the genetic diversity and spread of these bacteria and their neurotoxin genes. Core genome single-nucleotide polymorphism (SNP) analysis revealed two major lineages; C. botulinum Group I (most strains possessed botulinum neurotoxin gene(s) of types A, B and/or F) and C. sporogenes (some strains possessed a type B botulinum neurotoxin gene). Both lineages contained strains responsible for foodborne, infant and wound botulism. A new C. sporogenes cluster was identified that included five strains with a gene encoding botulinum neurotoxin sub-type B1. There was significant evidence of horizontal transfer of botulinum neurotoxin genes between distantly related bacteria. Population structure/diversity have been characterised, and novel associations discovered between whole genome lineage, botulinum neurotoxin sub-type variant, epidemiological links to foodborne, infant and wound botulism, and geographic origin. The impact of genomic and physiological variability on the botulism risk has been assessed. The genome sequences are a valuable resource for future research (e.g., pathogen biology, evolution of C. botulinum and its neurotoxin genes, improved pathogen detection and discrimination), and support enhanced risk assessments and the prevention of botulism.

Pan-Genomic Analysis of Clostridium botulinum Group II (Non-Proteolytic C. botulinum) Associated with Foodborne Botulism and Isolated from the Environment

The neurotoxin formed by Clostridium botulinum Group II is a major cause of foodborne botulism, a deadly intoxication. This study aims to understand the genetic diversity and spread of C. botulinum Group II strains and their neurotoxin genes. A comparative genomic study has been conducted with 208 highly diverse C. botulinum Group II strains (180 newly sequenced strains isolated from 16 countries over 80 years, 28 sequences from Genbank). Strains possessed a single type B, E, or F neurotoxin gene or were closely related strains with no neurotoxin gene. Botulinum neurotoxin subtype variants (including novel variants) with a unique amino acid sequence were identified. Core genome single-nucleotide polymorphism (SNP) analysis identified two major lineages-one with type E strains, and the second dominated by subtype B4 strains with subtype F6 strains. This study revealed novel details of population structure/diversity and established relationships between whole-genome lineage, botulinum neurotoxin subtype variant, association with foodborne botulism, epidemiology, and geographical source. Additionally, the genome sequences represent a valuable resource for the research community (e.g., understanding evolution of C. botulinum and its neurotoxin genes, dissecting key aspects of C. botulinum Group II biology). This may contribute to improved risk assessments and the prevention of foodborne botulism.

Fully Human Monoclonal Antibodies Effectively Neutralizing Botulinum Neurotoxin Serotype B

Botulinum neurotoxin (BoNT) is the most potent natural toxin known. Of the seven BoNT serotypes (A to G), types A, B, E, and F cause human botulism. Treatment of human botulism requires the development of effective toxin-neutralizing antibodies without side effects such as serum sickness and anaphylaxis. In this study, we generated fully human monoclonal antibodies (HuMAbs) against serotype B BoNT (BoNT/B1) using a murine–human chimera fusion partner cell line named SPYMEG. Of these HuMAbs, M2, which specifically binds to the light chain of BoNT/B1, showed neutralization activity in a mouse bioassay (approximately 10 i.p. LD₅₀/100 µg of antibody), and M4, which binds to the C-terminal of heavy chain, showed partial protection. The combination of two HuMAbs, M2 (1.25 µg) and M4 (1.25 µg), was able to completely neutralize BoNT/B1 (80 i.p. LD₅₀) with a potency greater than 80 i.p. LD₅₀/2.5 µg of antibodies, and was effective both prophylactically and therapeutically in the mouse model of botulism. Moreover, this combination showed broad neutralization activity against three type B subtypes, namely BoNT/B1, BoNT/B2, and BoNT/B6. These data demonstrate that the combination of M2 and M4 is promising in terms of a foundation for new human therapeutics for BoNT/B intoxication.

High prevalence of Clostridium botulinum in vegetarian sausages

Clostridium botulinum is a significant food safety concern due to its ability to produce highly potent neurotoxin and resistant endospores. Vegetarian sausages have become a popular source of plant protein and alternative for meat products. While vegetarian sausages have not been linked to botulism, numerous outbreaks due to preserved vegetables suggest a frequent occurrence of C. botulinum spores in the raw material. The product formulation of vegetarian sausages involves limited NaCl and preservatives, and shelf-lives may be several months. The safety of vegetarian sausages thus relies mainly on heat treatment and chilled storage. The main food safety concern is C. botulinum Group II that can grow and produce toxin at refrigeration temperatures. Here we show a high overall prevalence (32%) of C. botulinum in 74 samples of vegetarian sausages from seven producers. Both Groups I and II strains and genes for neurotoxin types A, B, E and F were detected in the products. The highest cell counts (1200 spores/kg) were observed for C. botulinum Group II in products with remaining shelf-lives of 6 months at the time of purchase. We conclude that vacuum-packaged vegetarian sausage products frequently contain C. botulinum spores and may possess a high risk of C. botulinum growth and toxin production. Chilled storage below 3°C and thorough reheating before consumption are warranted.

Closing Clostridium botulinum Group I Genomes Using a Combination of Short- and Long-Reads

Clostridium botulinum is a Gram-positive, spore-forming anaerobic bacterium that produces botulinum neurotoxin (BoNT). Closing their genomes provides information about their neurotoxin clusters' arrangement(s) and their location (e.g., chromosome or plasmid) which cannot be assessed using draft genomes. Therefore, we tested the use of long-read sequencing (nanopore sequencing) in combination with short-read sequencing to close two toxin-producing strains. These genomes could be used by the Public Health Emergency Preparedness and Response staff during botulism outbreaks. The genomes of two toxin-producing C. botulinum strains, one from an environmental sample (83F_CFSAN034202) and the other from a clinical sample (CDC51232_CFSAN034200) were sequenced using MinION and MiSeq devices. The genomes, including the chromosomes and the plasmids, were closed by a combination of long-read and short-read sequencing. They belonged to different C. botulinum sequence types (STs), with 83F belonging to ST4 and CDC51232 to ST7. A whole genome single nucleotide polymorphism (SNP) analysis clustered these two strains with strains in lineage 2 (e.g., 6CDC297) and 4 (e.g., NCTC2916) from Group I, respectively. These two strains were also bivalent strains with the BoNTB and BoNTA4 clusters located in the larger plasmid for CDC51232, and the BoNTB and BoNTA1 clusters located both in the chromosome for 83F. Overall, this study showed the advantage of combining these two sequencing methods to obtain high quality closed C. botulinum genomes that could be used for SNP phylogenies (source tracking) as well as for fast identification of BoNT clusters and their gene arrangements.

Public Health Risk Associated with Botulism as Foodborne Zoonoses

Botulism is a rare but severe neurological disease in man and animals that is caused by botulinum neurotoxins (BoNTs) produced by Clostridium botulinum and atypical strains from other Clostridium and non-Clostridium species. BoNTs are divided into more than seven toxinotypes based on neutralization with specific corresponding antisera, and each toxinotype is subdivided into subtypes according to amino acid sequence variations. Animal species show variable sensitivity to the different BoNT toxinotypes. Thereby, naturally acquired animal botulism is mainly due to BoNT/C, D and the mosaic variants CD and DC, BoNT/CD being more prevalent in birds and BoNT/DC in cattle, whereas human botulism is more frequently in the types A, B and E, and to a lower extent, F. Botulism is not a contagious disease, since there is no direct transmission from diseased animals or man to a healthy subject. Botulism occurs via the environment, notably from food contaminated with C. botulinum spores and preserved in conditions favorable for C. botulinum growth and toxin production. The high prevalence of botulism types C, D and variants DC and CD in farmed and wild birds, and to a lower extent in cattle, raises the risk of transmission to human beings. However, human botulism is much rarer than animal botulism, and botulism types C and D are exceptional in humans. Only 15 cases or suspected cases of botulism type C and one outbreak of botulism type D have been reported in humans to date. In contrast, animal healthy carriers of C. botulinum group II, such as C. botulinum type E in fish of the northern hemisphere, and C. botulinum B4 in pigs, represent a more prevalent risk of botulism transmission to human subjects. Less common botulism types in animals but at risk of transmission to humans, can sporadically be observed, such as botulism type E in farmed chickens in France (1998-2002), botulism type B in cattle in The Netherlands (1977-1979), botulism types A and B in horses, or botulism type A in dairy cows (Egypt, 1976). In most cases, human and animal botulisms have distinct origins, and cross transmissions between animals and human beings are rather rare, accidental events. But, due to the severity of this disease, human and animal botulism requires a careful surveillance.

Critical Analysis of Neuronal Cell and the Mouse Bioassay for Detection of Botulinum Neurotoxins

Botulinum Neurotoxins (BoNTs) are a large protein family that includes the most potent neurotoxins known to humankind. BoNTs delivered locally in humans at low doses are widely used pharmaceuticals. Reliable and quantitative detection of BoNTs is of paramount importance for the clinical diagnosis of botulism, basic research, drug development, potency determination, and detection in clinical, environmental, and food samples. Ideally, a definitive assay for BoNT should reflect the activity of each of the four steps in nerve intoxication. The in vivo mouse bioassay (MBA) is the 'gold standard' for the detection of BoNTs. The MBA is sensitive, robust, semi-quantitative, and reliable within its sensitivity limits. Potential drawbacks with the MBA include assay-to-assay potency variations, especially between laboratories, and false positives or negatives. These limitations can be largely avoided by careful planning and performance. Another detection method that has gained importance in recent years for research and potency determination of pharmaceutical BoNTs is cell-based assays, as these assays can be highly sensitive, quantitative, human-specific, and detect fully functional holotoxins at physiologically relevant concentrations. A myriad of other in vitro BoNT detection methods exist. This review focuses on critical factors and assay limitations of the mouse bioassay and cell-based assays for BoNT detection.

Structural insights into the interaction of botulinum neurotoxin a with its neuronal receptor SV2C

A dual-receptor interaction with a polysialoganglioside and synaptic vesicle glycoprotein 2 (SV2) is required for botulinum neurotoxin A (BoNT) toxicity. Here, we review what is currently known about the BoNT/A-SV2 interaction based on structural studies. Currently, five crystal structures of the receptor-binding domain (Hc) of BoNT subtypes A1 and A2 complexed to the large luminal domain (LD4) of SV2C have been determined. On the basis of the available structures, we will discuss the importance of protein-protein and protein-carbohydrate interactions for BoNT/A toxicity as well as the high plasticity of BoNT/A for receptor recognition by tolerating a variety of side-chain interactions at the interface. A plausible explanation how receptor-binding specificity of BoNT/A may be achieved without an extensive and conserved side chain-side chain interaction network will be provided.

Why Are Botulinum Neurotoxin-Producing Bacteria So Diverse and Botulinum Neurotoxins So Toxic?

Botulinum neurotoxins (BoNTs) are the most lethal toxins among all bacterial, animal, plant and chemical poisonous compounds. Although a great effort has been made to understand their mode of action, some questions are still open. Why, and for what benefit, have environmental bacteria that accidentally interact with their host engineered so diverse and so specific toxins targeting one of the most specialized physiological processes, the neuroexocytosis of higher organisms? The extreme potency of BoNT does not result from only one hyperactive step, but in contrast to other potent lethal toxins, from multi-step activity. The cumulative effects of the different steps, each having a limited effect, make BoNTs the most potent lethal toxins. This is a unique mode of evolution of a toxic compound, the high potency of which results from multiple steps driven by unknown selection pressure, targeting one of the most critical physiological process of higher organisms.

The Structure and Classification of Botulinum Toxins

Botulinum neurotoxins (BoNTs) are a family of bacterial protein toxins produced by various Clostridium species. They are traditionally classified into seven major serotypes (BoNT/A-G). Recent progress in sequencing microbial genomes has led to an ever-growing number of subtypes, chimeric toxins, BoNT-like toxins, and remotely related BoNT homologs, constituting an expanding BoNT superfamily. Recent structural studies of BoNTs, BoNT progenitor toxin complexes, tetanus neurotoxin (TeNT), toxin-receptor complexes, and toxin-substrate complexes have provided mechanistic understandings of toxin functions and the molecular basis for their variations. The growing BoNT superfamily of toxins present a natural repertoire that can be explored to develop novel therapeutic toxins, and the structural understanding of their variations provides a knowledge basis for engineering toxins to improve therapeutic efficacy and expand their clinical applications.

SERS detection of Clostridium botulinum neurotoxin serotypes A and B in buffer and serum: Towards the development of a biodefense test platform

Botulinum neurotoxins (BoNTs) are classified at a highest degree of threat in biodefense, due largely to their high lethality. With the growing risk of biowarfare, the shortcomings of the gold standard test for these neurotoxins, the mouse bioassay, have underscored the need to develop alternative diagnostic testing strategies. This paper reports on the detection of inactivated Clostridium botulinum neurotoxin serotype A (BoNT-A) and serotype B (BoNT-B), the two most important markers of botulism infection, by using a sandwich immunoassay, gold nanoparticle labels, and surface-enhanced Raman scattering (SERS) within the context of two threat scenarios. The first scenario mimics part of the analysis needed in response to a “white powder” threat by measuring both neurotoxins in phosphate-buffered saline (PBS), a biocompatible solvent often used to recover markers dispersed in a powdered matrix. The second scenario detects the two neurotoxins in spiked human serum to assess the clinical potential of the platform. The overall goal is to develop a test applicable to both scenarios in terms of projections of required levels of detection. We demonstrate the ability to measure BoNT-A and BoNT-B in PBS at a limit of detection (LoD) of 700 pg/mL (5 pM) and 84 pg/mL (0.6 pM), respectively, and in human serum at 1200 pg/mL (8 pM) and 91 pg/mL (0.6 pM), respectively, with a time to result under 24 h. The steps required to transform this platform into an onsite biodefense screening tool that can simultaneously and rapidly detect (<1 h) these and other agents are briefly discussed.

•

Raman-based immunoassays can successfully detect botulism neurotoxins.

•

Limits of detection for botulism neurotoxins A/B rival those of the mouse bioassay.

•

Serum and liquid extracts are suitable sample matrices for the Raman assay.

Botulinum and Tetanus Neurotoxins

Botulinum neurotoxins (BoNTs) and tetanus neurotoxin (TeNT) are the most potent toxins known and cause botulism and tetanus, respectively. BoNTs are also widely utilized as therapeutic toxins. They contain three functional domains responsible for receptor-binding, membrane translocation, and proteolytic cleavage of host proteins required for synaptic vesicle exocytosis. These toxins also have distinct features: BoNTs exist within a progenitor toxin complex (PTC), which protects the toxin and facilitates its absorption in the gastrointestinal tract, whereas TeNT is uniquely transported retrogradely within motor neurons. Our increasing knowledge of these toxins has allowed the development of engineered toxins for medical uses. The discovery of new BoNTs and BoNT-like proteins provides additional tools to understand the evolution of the toxins and to engineer toxin-based therapeutics. This review summarizes the progress on our understanding of BoNTs and TeNT, focusing on the PTC, receptor recognition, new BoNT-like toxins, and therapeutic toxin engineering.

Isolation and Characterization of the Novel Botulinum Neurotoxin A Subtype 6

Botulinum neurotoxins (BoNTs) have proved to be an effective treatment for a large number of neuropathic conditions. BoNTs comprise a large family of zinc metalloproteases, but BoNT/A1 is used nearly exclusively for pharmaceutical purposes. The genetic inactivation of a second BoNT gene in the native strain enabled expression and isolation of a single BoNT/A6 from cultures. Its characterization indicated that BoNT/A subtype A6 has a long duration of action comparable to A1, while it enters neurons faster and more efficiently and remains more localized after intramuscular injection. These characteristics of BoNT/A6 are of interest for potential use of BoNT/A6 as a novel BoNT-based therapeutic that is effective and has a fast onset, an improved safety profile, and a long duration of action. Use of BoNT/A6 as a pharmaceutical also has the potential to reveal novel treatment motifs compared to currently used treatments.

Botulinum neurotoxins (BoNTs), the most potent toxins known to humans and the causative agent of botulism, exert their effect by entering motor neurons and cleaving and inactivating SNARE proteins, which are essential for neurotransmitter release. BoNTs are proven, valuable pharmaceuticals used to treat more than 200 neuronal disorders. BoNTs comprise 7 serotypes and more than 40 isoforms (subtypes). BoNT/A1 is the only A-subtype used clinically due to its high potency and long duration of action. While other BoNT/A subtypes have been purified and described, only BoNT/A2 is being investigated as an alternative to BoNT/A1. Here we describe subtype BoNT/A6 with improved pharmacological properties compared to BoNT/A1. It was isolated from Clostridium botulinum CDC41370, which produces both BoNT/B2 and BoNT/A6. The gene encoding BoNT/B2 was genetically inactivated, and A6 was isolated to greater than 95% purity. A6 was highly potent in cultured primary rodent neuronal cultures and in human induced pluripotent stem cell-derived neurons, requiring 20-fold less toxin to cause 50% SNAP-25 cleavage than A1. Second, A6 entered hiPSCs faster and more efficiently than A1 and yet had a long duration of action similar to BoNT/A1. Third, BoNT/A6 had similar LD₅₀ as BoNT/A1 after intraperitoneal injection in mice; however, local intramuscular injection resulted in less systemic toxicity than BoNT/A1 and a higher (i.m.) LD₅₀, indicating its potential as a safer pharmaceutical. These data suggest novel characteristics of BoNT/A6 and its potential as an improved pharmaceutical due to more efficient neuronal cell entry, greater ability to remain localized at the injection site, and a long duration.

IMPORTANCE Botulinum neurotoxins (BoNTs) have proved to be an effective treatment for a large number of neuropathic conditions. BoNTs comprise a large family of zinc metalloproteases, but BoNT/A1 is used nearly exclusively for pharmaceutical purposes. The genetic inactivation of a second BoNT gene in the native strain enabled expression and isolation of a single BoNT/A6 from cultures. Its characterization indicated that BoNT/A subtype A6 has a long duration of action comparable to A1, while it enters neurons faster and more efficiently and remains more localized after intramuscular injection. These characteristics of BoNT/A6 are of interest for potential use of BoNT/A6 as a novel BoNT-based therapeutic that is effective and has a fast onset, an improved safety profile, and a long duration of action. Use of BoNT/A6 as a pharmaceutical also has the potential to reveal novel treatment motifs compared to currently used treatments.

Variations in the Botulinum Neurotoxin Binding Domain and the Potential for Novel Therapeutics

Botulinum neurotoxins (BoNTs) are categorised into immunologically distinct serotypes BoNT/A to /G). Each serotype can also be further divided into subtypes based on differences in amino acid sequence. BoNTs are ~150 kDa proteins comprised of three major functional domains: an N-terminal zinc metalloprotease light chain (LC), a translocation domain (H_N), and a binding domain (H_C). The H_C is responsible for targeting the BoNT to the neuronal cell membrane, and each serotype has evolved to bind via different mechanisms to different target receptors. Most structural characterisations to date have focussed on the first identified subtype within each serotype (e.g., BoNT/A1). Subtype differences within BoNT serotypes can affect intoxication, displaying different botulism symptoms in vivo, and less emphasis has been placed on investigating these variants. This review outlines the receptors for each BoNT serotype and describes the basis for the highly specific targeting of neuronal cell membranes. Understanding receptor binding is of vital importance, not only for the generation of novel therapeutics but also for understanding how best to protect from intoxication.

Botulinum Neurotoxin-Producing Bacteria. Isn't It Time that We Called a Species a Species?

Botulinum neurotoxins (BoNTs) are produced by a diverse set of seven clostridial species, though alternate naming systems have developed over the last 100 years. Starting in the 1950s, a single-species taxonomy where any bacterium producing BoNT would be designated Clostridium botulinum was introduced.

Botulinum neurotoxins (BoNTs) are produced by a diverse set of seven clostridial species, though alternate naming systems have developed over the last 100 years. Starting in the 1950s, a single-species taxonomy where any bacterium producing BoNT would be designated Clostridium botulinum was introduced. As the extreme diversity of these strains was recognized, a secondary system of taxonomic “groups” evolved. It became clear that these groups also had members that did not produce BoNT, and in some cases, they were given formal species names. Genomic analysis now clearly identifies species affiliations whether an isolate is toxigenic or not. It is clear that C. botulinum group nomenclature is no longer appropriate and that there are recognized species names for each clostridium. We advocate for the use of the scientific binomials and that the single-species group nomenclature be abandoned.

Therapeutic Antibodies for Biodefense

Diseases can be caused naturally by biological agents such as bacteria, viruses and toxins (natural risk). However, such biological agents can be intentionally disseminated in the environment by a State (military context) or terrorists to cause diseases in a population or livestock, to destabilize a nation by creating a climate of terror, destabilizing the economy and undermining institutions. Biological agents can be classified according to the severity of illness they cause, its mortality and how easily the agent can be spread. The Centers for Diseases Control and Prevention (CDC) classify biological agents in three categories (A, B and C); Category A consists of the six pathogens most suitable for use as bioweapons (Bacillus anthracis, Yersinia pestis, Francisella tularensis, botulinum neurotoxins, smallpox and viral hemorrhagic fevers). Antibodies represent a perfect biomedical countermeasure as they present both prophylactic and therapeutic properties, act fast and are highly specific to the target. This review focuses on the main biological agents that could be used as bioweapons, the history of biowarfare and antibodies that have been developed to neutralize these agents.

Botulinum Neurotoxin Diversity from a Gene-Centered View

Botulinum neurotoxins (BoNTs) rank amongst the most potent toxins known. The factors responsible for the emergence of the many known and yet unknown BoNT variants remain elusive. It also remains unclear why anaerobic bacteria that are widely distributed in our environment and normally do not pose a threat to humans, produce such deadly toxins. Even the possibility of accidental toxicity to humans has not been excluded. Here, I review the notion that BoNTs may have specifically evolved to target vertebrates. Considering the extremely complex molecular architecture of the toxins, which enables them to reach the bloodstream, to recognize and enter neurons, and to block neurotransmitter release, it seems highly unlikely that BoNT toxicity to vertebrates is a coincidence. The carcass⁻maggot cycle provides a plausible explanation for a natural role of the toxins: to enable mass reproduction of bacteria, spores, and toxins, using toxin-unaffected invertebrates, such as fly maggots, as the vectors. There is no clear correlation between toxigenicity and a selective advantage of clostridia in their natural habitat. Possibly, non-toxigenic strains profit from carcasses resulting from the action of toxigenic strains. Alternatively, a gene-centered view of toxin evolution would also explain this observation. Toxin-coding mobile genetic elements may have evolved as selfish genes, promoting their own propagation, similar to commensal viruses, using clostridia and other bacteria as the host. Research addressing the role of BoNTs in nature and the origin of toxin variability goes hand in hand with the identification of new toxin variants and the design of improved toxin variants for medical applications. These research directions may also reveal yet unknown natural antidotes against these extremely potent neurotoxins.