Improved detection methods by genetic and immunological techniques for botulinum C/D and D/C mosaic neurotoxins

A Comprehensive Structural Analysis of Clostridium botulinum Neurotoxin A Cell-Binding Domain from Different Subtypes

Botulinum neurotoxins (BoNTs) cause flaccid neuromuscular paralysis by cleaving one of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex proteins. BoNTs display high affinity and specificity for neuromuscular junctions, making them one of the most potent neurotoxins known to date. There are seven serologically distinct BoNTs (serotypes BoNT/A to BoNT/G) which can be further divided into subtypes (e.g., BoNT/A1, BoNT/A2…) based on small changes in their amino acid sequence. Of these, BoNT/A1 and BoNT/B1 have been utilised to treat various diseases associated with spasticity and hypersecretion. There are potentially many more BoNT variants with differing toxicological profiles that may display other therapeutic benefits. This review is focused on the structural analysis of the cell-binding domain from BoNT/A1 to BoNT/A6 subtypes (H_C/A1 to H_C/A6), including features such as a ganglioside binding site (GBS), a dynamic loop, a synaptic vesicle glycoprotein 2 (SV2) binding site, a possible Lys-Cys/Cys-Cys bridge, and a hinge motion between the H_CN and H_CC subdomains. Characterising structural features across subtypes provides a better understanding of how the cell-binding domain functions and may aid the development of novel therapeutics.

Human and animal botulism surveillance in France from 2008 to 2019

Botulism is a human and animal neurological disease caused by the action of bacterial neurotoxins (botulinum toxins) produced by bacteria from the genus Clostridium. This disease induces flaccid paralysis that can result in respiratory paralysis and heart failure. Due to its serious potential impact on public health, botulism is a closely monitored notifiable disease in France through a case-based passive surveillance system. In humans, this disease is rare, with an average of 10 outbreaks reported each year, mainly due to the consumption of contaminated foods. Type B and to a lesser extend type A are responsible for the majority of cases of foodborne botulism. Each year, an average of 30 outbreaks are recorded on poultry farms, about 20 cases in wild birds and about 10 outbreaks in cattle, involving a large number of animals. Mosaic forms C/D and D/C in birds and cattle, respectively, are the predominant types in animals in France. Types C and D have also been observed to a lesser extent in animals. With the exception of botulinum toxin E, which was exceptionally detected throughout the period in wild birds, the types of botulism found in animal outbreaks are different from those identified in human outbreaks over the last ten years in France and no human botulism outbreaks investigated have been linked to animal botulism. In line with the One Health concept, we present the first integrative approach to the routine surveillance of botulism in humans and animals in France.

Confirmation of botulism diagnosis in Australian bird samples by ELISA and RT rtPCR

We developed a sandwich ELISA that detects Clostridium botulinum C and D toxins and reverse-transcription real-time PCRs (RT-rtPCRs) that detect botulinum C and D toxin genes, respectively, to replace the mouse bioassay. The toxin genes were closely associated with the toxin molecules and used as surrogates for the presence of toxin. Samples (638) from 103 clinical cases of birds (302) with suspected botulinum toxicity came from wild birds and poultry (9 cases). Samples tested included blood serum, other body fluids, various tissues, gut contents, maggots, water, and sediment. Botulism was diagnosed in 34 cases (all of which had positive samples in the ELISA, the C toxin gene RT-rtPCR, or both assays). Botulism was suspected in 16 cases (each of which had 1 positive sample either in the ELISA or the C toxin gene RT-rtPCR). In the remaining 53 cases, no samples were positive, but botulism could not be excluded in 32 of these cases, whereas there was no indication of botulism or another diagnosis in 21 cases. The D toxin gene was not detected in any of the clinical samples. No C or D toxin genes were detected in 71 pooled cloacal swabs from 213 healthy migratory birds. The use of an ELISA that detects botulinum C and D toxins in combination with a RT-rtPCR for the botulinum C toxin gene can help confirm the diagnosis of botulism in birds.

A Case Report of a Botulism Outbreak in Beef Cattle Due to the Contamination of Wheat by a Roaming Cat Carcass: From the Suspicion to the Management of the Outbreak

Simple Summary

This study presents in detail a botulism outbreak in a beef cattle farm where the source of contamination was identified as a carcass of a roaming cat that had contaminated stored feed and initiated the botulism outbreak. In this case report, we present how the diagnosis was performed by describing the clinical signs and the analyses that were conducted; how the source of contamination was identified by analyzing samples collected on the farm, and finally, how the outbreak was managed so as to prevent recurrence of the disease and persistence of the contamination in the farm.

Abstract

We report a botulism outbreak in Charolais cattle fed with wheat flour contaminated by Clostridium botulinum type C and the management of the outbreak at each step from the clinical suspicion to the cleaning and disinfection operations. Diagnosis was based on typical suggestive clinical signs and detection of C. botulinum type C using real-time PCR in samples collected from three young affected bulls. All young exposed bulls and cows (18 animals) eventually died, but three young bulls and one cow were recovering when it was decided to euthanize them. C. botulinum type C was detected in the liver of these four animals. Analysis of the ration components demonstrated that wheat flour, wheat, and the mill used to make flour were positive for C. botulinum type C. A dead cat positive for C. botulinum type C was discovered in the silo where wheat grain was stored and was considered the source of contamination. The cat’s entire body was found mummified, well preserved, and not rotting in the silo. Specific measures, in particular, vaccination of the rest of the herd and cleaning and disinfection operations, were implemented to prevent any recurrence of the outbreak. The presence of wild animal carcasses in feed harboring anaerobic conditions like silage, in particular during harvesting, are known to be at risk for the initiation of a botulism outbreak. This outbreak is a reminder that the presence of an animal carcass in feed, regardless of the kind of feed and whenever the contamination occurs, either during harvesting or storage, is sufficient to induce a botulism outbreak.

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.

Substrate cleavage and duration of action of botulinum neurotoxin type FA ("H, HA")

Botulinum neurotoxin (BoNT) type FA is the only known naturally occurring chimeric BoNT of domains of BoNT/A and BoNT/F. BoNT/FA consists of an F5-like light chain (LC), a unique heavy chain (HC) translocation domain, and a HC receptor binding domain similar to BoNT/A1. Previous analyses of purified BoNT/FA have indicated a 5-10-fold greater potency in cultured human or rat neurons as compared to BoNT/A1 and a 400-500-fold greater potency compared to BoNT/B1. However, in vivo potency in mice was about 5-fold lower than BoNT/A1 or/B1. In this report, species specificity was examined by cell-based assays using primary neurons from mice and examining VAMP1 and 2 cleavage. The data indicated similar potency of BoNT/FA in primary mouse spinal cord neurons as previously observed in primary rat and human induced pluripotent stem cell (hiPSC) derived neuronal cell models, and equal enzymatic cleavage of mouse VAMP1 and 2 isoforms. Since the duration of action of BoNTs is due to continuous enzymatic activity of the LC in the neuronal cytosol, BoNT/FA was expected to have a short duration of action due to its F-type LC. In this report the duration of action of BoNT/FA was compared to that of BoNT/F1,/F5, and/B1 in both hiPSC derived neurons and in the in vivo mouse model. The data indicate a duration of action of BoNT/FA similar to BoNT/B1, while BoNT/F5 had a short duration of action similar to BoNT/F1.

A bovine botulism outbreak associated with a suspected cross-contamination from a poultry farm

In October 2014, an outbreak of botulism type D/C occurred on two cattle farms in close proximity. A poultry farm located nearby with no history of botulism had transferred poultry manure to both bovine farms before the beginning of the outbreak. Given this context, epidemiological investigation was conducted to determine if the poultry farm was a reservoir of C. botulinum type D/C and to identify the source of contamination on the cattle farms. Environmental samples were collected at three houses on the poultry farm (boot swabs from the surroundings, swabs from the ventilation system, boot swabs from the poultry litter and darkling beetles samples), and on the two cattle farms (silage samples, boot swabs from the cattle stalls, boot swabs from the cattle pasture and poultry manure samples). These samples were analyzed using real-time PCR after an enrichment step to detect C. botulinum type D/C. On the poultry farm, three boot swabs from the surroundings, two swabs from the ventilation system, one boot swab from the litter and one sample of darkling beetles were detected positive. On one cattle farm, C. botulinum type D/C was identified in a sample of silage made from grass grown on a field on which the poultry manure had previously been stored and in a boot swab from a pasture. On the other cattle farm, C. botulinum type D/C was detected in a sample of poultry manure stored on the cattle farm and in a boot swab from a pasture. This investigation shows that the healthy poultry farm might have been the reservoir of C. botulinum type D/C and that cross-contamination between poultry and cattle likely occurred, resulting in the botulism outbreak on the two cattle farms.

Detection, differentiation, and identification of botulinum neurotoxin serotypes C, CD, D, and DC by highly specific immunoassays and mass spectrometry

Botulinum neurotoxin (BoNT) serotypes C and D and their mosaic variants CD and DC cause severe cases of botulism in animal husbandry and wildlife. Epidemiological data on the exact serotype or toxin variant causing outbreaks are rarely available, mainly because of their high sequence identity and the lack of fast and specific screening tools to detect and differentiate the four similar toxins. To fill this gap, we developed four highly specific sandwich enzyme-linked immunosorbent assays (ELISAs) able to detect and differentiate botulinum neurotoxins type BoNT/C, D, CD, and DC based on four distinct combinations of specific monoclonal antibodies targeting both conserved and divergent subdomains of the four toxins. Here, highly sensitive detection with detection limits between 2 and 24 pg mL(-1) was achieved. The ELISAs were extensively validated and results were compared with data obtained by quantitative real-time PCR using a panel of Clostridium botulinum strains, real sample materials from veterinary botulism outbreaks, and non-BoNT-producing Clostridia. Additionally, in order to verify the results obtained by ELISA screening, the new monoclonal antibodies were used for BoNT enrichment and subsequent detection (i) on a functional level by endopeptidase mass spectrometry (Endopep-MS) assays and (ii) on a protein sequence level by LC-MS/MS spectrometry. Based on all technical information gathered in the validation study, the four differentiating ELISAs turned out to be highly reliable screening tools for the rapid analysis of veterinary botulism cases and should aid future field investigations of botulism outbreaks and the acquisition of epidemiological data.

Cytotoxicity of South-African medicinal plants towards sensitive and multidrug-resistant cancer cells

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional medicine plays a major role for primary health care worldwide. Cancer belongs to the leading disease burden in industrialized and developing countries. Successful cancer therapy is hampered by the development of resistance towards established anticancer drugs.

AIM

In the present study, we investigated the cytotoxicity of 29 extracts from 26 medicinal plants of South-Africa against leukemia cell lines, most of which are used traditionally to treat cancer and related symptoms.

MATERIAL AND METHODS

We have investigated the plant extracts for their cytotoxic activity towards drug-sensitive parental CCRF-CEM leukemia cells and their multidrug-resistant P-glycoprotein-overexpressing subline, CEM/ADR5000 by means of the resazurin assay. A panel of 60 NCI tumor cell lines have been investigated for correlations between selected phytochemicals from medicinal plants and the expression of resistance-conferring genes (ABC-transporters, oncogenes, tumor suppressor genes).

RESULTS

Seven extracts inhibited both cell lines (Acokanthera oppositifolia, Hypoestes aristata, Laurus nobilis, Leonotis leonurus, Plectranthus barbatus, Plectranthus ciliates, Salvia apiana). CEM/ADR5000 cells exhibited a low degree of cross-resistance (3.35-fold) towards the L. leonurus extract, while no cross-resistance was observed to other plant extracts, although CEM/ADR5000 cells were highly resistant to clinically established drugs. The log10IC50 values for two out of 14 selected phytochemicals from these plants (acovenoside A and ouabain) of 60 tumor cell lines were correlated to the expression of ABC-transporters (ABCB1, ABCB5, ABCC1, ABCG2), oncogenes (EGFR, RAS) and tumor suppressors (TP53). Sensitivity or resistance of the cell lines were not statistically associated with the expression of these genes, indicating that multidrug-resistant, refractory tumors expressing these genes may still respond to acovenoside A and ouabain.

CONCLUSION

The bioactivity of South African medicinal plants may represent a basis for the development of strategies to treat multidrug-resistant tumors either by phytotherapeutic approaches with whole plant preparations or by classical drug development with isolated compounds such as acovenoside A or ouabain.

Purification and Characterization of Botulinum Neurotoxin FA from a Genetically Modified Clostridium botulinum Strain

Botulinum neurotoxins (BoNTs), produced by neurotoxigenic clostridial species, are the cause of the severe disease botulism in humans and animals. Early research on BoNTs has led to their classification into seven serotypes (serotypes A to G) based upon the selective neutralization of their toxicity in mice by homologous antibodies. Recently, a report of a potential eighth serotype of BoNT, designated "type H," has been controversial. This novel BoNT was produced together with BoNT/B2 in a dual-toxin-producing Clostridium botulinum strain. The data used to designate this novel toxin as a new serotype were derived from culture supernatant containing both BoNT/B2 and novel toxin and from sequence information, although data from two independent laboratories indicated neutralization by antibodies raised against BoNT/A1, and classification as BoNT/FA was proposed. The sequence data indicate a chimeric structure consisting of a BoNT/A1 receptor binding domain, a BoNT/F5 light-chain domain, and a novel translocation domain most closely related to BoNT/F1. Here, we describe characterization of this toxin purified from the native strain in which expression of the second BoNT (BoNT/B) has been eliminated. Mass spectrometry analysis indicated that the toxin preparation contained only BoNT/FA and confirmed catalytic activity analogous to that of BoNT/F5. The in vivo mouse bioassay indicated a specific activity of this toxin of 3.8 × 10(7) mouse 50% lethal dose (mLD50) units/mg, whereas activity in cultured human neurons was very high (50% effective concentration [EC50] = 0.02 mLD50/well). Neutralization assays in cells and mice both indicated full neutralization by various antibodies raised against BoNT/A1, although at 16- to 20-fold-lower efficiency than for BoNT/A1. IMPORTANCE Botulinum neurotoxins (BoNTs), produced by anaerobic bacteria, are the cause of the potentially deadly, neuroparalytic disease botulism. BoNTs have been classified into seven serotypes, serotypes A to G, based upon their selective neutralization by homologous antiserum, which is relevant for clinical and diagnostic purposes. Even though supportive care dramatically reduces the death rate of botulism, the only pharmaceutical intervention to reduce symptom severity and recovery time is early administration of antitoxin (antiserum raised against BoNTs). A recent report of a novel BoNT serotype, serotype H, raised concern of a "treatment-resistant" and highly potent toxin. However, the toxin's chimeric structure and characteristics indicate a chimeric BoNT/FA. Here we describe the first characterization of this novel toxin in purified form. BoNT/FA was neutralized by available antitoxins, supporting classification as BoNT/FA. BoNT/FA required proteolytic activation to achieve full toxicity and had relatively low potency in mice compared to BoNT/A1 but surprisingly high activity in cultured neurons.

Recommended Immunological Strategies to Screen for Botulinum Neurotoxin-Containing Samples

Botulinum neurotoxins (BoNTs) cause the life-threatening neurological illness botulism in humans and animals and are divided into seven serotypes (BoNT/A–G), of which serotypes A, B, E, and F cause the disease in humans. BoNTs are classified as “category A” bioterrorism threat agents and are relevant in the context of the Biological Weapons Convention. An international proficiency test (PT) was conducted to evaluate detection, quantification and discrimination capabilities of 23 expert laboratories from the health, food and security areas. Here we describe three immunological strategies that proved to be successful for the detection and quantification of BoNT/A, B, and E considering the restricted sample volume (1 mL) distributed. To analyze the samples qualitatively and quantitatively, the first strategy was based on sensitive immunoenzymatic and immunochromatographic assays for fast qualitative and quantitative analyses. In the second approach, a bead-based suspension array was used for screening followed by conventional ELISA for quantification. In the third approach, an ELISA plate format assay was used for serotype specific immunodetection of BoNT-cleaved substrates, detecting the activity of the light chain, rather than the toxin protein. The results provide guidance for further steps in quality assurance and highlight problems to address in the future.

The long journey of botulinum neurotoxins into the synapse

Botulinum neurotoxins (BoNT) cause the disease botulism, a flaccid paralysis of the muscle. They are also very effective, widely used medicines applied locally in sub-nanogram quantities. BoNTs are released together with several non-toxic, associated proteins as progenitor toxin complexes (PCT) by Clostridium botulinum to become highly potent oral poisons ingested via contaminated food. They block the neurotransmission in susceptible animals and humans already in nanogram quantities due to their specific ability to enter motoneurons and to cleave only selected neuronal proteins involved in neuroexocytosis. BoNTs have developed a sophisticated strategy to passage the gastrointestinal tract and to be absorbed in the intestine of the host to finally attack neurons. A non-toxic non-hemagglutinin (NTNHA) forms a binary complex with BoNT to protect it from gastrointestinal degradation. This binary M-PTC is one component of the bi-modular 14-subunit ∼760 kDa large progenitor toxin complex. The other component is the structurally and functionally independent dodecameric hemagglutinin (HA) complex which facilitates the absorption on the intestinal epithelium by glycan binding. Subsequent to its transcytosis the HA complex disrupts the tight junction of the intestinal barrier from the basolateral side by binding to E-cadherin. Now, the L-PTC can also enter the circulation by paracellular routes in much larger quantities. From here, the dissociated BoNTs reach the neuromuscular junction and accumulate via interaction with polysialo gangliosides, complex glycolipids, on motoneurons at the neuromuscular junction. Subsequently, additional specific binding to luminal segments of synaptic vesicles proteins like SV2 and synaptotagmin leads to their uptake. Finally, the neurotoxins shut down the synaptic vesicle cycle, which they had exploited before to enter their target cells, via specific cleavage of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, which constitute the core components of the cellular membrane fusion machinery.

Molecular gene profiling of Clostridium botulinum group III and its detection in naturally contaminated samples originating from various European countries

We report the development of real-time PCR assays for genotyping Clostridium botulinum group III targeting the newly defined C. novyi sensu lato group; the nontoxic nonhemagglutinin (NTNH)-encoding gene ntnh; the botulinum neurotoxin (BoNT)-encoding genes bont/C, bont/C/D, bont/D, and bont/D/C; and the flagellin (fliC) gene. The genetic diversity of fliC among C. botulinum group III strains resulted in the definition of five major subgroups named fliC-I to fliC-V. Investigation of fliC subtypes in 560 samples, with various European origins, showed that fliC-I was predominant and found exclusively in samples contaminated by C. botulinum type C/D, fliC-II was rarely detected, no sample was recorded as fliC-III or fliC-V, and only C. botulinum type D/C samples tested positive for fliC-IV. The lack of genetic diversity of the flagellin gene of C. botulinum type C/D would support a clonal spread of type C/D strains in different geographical areas. fliC-I to fliC-III are genetically related (87% to 92% sequence identity), whereas fliC-IV from C. botulinum type D/C is more genetically distant from the other fliC types (with only 50% sequence identity). These findings suggest fliC-I to fliC-III have evolved in a common environment and support a different genetic evolution for fliC-IV. A combination of the C. novyi sensu lato, ntnh, bont, and fliC PCR assays developed in this study allowed better characterization of C. botulinum group III and showed the group to be less genetically diverse than C. botulinum groups I and II, supporting a slow genetic evolution of the strains belonging to C. botulinum group III.

Recombinant rabies virus particles presenting botulinum neurotoxin antigens elicit a protective humoral response in vivo

Botulinum neurotoxins are one of the most potent toxins found in nature, with broad medical applications from cosmetics to the treatment of various neuropathies. Additionally, these toxins are classified as Category A-Tier 1 agents, with human lethal doses calculated at as little as 90 ng depending upon the route of administration. Of the eight distinct botulinum neurotoxin serotypes, the most common causes of human illness are from serotypes /A, /B, and /E. Protection can be achieved by eliciting antibody responses against the receptor-binding domain of the neurotoxin. Our previous research has shown that recombinant rabies virus–based particles can effectively present heterologous antigens. Here, we describe a novel strategy using recombinant rabies virus particles that elicits a durable humoral immune response against the botulinum neurotoxin receptor binding domains from serotypes /A, /B, and /E. Following intramuscular administration of β-propiolactone-inactivated rabies virus particles, mice elicited specific immune responses against the cognate antigen. Administration of a combination of these vectors also demonstrated antibody responses against all three serotypes based on enzyme-linked immunosorbent assay (ELISA) measurements, with minimal decay within the study timeline. Complete protection was achieved against toxin challenge from the serotypes /A and /B and partial protection for /E, indicating that a multivalent approach is feasible.