Rapid Detection of Botulinum Neurotoxins-A Review

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.

Surface Acoustic Wave Immunosensor for Detection of Botulinum Neurotoxin

A Love-type acoustic wave sensor (AT-cut quartz substrate, SiO2 guiding layer) with a center frequency of approximately 120 MHz was used to detect a simulant of pathogenic botulinum neurotoxin type A-recombinant of BoNT-A light chain-in liquid samples. The sensor was prepared by immobilizing monoclonal antibodies specific for botulinum neurotoxin via a thiol monolayer deposited on a gold substrate. Studies have shown that the sensor enables selective analyte detection within a few minutes. In addition, the sensor can be used several times (regeneration of the sensor is possible using a low pH buffer). Nevertheless, the detectability of the analyte is relatively low compared to other analytical techniques that can be used for rapid detection of botulinum neurotoxin. The obtained results confirm the operation of the proposed sensor and give hope for further development of this label-free technique for detecting botulinum neurotoxin.

Biological and Immunological Characterization of a Functional L-HN Derivative of Botulinum Neurotoxin Serotype F

Botulinum neurotoxins (BoNTs) can cause nerve paralysis syndrome in mammals and other vertebrates. BoNTs are the most toxic biotoxins known and are classified as Class A biological warfare agents. BoNTs are mainly divided into seven serotypes A-G and new neurotoxins BoNT/H and BoNT/X, which have similar functions. BoNT proteins are 150 kDa polypeptide consisting of two chains and three domains: the light chain (L, catalytic domain, 50 kDa) and the heavy chain (H, 100 kDa), which can be divided into an N-terminal membrane translocation domain (HN, 50 kDa) and a C-terminal receptor binding domain (Hc, 50 kDa). In current study, we explored the immunoprotective efficacy of each functional molecule of BoNT/F and the biological characteristics of the light chain-heavy N-terminal domain (FL-HN). The two structure forms of FL-HN (i.e., FL-HN-SC: single chain FL-HN and FL-HN-DC: di-chain FL-HN) were developed and identified. FL-HN-SC could cleave the vesicle associated membrane protein 2 (VAMP2) substrate protein in vitro as FL-HN-DC or FL. While only FL-HN-DC had neurotoxicity and could enter neuro-2a cells to cleave VAMP2. Our results showed that the FL-HN-SC had a better immune protection effect than the Hc of BoNT/F (FHc), which indicated that L-HN-SC, as an antigen, provided the strongest protective effects against BoNT/F among all the tested functional molecules. Further in-depth research on the different molecular forms of FL-HN suggested that there were some important antibody epitopes at the L-HN junction of BoNT/F. Thus, FL-HN-SC could be used as a subunit vaccine to replace the FHc subunit vaccine and/or toxoid vaccine, and to develop antibody immune molecules targeting L and HN domains rather than the FHc domain. FL-HN-DC could be used as a new functional molecule to evaluate and explore the structure and activity of toxin molecules. Further exploration of the biological activity and molecular mechanism of the functional FL-HN or BoNT/F is warranted.

Optimization of Application-Driven Development of In Vitro Neuromuscular Junction Models

Neuromuscular junctions (NMJs) are specialized synapses responsible for signal transduction between motor neurons (MNs) and skeletal muscle tissue. Malfunction at this site can result from developmental disorders, toxic environmental exposures, and neurodegenerative diseases leading to severe neurological dysfunction. Exploring these conditions in human or animal subjects is restricted by ethical concerns and confounding environmental factors. Therefore, in vitro NMJ models provide exciting opportunities for advancements in tissue engineering. In the last two decades, multiple NMJ prototypes and platforms have been reported, and each model system design is strongly tied to a specific application: exploring developmental physiology, disease modeling, or high-throughput screening. Directing the differentiation of stem cells into mature MNs and/or skeletal muscle for NMJ modeling has provided critical cues to recapitulate early-stage development. Patient-derived inducible pluripotent stem cells provide a personalized approach to investigating NMJ disease, especially when disease etiology cannot be resolved down to a specific gene mutation. Having reproducible NMJ culture replicates is useful for high-throughput screening to evaluate drug toxicity and determine the impact of environmental threat exposures. Cutting-edge bioengineering techniques have propelled this field forward with innovative microfabrication and design approaches allowing both two-dimensional and three-dimensional NMJ culture models. Many of these NMJ systems require further validation for broader application by regulatory agencies, pharmaceutical companies, and the general research community. In this summary, we present a comprehensive review on the current state-of-art research in NMJ models and discuss their ability to provide valuable insight into cell and tissue interactions. Impact statement In vitro neuromuscular junction (NMJ) models reveal the specialized mechanisms of communication between neurons and muscle tissue. This site can be disrupted by developmental disorders, toxic environmental exposures, or neurodegenerative diseases, which often lead to fatal outcomes and is therefore of critical importance to the medical community. Many bioengineering approaches for in vitro NMJ modeling have been designed to mimic development and disease; other approaches include in vitro NMJ models for high-throughput toxicology screening, providing a platform to limit or replace animal testing. This review describes various NMJ applications and the bioengineering advancements allowing for human NMJ characteristics to be more accurately recapitulated. While the extensive range of NMJ device structures has hindered standardization attempts, there is still a need to harmonize these devices for broader application and to continue advancing the field of NMJ modeling.

Salmonella spp. in Chicken: Prevalence, Antimicrobial Resistance, and Detection Methods

Multidrug-resistant Salmonella spp. is one of the leading causes of worldwide foodborne disease outbreaks. Animal-derived foods, particularly chicken and poultry products, are the most likely source of Salmonella transmission to humans. The increasing demand for chicken meat has raised a global food safety issue. This review aims to determine the prevalence and antimicrobial resistance of Salmonella spp. in chickens from various countries in Asia. The methods for detecting Salmonella will also be discussed in this review. The prevalence of Salmonella spp. in chicken and poultry products is lower in developed countries than in developing countries. In addition, the incidence of Salmonella spp. in chicken and poultry products from fresh markets is higher than those from supermarkets. Furthermore, this review also reported the presence of multidrug-resistant Salmonella strains in various Asian countries. Rapid Salmonella detection based on immunological assays, molecular-based assays, and biosensors can provide more accurate results with high sensitivity and specificity. These methods also require a shorter time than the cultural-based Salmonella detection method. The use of suitable detection methods to determine the presence of Salmonella spp. in chicken and poultry products is important to ensure food safety.

Metallic and Metal Oxides Nanoparticles for Sensing Food Pathogens—An Overview of Recent Findings and Future Prospects

Nowadays, special importance is given to quality control and food safety. Food quality currently creates significant problems for the industry and implicitly for consumers and society. The effects materialize in economic losses, alterations of the quality and organoleptic properties of the commercial products, and, last but not least, they constitute risk factors for the consumer’s health. In this context, the development of analytical systems for the rapid determination of the sanitary quality of food products by detecting possible pathogenic microorganisms (such as Escherichia coli or Salmonella due to the important digestive disorders that they can cause in many consumers) is of major importance. Using efficient and environmentally friendly detection systems for identification of various pathogens that modify food matrices and turn them into food waste faster will also improve agri-food quality throughout the food chain. This paper reviews the use of metal nanoparticles used to obtain bio nanosensors for the purpose mentioned above. Metallic nanoparticles (Au, Ag, etc.) and their oxides can be synthesized by several methods, such as chemical, physical, physico-chemical, and biological, each bringing advantages and disadvantages in their use for developing nanosensors. In the “green chemistry” approach, a particular importance is given to the metal nanoparticles obtained by phytosynthesis. This method can lead to the development of good quality nanoparticles, at the same time being able to use secondary metabolites from vegetal wastes, as such providing a circular economy character. Considering these aspects, the use of phytosynthesized nanoparticles in other biosensing applications is also presented as a glimpse of their potential, which should be further explored.

A colorimetric aptasensor based on gold nanoparticles for detection of microbial toxins: an alternative approach to conventional methods

Frequent contamination of foods with microbial toxins produced by microorganisms such as bacteria, fungi, and algae represents an increasing public health problem that requires the development of quick and easy tools to detect them at trace levels. Recently, it has been found that colorimetric detection methods may replace traditional methods in the field because of their ease of use, quick response, ease of manufacture, low cost, and naked-eye visibility. Therefore, it is suitable for fieldwork, especially for work in remote areas of the world. However, the development of colorimetric detection methods with low detection limits is a challenge that limits their wide applicability in the detection of food contaminants. To address these challenges, nanomaterial-based transduction systems are used to construct colorimetric biosensors. For example, gold nanoparticles (AuNPs) provide an excellent platform for the development of colorimetric biosensors because they offer the advantages of easy synthesis, biocompatibility, advanced surface functionality, and adjustable physicochemical properties. The selectivity of the colorimetric biosensor can be achieved by the combination of aptamers and gold nanoparticles, which provides an unprecedented opportunity to detect microbial toxins. Compared to antibodies, aptamers have significant advantages in the analysis of microbial toxins due to their smaller size, higher binding affinity, reproducible chemical synthesis and modification, stability, and specificity. Two colorimetric mechanisms for the detection of microbial toxins based on AuNPs have been described. First, sensors that use the localized surface plasmon resonance (LSPR) phenomenon of gold nanoparticles can exhibit very strong colors in the visible range because of changes caused by aggregation or disaggregation. Second, the detection mechanism of AuNPs is based on their enzyme mimetic properties and it is possible to construct a colorimetric biosensor based on the 3,3',5,5'-tetramethylbenzidine/Hydrogen peroxide, TMB/H₂O₂ reaction to detect microbial toxins. Therefore, this review summarizes the recent applications of AuNP-based colorimetric aptasensors for detecting microbial toxins, including bacterial toxins, fungal toxins, and algal toxins focusing on selectivity, sensitivity, and practicality. Finally, the most important current challenges in this field and future research opportunities are discussed.

Recent Developments in Botulinum Neurotoxins Detection

Botulinum neurotoxins (BoNTs) are produced as protein complexes by bacteria of the genus Clostridium that are Gram-positive, anaerobic and spore forming (Clostridium botulinum, C. butyricum, C. baratii and C. argentinense spp.). BoNTs show a high immunological and genetic diversity. Therefore, fast, precise, and more reliable detection methods are still required to monitor outbreaks and ensure surveillance of botulism. The botulinum toxin field also comprises therapeutic uses, basic research studies and biodefense issues. This review presents currently available detection methods, and new methods offering the potential of enhanced precision and reproducibility. While the immunological methods offer a range of benefits, such as rapid analysis time, reproducibility and high sensitivity, their implementation is subject to the availability of suitable tools and reagents, such as specific antibodies. Currently, the mass spectrometry approach is the most sensitive in vitro method for a rapid detection of active or inactive forms of BoNTs. However, these methods require inter-laboratory validation before they can be more widely implemented in reference laboratories. In addition, these surrogate in vitro models also require full validation before they can be used as replacement bioassays of potency. Cell-based assays using neuronal cells in culture recapitulate all functional steps of toxin activity, but are still at various stages of development; they are not yet sufficiently robust, due to high batch-to-batch cell variability. Cell-based assays have a strong potential to replace the mouse bioassay (MBA) in terms of BoNT potency determination in pharmaceutical formulations; they can also help to identify suitable inhibitors while reducing the number of animals used. However, the development of safe countermeasures still requires the use of in vivo studies to complement in vitro immunological or cell-based approaches.

Detection of VAMP Proteolysis by Tetanus and Botulinum Neurotoxin Type B In Vivo with a Cleavage-Specific Antibody

Tetanus and Botulinum type B neurotoxins are bacterial metalloproteases that specifically cleave the vesicle-associated membrane protein VAMP at an identical peptide bond, resulting in inhibition of neuroexocytosis. The minute amounts of these neurotoxins commonly used in experimental animals are not detectable, nor is detection of their VAMP substrate sensitive enough. The immune detection of the cleaved substrate is much more sensitive, as we have previously shown for botulinum neurotoxin type A. Here, we describe the production in rabbit of a polyclonal antibody raised versus a peptide encompassing the 13 residues C-terminal with respect to the neurotoxin cleavage site. The antibody was affinity purified and found to recognize, with high specificity and selectivity, the novel N-terminus of VAMP that becomes exposed after cleavage by tetanus toxin and botulinum toxin type B. This antibody recognizes the neoepitope not only in native and denatured VAMP but also in cultured neurons and in neurons in vivo in neurotoxin-treated mice or rats, suggesting the great potential of this novel tool to elucidate tetanus and botulinum B toxin activity in vivo.

Botulinum Neurotoxin C Dual Detection through Immunological Recognition and Endopeptidase Activity Using Porous Silicon Interferometers

Botulinum neurotoxins (BoNTs) are the most potent toxins known in nature produced by Clostridium botulinum strains, which can cause life-threatening diseases in both humans and animals. The latter is of serious environmental and economic concern, resulting in high mortality, production losses, and rejection of contaminated animal feed. The available in vivo mouse assay is inadequate for real-time and on-site assessment of outbreaks. Herein, we present a reflective-based approach for the detection of BoNT/C while estimating its activity. Two adjacent porous Si Fabry-Pérot interferometers are simultaneously utilized to quantify minute BoNT/C concentrations by a competitive immunoassay and to assess their endopeptidase activity. The reflectivity signals of each interferometer are amplified by biochemical reaction products infiltration into the scaffold or by peptide fragments detachment from the nanostructure. The optical assay is highly sensitive in compliance with the in vivo approach by presenting a detection limit of 4.24 pg mL^-1. The specificity and selectivity of the designed platform are cross-validated against BoNT/B and BoNT/D, also relevant to animal health. Finally, the analytical performances of both interferometers for real-life scenarios are confirmed using actual toxins while depicting excellent compliance to complex media analysis. Overall, the presented sensing scheme offers an efficient, rapid, and label-free approach for potential biodiagnostic elucidation of botulism outbreaks.

Discrimination of the Activity of Low-Affinity Wild-Type and High-Affinity Mutant Recombinant BoNT/B by a SIMA Cell-Based Reporter Release Assay

Botulinum neurotoxin (BoNT) is used for the treatment of a number of ailments. The activity of the toxin that is isolated from bacterial cultures is frequently tested in the mouse lethality assay. Apart from the ethical concerns inherent to this assay, species-specific differences in the affinity for different BoNT serotypes give rise to activity results that differ from the activity in humans. Thus, BoNT/B is more active in mice than in humans. The current study shows that the stimulus-dependent release of a luciferase from a differentiated human neuroblastoma–based reporter cell line (SIMA-hPOMC1-26-Gluc) was inhibited by clostridial and recombinant BoNT/A to the same extent, whereas both clostridial and recombinant BoNT/B inhibited the release to a lesser extent and only at much higher concentrations, reflecting the low activity of BoNT/B in humans. By contrast, the genetically modified BoNT/B-MY, which has increased affinity for human synaptotagmin, and the BoNT/B protein receptor inhibited luciferase release effectively and with an EC50 comparable to recombinant BoNT/A. This was due to an enhanced uptake into the reporter cells of BoNT/B-MY in comparison to the recombinant wild-type toxin. Thus, the SIMA-hPOMC1-26-Gluc cell assay is a versatile tool to determine the activity of different BoNT serotypes providing human-relevant dose-response data.

Optogenetically controlled human functional motor endplate for testing botulinum neurotoxins

Background

The lack of physiologically relevant and predictive cell-based assays is one of the major obstacles for testing and developing botulinum neurotoxins (BoNTs) therapeutics. Human-induced pluripotent stem cells (hiPSCs)-derivatives now offer the opportunity to improve the relevance of cellular models and thus the translational value of preclinical data.

Methods

We investigated the potential of hiPSC-derived motor neurons (hMNs) optical stimulation combined with calcium imaging in cocultured muscle cells activity to investigate BoNT-sensitivity of an in vitro model of human muscle-nerve system.

Results

Functional muscle-nerve coculture system was developed using hMNs and human immortalized skeletal muscle cells. Our results demonstrated that hMNs can innervate myotubes and induce contractions and calcium transient in muscle cells, generating an in vitro human motor endplate showing dose-dependent sensitivity to BoNTs intoxication. The implementation of optogenetics combined with live calcium imaging allows to monitor the impact of BoNTs intoxication on synaptic transmission in human motor endplate model.

Conclusions

Altogether, our findings demonstrate the promise of optogenetically hiPSC-derived controlled muscle-nerve system for pharmaceutical BoNTs testing and development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02665-3.

Current Developments in Diagnostic Assays for Laboratory Confirmation and Investigation of Botulism

Detection of botulinum neurotoxin or isolation of the toxin producing organism is required for the laboratory confirmation of botulism in clinical specimens. In an effort to reduce animal testing required by the gold standard method of botulinum neurotoxin detection, the mouse bioassay, many technologies have been developed to detect and characterize the causative agent of botulism. Recent advancements in these technologies have led to improvements in technical performance of diagnostic assays; however, many emerging assays have not been validated for the detection of all serotypes in complex clinical and environmental matrices. Improvements to culture protocols, endopeptidase-based assays, and a variety of immunological and molecular methods have provided laboratories with a variety of testing options to evaluate and incorporate into their testing algorithms. While significant advances have been made to improve these assays, additional work is necessary to evaluate these methods in various clinical matrices and to establish standardized criteria for data analysis and interpretation.

Botulinum Neurotoxin-C Detection Using Nanostructured Porous Silicon Interferometer

Botulinum neurotoxins (BoNT) are the most potent toxins, which are produced by Clostridium bacteria and cause the life-threatening disease of botulism in all vertebrates. Specifically, animal botulism represents a serious environmental and economic concern in animal production due to the high mortality rates observed during outbreaks. Despite the availability of vaccines against BoNT, there are still many outbreaks of botulism worldwide. Alternative assays capable of replacing the conventional in vivo assay in terms of rapid and sensitive quantification, and the applicability for on-site analysis, have long been perused. Herein, we present a simple, highly sensitive and label-free optical biosensor for real-time detection of BoNT serotype C using a porous silicon Fabry–Pérot interferometer. A competitive immunoassay coupled to a biochemical cascade reaction was adapted for optical signal amplification. The resulting insoluble precipitates accumulated within the nanostructure changed the reflectivity spectra by alternating the averaged refractive index. The augmented optical performance allowed for a linear response within the range of 10 to 10,000 pg mL−1 while presenting a detection limit of 4.8 pg mL−1. The practical aspect of the developed assay was verified using field BoNT holotoxins to exemplify the potential use of the developed optical approach for rapid bio-diagnosis of BoNT. The specificity and selectivity of the assay were successfully validated using an adjacent holotoxin relevant for farm animals (BoNT serotype D). Overall, this work sets the foundation for implementing a miniaturized interferometer for routine on-site botulism diagnosis, thus significantly reducing the need for animal experimentation and shortening analysis turnaround for early evidence-based therapy.

Fluorescent detection of target proteins via a molecularly imprinted hydrogel

Proteins are typically separated by an immune reaction, such as an enzyme-linked immunosorbent assay, and are detected by selective fluorescent labeling. This has potential for complicated procedures and the denaturation of proteins by labeling, and is cost consuming. In this study, we propose a technique for the selective separation and detection of a target protein using a molecularly imprinted hydrogel (PI gel) with fluorescent monomers. We focused on 8-anilino-1-naphthalenesulfonic acid (ANS), where the fluorescence intensity is easily changed by the interaction with proteins, and successfully synthesized the ANS monomer and a poly(ethylene glycol) (PEG) conjugated ANS monomer. The PI gel with the ANS monomers using bovine serum albumin (BSA) as a template showed the selective adsorption of BSA and the fluorescence intensity increased due to the adsorption of BSA.

Emerging Opportunities in Human Pluripotent Stem-Cells Based Assays to Explore the Diversity of Botulinum Neurotoxins as Future Therapeutics

Botulinum neurotoxins (BoNTs) are produced by Clostridium botulinum and are responsible for botulism, a fatal disorder of the nervous system mostly induced by food poisoning. Despite being one of the most potent families of poisonous substances, BoNTs are used for both aesthetic and therapeutic indications from cosmetic reduction of wrinkles to treatment of movement disorders. The increasing understanding of the biology of BoNTs and the availability of distinct toxin serotypes and subtypes offer the prospect of expanding the range of indications for these toxins. Engineering of BoNTs is considered to provide a new avenue for improving safety and clinical benefit from these neurotoxins. Robust, high-throughput, and cost-effective assays for BoNTs activity, yet highly relevant to the human physiology, have become indispensable for a successful translation of engineered BoNTs to the clinic. This review presents an emerging family of cell-based assays that take advantage of newly developed human pluripotent stem cells and neuronal function analyses technologies.

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.

De novo design of modular and tunable protein biosensors

Naturally occurring protein switches have been repurposed for the development of biosensors and reporters for cellular and clinical applications¹. However, the number of such switches is limited, and reengineering them is challenging. Here we show that a general class of protein-based biosensors can be created by inverting the flow of information through de novo designed protein switches in which the binding of a peptide key triggers biological outputs of interest². The designed sensors are modular molecular devices with a closed dark state and an open luminescent state; analyte binding drives the switch from the closed to the open state. Because the sensor is based on the thermodynamic coupling of analyte binding to sensor activation, only one target binding domain is required, which simplifies sensor design and allows direct readout in solution. We create biosensors that can sensitively detect the anti-apoptosis protein BCL-2, the IgG1 Fc domain, the HER2 receptor, and Botulinum neurotoxin B, as well as biosensors for cardiac troponin I and an anti-hepatitis B virus antibody with the high sensitivity required to detect these molecules clinically. Given the need for diagnostic tools to track the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)³, we used the approach to design sensors for the SARS-CoV-2 spike protein and antibodies against the membrane and nucleocapsid proteins. The former, which incorporates a de novo designed spike receptor binding domain (RBD) binder⁴, has a limit of detection of 15 pM and a luminescence signal 50-fold higher than the background level. The modularity and sensitivity of the platform should enable the rapid construction of sensors for a wide range of analytes, and highlights the power of de novo protein design to create multi-state protein systems with new and useful functions.

Toxemia in Human Naturally Acquired Botulism

Human botulism is a severe disease characterized by flaccid paralysis and inhibition of certain gland secretions, notably salivary secretions, caused by inhibition of neurotransmitter release. Naturally acquired botulism occurs in three main forms: food-borne botulism by ingestion of preformed botulinum neurotoxin (BoNT) in food, botulism by intestinal colonization (infant botulism and intestinal toxemia botulism in infants above one year and adults), and wound botulism. A rapid laboratory confirmation of botulism is required for the appropriate management of patients. Detection of BoNT in the patient's sera is the most direct way to address the diagnosis of botulism. Based on previous published reports, botulinum toxemia was identified in about 70% of food-borne and wound botulism cases, and only in about 28% of infant botulism cases, in which the diagnosis is mainly confirmed from stool sample investigation. The presence of BoNT in serum depends on the BoNT amount ingested with contaminated food or produced locally in the intestine or wound, and the timeframe between serum sampling and disease onset. BoNT levels in patient's sera are most frequently low, requiring a highly sensitive method of detection. Mouse bioassay is still the most used method of botulism identification from serum samples. However, in vitro methods based on BoNT endopeptidase activity with detection by mass spectrometry or immunoassay have been developed and depending on BoNT type, are more sensitive than the mouse bioassay. These new assays show high specificity for individual BoNT types and allow more accurate differentiation between positive toxin sera from botulism and autoimmune neuropathy patients.

Docking Simulation and Sandwich Assay for Aptamer-Based Botulinum Neurotoxin Type C Detection

Aptamers are biomaterials that bind to a target molecule through a unique structure, and have high applicability in the diagnostic and medical fields. To effectively utilize aptamers, it is important to analyze the structure of the aptamer binding to the target molecule; however, there are difficulties in experimentally identifying this structure. In the modern pharmaceutical industry, computer-driven docking simulations that predict intermolecular binding models are used to select candidates that effectively bind target molecules. Botulinum toxin (BoNT) is the most poisonous neurotoxin produced from the Clostridium botulinum bacteria, and BoNT/C, one of the eight serotypes, causes paralysis in livestock. In this study, the aptamers that bound to BoNT/C were screened via the systematic evolution of ligands by exponential enrichment, and the binding affinity analysis and binding model were evaluated to select optimal aptamers. Based on surface plasmon resonance analysis and molecular operating environment docking simulation, a pair of aptamers that had high binding affinity to BoNT/C and were bound to different BoNT/C sites were selected. A sandwich assay based on this aptamer pair detected the BoNT/C protein to a concentration as low as ~0.2 ng Ml^-1. These results show that docking simulations are a useful strategy for screening aptamers that bind to specific targets.

De novo design of modular and tunable allosteric biosensors

Naturally occurring allosteric protein switches have been repurposed for developing novel biosensors and reporters for cellular and clinical applications ¹ , but the number of such switches is limited, and engineering them is often challenging as each is different. Here, we show that a very general class of allosteric protein-based biosensors can be created by inverting the flow of information through de novo designed protein switches in which binding of a peptide key triggers biological outputs of interest ² . Using broadly applicable design principles, we allosterically couple binding of protein analytes of interest to the reconstitution of luciferase activity and a bioluminescent readout through the association of designed lock and key proteins. Because the sensor is based purely on thermodynamic coupling of analyte binding to switch activation, only one target binding domain is required, which simplifies sensor design and allows direct readout in solution. We demonstrate the modularity of this platform by creating biosensors that, with little optimization, sensitively detect the anti-apoptosis protein Bcl-2, the hIgG1 Fc domain, the Her2 receptor, and Botulinum neurotoxin B, as well as biosensors for cardiac Troponin I and an anti-Hepatitis B virus (HBV) antibody that achieve the sub-nanomolar sensitivity necessary to detect clinically relevant concentrations of these molecules. Given the current need for diagnostic tools for tracking COVID-19 ³ , we use the approach to design sensors of antibodies against SARS-CoV-2 protein epitopes and of the receptor-binding domain (RBD) of the SARS-CoV-2 Spike protein. The latter, which incorporates a de novo designed RBD binder, has a limit of detection of 15pM with an up to seventeen fold increase in luminescence upon addition of RBD. The modularity and sensitivity of the platform should enable the rapid construction of sensors for a wide range of analytes and highlights the power of de novo protein design to create multi-state protein systems with new and useful functions.

Development of an alternative approach for detecting botulinum neurotoxin type A in honey: Analysis of non-toxic peptides with a reference labelled protein via liquid chromatography-tandem mass spectrometry

In this study, we developed a reference labelled protein containing the partial amino acid sequence of botulinum neurotoxin type A (BoNTA). We also applied it as an internal standard to detect specific and non-toxic peptides originated from BoNTA in honey with the use of liquid chromatography-tandem mass spectrometry (LC-MS/MS). Original proteins in the honey sample were collected through a two-step process that included solubilisation and trichloroacetic acid (TCA) precipitation. Solubilisation by adding water enabled processing of proteins in honey. TCA precipitation collected proteins without specific binding. The combination of protein alkylation and an appropriate enzyme-to-protein ratio ensured feasibility of tryptic digestion. A desalting process eliminated a large amount of salts and other tryptic peptides in the honey sample. The use of the reference labelled protein enabled compensation for tryptic digestion efficiency and electrospray ionisation efficiency based on LC-MS/MS measurement. After the peptide selection and protein BlastP analysis, five unique peptides were chosen. The non-toxic peptides originating from BoNTA were reliably detected using LC-MS/MS based on a multiple-reaction monitoring mode. Detection of several peptides ensured screening of BoNTA in honey samples. Based on the responses, the proteotypic peptide LYGIAINPNR was selected as the quantitative peptide. Due to maintaining the relative ion ratios, the selective transition completely identified the non-toxic peptides. The intensity of the transitions established a detection limit of BoNTA estimated to be 9.4 ng mL^-1. Although extraction efficiency was not evaluated using the BoNTA standard, the results suggested this method may be used for quantification of BoNTA in honey. The method was applied to 19 honey samples purchased in Tokyo; none of them was found to contain the target toxin. Overall, the method is expected to accelerate BoNTA monitoring for food safety.

Metody stosowane do wykrywania i identyfikacji toksyn botulinowych w próbkach klinicznych i żywności*

Botulism is a severe neuroparalytic illness, which affects the nervous system. It is caused by botulinum neurotoxins (BoNTs), produced by anaerobic gram-positive bacteria Clostridium botulinum. There are 7 serotypes of BoNT A-G, but BoNT A/B/D/E plays a major role in botulism affecting humans. Foodborne botulism (classic botulism) is the most frequent clinical manifestation occurring after consumption of food containing botulinum neurotoxins. The diagnosis of botulism is based on clinical symptoms; however, recommended and alternative laboratory methods are used to confirm the etiology of symptoms and the identification of BoNT toxin type. The aim of this work was to present the epidemiology of foodborne botulism in Poland and to gather and analyze the available diagnostic methods that allow us to detect BoNT in clinical samples. Using the epidemiological reports of National Institute of Hygiene in Poland and findings presented in the Przegląd Epidemiologiczny, the incidence of classical botulism in Poland has been presented over a period of recent 18 years. Searching for the optimal diagnostic method for BoNT identification in various samples, we have confronted the sensitivity and specificity of recently available alternative methods with classical biological assay.

Analysis of Motor Neurons Differentiated from Human Induced Pluripotent Stem Cells for the Use in Cell-Based Botulinum Neurotoxin Activity Assays

Botulinum neurotoxins (BoNTs) are potent neurotoxins produced by bacteria, which inhibit neurotransmitter release, specifically in their physiological target known as motor neurons (MNs). For the potency assessment of BoNTs produced for treatment in traditional and aesthetic medicine, the mouse lethality assay is still used by the majority of manufacturers, which is ethically questionable in terms of the 3Rs principle. In this study, MNs were differentiated from human induced pluripotent stem cells based on three published protocols. The resulting cell populations were analyzed for their MN yield and their suitability for the potency assessment of BoNTs. MNs produce specific gangliosides and synaptic proteins, which are bound by BoNTs in order to be taken up by receptor-mediated endocytosis, which is followed by cleavage of specific soluble N-ethylmaleimide-sensitive-factor attachment receptor (SNARE) proteins required for neurotransmitter release. The presence of receptors and substrates for all BoNT serotypes was demonstrated in MNs generated in vitro. In particular, the MN differentiation protocol based on Du et al. yielded high numbers of MNs in a short amount of time with high expression of BoNT receptors and targets. The resulting cells are more sensitive to BoNT/A1 than the commonly used neuroblastoma cell line SiMa. MNs are, therefore, an ideal tool for being combined with already established detection methods.

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.

Biosecurity Threat Posed by Botulinum Toxin

The deliberate release of biological agents with terrorist or criminal intent continues to pose concerns in the current geopolitical situation. Therefore, attention is still needed to ensure preparedness against the potential use of pathogens as unconventional weapons. Botulinum neurotoxin (BoNT) is one such biological threat, characterized by an extremely low lethal dose, high morbidity and mortality when appropriately disseminated, and the capacity to cause panic and social disruption. This paper addresses the risks of a potential release of the botulinum neurotoxin and summarizes the relevant aspects of the threat.

Hospital-acquired intestinal toxemia botulism in a newly diagnosed adult colon cancer patient

This manuscript reports a case of intestinal toxemia botulism in an adult with recently diagnosed metastatic colon cancer in whom botulism symptoms began 23 days after hospital admission. Representing the rarest form of botulism presentation in clinical practice, this infectious disease may have developed due to a cluster of predisposing factors that favored Clostridium botulinum colonization and the endogenous production of neurotoxins, among which are previous use of broad-spectrum antibiotics and colon changes related to the development of the neoplasia. This case highlights the importance of considering intestinal toxemia botulism in the differential diagnosis of a patient presenting with symmetrical descending flaccid paralysis, since immediate treatment with botulinum antitoxin may improve clinical outcomes.