The safety of pasteurised in-pack chilled meat products with respect to the foodborne botulism hazard

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.

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.

Challenge tests reveal limited outgrowth of proteolytic Clostridium botulinum during the production of nitrate- and nitrite-free fermented sausages

Nitrate and nitrite salts perform a versatile role in fermented meats, including the inhibition of food pathogens (in particular proteolytic group I Clostridium botulinum). Despite the increasing interest in clean-label products, little is known about the behaviour of this pathogen in response to the removal of chemical preservatives from fermented meat formulations. Therefore, challenge tests with a cocktail of nontoxigenic group I C. botulinum strains were performed to produce nitrate/nitrite-free fermented sausages under different acidification conditions and starter culture formulations, including the use of an anticlostridial Mammaliicoccus sciuri strain. Results showed limited outgrowth of C. botulinum, even in the absence of acidification. The anticlostridial starter culture did not lead to an additional inhibitory effect. The selective plating procedure adopted within this study proofed robust to follow germination and growth of C. botulinum, inhibiting common fermentative meat microbiota. The challenge tests constitute a suitable tool to assess the behaviour of this food pathogen within fermented meats upon nitrate- and nitrite omission.

Bacterial species diversity of traditionally ripened sheep legs from the Faroe Islands (skerpikjøt)

Skerpikjøt is a traditionally ripened sheep leg product from the Faroe Islands, constituting a relatively underexplored microbial ecosystem. The objective of this study is to achieve a deeper understanding of the microbial composition of this artisanal product. Nine ripened hind legs, obtained from three different producers, were assessed regarding their bacterial communities and contents of biogenic amines, including both surface and core samples. Biogenic amine concentrations were generally low, although one sample had a somewhat elevated concentration of cadaverine. Bacterial diversity was investigated by culture-dependent and culture-independent techniques. Gram-positive catalase-positive cocci (GCC) constituted the most abundant group. Within this group, Staphylococcus equorum was the most prevailing species, followed by Kocuria sp., Mammaliicoccus vitulinus, and Staphylococcus saprophyticus. Lactic acid bacteria prevailed in only one sample and were mainly represented by Latilactobacillus curvatus. Enterobacterial communities were characterised by the prevalence of Serratia proteamaculans. Despite the majority of GCC, Clostridium putrefaciens was the most abundant bacterial species in some core samples. Taken together, the culture-dependent and culture-independent identification methods gave complementary results.

High-pressure pasteurization of low-acid chilled ready-to-eat food

The working population growth have created greater consumer demand for ready-to-eat (RTE) foods. Pasteurization is one of the most common preservation methods for commercial production of low-acid RTE cold-chain products. Proper selection of a pasteurization method plays an important role not only in ensuring microbial safety but also in maintaining food quality during storage. Better retention of flavor, color, appearance, and nutritional value of RTE products is one of the reasons for the food industry to adopt novel technologies such as high-pressure processing (HPP) as a substitute or complementary technology for thermal pasteurization. HPP has been used industrially for the pasteurization of high-acid RTE products. Yet, this method is not commonly used for pasteurization of low-acid RTE food products, due primarily to the need of additional heating to thermally inactivate spores, coupled with relatively long treatment times resulting in high processing costs. Practical Application: Food companies would like to adopt novel technologies such as HPP instead of using conventional thermal processes, yet there is a lack of information on spoilage and the shelf-life of pasteurized low-acid RTE foods (by different novel pasteurization methods including HPP) in cold storage. This article provides an overview of the microbial concerns and related regulatory guidelines for the pasteurization of low-acid RTE foods and summarizes the effects of HPP in terms of microbiology (both pathogens and spoilage microorganisms), quality, and shelf-life on low-acid RTE foods. This review also includes the most recent research articles regarding a comparison between HPP pasteurization and thermal pasteurization treatments and the limitations of HPP for low-acid chilled RTE foods.

Selection and Development of Nontoxic Nonproteolytic Clostridium botulinum Surrogate Strains for Food Challenge Testing

Clostridium botulinum causes severe foodborne intoxications by producing a potent neurotoxin. Challenge studies with this pathogen are an important tool to ensure the safety of new processing techniques and newly designed or modified foods, but they are hazardous and complicated by the lack of an effective selective counting medium. Therefore, this study aimed to develop selectable nontoxic surrogate strains for group II, or nonproteolytic, C. botulinum, which are psychotropic and hence of particular concern in mildly treated, refrigerated foods. Thirty-one natural nontoxic nonproteolytic strains, 16 of which were isolated in this work, were characterized in detail, revealing that 28 strains were genomically and phenotypically indistinguishable from toxic strains. Five strains, representing the genomic and phenotypic diversity of group II C. botulinum, were selected and successfully equipped with an erythromycin (Em) resistance marker in a defective structural phage gene without altering phenotypic features. Finally, a selective medium containing Em, cycloserine (Cs), gentamicin (Gm), and lysozyme (Ly) was developed, which inhibited the background microbiota of commercial cooked ham, chicken filet, and salami, but supported spore germination and growth of the Em-resistant surrogate strains. The surrogates developed in this work are expected to facilitate food challenge studies with nonproteolytic C. botulinum for the food industry and can also provide a safe alternative for basic C. botulinum research.

Novel Lytic Enzyme of Prophage Origin from Clostridium botulinum E3 Strain Alaska E43 with Bactericidal Activity against Clostridial Cells

Clostridium botulinum is a Gram-positive, anaerobic, spore-forming bacterium capable of producing botulinum toxin and responsible for botulism of humans and animals. Phage-encoded enzymes called endolysins, which can lyse bacteria when exposed externally, have potential as agents to combat bacteria of the genus Clostridium. Bioinformatics analysis revealed in the genomes of several Clostridium species genes encoding putative N-acetylmuramoyl-l-alanine amidases with anti-clostridial potential. One such enzyme, designated as LysB (224-aa), from the prophage of C. botulinum E3 strain Alaska E43 was chosen for further analysis. The recombinant 27,726 Da protein was expressed and purified from E. coli Tuner(DE3) with a yield of 37.5 mg per 1 L of cell culture. Size-exclusion chromatography and analytical ultracentrifugation experiments showed that the protein is dimeric in solution. Bioinformatics analysis and results of site-directed mutagenesis studies imply that five residues, namely H25, Y54, H126, S132, and C134, form the catalytic center of the enzyme. Twelve other residues, namely M13, H43, N47, G48, W49, A50, L73, A75, H76, Q78, N81, and Y182, were predicted to be involved in anchoring the protein to the lipoteichoic acid, a significant component of the Gram-positive bacterial cell wall. The LysB enzyme demonstrated lytic activity against bacteria belonging to the genera Clostridium, Bacillus, Staphylococcus, and Deinococcus, but did not lyse Gram-negative bacteria. Optimal lytic activity of LysB occurred between pH 4.0 and 7.5 in the absence of NaCl. This work presents the first characterization of an endolysin derived from a C. botulinum Group II prophage, which can potentially be used to control this important pathogen.

Cardinal parameter growth and growth boundary model for non-proteolytic Clostridium botulinum - Effect of eight environmental factors

A new cardinal parameter growth and growth boundary model for non-proteolytic C. botulinum was developed and validated for fresh and lightly preserved seafood and poultry products. 523 growth rates in broth were used to determine cardinal parameter values and terms for temperature, pH, NaCl/water activity, acetic, benzoic, citric, lactic and sorbic acids. The new growth and growth boundary model included the inhibiting interactive effect between these factors and it was calibrated using growth curves from 10 challenge tests with unprocessed seafood. For model evaluation, 40 challenge tests with well characterized fresh and lightly preserved seafood were performed. Comparison of these observed growth curves and growth rates (μ_max-values) predicted by the new model resulted in a bias factor (B_f) of 1.12 and an accuracy factor (A_f) of 1.40. Furthermore, the new model was evaluated with 94 growth rates and 432 time to toxin formation data extracted from the scientific literature for seafood, poultry, meat, pasta and prepared meals. These data included responses for 36 different toxigenic strains of non-proteolytic C. botulinum. The obtained B_f-/A_f-values were 0.97/2.04 for μ_max-values and 0.96/1.80 for time to toxin formation. The model correctly predicted 93.8% of the growth responses with 5.6% being fail-safe and <1% fail-dangerous. A cocktail of four non-toxin producing Clostridium spp. isolates was used to develop the new model and these isolates had more than 99.8% 16S rRNA gene similarity to non-proteolytic C. botulinum (Group II). The high number of environmental factors included in the new model makes it a flexible tool to facilitate development or reformulation of seafood and poultry products that do not support the growth of non-proteolytic C. botulinum. Further, evaluation of the new model with well characterized products is desirable particularly for meat, vegetables, pasta and prepared meals as well as for dairy products that was not included in the present study.

The orphan germinant receptor protein GerXAO (but not GerX3b) is essential for L-alanine induced germination in Clostridium botulinum Group II

Clostridium botulinum is an anaerobic spore forming bacterium that produces the potent botulinum neurotoxin that causes a severe and fatal neuro-paralytic disease of humans and animals (botulism). C. botulinum Group II is a psychrotrophic saccharolytic bacterium that forms spores of moderate heat resistance and is a particular hazard in minimally heated chilled foods. Spore germination is a fundamental process that allows the spore to transition to a vegetative cell and typically involves a germinant receptor (GR) that responds to environmental signals. Analysis of C. botulinum Group II genomes shows they contain a single GR cluster (gerX3b), and an additional single gerA subunit (gerXAO). Spores of C. botulinum Group II strain Eklund 17B germinated in response to the addition of L-alanine, but did not germinate following the addition of exogenous Ca²⁺-DPA. Insertional inactivation experiments in this strain unexpectedly revealed that the orphan GR GerXAO is essential for L-alanine stimulated germination. GerX3bA and GerX3bC affected the germination rate but were unable to induce germination in the absence of GerXAO. No role could be identified for GerX3bB. This is the first study to identify the functional germination receptor of C. botulinum Group II.

Low-temperature long-time cooking of meat: Eating quality and underlying mechanisms

Heat treatment of meat at temperatures between 50 and 65 °C, for extended periods of time, is known as low-temperature long-time (LTLT) cooking. This cooking method produces meat that has increased tenderness and better appearance than when cooked at higher temperatures. Public concerns regarding this method have focused on the ability to design heat treatments that can reach microbiological safety. The heat treatment induces modification of the meat structure and its constituents, which can explain the desirable eating quality traits obtained. Denaturation, aggregation, and degradation of myofibrillar, sarcoplasmic and connective tissue proteins occur depending on the combination of time and temperature during the heat treatment. The protein changes, especially in relation to collagen denaturation, along with proteolytic activity, have often been regarded to be the main contributors to the increased meat tenderness. The mechanisms involved and the possible contribution of other factors are reviewed and discussed.

Canonical germinant receptor is dispensable for spore germination in Clostridium botulinum group II strain NCTC 11219

Clostridium botulinum is an anaerobic sporeforming bacterium that is notorious for producing a potent neurotoxin. Spores of C. botulinum can survive mild food processing treatments and subsequently germinate, multiply, produce toxin and cause botulism. Control of spore germination and outgrowth is therefore essential for the safety of mildly processed foods. However, little is known about the process of spore germination in group II C. botulinum (gIICb), which are a major concern in chilled foods because they are psychrotrophic. The classical model of spore germination states that germination is triggered by the binding of a germinant molecule to a cognate germinant receptor. Remarkably, unlike many other sporeformers, gIICb has only one predicted canonical germinant receptor although it responds to multiple germinants. Therefore, we deleted the gerBAC locus that encodes this germinant receptor to determine its role in germination. Surprisingly, the deletion did not affect germination by any of the nutrient germinants, nor by the non-nutrient dodecylamine. We conclude that one or more other, so far unidentified, germinant receptors must be responsible for nutrient induced germination in gIICb. Furthermore, the gerBAC locus was strongly conserved with intact open reading frames in 159 gIICb genomes, suggesting that it has nevertheless an important function.

An Integrative Approach to Computational Modelling of the Gene Regulatory Network Controlling Clostridium botulinum Type A1 Toxin Production

Clostridium botulinum produces botulinum neurotoxins (BoNTs), highly potent substances responsible for botulism. Currently, mathematical models of C. botulinum growth and toxigenesis are largely aimed at risk assessment and do not include explicit genetic information beyond group level but integrate many component processes, such as signalling, membrane permeability and metabolic activity. In this paper we present a scheme for modelling neurotoxin production in C. botulinum Group I type A1, based on the integration of diverse information coming from experimental results available in the literature. Experiments show that production of BoNTs depends on the growth-phase and is under the control of positive and negative regulatory elements at the intracellular level. Toxins are released as large protein complexes and are associated with non-toxic components. Here, we systematically review and integrate those regulatory elements previously described in the literature for C. botulinum Group I type A1 into a population dynamics model, to build the very first computational model of toxin production at the molecular level. We conduct a validation of our model against several items of published experimental data for different wild type and mutant strains of C. botulinum Group I type A1. The result of this process underscores the potential of mathematical modelling at the cellular level, as a means of creating opportunities in developing new strategies that could be used to prevent botulism; and potentially contribute to improved methods for the production of toxin that is used for therapeutics.

Clostridium botulinum produces botulinum neurotoxins (BoNTs), highly potent substances responsible for botulism. Currently, mathematical models of C. botulinum growth and toxigenesis are largely aimed at risk assessment and do not include explicit genetic information. In this paper we present modelling based on the integration of diverse information from experimental results available in the literature. Experiments show that production of BoNTs depends on the growth-phase and is under the control of positive and negative regulatory elements at the intracellular level. Here, we integrate these regulatory elements into a combined model of population dynamics and gene regulation to build the first computational model of toxin production at the molecular level. We conduct a validation of our model against several items of published experimental data for different wild type and mutant strains of C. botulinum Group I type A1. The result of this process underscores the potential of mathematical modelling at the cellular level, as a means of creating opportunities that could be used to prevent botulism, and potentially contribute to improved methods for the production of toxin used for therapeutics.

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

Clostridium botulinum is a highly dangerous pathogen that forms very resistant endospores that are ubiquitous in the environment, and which, under favorable conditions germinate to produce vegetative cells that multiply and form the exceptionally potent botulinum neurotoxin. To improve the control of botulinum neurotoxin-forming clostridia, it is important to understand the mechanisms involved in spore germination. Here we present models for spore germination in C. botulinum based on comparative genomics analyses, with C. botulinum Groups I and III sharing similar pathways, which differ from those proposed for C. botulinum Groups II and IV. All spores germinate in response to amino acids interacting with a germinant receptor, with four types of germinant receptor identified [encoded by various combinations of gerA, gerB, and gerC genes (gerX)]. There are three gene clusters with an ABC-like configuration; ABC [gerX1], ABABCB [gerX2] and ACxBBB [gerX4], and a single CA-B [gerX3] gene cluster. Subtypes have been identified for most germinant receptor types, and the individual GerX subunits of each cluster show similar grouping in phylogenetic trees. C. botulinum Group I contained the largest variety of gerX subtypes, with three gerX1, three gerX2, and one gerX3 subtypes, while C. botulinum Group III contained two gerX1 types and one gerX4. C. botulinum Groups II and IV contained a single germinant receptor, gerX3 and gerX1, respectively. It is likely that all four C. botulinum Groups include a SpoVA channel involved in dipicolinic acid release. The cortex-lytic enzymes present in C. botulinum Groups I and III appear to be CwlJ and SleB, while in C. botulinum Groups II and IV, SleC appears to be important.

Systematic Assessment of Nonproteolytic Clostridium botulinum Spores for Heat Resistance

Heat treatment is an important controlling factor that, in combination with other hurdles (e.g., pH, aw), is used to reduce numbers and prevent the growth of and associated neurotoxin formation by nonproteolytic C. botulinum in chilled foods. It is generally agreed that a heating process that reduces the spore concentration by a factor of 10(6) is an acceptable barrier in relation to this hazard. The purposes of the present study were to review the available data relating to heat resistance properties of nonproteolytic C. botulinum spores and to obtain an appropriate representation of parameter values suitable for use in quantitative microbial risk assessment. In total, 753 D values and 436 z values were extracted from the literature and reveal significant differences in spore heat resistance properties, particularly those corresponding to recovery in the presence or absence of lysozyme. A total of 503 D and 338 z values collected for heating temperatures at or below 83°C were used to obtain a probability distribution representing variability in spore heat resistance for strains recovered in media that did not contain lysozyme.

IMPORTANCE

In total, 753 D values and 436 z values extracted from literature sources reveal significant differences in spore heat resistance properties. On the basis of collected data, two z values have been identified, z = 7°C and z = 9°C, for spores recovered without and with lysozyme, respectively. The findings support the use of heat treatment at 90°C for 10 min to reduce the spore concentration by a factor of 10(6), providing that lysozyme is not present during recovery. This study indicates that greater heat treatment is required for food products containing lysozyme, and this might require consideration of alternative recommendation/guidance. In addition, the data set has been used to test hypotheses regarding the dependence of spore heat resistance on the toxin type and strain, on the heating technique used, and on the method of D value determination used.

Construction of Nontoxigenic Mutants of Nonproteolytic Clostridium botulinum NCTC 11219 by Insertional Mutagenesis and Gene Replacement

Group II nonproteolytic Clostridium botulinum (gIICb) strains are an important concern for the safety of minimally processed ready-to-eat foods, because they can grow and produce botulinum neurotoxin during refrigerated storage. The principles of control of gIICb by conventional food processing and preservation methods have been well investigated and translated into guidelines for the food industry; in contrast, the effectiveness of emerging processing and preservation techniques has been poorly documented. The reason is that experimental studies with C. botulinum are cumbersome because of biosafety and biosecurity concerns. In the present work, we report the construction of two nontoxigenic derivatives of the type E gIICb strain NCTC 11219. In the first strain, the botulinum toxin gene (bont/E) was insertionally inactivated with a retargeted intron using the ClosTron system. In the second strain, bont/E was exchanged for an erythromycin resistance gene using a new gene replacement strategy that makes use of pyrE as a bidirectional selection marker. Growth under optimal and stressed conditions, sporulation efficiency, and spore heat resistance of the mutants were unaltered, except for small differences in spore heat resistance at 70°C and in growth at 2.3% NaCl. The mutants described in this work provide a safe alternative for basic research as well as for food challenge and process validation studies with gIICb. In addition, this work expands the clostridial genetic toolbox with a new gene replacement method that can be applied to replace any gene in gIICb and other clostridia.

IMPORTANCE

The nontoxigenic mutants described in this work provide a safe alternative for basic research as well as for food challenge and process validation studies with psychrotrophic Clostridium botulinum In addition, this work expands the clostridial genetic toolbox with a new gene replacement method that can be applied to replace any gene in clostridia.

Study on Potential Clostridium Botulinum Growth and Toxin Production in Parma Ham

The objective of this study was to investigate Clostridium botulinum growth and toxin production in the industrially manufactured Italian Parma ham. The study focuses on the Parma ham production phase identified as maximum risk to C. botulinum proliferation, i.e. the transition from cold phase (salting and resting) to a phase carried out at temperature between 15 and 23°C (drying). A preliminary in vitro test was carried out in order to verify the capability of 6 C. botulinum strains (1 type A, 4 type B, and 1 type E strains) to grow in conditions of temperature, pH and NaCl concentration comparable to those of the beginning stage of ham drying. Five C. botulinum strains grew at 20°C and pH 6, four strains produced toxin when inoculated at a concentration equal to 103 cfu/mL at NaCl concentration of 4%, while when the inoculum concentration was 10 cfu/mL, NaCl concentration of 3% resulted the toxin-genesis limiting factor. An experimental contamination with a mixture of the 5 C. botulinum strains selected by the preliminary in vitro test was performed on 9 thighs inoculated at the end of the resting phase. The study was designed to evaluate the potential growth and toxin production in extremely favourable conditions for the bacterium. Type B proteolytic C. botulinum toxin was produced after 14 days of incubation at 20°C in 2 thighs characterised by high weight, low number of days of resting and anomalous physiochemical characteristics [one for very low NaCl concentration (1.59%), the other for elevated pH (6.27) and both for high water activity values (>0.970)]. The results of this research confirm that the cold resting step is a critical phase in the production process of Parma ham for the investigated hazard. Based on the present study, the long resting phase adopted in the manufacturing of Parma ham is proven effective to prevent the growth of C. botulinum, an event which could not otherwise be excluded if the hams were processed under less stringent technological conditions.

New Elements To Consider When Modeling the Hazards Associated with Botulinum Neurotoxin in Food

Botulinum neurotoxins (BoNTs) produced by the anaerobic bacterium Clostridium botulinum are the most potent biological substances known to mankind. BoNTs are the agents responsible for botulism, a rare condition affecting the neuromuscular junction and causing a spectrum of diseases ranging from mild cranial nerve palsies to acute respiratory failure and death. BoNTs are a potential biowarfare threat and a public health hazard, since outbreaks of foodborne botulism are caused by the ingestion of preformed BoNTs in food. Currently, mathematical models relating to the hazards associated with C. botulinum, which are largely empirical, make major contributions to botulinum risk assessment. Evaluated using statistical techniques, these models simulate the response of the bacterium to environmental conditions. Though empirical models have been successfully incorporated into risk assessments to support food safety decision making, this process includes significant uncertainties so that relevant decision making is frequently conservative and inflexible. Progression involves encoding into the models cellular processes at a molecular level, especially the details of the genetic and molecular machinery. This addition drives the connection between biological mechanisms and botulism risk assessment and hazard management strategies. This review brings together elements currently described in the literature that will be useful in building quantitative models of C. botulinum neurotoxin production. Subsequently, it outlines how the established form of modeling could be extended to include these new elements. Ultimately, this can offer further contributions to risk assessments to support food safety decision making.

Quantification of Nonproteolytic Clostridium botulinum Spore Loads in Food Materials

We have produced data and developed analysis to build representations for the concentration of spores of nonproteolytic Clostridium botulinum in materials that are used during the manufacture of minimally processed chilled foods in the United Kingdom. Food materials are categorized into homogenous groups which include meat, fish, shellfish, cereals, fresh plant material, dairy liquid, dairy nonliquid, mushroom and fungi, and dried herbs and spices. Models are constructed in a Bayesian framework and represent a combination of information from a literature survey of spore loads from positive-control experiments that establish a detection limit and from dedicated microbiological tests for real food materials. The detection of nonproteolytic C. botulinum employed an optimized protocol that combines selective enrichment culture with multiplex PCR, and the majority of tests on food materials were negative. Posterior beliefs about spore loads center on a concentration range of 1 to 10 spores kg(-1). Posterior beliefs for larger spore loads were most significant for dried herbs and spices and were most sensitive to the detailed results from control experiments. Probability distributions for spore loads are represented in a convenient form that can be used for numerical analysis and risk assessments.

The pattern of growth observed for Clostridium botulinum type A1 strain ATCC 19397 is influenced by nutritional status and quorum sensing: a modelling perspective

Botulinum neurotoxins (BoNTs) produced by the anaerobic bacterium Clostridium botulinum are the most poisonous substances known to mankind. However, toxin regulation and signals triggering synthesis as well as the regulatory network and actors controlling toxin production are unknown. Experiments show that the neurotoxin gene is growth phase dependent for C. botulinum type A1 strain ATCC 19397, and toxin production is influenced both by culture conditions and nutritional status of the medium. Building mathematical models to describe the genetic and molecular machinery that drives the synthesis and release of BoNT requires a simultaneous description of the growth of the bacterium in culture. Here, we show four plausible modelling options which could be considered when constructing models describing the pattern of growth observed in a botulinum growth medium. Commonly used bacterial growth models are unsuitable to fit the pattern of growth observed, since they only include monotonic growth behaviour. We find that a model that includes both the nutritional status and the ability of the cells to sense their surroundings in a quorum-sensing manner is most successful at explaining the pattern of growth obtained for C. botulinum type A1 strain ATCC 19397.

Morphological and genetic characterization of group I Clostridium botulinum type B strain 111 and the transcriptional regulator spoIIID gene knockout mutant in sporulation

Clostridium botulinum is a heat-resistant spore-forming bacterium that causes the serious paralytic illness botulism. Heat-resistant spores may cause food sanitation hazards and sporulation plays a central role in the survival of C. botulinum. We observed morphological changes and investigated the role of the transcriptional regulator SpoIIID in the sporulation of C. botulinum type B strain 111 in order to elucidate the molecular mechanism in C. botulinum. C. botulinum type B formed heat-resistant spores through successive morphological changes corresponding to those of Bacillus subtilis, a spore-forming model organism. An analysis of the spoIIID gene knockout mutant revealed that the transcriptional regulator SpoIIID contributed to heat-resistant spore formation by C. botulinum type B and activated the transcription of the sigK gene later during sporulation. Transcription of the spoIIID gene, which differed from that in B. subtilis and Clostridium difficile, was observed in the sigE gene knockout mutant of C. botulinum type B. An analysis of the sigF gene knockout mutant showed that the sporulation-specific sigma factor SigF was essential for transcription of the spoIIID gene in C. botulinum type B. These results suggest that the regulation of sporulation in C. botulinum is not similar to that in B. subtilis and other clostridia.

Draft Genome Sequences of Two Clostridium botulinum Group II (Nonproteolytic) Type B Strains (DB-2 and KAPB-3)

Clostridium botulinum is important for food safety and studies of neurotoxins associated with human botulism. We present the draft genome sequences of two strains belonging to group II type B: one collected from Pacific Ocean sediments (DB-2) and another obtained during a botulism outbreak (KAPB-3).

Genomes, neurotoxins and biology of Clostridium botulinum Group I and Group II

Recent developments in whole genome sequencing have made a substantial contribution to understanding the genomes, neurotoxins and biology of Clostridium botulinum Group I (proteolytic C. botulinum) and C. botulinum Group II (non-proteolytic C. botulinum). Two different approaches are used to study genomics in these bacteria; comparative whole genome microarrays and direct comparison of complete genome DNA sequences. The properties of the different types of neurotoxin formed, and different neurotoxin gene clusters found in C. botulinum Groups I and II are explored. Specific examples of botulinum neurotoxin genes are chosen for an in-depth discussion of neurotoxin gene evolution. The most recent cases of foodborne botulism are summarised.

Role of csp genes in NaCl, pH, and ethanol stress response and motility in Clostridium botulinum ATCC 3502

Clostridium botulinum is a notable food pathogen and responsible for botulism due to production of botulinum neurotoxin. Strains of C. botulinum can adapt to and survive in stress conditions and food processing. The cold shock protein coding genes (csp) are involved in growth at low temperature, but they may also possess other functions. In this mutational analysis we show that cspB and cspC, but not cspA, are important for NaCl, pH and ethanol stress responses and for motility of C. botulinum ATCC 3502. In all NaCl concentrations tested, the cspB mutant had lower maximum growth rate and, together with the cspC mutant, a longer lag phase compared to the wild-type strain. At low pH, the cspB and cspC mutants showed either lower maximum growth rates or longer lag phases compared to the wild type. In all ethanol concentrations tested, the cspB mutant had lower maximum growth rates and the cspC mutant had a longer lag phase than the wild-type strain. Motility was reduced in cspA and cspC mutants, and flagella formation was affected. The results suggest that cspB plays a universal role in stress response and cspC aids C. botulinum in NaCl, pH and ethanol stress in C. botulinum ATCC 3502.

New targets in the search for preventive and therapeutic agents for botulism

Botulism is a severe neuroparalytic disease resulting from exposure to one of the most poisonous toxins to humans. Because of this high potency and the use of toxins as biological weapons, botulism is a public health concern and each case represents an emergency. Current therapy involves respiratory supportive care and anti-toxins administration. As a preventive measure, vaccination against toxins represents an effective strategy but is undesirable due the rarity of botulism and the effectiveness of toxins in treating several neuromuscular disorders. This paper summarizes the current issues in botulism treatment and prevention, highlighting the challenge for future researches.

Alternative sigma factors SigF, SigE, and SigG are essential for sporulation in Clostridium botulinum ATCC 3502

Clostridium botulinum produces heat-resistant endospores that may germinate and outgrow into neurotoxic cultures in foods. Sporulation is regulated by the transcription factor Spo0A and the alternative sigma factors SigF, SigE, SigG, and SigK in most spore formers studied to date. We constructed mutants of sigF, sigE, and sigG in C. botulinum ATCC 3502 and used quantitative reverse transcriptase PCR and electron microscopy to assess their expression of the sporulation pathway on transcriptional and morphological levels. In all three mutants the expression of spo0A was disrupted. The sigF and sigE mutants failed to induce sigG and sigK beyond exponential-phase levels and halted sporulation during asymmetric cell division. In the sigG mutant, peak transcription of sigE was delayed and sigK levels remained lower than that in the parent strain. The sigG mutant forespore was engulfed by the mother cell and possessed a spore coat but no peptidoglycan cortex. The findings suggest that SigF and SigE of C. botulinum ATCC 3502 are essential for early sporulation and late-stage induction of sigK, whereas SigG is essential for spore cortex formation but not for coat formation, as opposed to previous observations in B. subtilis sigG mutants. Our findings add to a growing body of evidence that regulation of sporulation in C. botulinum ATCC 3502, and among the clostridia, differs from the B. subtilis model.

Genomic and physiological variability within Group II (non-proteolytic) Clostridium botulinum

Background

Clostridium botulinum is a group of four physiologically and phylogenetically distinct bacteria that produce botulinum neurotoxin. While studies have characterised variability between strains of Group I (proteolytic) C. botulinum, the genetic and physiological variability and relationships between strains within Group II (non-proteolytic) C. botulinum are not well understood. In this study the genome of Group II strain C. botulinum Eklund 17B (NRP) was sequenced and used to construct a whole genome DNA microarray. This was used in a comparative genomic indexing study to compare the relatedness of 43 strains of Group II C. botulinum (14 type B, 24 type E and 5 type F). These results were compared with characteristics determined from physiological tests.

Results

Whole genome indexing showed that strains of Group II C. botulinum isolated from a wide variety of environments over more than 75 years clustered together indicating the genetic background of Group II C. botulinum is stable. Further analysis showed that strains forming type B or type F toxin are closely related with only toxin cluster genes targets being unique to either type. Strains producing type E toxin formed a separate subset. Carbohydrate fermentation tests supported the observation that type B and F strains form a separate subset to type E strains. All the type F strains and most of type B strains produced acid from amylopectin, amylose and glycogen whereas type E strains did not. However, these two subsets did not differ strongly in minimum growth temperature or maximum NaCl concentration for growth. No relationship was found between tellurite resistance and toxin type despite all the tested type B and type F strains carrying tehB, while the sequence was absent or diverged in all type E strains.

Conclusions

Although Group II C. botulinum form a tight genetic group, genomic and physiological analysis indicates there are two distinct subsets within this group. All type B strains and type F strains are in one subset and all type E strains in the other.

Development of a time-to-detect growth model for heat-treated Bacillus cereus spores

The microbiological safety and quality of Refrigerated Processed Foods of Extended Durability (REPFEDs) relies on a combination of mild heat treatment and refrigeration, sometimes in combination with other inhibitory agents that are ineffective when used alone. In this context, a predictive model describing the time-to-detect growth (measured by turbidimetry) of psychrotrophic Bacillus cereus spores submitted to various combinations of pH, water activity (aw), heat treatment and storage temperature was developed. As the inoculum was high, the time-to-detect growth was the sum of two times: for a large part of the spore lag time (time before germination and outgrowth) and to a lesser extent of the time to have subsequent vegetative cells growing up to a detectable level. A dataset of 434 combinations (of pH, aw, heat treatment, storage temperature and B. cereus strain), originally collected at Ghent University to build a growth/no-growth model for two Bacillus cereus strains, was re-interpreted as time-to-detect growth values. In the growth area (223 combinations) the time-to-detect growth was set as the longest time where none, or only one, of the 8 replicated wells showed growth. In the no-growth area (211 combinations) the time-to-detect growth was set as longer than the time where the experiment was stopped (60days or more) and analysed as a censored response. The factors of variation were heat-treatment intensity (85°C, 87°C and 90°C in a time range of 1 to 38min), storage temperature (8-30°C), pH (5.2-6.4) and aw (0.973-0.995). Two different strains were analysed. The model had a Gamma multiplicative structure; it was solved by Bayesian inference with informative prior distributions. To be implemented in a decision tool, for instance to calculate the process and formulation conditions required to achieve a given detection time, each Gamma term had some constraints: they had to be monotonous, continuous and algebraically simple mathematical functions (i.e. having analytical solution). Overall, the cumulative effect of various stressful conditions (pasteurisation process, low temperature, and low pH) enables to extend the time-to-detect growth up to 60days or more, whereas the heat-treatment on its own did not have a similar effect. For example, with the most heat resistant strain (strain 1, FF140), for a product at aw0.99, stored at 10°C, heat-treated at 90°C for 10min, a time-to-detect growth of 2days was expected when the pH equalled 6.5. Under the same conditions, if the pH was reduced to 5.8, the time-to-detect growth was predicted to be 11days (and 33days at pH5.5). After a pasteurisation at 90°C for 10min, for a product kept at 10°C, combinations of pH and aw such as pH6.0-aw0.97, pH5.7-aw0.98 or pH5.5-aw0.99 were predicted to extend the time-to-detect growth up to 30days. The developed model is a useful tool for REPFED producers to guarantee the safety of their products towards psychrotrophic B. cereus.

Array-based transcriptional analysis of Clostridium sporogenes UC9000 during germination, cell outgrowth and vegetative life

The members of the genus Clostridium, including the spore-forming anaerobic bacteria, have a complex and strictly regulated life cycle, but very little is known about the genetic pathways involved in the different stages of their life cycle. Clostridium sporogenes, a Gram-positive bacterium usually involved in food spoilage and frequently isolated from late blowing cheese, is genetically indistinguishable from the proteolytic Clostridium botulinum. As the non-neurotoxic counterpart, it is often used as an exemplar for the toxic subtypes. In this work, we performed a microscopic study combined with a custom array-based analysis of the C. sporogenes cycle, from dormant spores to the early stationary phase. We identified a total of 211 transcripts in spores, validating the hypothesis that mRNAs are abundant in spores and the pattern of mRNA expression is strikingly different from that present in growing cells. The spore transcripts included genes responsible for different life-sustaining functions, suggesting there was transcript entrapment or basic poly-functional gene activation for future steps. In addition, 3 h after the beginning of the germination process, 20% of the total up-regulated genes were temporally expressed in germinating spores. The vegetative condition appeared to be more active in terms of gene transcription and protein synthesis than the spore, and genes coding for germination and sporulation factors seemed to be expressed at this point. These results suggest that spores are not silent entities, and a broader knowledge of the genetic pathways involved in the Clostridium life cycle could provide a better understanding of pathogenic clostridia types.

Does proximity to neighbours affect germination of spores of non-proteolytic Clostridium botulinum?

It is recognised that inoculum size affects the rate and extent of bacterial spore germination. It has been proposed that this is due to spores interacting: molecules released from germinated spores trigger germination of dormant neighbours. This study investigated whether changes to the total number of spores in a system or proximity to other spores (local spore density) had a more significant effect on interaction between spores of non-proteolytic Clostridium botulinum strain Eklund 17B attached to defined areas of microscope slides. Both the number of spores attached to the slides and local spore density (number of spores per mm(2)) were varied by a factor of nine. Germination was observed microscopically at 15 °C for 8 h and the probability of, and time to, germination calculated from image analysis measurements. Statistical analysis revealed that the effect of total spore number on the probability of germination within 8 h was more significant than that of proximity to neighbours (local spore density); its influence on germination probability was approximately four-times greater. Total spore number had an even more significant affect on time to germination; it had a nine-fold greater influence than proximity to neighbours. The applied models provide a means to characterise, quantitatively, the effect of the total spore number on spore germination relative to the effect of proximity to neighbouring spores.

Epidemiology of influenza A 2009 H1N1 virus pandemic in the U.S

A novel influenza, Influenza A 2009 H1N1 virus outbreak, emerged in mid-April 2009, and by December 2009, spread across the world. This epidemiologic analysis uses the epidemiologic problem oriented approach to gather information for, and develop a quantitative risk assessment model that evaluates the likelihood of an influenza outbreak in the U.S., utilizing weekly incidence rates (WIR) and case fatality rates (CFR) stratified by age and the 10 U.S. regions. In addition ArcGIS was used to show variability regarding morbidity rate and WIR. Using data from the Centers for Disease Control and Prevention (CDC) Emerging Infections Program and applying Monte Carlo simulations with @Risk software, the analysis revealed morbidity to be highest in region 8 (1.27×10-2) and lowest in region 2 (0.13×10-2). Heat map results revealed WIR were higher in younger children, for all regions. Case fatality rates were 10 times greater in younger people than in older people and children. Using these different temporal-spatial epidemiologic methods can help decision-makers identify high-risk population clusters and help in prevention and control of pandemics.

Food safety objective approach for controlling Clostridium botulinum growth and toxin production in commercially sterile foods

As existing technologies are refined and novel microbial inactivation technologies are developed, there is a growing need for a metric that can be used to judge equivalent levels of hazard control stringency to ensure food safety of commercially sterile foods. A food safety objective (FSO) is an output-oriented metric that designates the maximum level of a hazard (e.g., the pathogenic microorganism or toxin) tolerated in a food at the end of the food supply chain at the moment of consumption without specifying by which measures the hazard level is controlled. Using a risk-based approach, when the total outcome of controlling initial levels (H(0)), reducing levels (ΣR), and preventing an increase in levels (ΣI) is less than or equal to the target FSO, the product is considered safe. A cross-disciplinary international consortium of specialists from industry, academia, and government was organized with the objective of developing a document to illustrate the FSO approach for controlling Clostridium botulinum toxin in commercially sterile foods. This article outlines the general principles of an FSO risk management framework for controlling C. botulinum growth and toxin production in commercially sterile foods. Topics include historical approaches to establishing commercial sterility; a perspective on the establishment of an appropriate target FSO; a discussion of control of initial levels, reduction of levels, and prevention of an increase in levels of the hazard; and deterministic and stochastic examples that illustrate the impact that various control measure combinations have on the safety of well-established commercially sterile products and the ways in which variability all levels of control can heavily influence estimates in the FSO risk management framework. This risk-based framework should encourage development of innovative technologies that result in microbial safety levels equivalent to those achieved with traditional processing methods.

Towards an international standard for detection and typing botulinum neurotoxin-producing Clostridia types A, B, E and F in food, feed and environmental samples: a European ring trial study to evaluate a real-time PCR assay

A real-time PCR method for detection and typing of BoNT-producing Clostridia types A, B, E, and F was developed on the framework of the European Research Project "Biotracer". A primary evaluation was carried out using 104 strains and 17 clinical and food samples linked to botulism cases. Results showed 100% relative accuracy, 100% relative sensitivity, 100% relative specificity, and 100% selectivity (inclusivity on 73 strains and exclusivity on 31 strains) of the real-time PCR against the reference cultural method combined with the standard mouse bioassay. Furthermore, a ring trial study performed at four different European laboratories in Italy, France, the Netherlands, and Sweden was carried out using 47 strains, and 30 clinical and food samples linked to botulism cases. Results showed a concordance of 95.7% among the four laboratories. The reproducibility generated a relative standard deviation in the range of 2.18% to 13.61%. Considering the high level of agreement achieved between the laboratories, this real-time PCR is a suitable method for rapid detection and typing of BoNT-producing Clostridia in clinical, food and environmental samples and thus support the use of it as an international standard method.

Development and application of a new method for specific and sensitive enumeration of spores of nonproteolytic Clostridium botulinum types B, E, and F in foods and food materials

The highly potent botulinum neurotoxins are responsible for botulism, a severe neuroparalytic disease. Strains of nonproteolytic Clostridium botulinum form neurotoxins of types B, E, and F and are the main hazard associated with minimally heated refrigerated foods. Recent developments in quantitative microbiological risk assessment (QMRA) and food safety objectives (FSO) have made food safety more quantitative and include, as inputs, probability distributions for the contamination of food materials and foods. A new method that combines a selective enrichment culture with multiplex PCR has been developed and validated to enumerate specifically the spores of nonproteolytic C. botulinum. Key features of this new method include the following: (i) it is specific for nonproteolytic C. botulinum (and does not detect proteolytic C. botulinum), (ii) the detection limit has been determined for each food tested (using carefully structured control samples), and (iii) a low detection limit has been achieved by the use of selective enrichment and large test samples. The method has been used to enumerate spores of nonproteolytic C. botulinum in 637 samples of 19 food materials included in pasta-based minimally heated refrigerated foods and in 7 complete foods. A total of 32 samples (5 egg pastas and 27 scallops) contained spores of nonproteolytic C. botulinum type B or F. The majority of samples contained <100 spores/kg, but one sample of scallops contained 444 spores/kg. Nonproteolytic C. botulinum type E was not detected. Importantly, for QMRA and FSO, the construction of probability distributions will enable the frequency of packs containing particular levels of contamination to be determined.

Rapid affinity immunochromatography column-based tests for sensitive detection of Clostridium botulinum neurotoxins and Escherichia coli O157

Existing methods for detection of food-borne pathogens and their toxins are frequently time-consuming, require specialized equipment, and involve lengthy culture procedures and/or animal testing and are thus unsuitable for a rapid response to an emergency public health situation. A series of simple and rapid affinity immunochromatography column (AICC) assays were developed to detect Clostridium botulinum neurotoxin types A, B, E, and F and Escherichia coli O157 in food matrices. Specifically, for milk, grape juice with peach juice, and bottled water, the detection limit for the botulinum neurotoxin type A complex was 0.5 ng. Use of this method with a 10-ml sample would therefore result in a detection limit of 50 pg ml(-l). Thus, this assay is approximately 2 orders of magnitude more sensitive than a comparable lateral-flow assay. For botulinum neurotoxin complex types B, E, and F, the minimum detection limit was 5 ng to 50 ng. Sensitive detection of E. coli O157 was achieved, and the detection limit was 500 cells. The AICC test was also shown to be specific, rapid, and user friendly. This test takes only 15 to 30 min to complete without any specialized equipment and thus is suitable for use in the field. It has the potential to replace existing methods for presumptive detection of botulinum neurotoxin types A, B, E, and F and E. coli O157 in contaminated matrices without a requirement for preenrichment.

Lag time variability in individual spores of Clostridium botulinum

Quantifying lag times from individual spores and the associated variability is an important part of understanding the hazard associated with spore-forming pathogens such as Clostridium botulinum. Knowledge of the underlying distribution would allow greater refinement of risk assessments. To date most studies have either examined lag time indirectly by measuring time to growth or have only examined the first stage of lag, germination. Recent studies have attempted to quantify the variability of spores during the different stages of lag phase and to examine the relationships between these stages. The effect of incubation temperature (22 °C, 15 °C, 10 °C or 8 °C), heat treatment (unheated or 80 °C for 20 s) and sodium chloride concentration in both the sporulation medium (0 or 3% w/v) or growth medium (0 or 2% w/v) on growth from individual spores has been examined. These studies found spores within a single population are very heterogeneous with large variability in all stages of lag. The duration and variability of times for germination, outgrowth and first doubling depended on both the historic treatment of the spores and the prevailing growth conditions, and the stage of lag most affected was treatment dependant.

Clostridium botulinum in the post-genomic era

Foodborne botulism is a severe neuroparalytic disease caused by consumption of botulinum neurotoxin formed by strains of proteolytic Clostridium botulinum and non-proteolytic C. botulinum during their growth in food. The botulinum neurotoxin is the most potent substance known, with as little as 30-100 ng potentially fatal, and consumption of just a few milligrams of neurotoxin-containing food is likely to be sufficient to cause illness and potentially death. In order to minimise the foodborne botulism hazard, it is necessary to extend understanding of the biology of these bacteria. This process has been recently advanced by genome sequencing and subsequent analysis. In addition to neurotoxin formation, endospore formation is also critical to the success of proteolytic C. botulinum and non-proteolytic C. botulinum as foodborne pathogens. The endospores are highly resistant, and enable survival of adverse treatments such as heating. To better control the botulinum neurotoxin-forming clostridia, it is important to understand spore resistance mechanisms, and the physiological processes involved in germination and lag phase during recovery from this dormant state.

Effects of carbon dioxide on growth of proteolytic Clostridium botulinum, its ability to produce neurotoxin, and its transcriptome

The antimicrobial gas carbon dioxide is frequently used in modified atmosphere packaging. In the present study, the effects of CO2 (10 to 70%, vol/vol) on gene expression (measured using quantitative reverse transcription-PCR and a whole-genome DNA microarray) and neurotoxin formation (measured using an enzyme-linked immunosorbent assay [ELISA]) by proteolytic Clostridium botulinum type A1 strain ATCC 3502 were studied during the growth cycle. Interestingly, in marked contrast to the situation with nonproteolytic C. botulinum types B and E, CO2 had little effect on any of these parameters. At all CO2 concentrations, relative expression of neurotoxin cluster genes peaked in the transition between exponential and stationary phases, with evidence of a second rise in expression in late stationary phase. Microarray analysis enabled identification of coding sequences whose expression profiles matched those of the neurotoxin cluster. Further research is needed to determine whether these are connected to neurotoxin formation or are merely growth phase associated.

Multiplex PCR for detection of botulinum neurotoxin-producing clostridia in clinical, food, and environmental samples

Botulinum neurotoxin (BoNT), the most toxic substance known, is produced by the spore-forming bacterium Clostridium botulinum and, in rare cases, also by some strains of Clostridium butyricum and Clostridium baratii. The standard procedure for definitive detection of BoNT-producing clostridia is a culture method combined with neurotoxin detection using a standard mouse bioassay (SMB). The SMB is highly sensitive and specific, but it is expensive and time-consuming and there are ethical concerns due to use of laboratory animals. PCR provides a rapid alternative for initial screening for BoNT-producing clostridia. In this study, a previously described multiplex PCR assay was modified to detect all type A, B, E, and F neurotoxin genes in isolated strains and in clinical, food, environmental samples. This assay includes an internal amplification control. The effectiveness of the multiplex PCR method for detecting clostridia possessing type A, B, E, and F neurotoxin genes was evaluated by direct comparison with the SMB. This method showed 100% inclusivity and 100% exclusivity when 182 BoNT-producing clostridia and 21 other bacterial strains were used. The relative accuracy of the multiplex PCR and SMB was evaluated using 532 clinical, food, and environmental samples and was estimated to be 99.2%. The multiplex PCR was also used to investigate 110 freshly collected food and environmental samples, and 4 of the 110 samples (3.6%) were positive for BoNT-encoding genes.

Independent evolution of neurotoxin and flagellar genetic loci in proteolytic Clostridium botulinum

Background

Proteolytic Clostridium botulinum is the causative agent of botulism, a severe neuroparalytic illness. Given the severity of botulism, surprisingly little is known of the population structure, biology, phylogeny or evolution of C. botulinum. The recent determination of the genome sequence of C. botulinum has allowed comparative genomic indexing using a DNA microarray.

Results

Whole genome microarray analysis revealed that 63% of the coding sequences (CDSs) present in reference strain ATCC 3502 were common to all 61 widely-representative strains of proteolytic C. botulinum and the closely related C. sporogenes tested. This indicates a relatively stable genome. There was, however, evidence for recombination and genetic exchange, in particular within the neurotoxin gene and cluster (including transfer of neurotoxin genes to C. sporogenes), and the flagellar glycosylation island (FGI). These two loci appear to have evolved independently from each other, and from the remainder of the genetic complement. A number of strains were atypical; for example, while 10 out of 14 strains that formed type A1 toxin gave almost identical profiles in whole genome, neurotoxin cluster and FGI analyses, the other four strains showed divergent properties. Furthermore, a new neurotoxin sub-type (A5) has been discovered in strains from heroin-associated wound botulism cases. For the first time, differences in glycosylation profiles of the flagella could be linked to differences in the gene content of the FGI.

Conclusion

Proteolytic C. botulinum has a stable genome backbone containing specific regions of genetic heterogeneity. These include the neurotoxin gene cluster and the FGI, each having evolved independently of each other and the remainder of the genetic complement. Analysis of these genetic components provides a high degree of discrimination of strains of proteolytic C. botulinum, and is suitable for clinical and forensic investigations of botulism outbreaks.

Contrasting effects of heat treatment and incubation temperature on germination and outgrowth of individual spores of nonproteolytic Clostridium botulinum bacteria

In this study, we determined the effects of incubation temperature and prior heat treatment on the lag-phase kinetics of individual spores of nonproteolytic Clostridium botulinum Eklund 17B. The times to germination (t(germ)), one mature cell (t(C1)), and two mature cells (t(C2)) were measured for individual unheated spores incubated at 8, 10, 15, or 22 degrees C and used to calculate the t(germ), the outgrowth time (t(C1) - t(germ)), and the first doubling time (t(C2) - t(C1)). Measurements were also made at 22 degrees C of spores that had previously been heated at 80 degrees C for 20 s. For unheated spores, outgrowth made a greater contribution to the duration and variability of the lag phase than germination. Decreasing incubation temperature affected germination less than outgrowth; thus, the proportion of lag associated with germination was less at lower incubation temperatures. Heat treatment at 80 degrees C for 20 s increased the median germination time of surviving spores 16-fold and greatly increased the variability of spore germination times. The shape of the lag-time (t(C1)) and outgrowth (t(C1) - t(germ)) distributions were the same for unheated spores, but heat treatment altered the shape of the lag-time distribution, so it was no longer homogeneous with the outgrowth distribution. Although heat treatment mainly extended germination, there is also evidence of damage to systems required for outgrowth. However, this damage was quickly repaired and was not evident by the time the cells started to double. The results presented here combined with previous findings show that the stage of lag most affected, and the extent of any effect in terms of duration or variability, differs with both historical treatment and the growth conditions.