Comparison of cytotoxin cytK promoters from Bacillus cereus strain ATCC 14579 and from a B. cereus food-poisoning strain

Colorimetric aptasensor for detection of Bacillus cytotoxicus spores in milk and ready-to-use food

The high incidence of foodborne diseases caused by pathogenic bacteria raises concerns worldwide and imposes considerable public healthcare challenges. This is especially observed with dormant spores of Bacilli, which can often survive treatments used by the food industry to kill growing bacteria. The early and rapid detection of bacterial spores is essential to ensure food safety. Commercial availability of such a test will present a high potential for food sector. We present a point-of-need colorimetric assay for detection of Bacillus cytotoxicus spores in food. The detection principle is based on spore-enhanced peroxidase-like catalytic activity of gold nanoparticles. The sensing platform consists of a microtube containing gold nanoparticles (AuNPs), and magnetic particles (MPs), both conjugated with specific aptamer BAS6R that recognize B. cytotoxicus spores. Upon the addition of the sample, spores were determined as present by the enhanced color change of the solution, due to the oxidation of tetramethylbenidine (TMB) with H₂O₂. The assay was evaluated by the naked eye (on/off) and quantitatively with use of a spectrophotometer. BAS6R@AuNPs aptasensor coupled to BAS6R@MPs proved to be highly sensitive, achieving the naked-eye limit of detection as low as 10² cfu/mL in water and milk, and 10⁴ cfu/mL in mashed potatoes. Moreover, discrimination between spores of B. cytotoxicus and B. subtilis as well as bacterial vegetative cells was achieved in contaminated food samples, providing a good selectivity. This work provides a promising proof of concept for the development of instrument-free, low-cost and rapid assay for Bacillus cytotoxicus spore detection, which is able to compete in sensitivity with conventional costly and time-consuming laboratory analyses.

Pore-Forming Toxins During Bacterial Infection: Molecular Mechanisms and Potential Therapeutic Targets

Bacterial infections are predominantly treated with antibiotics, and resistance to antibiotics is becoming an increasing threat to our health. Pore-forming toxins (PFTs) are virulence factors secreted by many pathogenic bacterial strains, both in acute and chronic infections. They are special membrane-targeting proteins that exert toxic effects by forming pores in the cell membrane. Recent studies have elucidated the structure of PFTs and the detailed molecular mechanisms of their pathogenicity. Here, we discuss recent findings that highlight the regulatory mechanisms and important roles of two types of PFTs, α-PFTs and β-PFTs, in mediating the virulence of bacteria, and the therapeutic potential of targeting PFTs for antibacterial treatment. Therapeutic strategies based on PFTs are highly specific and may alleviate the issue of increasing resistance to antibiotics.

The Food Poisoning Toxins of Bacillus cereus

Bacillus cereus is a ubiquitous soil bacterium responsible for two types of food-associated gastrointestinal diseases. While the emetic type, a food intoxication, manifests in nausea and vomiting, food infections with enteropathogenic strains cause diarrhea and abdominal pain. Causative toxins are the cyclic dodecadepsipeptide cereulide, and the proteinaceous enterotoxins hemolysin BL (Hbl), nonhemolytic enterotoxin (Nhe) and cytotoxin K (CytK), respectively. This review covers the current knowledge on distribution and genetic organization of the toxin genes, as well as mechanisms of enterotoxin gene regulation and toxin secretion. In this context, the exceptionally high variability of toxin production between single strains is highlighted. In addition, the mode of action of the pore-forming enterotoxins and their effect on target cells is described in detail. The main focus of this review are the two tripartite enterotoxin complexes Hbl and Nhe, but the latest findings on cereulide and CytK are also presented, as well as methods for toxin detection, and the contribution of further putative virulence factors to the diarrheal disease.

Prevalence and Diversity of the Thermotolerant Bacterium Bacillus cytotoxicus among Dried Food Products

HIGHLIGHTS

Bacillus cytotoxicus was found in all tested potato flakes but at loads lower than 10² CFU/g. B. cytotoxicus was observed in other potato-containing products and in millet flour. B. cytotoxicus isolates (n = 57) fell into six RAPD patterns and 11 plasmid profiles. A large proportion of B. cytotoxicus isolates contained small and/or large plasmids.

Comparative Bioinformatics and Experimental Analysis of the Intergenic Regulatory Regions of Bacillus cereus hbl and nhe Enterotoxin Operons and the Impact of CodY on Virulence Heterogeneity

Bacillus cereus is a food contaminant with greatly varying enteropathogenic potential. Almost all known strains harbor the genes for at least one of the three enterotoxins Nhe, Hbl, and CytK. While some strains show no cytotoxicity, others have caused outbreaks, in rare cases even with lethal outcome. The reason for these differences in cytotoxicity is unknown. To gain insight into the origin of enterotoxin expression heterogeneity in different strains, the architecture and role of 5′ intergenic regions (5′ IGRs) upstream of the nhe and hbl operons was investigated. In silico comparison of 142 strains of all seven phylogenetic groups of B. cereus sensu lato proved the presence of long 5′ IGRs upstream of the nheABC and hblCDAB operons, which harbor recognition sites for several transcriptional regulators, including the virulence regulator PlcR, redox regulators ResD and Fnr, the nutrient-sensitive regulator CodY as well as the master regulator for biofilm formation SinR. By determining transcription start sites, unusually long 5′ untranslated regions (5′ UTRs) upstream of the nhe and hbl start codons were identified, which are not present upstream of cytK-1 and cytK-2. Promoter fusions lacking various parts of the nhe and hbl 5′ UTR in B. cereus INRA C3 showed that the entire 331 bp 5′ UTR of nhe is necessary for full promoter activity, while the presence of the complete 606 bp hbl 5′ UTR lowers promoter activity. Repression was caused by a 268 bp sequence directly upstream of the hbl transcription start. Luciferase activity of reporter strains containing nhe and hbl 5′ IGR lux fusions provided evidence that toxin gene transcription is upregulated by the depletion of free amino acids. Electrophoretic mobility shift assays showed that the branched-chain amino acid sensing regulator CodY binds to both nhe and hbl 5′ UTR downstream of the promoter, potentially acting as a nutrient-responsive roadblock repressor of toxin gene transcription. PlcR binding sites are highly conserved among all B. cereus sensu lato strains, indicating that this regulator does not significantly contribute to the heterogeneity in virulence potentials. The CodY recognition sites are far less conserved, perhaps conferring varying strengths of CodY binding, which might modulate toxin synthesis in a strain-specific manner.

Fatty acid profiles and desaturase-encoding genes are different in thermo- and psychrotolerant strains of the Bacillus cereus Group

BACKGROUND

The Bacillus cereus Group consists of closely-related bacteria, including pathogenic or harmless strains, and whose species can be positioned along the seven phylogenetic groups of Guinebretière et al. (I-VII). They exhibit different growth-temperature ranges, through thermotolerant to psychrotolerant thermotypes. Among these, B. cytotoxicus is an atypical thermotolerant and food-poisoning agent affiliated to group VII whose thermotolerance contrasts with the mesophilic and psychrotolerant thermotypes associated to the remaining groups I-VI. To understand the role of fatty acid (FA) composition in these variable thermotypes (i.e. growth behavior vs temperatures), we report specific features differentiating the FA pattern of B. cytotoxicus (group VII) from its counterparts (groups I-VI).

FINDINGS

The FA pattern of thermotolerant group VII (B. cytotoxicus) displayed several specific features. Most notably, we identified a high ratio of the branched-chain FAs iso-C15/iso-C13 (i15/i13) and the absence of the unsaturated FA (UFA) C16:1(5) consistent with the absence of ∆5 desaturase DesA. Conversely, phylogenetic groups II-VI were characterized by lower i15/i13 ratios and variable proportions of C16:1(5) depending on thermotype, and presence of the DesA desaturase. In mesophilic group I, thermotype seemed to be related to an atypically high amount of C16:1(10) that may involve ∆10 desaturase DesB.

CONCLUSION

The levels of i15/i13 ratio, C16:1(5) and C16:1(10) UFAs were related to growth temperature variations recorded between thermotypes and/or phylogenetic groups. These FA are likely to play a role in membrane fluidity and may account for the differences in temperature tolerance observed in B. cereus Group strains.

Cytochrome c551 and the cytochrome c maturation pathway affect virulence gene expression in Bacillus cereus ATCC 14579

Loss of the cytochrome c maturation system in Bacillus cereus results in increased transcription of the major enterotoxin genes nhe, hbl, and cytK and the virulence regulator plcR. Increased virulence factor production occurs at 37°C under aerobic conditions, similar to previous findings in Bacillus anthracis. Unlike B. anthracis, much of the increased virulence gene expression can be attributed to loss of only c551, one of the two small c-type cytochromes. Additional virulence factor expression occurs with loss of resBC, encoding cytochrome c maturation proteins, independently of the presence of the c-type cytochrome genes. Hemolytic activity of strains missing either cccB or resBC is increased relative to that in the parental strain, while sporulation efficiency is unaffected in the mutants. Increased virulence gene expression in the ΔcccB and ΔresBC mutants occurs only in the presence of an intact plcR gene, indicating that this process is PlcR dependent. These findings suggest a new mode of regulation of B. cereus virulence and reveal intriguing similarities and differences in virulence regulation between B. cereus and B. anthracis.

The CasKR two-component system is required for the growth of mesophilic and psychrotolerant Bacillus cereus strains at low temperatures

The different strains of Bacillus cereus can grow at temperatures covering a very diverse range. Some B. cereus strains can grow in chilled food and consequently cause food poisoning. We have identified a new sensor/regulator mechanism involved in low-temperature B. cereus growth. Construction of a mutant of this two-component system enabled us to show that this system, called CasKR, is required for growth at the minimal temperature (Tmin). CasKR was also involved in optimal cold growth above Tmin and in cell survival below Tmin. Microscopic observation showed that CasKR plays a key role in cell shape during cold growth. Introducing the casKR genes in a ΔcasKR mutant restored its ability to grow at Tmin. Although it was first identified in the ATCC 14579 model strain, this mechanism has been conserved in most strains of the B. cereus group. We show that the role of CasKR in cold growth is similar in other B. cereus sensu lato strains with different growth temperature ranges, including psychrotolerant strains.

PCR detection of cytK gene in Bacillus cereus group strains isolated from food samples

A method for detection of the cytotoxin K cytK structural gene and its active promoter preceded by the PlcR-binding box, controlling the expression level of this enterotoxin, was developed. The method was applied for the purpose of the analysis of 47 bacterial strains belonging to the Bacillus cereus group isolated from different food products. It was found that the majority of the analyzed strains carried the fully functional cytK gene with its PlcR regulated promoter. The cytK gene was not detected in four emetic strains of Bacillus cereus carrying the cesB gene and potentially producing an emetic toxin - cereulide. The cytotoxin K gene was detected in 4 isolates classified as Bacillus mycoides and one reference strain B. mycoides PCM 2024. The promoter region and the N-terminal part of the cytK gene from two strains of B. mycoides (5D and 19E) showed similarities to the corresponding sequences of Bacillus cereus W23 and Bacillus thuringiensis HD-789, respectively. It was shown for the first time that the cytK gene promoter region from strains 5D and 19E of Bacillus mycoides had a similar arrangement to the corresponding sequence of Bacillus cereus ATCC 14579. The presence of the cytK gene in Bacillus mycoides shows that this species, widely recognized as nonpathogenic, may pose potential biohazard to human beings.

The pore-forming haemolysins of bacillus cereus: a review

The Bacillus cereus sensu lato group contains diverse Gram-positive spore-forming bacteria that can cause gastrointestinal diseases and severe eye infections in humans. They have also been incriminated in a multitude of other severe, and frequently fatal, clinical infections, such as osteomyelitis, septicaemia, pneumonia, liver abscess and meningitis, particularly in immuno-compromised patients and preterm neonates. The pathogenic properties of this organism are mediated by the synergistic effects of a number of virulence products that promote intestinal cell destruction and/or resistance to the host immune system. This review focuses on the pore-forming haemolysins produced by B. cereus: haemolysin I (cereolysin O), haemolysin II, haemolysin III and haemolysin IV (CytK). Haemolysin I belongs to the cholesterol-dependent cytolysin (CDC) family whose best known members are listeriolysin O and perfringolysin O, produced by L. monocytogenes and C. perfringens respectively. HlyII and CytK are oligomeric ß-barrel pore-forming toxins related to the α-toxin of S. aureus or the ß-toxin of C. perfringens. The structure of haemolysin III, the least characterized haemolytic toxin from the B. cereus, group has not yet been determined.

Bacillus cytotoxicus sp. nov. is a novel thermotolerant species of the Bacillus cereus Group occasionally associated with food poisoning

An aerobic endospore-forming bacillus (NVH 391-98T) was isolated during a severe food poisoning outbreak in France in 1998, and four other similar strains have since been isolated, also mostly from food poisoning cases. Based on 16S rRNA gene sequence similarity, these strains were shown to belong to the Bacillus cereus Group (over 97 % similarity with the current Group species) and phylogenetic distance from other validly described species of the genus Bacillus was less than 95 %. Based on 16S rRNA gene sequence similarity and MLST data, these novel strains were shown to form a robust and well-separated cluster in the B. cereus Group, and constituted the most distant cluster from species of this Group. Major fatty acids (iso-C15 : 0, C16 : 0, iso-C17 : 0, anteiso-C15 : 0, iso-C16 : 0, iso-C13 : 0) supported the affiliation of these strains to the genus Bacillus , and more specifically to the B. cereus Group. NVH 391-98T taxon was more specifically characterized by an abundance of iso-C15 : 0 and low amounts of iso-C13 : 0 compared with other members of the B. cereus Group. Genome similarity together with DNA–DNA hybridization values and physiological and biochemical tests made it possible to genotypically and phenotypically differentiate NVH 391-98T taxon from the six current B. cereus Group species. NVH 391-98T therefore represents a novel species, for which the name Bacillus cytotoxicus sp. nov. is proposed, with the type strain NVH 391-98T ( = DSM 22905T = CIP 110041T).

Sequence Analysis of Inducible Prophage phIS3501 Integrated into the Haemolysin II Gene of Bacillus thuringiensis var israelensis ATCC35646

Diarrheic food poisoning by bacteria of the Bacillus cereus group is mostly due to several toxins encoded in the genomes. One of them, cytotoxin K, was recently identified as responsible for severe necrotic syndromes. Cytotoxin K is similar to a class of proteins encoded by genes usually annotated as haemolysin II (hlyII) in the majority of genomes of the B. cereus group. The partially sequenced genome of Bacillus thuringiensis var israelensis ATCC35646 contains several potentially induced prophages, one of them integrated into the hlyII gene. We determined the complete sequence and established the genomic organization of this prophage-designated phIS3501. During induction of excision of this prophage with mitomycin C, intact hlyII gene is formed, thus providing to cells a genetic ability to synthesize the active toxin. Therefore, this prophage, upon its excision, can be implicated in the regulation of synthesis of the active toxin and thus in the virulence of bacterial host. A generality of selection for such systems in bacterial pathogens is indicated by the similarity of this genetic arrangement to that of Staphylococcus aureus  β-haemolysin.

Molecular and toxigenic characterization of Bacillus cereus and Bacillus thuringiensis strains isolated from commercial ground roasted coffee

Thirty samples of roasted ground coffee beans from 10 different commercial brands were analyzed to investigate the occurrence and levels of Bacillus cereus and Bacillus thuringiensis strains. Strains were evaluated for their genetic diversity by repetitive element sequence polymorphism PCR (Rep-PCR) and for their toxigenic profiles, i.e., the presence of hblA, hblC, hblD, nheA, nheB, nheC, cytK, ces, and entFM. Survival and multiplication of B. cereus sensu lato in the ready-to-drink coffee was determined to evaluate this beverage as a possible vehicle for B. cereus infection. B. cereus was detected in 17 (56.7%) of the 30 samples, and B. thuringiensis was detected in 8 (26.7%) of the 30 samples. Five samples did not produce any characteristic growth. The most common gene, entFM, was detected in 23 strains (92%). The NHE complex (nheA, nheB, and nheC genes) was found in 19 strains (76%). The HBL complex (hblA, hblC, and hblD) was found in 16 strains (64%). All strains were negative for ces. The cytK gene was found in 16 strains (64%). The computer-assisted cluster analysis of Rep-PCR profiles using a clustering criterion of 80% similarity revealed four main clusters. Cluster 1 was the predominant and comprised three B. thuringiensis strains with 100% similarity, cluster 2 comprised two B. cereus strains (100% similarity), cluster 3 comprised two B. thuringiensis strains (90% similarity), and cluster 4 comprised one B. thuringiensis strain and one B. cereus strain (85% similarity). The cluster analysis of fingerprints generated by Rep-PCR revealed a high genetic diversity among the B. cereus strains, suggesting that the contamination could have originated from different sources. In our experiments, when sugar was added and the beverage was kept in thermic bottles there was a significant increase in B. cereus sensu lato levels, which may increase the risk of food poisoning. These results highlight the need for additional studies on this subject to better evaluate coffee as a food poisoning vehicle.

Regulation of toxin production by Bacillus cereus and its food safety implications

Toxin expression is of utmost importance for the food-borne pathogen B. cereus, both in food poisoning and non-gastrointestinal host infections as well as in interbacterial competition. Therefore it is no surprise that the toxin gene expression is tightly regulated by various internal and environmental signals. An overview of the current knowledge regarding emetic and diarrheal toxin transcription and expression is presented in this review. The food safety aspects and management tools such as temperature control, food preservatives and modified atmosphere packaging are discussed specifically for B. cereus emetic and diarrheal toxin production.

Genetic diversity, antimicrobial resistance and toxigenic profiles of Bacillus cereus isolated from food in Brazil over three decades

Bacillus cereus is an ever-present problem. It is widely distributed in several environments such as soil and plants and is commonly isolated from food and additives. In this study we analyzed 97 foodborne B. cereus sensu stricto strains isolated in Brazil in the 1980's, 1990's and 2000's in order to investigate the genetic diversity (assessed by Rep-PCR), antimicrobial resistance and toxigenic profiles (presence of hblA, hblC and hblD; nheA, nheB and nheC as well as cytK, ces and entFM genes) of such strains. The majority of the strains (79, 81.4%) were β-hemolytic. The NHE complex was found in 82 strains (84.5%) and HBL complex was found in 61 (62.9%) strains. All strains were negative to ces. The cytK-2 gene was found in 44 (45.4%) strains. The predominant toxigenic pattern was type I (32, 33%) which included strains positive for all toxin genes but ces. Computer assisted cluster analysis of Rep-PCR profiles showed a high genetic diversity. Seven major clusters comprising two or more strains were found and cluster 1 was predominant (ten strains, nine of them showing 100% similarity). This cluster included strains isolated in the 1980's and the 1990's. Cluster analysis of Rep-PCR profiles based on decade of isolation, source, hemolytic pattern, toxigenic and antibiotic resistance patterns revealed a similar clustering pattern as found in the analysis including all strains. The inability to observe a predominant band pattern when Rep-PCR cluster analysis was based on decade of isolation suggests that this diversity has been maintained over time. All strains were susceptible to gentamicin. We detected resistance to tetracycline (11 strains showing intermediate resistance and nine completely resistant strains), clindamycin (ten intermediate strains) and vancomycin (one strain). Clindamycin resistance showed statistical association with strains isolated in 2000's. The predominant resistance pattern was type A (72, 72.2%) which included strains susceptible to all drugs tested. Our results suggest that the majority of the strains present in several types of food in Brazil pose a potential risk to cause food poisoning due to the high prevalence of toxin genes found in these strains. However, additional studies involving cytotoxicity tests and affiliation of these strains to phylogenetic groups based on molecular data would be useful to better evaluate this potential and could provide a more accurate indication of the risk.

Bacillus cereus induces permeability of an in vitro blood-retina barrier

Most Bacillus cereus toxin production is controlled by the quorum-sensing-dependent, pleiotropic global regulator plcR, which contributes to the organism's virulence in the eye. The purpose of this study was to analyze the effects of B. cereus infection and plcR-regulated toxins on the barrier function of retinal pigment epithelium (RPE) cells, the primary cells of the blood-retina barrier. Human ARPE-19 cells were apically inoculated with wild-type or quorum-sensing-deficient B. cereus, and cytotoxicity was analyzed. plcR-regulated toxins were not required for B. cereus-induced RPE cytotoxicity, but these toxins did increase the rate of cell death, primarily by necrosis. B. cereus infection of polarized RPE cell monolayers resulted in increased barrier permeability, independent of plcR-regulated toxins. Loss of both occludin and ZO-1 expression occurred by 8 h postinfection, but alterations in tight junctions appeared to precede cytotoxicity. Of the several proinflammatory cytokines analyzed, only interleukin-6 was produced in response to B. cereus infection. These results demonstrate the deleterious effects of B. cereus infection on RPE barrier function and suggest that plcR-regulated toxins may not contribute significantly to RPE barrier permeability during infection.

The genetically remote pathogenic strain NVH391-98 of the Bacillus cereus group is representative of a cluster of thermophilic strains

Bacteria of the Bacillus cereus group are known to cause food poisoning. A rare phylogenetically remote strain, NVH391-98, was recently characterized to encode a particularly efficient cytotoxin K presumably responsible for food poisoning. This pathogenic strain and its close relatives can be phenotypically distinguished from other strains of the B. cereus group by the inability to grow at temperatures below 17 degrees C and by the ability to grow at temperatures from 48 to 53 degrees C. A temperate phage, phBC391A2, residing in the genome of NVH391-98 allows us to distinguish the three known members of this thermophilic strain cluster.

Growth-related variations in the Bacillus cereus secretome

Using 2-DE, transcriptional gene fusions and cell cytotoxicity assays, we followed changes in the Bacillus cereus strain ATCC14579 secretome, gene expression and culture supernatant cytotoxicity from the end of the vegetative phase up to 5 h after entry into the stationary phase. The concentration of each of the 22 proteins in the culture supernatant was determined at various times. In addition, the stability of the proteins was studied. Fifteen of these proteins, including 14 members of the virulence regulon PlcR, were known or predicted to be secreted. All of the secreted proteins reached a maximum concentration during early stationary phase, but there were significant differences in the kinetics of their concentrations. The time courses of protein concentrations were in agreement with gene expression data, except for cytotoxin CytK, which was unstable, and for the metalloprotease InhA1. Supernatant cytoxicity also peaked in early stationary phase, and the kinetics of cytotoxicity paralleled the time course of concentration of the PlcR-controlled toxin, CytK. Our concomitant study of the time course of protein concentrations, gene expression and supernatant cytotoxicity reveals that the pathogenic potential of B. cereus peaks during the transition state. It also suggests that there is diversity in the regulation of gene expression within the PlcR regulon.

Low concentrations of bile salts induce stress responses and reduce motility in Bacillus cereus ATCC 14579 [corrected]

Tolerance to bile salts was investigated in forty Bacillus cereus strains, including 17 environmental isolates, 11 dairy isolates, 3 isolates from food poisoning outbreaks, and 9 other clinical isolates. Growth of all strains was observed at low bile salt concentrations, but no growth was observed on LB agar plates containing more than 0.005% bile salts. Preincubation of the B. cereus type strain, ATCC 14579, in low levels of bile salts did not increase tolerance levels. B. cereus ATCC 14579 was grown to mid-exponential growth phase and shifted to medium containing bile salts (0.005%). Global expression patterns were determined by hybridization of total cDNA to a 70-mer oligonucleotide microarray. A general stress response and a specific response to bile salts were observed. The general response was similar to that observed in cultures grown in the absence of bile salts but at a higher (twofold) cell density. Up-regulation of several putative multidrug exporters and transcriptional regulators and down-regulation of most motility genes were observed as part of the specific response. Motility experiments in soft agar showed that motility decreased following bile salts exposure, in accordance with the transcriptional data. Genes encoding putative virulence factors were either unaffected or down-regulated.

Biology and taxonomy ofBacillus cereus,Bacillus anthracis, andBacillus thuringiensis

Three species of the Bacillus cereus group (Bacillus cereus, Bacillus anthracis , and Bacillus thuringiensis ) have a marked impact on human activity. Bacillus cereus and B. anthracis are important pathogens of mammals, including humans, and B. thuringiensis is extensively used in the biological control of insects. The microbiological, biochemical, and genetic characteristics of these three species are reviewed, together with a discussion of several genomic studies conducted on strains of B. cereus group. Using bacterial systematic concepts, we speculate that to understand the taxonomic relationship within this group of bacteria, special attention should be devoted also to the ecology and the population genetics of these species.

Toxin production in a rare and genetically remote cluster of strains of the Bacillus cereus group

Background

Three enterotoxins are implicated in diarrhoeal food poisoning due to Bacillus cereus: Haemolysin BL (Hbl), Non-haemolytic enterotoxin (Nhe), and Cytotoxin K (CytK). Toxin gene profiling and assays for detection of toxin-producing stains have been used in attempts to evaluate the enterotoxic potential of B. cereus group strains. B. cereus strain NVH 391/98, isolated from a case of fatal enteritis, was genetically remote from other B. cereus group strains. This strain lacked the genes encoding Hbl and Nhe, but contains CytK-1. The high virulence of this strain is thought to be due to the greater cytotoxic activity of CytK-1 compared to CytK-2, and to a high level of cytK expression. To date, only three strains containing cytK-1 have been identified; B. cereus strains NVH 391/98, NVH 883/00, and INRA AF2.

Results

A novel gene variant encoding Nhe was identified in these three strains, which had an average of 80% identity in protein sequence with previously identified Nhe toxins. While culture supernatants containing CytK and Nhe from NVH 391/98 and INRA AF2 were highly cytotoxic, NVH 883/00 expressed little or no CytK and Nhe and was non-cytotoxic. Comparative sequence and expression studies indicated that neither the PlcR/PapR quorum sensing system, nor theYvrGH and YvfTU two-component systems, were responsible for the observed difference in toxin production. Additionally, phylogenetic analysis of 13 genes showed that NVH 391/98, NVH 883/00, and INRA AF2 comprise a novel cluster of strains genetically distant from other B. cereus group strains.

Conclusion

Due to its divergent sequence, the novel nhe operon had previously not been detected in NVH 391/98 using PCR and several monoclonal antibodies. Thus, toxigenic profiling based on the original nhe sequence will fail to detect the toxin in this group of strains. The observation that strain NVH 883/00 carries cytK-1 but is non-cytotoxic indicates that the detection of this gene variant is not a sufficient criterion for identification of highly cytotoxic strains. The presence of the novel nhe operon and the cytK-1 gene variant in this cluster of strains reflect their phylogenetically remote relationship towards other B. cereus group strains.

Extending the Bacillus cereus group genomics to putative food-borne pathogens of different toxicity

The Bacillus cereus group represents sporulating soil bacteria containing pathogenic strains which may cause diarrheic or emetic food poisoning outbreaks. Multiple locus sequence typing revealed a presence in natural samples of these bacteria of about 30 clonal complexes. Application of genomic methods to this group was however biased due to the major interest for representatives closely related to Bacillus anthracis. Albeit the most important food-borne pathogens were not yet defined, existing data indicate that they are scattered all over the phylogenetic tree. The preliminary analysis of the sequences of three genomes discussed in this paper narrows down the gaps in our knowledge of the B. cereus group. The strain NVH391-98 is a rare but particularly severe food-borne pathogen. Sequencing revealed that the strain should be a representative of a novel bacterial species, for which the name Bacillus cytotoxis or Bacillus cytotoxicus is proposed. This strain has a reduced genome size compared to other B. cereus group strains. Genome analysis revealed absence of sigma B factor and the presence of genes encoding diarrheic Nhe toxin, not detected earlier. The strain B. cereus F837/76 represents a clonal complex close to that of B. anthracis. Including F837/76, three such B. cereus strains had been sequenced. Alignment of genomes suggests that B. anthracis is their common ancestor. Since such strains often emerge from clinical cases, they merit a special attention. The third strain, KBAB4, is a typical facultative psychrophile generally found in soil. Phylogenic studies show that in nature it is the most active group in terms of gene exchange. Genomic sequence revealed high presence of extra-chromosomal genetic material (about 530kb) that may account for this phenomenon. Genes coding Nhe-like toxin were found on a big plasmid in this strain. This may indicate a potential mechanism of toxicity spread from the psychrophile strain community. The results of this genomic work and ecological compartments of different strains incite to consider a necessity of creating prophylactic vaccines against bacteria closely related to NVH391-98 and F837/76. Presumably developing of such vaccines can be based on the properties of non-pathogenic strains such as KBAB4 or ATCC14579 reported here or earlier. By comparing the protein coding genes of strains being sequenced in this project to others we estimate the shared proteome, or core genome, in the B. cereus group to be 3000+/-200 genes and the total proteome, or pan-genome, to be 20-25,000 genes.

Characterization of mesophilic bacilli in faeces of feedlot cattle

AIMS

To determine the identity and composition of mesophilic Bacillus spp. in faeces sampled from feedlot cattle.

METHODS AND RESULTS

Faecal samples from 10 feedlot cattle were analysed. The total aerobic spore count increased from 4.6 x 10(4) CFU g(-1) (before feedlotting, day 0) to 1.6 x 10(6) CFU g(-1) (feedlot for day 76). A total of 150 randomly selected spore isolates (60 each from days 0 and 76 cattle, 30 from feed) were speciated using a Bacillus group-specific PCR-amplified ribosomal DNA restriction analysis technique (Wu et al. 2006). At day 0, Bacillus subtilis and Bacillus cereus predominated with a prevalence of 58.3% and 26.7%, respectively, whereas three species, B. subtilis (50.0%), Bacillus licheniformis (27.6%) and Bacillus clausii (20.0%) predominated in day 76 faecal samples. Of these, only the first two species were present in feed samples at a frequency of 70% and 30% respectively. All B. cereus isolates on day 0, possessed at least one of three enterotoxin genes (nheA, nheB and nheC) but these were completely eliminated after a period of feedlotting. All isolates of B. licheniformis were genotypically heterogeneous according to pulsed-field gel electrophoresis analysis.

CONCLUSIONS

Cattle faeces contain large numbers of Bacillus spores representing different mesophilic species. Stable faecal populations of particular Bacillus spp. mimicking those found in feed, were subsequently established by feedlotting.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results obtained and methods used in this study will help to investigate the indigenous Bacillus composition in the gastrointestinal tract of cattle and will further guide the administration of Bacillus probiotics.

Biofilm formation by Bacillus cereus is influenced by PlcR, a pleiotropic regulator

The DeltaplcR mutant of Bacillus cereus strain ATCC 14579 developed significantly more biofilm than the wild type and produced increased amounts of biosurfactant. Biosurfactant production is required for biofilm formation and may be directly or indirectly repressed by PlcR, a pleiotropic regulator. Coating polystyrene plates with surfactin, a biosurfactant from Bacillus subtilis, rescued the deficiency in biofilm formation by the wild type.

Occurrence of natural Bacillus thuringiensis contaminants and residues of Bacillus thuringiensis-based insecticides on fresh fruits and vegetables

A total of 128 Bacillus cereus-like strains isolated from fresh fruits and vegetables for sale in retail shops in Denmark were characterized. Of these strains, 39% (50/128) were classified as Bacillus thuringiensis on the basis of their content of cry genes determined by PCR or crystal proteins visualized by microscopy. Random amplified polymorphic DNA analysis and plasmid profiling indicated that 23 of the 50 B. thuringiensis strains were of the same subtype as B. thuringiensis strains used as commercial bioinsecticides. Fourteen isolates were indistinguishable from B. thuringiensis subsp. kurstaki HD1 present in the products Dipel, Biobit, and Foray, and nine isolates grouped with B. thuringiensis subsp. aizawai present in Turex. The commercial strains were primarily isolated from samples of tomatoes, cucumbers, and peppers. A multiplex PCR method was developed to simultaneously detect all three genes in the enterotoxin hemolysin BL (HBL) and the nonhemolytic enterotoxin (NHE), respectively. This revealed that the frequency of these enterotoxin genes was higher among the strains indistinguishable from the commercial strains than among the other B. thuringiensis and B. cereus-like strains isolated from fruits and vegetables. The same was seen for a third enterotoxin, CytK. In conclusion, the present study strongly indicates that residues of B. thuringiensis-based insecticides can be found on fresh fruits and vegetables and that these are potentially enterotoxigenic.

Cereulide synthetase gene cluster from emetic Bacillus cereus: structure and location on a mega virulence plasmid related to Bacillus anthracis toxin plasmid pXO1

BACKGROUND

Cereulide, a depsipeptide structurally related to valinomycin, is responsible for the emetic type of gastrointestinal disease caused by Bacillus cereus. Recently, it has been shown that this toxin is produced by a nonribosomal peptide synthetase (NRPS), but its exact genetic organization and biochemical synthesis is unknown.

RESULTS

The complete sequence of the cereulide synthetase (ces) gene cluster, which encodes the enzymatic machinery required for the biosynthesis of cereulide, was dissected. The 24 kb ces gene cluster comprises 7 CDSs and includes, besides the typical NRPS genes like a phosphopantetheinyl transferase and two CDSs encoding enzyme modules for the activation and incorporation of monomers in the growing peptide chain, a CDS encoding a putative hydrolase in the upstream region and an ABC transporter in the downstream part. The enzyme modules responsible for incorporation of the hydroxyl acids showed an unusual structure while the modules responsible for the activation of the amino acids Ala and Val showed the typical domain organization of NRPS. The ces gene locus is flanked by genetic regions with high homology to virulence plasmids of B. cereus, Bacillus thuringiensis and Bacillus anthracis. PFGE and Southern hybridization showed that the ces genes are restricted to emetic B. cereus and indeed located on a 208 kb megaplasmid, which has high similarities to pXO1-like plasmids.

CONCLUSION

The ces gene cluster that is located on a pXO1-like virulence plasmid represents, beside the insecticidal and the anthrax toxins, a third type of B. cereus group toxins encoded on megaplasmids. The ces genes are restricted to emetic toxin producers, but pXO1-like plasmids are also present in emetic-like strains. These data might indicate the presence of an ancient plasmid in B. cereus which has acquired different virulence genes over time. Due to the unusual structure of the hydroxyl acid incorporating enzyme modules of Ces, substantial biochemical efforts will be required to dissect the complete biochemical pathway of cereulide synthesis.

Anaerobic cells of Bacillus cereus F4430/73 respond to low oxidoreduction potential by metabolic readjustments and activation of enterotoxin expression

In the present study, a food-borne pathogen strain of Bacillus cereus (F4430/73) was anaerobically grown in controlled-batch conditions under low initial oxidoreduction potential (ORP=-148 mV) using hydrogen gas as reducing agent. Its physiological characteristics, including growth, glucose fermentation capacity and enterotoxin production, were compared with anaerobic conditions generated by nitrogen gas (ORP=+ 45 mV). The results showed that low ORP affected growth mainly during the early stages. Maximal specific rates of growth and glucose consumption were reduced, and drastic changes in time profiles of fermentation product concentration were observed. Production of lactate was promoted at the expense of acetate. Nevertheless, low ORP did not affect final biomass yield. Under both ORP conditions, Non-haemolytic enterotoxin (Nhe) was produced early during the exponential growth phase as a first enterotoxin and Haemolysin BL (Hbl) later during the early stationary growth phase as a second enterotoxin. The major effect of low ORP was the strong stimulation of Hbl production and, to a lesser extent, Nhe production. This control was complex, involving different levels of regulation. We discussed the regulation of enterotoxin expression and the involvement of the pleiotropic regulator PlcR.

FlhA influences Bacillus thuringiensis PlcR-regulated gene transcription, protein production, and virulence

Bacillus thuringiensis and Bacillus cereus are closely related. B. thuringiensis is well known for its entomopathogenic properties, principally due to the synthesis of plasmid-encoded crystal toxins. B. cereus appears to be an emerging opportunistic human pathogen. B. thuringiensis and B. cereus produce many putative virulence factors which are positively controlled by the pleiotropic transcriptional regulator PlcR. The inactivation of plcR decreases but does not abolish virulence, indicating that additional factors like flagella may contribute to pathogenicity. Therefore, we further analyzed a mutant (B. thuringiensis 407 Cry(-) DeltaflhA) previously described as being defective in flagellar apparatus assembly and in motility as well as in the production of hemolysin BL and phospholipases. A large picture of secreted proteins was obtained by two-dimensional electrophoresis analysis, which revealed that flagellar proteins are not secreted and that production of several virulence-associated factors is reduced in the flhA mutant. Moreover, we quantified the effect of FlhA on plcA and hblC gene transcription. The results show that the flhA mutation results in a significant reduction of plcA and hblC transcription. These results indicate that the transcription of several PlcR-regulated virulence factors is coordinated with the flagellar apparatus. Consistently, the flhA mutant also shows a strong decrease in cytotoxicity towards HeLa cells and in virulence against Galleria mellonella larvae following oral and intrahemocoelic inoculation. The decrease in virulence may be due to both a lack of flagella and a lower production of secreted factors. Hence, FlhA appears to be an essential virulence factor with a pleiotropic role.

Occurrence and significance ofBacillus cereusandBacillus thuringiensisin ready-to-eat food

Among 48,901 samples of ready-to-eat food products at the Danish retail market, 0.5% had counts of Bacillus cereus-like bacteria above 10(4) cfu g(-1). The high counts were most frequently found in starchy, cooked products, but also in fresh cucumbers and tomatoes. Forty randomly selected strains had at least one gene or component involved in human diarrhoeal disease, while emetic toxin was related to only one B. cereus strain. A new observation was that 31 out of the 40 randomly selected B. cereus-like strains could be classified as Bacillus thuringiensis due to crystal production and/or content of cry genes. Thus, a large proportion of the B. cereus-like organisms present in food may belong to B. thuringiensis.