Sequence analysis of three Bacillus cereus loci carrying PIcR-regulated genes encoding degradative enzymes and enterotoxin

Bacillus cereus: A review of "fried rice syndrome" causative agents

"Fried rice syndrome" originated from the first exposure to a fried rice dish contaminated with Bacillus cereus. This review compiles available data on the prevalence of B. cereus outbreak cases that occurred between 1984 and 2019. The outcome of B. cereus illness varies dramatically depending on the pathogenic strain encounter and the host's immune system. B. cereus causes a self-limiting, diarrheal illness caused by heat-resistant enterotoxin proteins, and an emetic illness caused by the deadly toxin named cereulide. The toxins together with their extrinsic factors are discussed. The possibility of more contamination of B. cereus in protein-rich food has also been shown. Therefore, the aim of this review is to summarize the available data, focusing mainly on B. cereus physiology as the causative agent for "fried rice syndrome." This review emphasizes the prevalence of B. cereus in starchy food contamination and outbreak cases reported, the virulence of both enterotoxins and emetic toxins produced, and the possibility of contaminated in protein-rich food. The impact of emetic or enterotoxin-producing B. cereus on public health cannot be neglected. Thus, it is essential to constantly monitor for B. cereus contamination during food handling and hygiene practices for food product preparation.

The importance of the PapR7 C-terminus and amide protons in mediating quorum sensing in Bacilluscereus

The opportunistic human pathogen Bacillus cereus controls the expression of key infection-promoting phenotypes using bacterial quorum sensing (QS). QS signal transduction within the species is controlled by an autoinducing peptide, PapR7, and its cognate receptor, PlcR, indicating that the PlcR:PapR interface is a prime target for QS inhibitor development. The C-terminal region of the peptide (PapR7; ADLPFEF) has been successfully employed as a scaffold to develop potent QS modulators. Despite the noted importance of the C-terminal carboxylate and amide protons in crystallographic data, their role in QS activity has yet to be explored. In this study, an N-methyl scan of PapR7 was conducted in conjunction with a C-terminal modification of previously identified B. cereus QS inhibitors. The results indicate that the amide proton at Glu6 and the C-terminal carboxylate are important for effective QS inhibition of the PlcR regulon. Through β-galactosidase and hemolysis assays, a series of QS inhibitors were discovered, including several capable of inhibiting QS with nanomolar potency. These inhibitors, along with the structure-activity data reported, will serve as valuable tools for disrupting the B. cereus QS pathway towards developing novel anti-infective strategies.

From cereus to anthrax and back again: Assessment of the temperature-dependent phenotypic switching in the "cross-over" strain Bacillus cereus G9241

Bacillus cereus G9241 was isolated from a Louisiana welder suffering from an anthrax-like infection. The organism carries two transcriptional regulators that have previously been proposed to be incompatible with each other in Bacillus anthracis: the pleiotropic transcriptional regulator PlcR found in most members of the Bacillus cereus group but truncated in all B. anthracis isolates, and the anthrax toxin regulator AtxA found in all B. anthracis strains and a few B. cereus sensu stricto strains. Here we report cytotoxic and hemolytic activity of cell free B. cereus G9241 culture supernatants cultured at 25°C to various eukaryotic cells. However, this is not observed at the mammalian infection relevant temperature 37°C, behaving much like the supernatants generated by B. anthracis. Using a combination of genetic and proteomic approaches to understand this unique phenotype, we identified several PlcR-regulated toxins to be secreted highly at 25°C compared to 37°C. Furthermore, results suggest that differential expression of the protease involved in processing the PlcR quorum sensing activator molecule PapR appears to be the limiting step for the production of PlcR-regulated toxins at 37°C, giving rise to the temperature-dependent hemolytic and cytotoxic activity of the culture supernatants. This study provides an insight on how B. cereus G9241 is able to "switch" between B. cereus and B. anthracis-like phenotypes in a temperature-dependent manner, potentially accommodating the activities of both PlcR and AtxA.

Presence and function of Hbl B', the fourth protein component encoded by the hbl operon in Bacillus cereus

The genes hblC, hblD and hblA encode the components Hbl L2, L1 and B of the pore forming enterotoxin haemolysin BL of Bacillus cereus. Two variants of the operon existand the more common one additionally contains hblB downstream of hblCDA. Up to now, it was completely unclear whether the corresponding protein, Hbl B', is widely expressed among B. cereus strains and if it has a distinct function. In the present study, it was shown that the hblB gene is indeed expressed and the Hbl B' protein is secreted by nearly all analysed B. cereus strains. For the latter, a detection system was developed based on monoclonal antibody 11A5. Further, a distinct reduction of cytotoxic and haemolytic activity was observed when recombinant (r)Hbl B' was applied simultaneously with L2, L1 and B. This effect was due to direct interaction of rHbl B' with L1. _D^-6B. cereusAltogether, we present the first simple tool for the detection of Hbl B' in B. cereus culture supernatants. Moreover, an important regulatory function of Hbl B' in the mechanism of Hbl was determined, which is best described as an additional control of complex formation, balancing the amounts of Hbl B-L1 complexes and the corresponding free subunits.

The genomic characterisation and comparison of Bacillus cereus strains isolated from indoor air

Background

Bacillus cereus is ubiquitous in nature, found in environments such as soil, plants, air, and part of the insect and human gut microbiome. The ability to produce endospores and biofilms contribute to their pathogenicity, classified in two types of food poisoning: diarrheal and emetic syndromes. Here we report gap-free, whole-genome sequences of two B. cereus strains isolated from air samples and analyse their emetic and diarrheal potential.

Results

Genome assemblies of the B. cereus strains consist of one chromosome and seven plasmids each. The genome size of strain SGAir0260 is 6.30-Mb with 6590 predicted coding sequences (CDS) and strain SGAir0263 is 6.47-Mb with 6811 predicted CDS. Macrosynteny analysis showed 99% collinearity between the strains isolated from air and 90.2% with the reference genome. Comparative genomics with 57 complete B. cereus genomes suggests these strains from air are closely associated with strains isolated from foodborne illnesses outbreaks. Due to virulence potential of B. cereus and its reported involvement in nosocomial infections, antibiotic resistance analyses were performed and confirmed resistance to ampicillin and fosfomycin, with susceptibility to ciprofloxacin, tetracycline and vancomycin in both strains.

Conclusion

Phylogenetic analysis combined with detection of haemolytic (hblA, hblC, and hblD) and non-haemolytic (nheA, nheB, and nheC) enterotoxin genes in both air-isolated strains point to the diarrheic potential of the air isolates, though not emetic. Characterization of these airborne strains and investigation of their potential disease-causing genes could facilitate identification of environmental sources of contamination leading to foodborne illnesses and nosocomial infections transported by air.

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.

Characteristics of the Protein Complexes and Pores Formed by Bacillus cereus Hemolysin BL

Bacillus cereus Hemolysin BL is a tripartite toxin responsible for a diarrheal type of food poisoning. Open questions remain regarding its mode of action, including the extent to which complex formation prior to cell binding contributes to pore-forming activity, how these complexes are composed, and the properties of the pores formed in the target cell membrane. Distinct complexes of up to 600 kDa were found on native gels, whose structure and size were primarily defined by Hbl B. Hbl L1 and L2 were also identified in these complexes using Western blotting and an LC-MS approach. LC-MS also revealed that many other proteins secreted by B. cereus exist in complexes. Further, a decrease of toxic activity at temperatures ≥60 °C was shown, which was unexpectedly restored at higher temperatures. This could be attributed to a release of Hbl B monomers from tight complexation, resulting in enhanced cell binding. In contrast, Hbl L1 was rather susceptible to heat, while heat treatment of Hbl L2 seemed not to be crucial. Furthermore, Hbl-induced pores had a rather small single-channel conductance of around 200 pS and a probable channel diameter of at least 1 nm on planar lipid bilayers. These were highly instable and had a limited lifetime, and were also slightly cation-selective. Altogether, this study provides astonishing new insights into the complex mechanism of Hbl pore formation, as well as the properties of the pores.

Elucidating the Hot Spot Residues of Quorum Sensing Peptidic Autoinducer PapR by Multiple Amino Acid Replacements

The quorum sensing (QS) system of Bacillus cereus, an opportunistic human pathogen, utilizes the autoinducing PapR peptide signal that mediates the activation of the pleiotropic virulence regulator PlcR. A set of synthetic 7-mer PapR-derived peptides (PapR₇; ADLPFEF) have been shown to inhibit efficiently the PlcR regulon activity and the production of virulence factors, reflected by a loss in hemolytic activity without affecting bacterial growth. Interestingly, these first potent synthetic inhibitors involved D-amino acid or alanine replacements of three amino acids; proline, glutamic acid, and phenylalanine of the heptapeptide PapR. To better understand the role of these three positions in PlcR activity, we report herein the second generation design, synthesis, and characterization of PapR₇-derived combinations, alternate double and triple alanine and D-amino acids replacement at these positions. Our findings generate a new set of non-native PapR₇-derived peptides that inhibit the PlcR regulon activity and the production of virulence factors. Using the amino acids substitution strategy, we revealed the role of proline and glutamic acid on PlcR regulon activation. Moreover, we demonstrated that the D-Glutamic acid substitution was crucial for the design of stronger PlcR antagonists. These peptides represent potent synthetic inhibitors of B. cereus QS and constitute new and readily accessible chemical tools for the study of the PlcR system. Our method might be applied to other quorum sensing systems to design new anti-virulence agents.

Binding to The Target Cell Surface Is The Crucial Step in Pore Formation of Hemolysin BL from Bacillus cereus

A major virulence factor involved in Bacillus cereus food poisoning is the three-component enterotoxin hemolysin BL. It consists of the binding component B and the two lytic components L₁ and L₂. Studying its mode of action has been challenging, as natural culture supernatants additionally contain Nhe, the second three-component enterotoxin, and purification of recombinant (r) Hbl components has been difficult. In this study, we report on pore-forming, cytotoxic, cell binding and hemolytic activity of recently generated rHbl components expressed in E. coli. It is known that all three Hbl components are necessary for cytotoxicity and pore formation. Here we show that an excess of rHbl B enhances, while an excess of rHbl L₁ hinders, the velocity of pore formation. Most rapid pore formation was observed with ratios L₂:L₁:B = 1:1:10 and 10:1:10. It was further verified that Hbl activity is due to sequential binding of the components B - L₁ - L₂. Accordingly, all bioassays proved that binding of Hbl B to the cell surface is the crucial step for pore formation and cytotoxic activity. Binding of Hbl B took place within minutes, while apposition of the following L₁ and L₂ occurred immediately. Further on, applying toxin components simultaneously, it seemed that Hbl L₁ enhanced binding of B to the target cell surface. Overall, these data contribute significantly to the elucidation of the mode of action of Hbl, and suggest that its mechanism of pore formation differs substantially from that of Nhe, although both enterotoxin complexes are sequentially highly related.

Quorum sensing intervened bacterial signaling: Pursuit of its cognizance and repression

Bacteria communicate within a system by means of a density dependent mechanism known as quorum sensing which regulate the metabolic and behavioral activities of a bacterial community. This sort of interaction occurs through a dialect of chemical signals called as autoinducers synthesized by bacteria. Bacterial quorum sensing occurs through various complex pathways depending upon specious diversity. Therefore the cognizance of quorum sensing mechanism will enable the regulation and thereby constrain bacterial communication. Inhibition strategies of quorum sensing are collectively called as quorum quenching; through which bacteria are incapacitated of its interaction with each other. Many virulence mechanism such as sporulation, biofilm formation, toxin production can be blocked by quorum quenching. Usually quorum quenching mechanisms can be broadly classified into enzymatic methods and non-enzymatic methods. Substantial understanding of bacterial communication and its inhibition enhances the development of novel antibacterial therapeutic drugs. In this review we have discussed the types and mechanisms of quorum sensing and various methods to inhibit and regulate density dependent bacterial communication.

RNA-Seq Analysis of Antibiotic-Producing Bacillus subtilis SC-8 Reveals a Role for Small Peptides in Controlling PapR Signaling

Bacillus subtilis SC-8 (BSSC8) shows a narrow antimicrobial activity against the Bacillus cereus group. Previously, B. cereus-derived PapR as a signal peptide to stimulate PlcR, which plays a significant role in regulating the transcription of virulence factors, was assumed to stimulate antibiotic production in BSSC8. To better understand the functional role of PapR in the antibiotic production of BSSC8 and the interspecies interaction, the global transcriptomic profiling of BSSC8 was investigated using RNA-Seq in this study. Small peptides derived from B. cereus wild type (WTBC) and a papR-deleted mutant strain (MTBC) were individually supplied to BSSC8 cultures, and changes in global transcription levels were compared by RNA-Seq. In the presence of WTBC small peptides, more genes (80.9%) were significantly upregulated than in cells exposed to MTBC small peptides. Specifically, 48.8 and 83.4% of genes involved in glycolysis and the TCA cycle, respectively, showed changes in transcription levels in response to small peptides from both strains. Of the genes showing the alterations, 35.0% (glycolysis) and 60.0% (TCA cycle) of transcripts were significantly regulated only in response to WTBC-derived small peptides. Furthermore, the expression of biosynthetic genes encoding several known antibiotics in BSSC8 was further decreased in response to WTBC small peptides.

Evidence for Complex Formation of the Bacillus cereus Haemolysin BL Components in Solution

Haemolysin BL is an important virulence factor regarding the diarrheal type of food poisoning caused by Bacillus cereus. However, the pathogenic importance of this three-component enterotoxin is difficult to access, as nearly all natural B. cereus culture supernatants additionally contain the highly cytotoxic Nhe, the second three-component toxin involved in the aetiology of B. cereus-induced food-borne diseases. To better address the toxic properties of the Hbl complex, a system for overexpression and purification of functional, cytotoxic, recombinant (r)Hbl components L₂, L₁ and B from E. coli was established and an nheABC deletion mutant was constructed from B. cereus reference strain F837/76. Furthermore, 35 hybridoma cell lines producing monoclonal antibodies (mAbs) against Hbl L₂, L₁ and B were generated. While mAbs 1H9 and 1D8 neutralized Hbl toxicity and thus, represent important tools for future investigations of the mode-of-action of Hbl on the target cell surface, mAb 1D7, in contrast, even enhanced Hbl toxicity by supporting the binding of Hbl B to the cell surface. By using the specific mAbs in Dot blots, indirect and hybrid sandwich enzyme immuno assays (EIAs), complex formation between Hbl L₁ and B, as well as L₁ and L₂ in solution could be shown for the first time. Surface plasmon resonance experiments with the rHbl components confirmed these results with K_D values of 4.7 × 10⁻⁷ M and 1.5 × 10⁻⁷ M, respectively. These findings together with the newly created tools lay the foundation for the detailed elucidation of the molecular mode-of-action of the highly complex three-component Hbl toxin.

Massive horizontal gene transfer, strictly vertical inheritance and ancient duplications differentially shape the evolution of Bacillus cereus enterotoxin operons hbl, cytK and nhe

Background

Bacillus cereus sensu lato comprises eight closely related species including the human pathogens Bacillus anthracis and Bacillus cereus. Within B. cereus sensu lato, chromosomally and plasmid-encoded toxins exist. While plasmid-mediated horizontal gene transfer of the emetic toxin, anthrax and insecticidal toxins is known, evolution of enterotoxin genes within the group has not been studied.

Results

We report draft genome assemblies of 25 strains, a phylogenetic network of 142 strains based on ANI derived from genome sequences and a phylogeny based on whole-genome SNP analysis. The data clearly support subdivision of B. cereus sensu lato into seven phylogenetic groups. While group I, V and VII represent B. pseudomycoides, B. toyonensis and B. cytotoxicus, which are distinguishable at species level (ANI border ≥ 96 %), strains ascribed to the other five species do not match phylogenic groups. The chromosomal enterotoxin operons nheABC and hblCDAB are abundant within B. cereus both isolated from infections and from the environment. While the duplicated hbl variant hbl_a is present in 22 % of all strains investigated, duplication of nheABC is extremely rare (0.02 %) and appears to be phylogenetically unstable. Distribution of toxin genes was matched to a master tree based on seven concatenated housekeeping genes, which depicts species relationships in B. cereus sensu lato as accurately as whole-genome comparisons. Comparison to the phylogeny of enterotoxin genes uncovered ample evidence for horizontal transfer of hbl, cytK and plcR, as well as frequent deletion of both toxins and duplication of hbl. No evidence for nhe deletion was found and stable horizontal transfer of nhe is rare. Therefore, evolution of B. cereus enterotoxin operons is shaped unexpectedly different for yet unknown reasons.

Conclusions

Frequent exchange of the pathogenicity factors hbl, cytK and plcR in B. cereus sensu lato appears to be an important mechanism of B. cereus virulence evolution, including so-called probiotic or non-pathogenic species, which might have consequences for risk assessment procedures. In contrast, exclusively vertical inheritance of nhe was observed, and since nhe-negative strains appear to be extremely rare, we suggest that fitness loss may be associated with deletion or horizontal transfer of the nhe operon.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0529-4) contains supplementary material, which is available to authorized users.

Capsules, toxins and AtxA as virulence factors of emerging Bacillus cereus biovar anthracis

Emerging B. cereus strains that cause anthrax-like disease have been isolated in Cameroon (CA strain) and Côte d’Ivoire (CI strain). These strains are unusual, because their genomic characterisation shows that they belong to the B. cereus species, although they harbour two plasmids, pBCXO1 and pBCXO2, that are highly similar to the pXO1 and pXO2 plasmids of B. anthracis that encode the toxins and the polyglutamate capsule respectively. The virulence factors implicated in the pathogenicity of these B. cereus bv anthracis strains remain to be characterised. We tested their virulence by cutaneous and intranasal delivery in mice and guinea pigs; they were as virulent as wild-type B. anthracis. Unlike as described for pXO2-cured B. anthracis, the CA strain cured of the pBCXO2 plasmid was still highly virulent, showing the existence of other virulence factors. Indeed, these strains concomitantly expressed a hyaluronic acid (HA) capsule and the B. anthracis polyglutamate (PDGA) capsule. The HA capsule was encoded by the hasACB operon on pBCXO1, and its expression was regulated by the global transcription regulator AtxA, which controls anthrax toxins and PDGA capsule in B. anthracis. Thus, the HA and PDGA capsules and toxins were co-regulated by AtxA. We explored the respective effect of the virulence factors on colonisation and dissemination of CA within its host by constructing bioluminescent mutants. Expression of the HA capsule by itself led to local multiplication and, during intranasal infection, to local dissemination to the adjacent brain tissue. Co-expression of either toxins or PDGA capsule with HA capsule enabled systemic dissemination, thus providing a clear evolutionary advantage. Protection against infection by B. cereus bv anthracis required the same vaccination formulation as that used against B. anthracis. Thus, these strains, at the frontier between B. anthracis and B. cereus, provide insight into how the monomorphic B. anthracis may have emerged.

Anthrax is caused by the bacterium Bacillus anthracis that affects all mammals worldwide. It emerged more than 10,000 years ago from a Bacillus cereus precursor. In the past decade, B. cereus bacteria were isolated in the USA from anthrax-like pneumonia cases. They harbour one virulence plasmid very similar to the toxin–encoding plasmid of B. anthracis. Recently, an anthrax-like disease in great apes in Africa was caused by emerging B. cereus strains, named B. cereus biovar anthracis. These strains are atypical as they possess both plasmids coding for toxin and capsule similar to those so far found only in B. anthracis. These unusual pathogenic B. cereus are currently neglected. We explored the virulence of these pathogens and their colonisation and dissemination capacity within the murine host. We found that these toxinogenic strains harbour two capsules, the classical B. anthracis capsule and an additional polysaccharidic capsule. This latter capsule confers virulence alone or in combination with toxins. Both capsules are concomitantly expressed, under the control of a common global regulator and host signals. Our results show that acquisition of new genetic information by these B. cereus clearly gives them a selective advantage, favouring their dissemination within infected hosts and the environment.

Bacillus thuringiensis membrane-damaging toxins acting on mammalian cells

Bacillus thuringiensis is widely used as a biopesticide in forestry and agriculture, being able to produce potent species-specific insecticidal toxins and considered nonpathogenic to other animals. More recently, however, repeated observations are documenting the association of this microorganism with various infectious diseases in humans, such as food-poisoning-associated diarrheas, periodontitis, bacteremia, as well as ocular, burn, and wound infections. Similar to B. cereus, B. thuringiensis produces an array of virulence factors acting against mammalian cells, such as phosphatidylcholine- and phosphatidylinositol-specific phospholipase C (PC-PLC and PI-PLC), hemolysins, in particular hemolysin BL (HBL), and various enterotoxins. The contribution of some of these toxins to B. thuringiensis pathogenicity has been studied in animal models of infection, following intravitreous, intranasal, or intratracheal inoculation. These studies lead to the speculation that the activities of PC-PLC, PI-PLC, and HBL are responsible for most of the pathogenic properties of B. thuringiensis in nongastrointestinal infections in mammals. This review summarizes data regarding the biological activity, the genetic basis, and the structural features of these membrane-damaging toxins.

SinR controls enterotoxin expression in Bacillus thuringiensis biofilms

The entomopathogen Bacillus thuringiensis produces dense biofilms under various conditions. Here, we report that the transition phase regulators Spo0A, AbrB and SinR control biofilm formation and swimming motility in B. thuringiensis, just as they control biofilm formation and swarming motility in the closely related saprophyte species B. subtilis. However, microarray analysis indicated that in B. thuringiensis, in contrast to B. subtilis, SinR does not control an eps operon involved in exopolysaccharides production, but regulates genes involved in the biosynthesis of the lipopeptide kurstakin. This lipopeptide is required for biofilm formation and was previously shown to be important for survival in the host cadaver (necrotrophism). Microarray analysis also revealed that the SinR regulon contains genes coding for the Hbl enterotoxin. Transcriptional fusion assays, Western blots and hemolysis assays confirmed that SinR controls Hbl expression, together with PlcR, the main virulence regulator in B. thuringiensis. We show that Hbl is expressed in a sustained way in a small subpopulation of the biofilm, whereas almost all the planktonic population transiently expresses Hbl. The gene coding for SinI, an antagonist of SinR, is expressed in the same biofilm subpopulation as hbl, suggesting that hbl transcription heterogeneity is SinI-dependent. B. thuringiensis and B. cereus are enteric bacteria which possibly form biofilms lining the host intestinal epithelium. Toxins produced in biofilms could therefore be delivered directly to the target tissue.

The Bacillus cereus Hbl and Nhe tripartite enterotoxin components assemble sequentially on the surface of target cells and are not interchangeable

Bacillus cereus is a spore-forming, Gram-positive bacterium commonly associated with outbreaks of food poisoning. It is also known as an opportunistic pathogen causing clinical infections such as bacteremia, meningitis, pneumonia, and gas gangrene-like cutaneous infections, mostly in immunocompromised patients. B. cereus secretes a plethora of toxins of which four are associated with the symptoms of food poisoning. Two of these, the non-hemolytic enterotoxin Nhe and the hemolysin BL (Hbl) toxin, are predicted to be structurally similar and are unique in that they require the combined action of three toxin proteins to induce cell lysis. Despite their dominant role in disease, the molecular mechanism of their toxic function is still poorly understood. We report here that B. cereus strain ATCC 10876 harbors not only genes encoding Nhe, but also two copies of the hbl genes. We identified Hbl as the major secreted toxin responsible for inducing rapid cell lysis both in cultured cells and in an intraperitoneal mouse toxicity model. Antibody neutralization and deletion of Hbl-encoding genes resulted in significant reductions of cytotoxic activity. Microscopy studies with Chinese Hamster Ovary cells furthermore showed that pore formation by both Hbl and Nhe occurs through a stepwise, sequential binding of toxin components to the cell surface and to each other. This begins with binding of Hbl-B or NheC to the eukaryotic membrane, and is followed by the recruitment of Hbl-L₁ or NheB, respectively, followed by the corresponding third protein. Lastly, toxin component complementation studies indicate that although Hbl and Nhe can be expressed simultaneously and are predicted to be structurally similar, they are incompatible and cannot complement each other.

Bacterial quorum sensing: its role in virulence and possibilities for its control

Quorum sensing is a process of cell-cell communication that allows bacteria to share information about cell density and adjust gene expression accordingly. This process enables bacteria to express energetically expensive processes as a collective only when the impact of those processes on the environment or on a host will be maximized. Among the many traits controlled by quorum sensing is the expression of virulence factors by pathogenic bacteria. Here we review the quorum-sensing circuits of Staphylococcus aureus, Bacillus cereus, Pseudomonas aeruginosa, and Vibrio cholerae. We outline these canonical quorum-sensing mechanisms and how each uniquely controls virulence factor production. Additionally, we examine recent efforts to inhibit quorum sensing in these pathogens with the goal of designing novel antimicrobial therapeutics.

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.

Comparative proteome analysis of Bacillus anthracis with pXO1 plasmid content

Bacillus anthracis the causative agent of anthrax, is an important pathogen among the Bacillus cereus group of species because of its physiological characteristics and its importance as a biological warfare agent. Tripartite anthrax toxin proteins and a poly-D-glutamic acid capsule are produced by B. anthracis vegetative cells during mammalian hosts infection and when cultured in conditions that are thought to mimic the host environment. To identify the factors regulating virulence in B. anthracis the whole cell proteins were extracted from two B. anthracis strains and separated by narrow range immobilized pH gradient (IPG) strips (pH 4-7), followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Proteins that were differentially expressed were identified by the peptide fingerprinting using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS). A total of 23 proteins were identified as being either upregulated or downregulated in the presence or absence of the virulence plasmid pXO1. Two plasmid encoded proteins and 12 cellular proteins were identified and documented as potential virulence factors.

Activation of the latent PlcR regulon in Bacillus anthracis

Many genes in Bacillus cereus and Bacillus thuringiensis are under the control of the transcriptional regulator PlcR and its regulatory peptide, PapR. In Bacillus anthracis, the causative agent of anthrax, PlcR is inactivated by truncation, and consequently genes having PlcR binding sites are expressed at very low levels when compared with B. cereus. We found that activation of the PlcR regulon in B. anthracis by expression of a PlcR–PapR fusion protein does not alter sporulation in strains containing the virulence plasmid pXO1 and thereby the global regulator AtxA. Using comparative 2D gel electrophoresis, we showed that activation of the PlcR regulon in B. anthracis leads to upregulation of many proteins found in the secretome of B. cereus, including phospholipases and proteases, such as the putative protease BA1995. Transcriptional analysis demonstrated expression of BA1995 to be dependent on PlcR–PapR, even though the putative PlcR recognition site of the BA1995 gene does not exactly match the PlcR consensus sequence, explaining why this protein had escaped recognition as belonging to the PlcR regulon. Additionally, while transcription of major PlcR-dependent haemolysins, sphingomyelinase and anthrolysin O is enhanced in response to PlcR activation in B. anthracis, only anthrolysin O contributes significantly to lysis of human erythrocytes. In contrast, the toxicity of bacterial culture supernatants from a PlcR-positive strain towards murine macrophages occurred independently of anthrolysin O expression in vitro and in vivo.

Expanding the known repertoire of virulence factors produced by Bacillus cereus through early secretome profiling in three redox conditions

The pathogen Bacillus cereus causes diarrheal disease in humans. In the small intestine, B. cereus has to deal with anaerobiosis, low oxidoreduction potential, and carbohydrate limitation conditions. To gain insight into the virulence potential of low density B. cereus cells in such an environment, we cultured bacteria in low and high oxidoreduction potential anoxic conditions and in fully oxic conditions and compared their full secretomes. A unique pattern of proteins assigned to virulence factors was revealed. Among the 57 virulence-related factors, 31 were found for the first time in the B. cereus secretome. The putative fourth component of hemolysin BL (HblB'), enterotoxin FM, hemolysin II, and three new putative conserved enterotoxins were uncovered. Cross-comparison of the relative abundance of secreted proteins reveals that a restricted set comprising 19 proteins showed significant changes in response to redox condition changes. We complemented these results with transcriptomics data and confirmed the cytotoxicity of the B. cereus secretome toward Caco-2 human epithelial cells. Our data suggest that (i) the redox-dependent regulatory pathway may modulate the expression of a subset of virulence factors to ensure an appropriate response in a specific redox environment, and (ii) an early growth phase-dependent pathway could regulate the expression of several virulence factors, allowing B. cereus to infect a host whatever the redox conditions. This early growth phase-dependent pathway may function, at least partially, independently of the pleiotropic virulence gene regulator PlcR and may therefore be more specific to the B. cereus group.

Identification of the main promoter directing cereulide biosynthesis in emetic Bacillus cereus and its application for real-time monitoring of ces gene expression in foods

Cereulide, the emetic Bacillus cereus toxin, is synthesized by cereulide synthetase via a nonribosomal peptide synthetase (NRPS) mechanism. Previous studies focused on the identification, structural organization, and biochemical characterization of the ces gene locus encoding cereulide synthetase; however, detailed information about the transcriptional organization of the ces genes was lacking. The present study shows that the cesPTABCD genes are transcribed as a 23-kb polycistronic transcript, while cesH, encoding a putative hydrolase, is transcribed from its own promoter. Transcription initiation was mapped by primer extension and rapid amplification of cDNA ends (RACE). Deletion analysis of promoter elements revealed a main promoter located upstream of the cesP coding sequence, encoding a 4'-phosphopantetheinyl transferase. This promoter drives transcription of cesPTABCD. In addition, intracistronic promoter regions in proximity to the translational start sites of cesB and cesT were identified but were only weakly active under the chosen assay conditions. The identified main promoter was amplified from the emetic reference strain B. cereus F4810/72 and fused to luciferase genes in order to study promoter activity in complex environments and to establish a biomonitoring system to assess cereulide production in different types of foods. ces promoter activity was strongly influenced by the food matrix and varied by 5 orders of magnitude. The amount of cereulide toxin extracted from spiked foods correlated well with the bioluminescence data, thus illustrating the potential of the established reporter system for monitoring of ces gene expression in complex matrices.

What sets Bacillus anthracis apart from other Bacillus species?

Bacillus anthracis is the cause of anthrax, and two large plasmids are essential for toxicity: pXO1, which contains the toxin genes, and pXO2, which encodes a capsule. B. anthracis forms a highly monomorphic lineage within the B. cereus group, but strains of Bacillus thuringiensis and B. cereus exist that are genetically closely related to the B. anthracis cluster. During the past five years B. cereus strains that contain the pXO1 virulence plasmid were discovered, and strains with both pXO1 and pXO2 have been isolated from great apes in Africa. Therefore, the presence of pXO1 and pXO2 no longer principally separates B. anthracis from other Bacilli. The B. anthracis lineage carries a specific mutation in the global regulator PlcR, which controls the transcription of secreted virulence factors in B. cereus and B. thuringiensis. Coevolution of the B. anthracis chromosome with its plasmids may be the basis for the successful development and uniqueness of the B. anthracis lineage.

Proteomic studies of Bacillus anthracis

Bacillus anthracis is a Gram-positive, spore-forming bacterium representing the etiological cause of anthrax, a rare lethal disease of animals and humans. Development of anthrax countermeasures has gained increasing attention owing to the potential use of B. anthracis spores as a bioterror weapon. The various forms of infection by B. anthracis are characterized both by toxemia and septicemia, both of which are the result of spore entry into the host followed by their germination into rapidly multiplying, toxin-producing bacilli. Following the publication of the bacterial genome, proteomic studies were carried out to determine the protein composition of the spore and identify exposed vegetative (membrane-located or secreted) proteins. These studies included comparison of strains differing in their virulence, cultured under different conditions and, in some cases, were complemented by serological inspection, which addressed expression during infection of proteomically identified proteins and their immunogenicity. The proteomic approach emerged as a valuable strategy for the generation of a pool of potential B. anthracis protein targets for further evaluation in detection, diagnostics, therapy and prophylaxis, and contributed to the elucidation of some aspects of the pathogenesis of the disease.

Transcriptional regulation of metabolic pathways, alternative respiration and enterotoxin genes in anaerobic growth of Bacillus cereus ATCC 14579

AIMS

To assess genes specifically activated during anaerobic growth that are involved in metabolism and pathogenesis of the foodborne pathogen Bacillus cereus.

METHODS AND RESULTS

Growth under anaerobic conditions in Brain Heart Infusion (BHI) broth revealed a reduced growth rate and lower yield as compared to growth under aerobic conditions. Subsequently, comparative transcriptome analysis showed specific genes induced under anaerobic conditions. These included novel genes identified for anaerobic growth of B. cereus, encoding metabolic pathways, such as the arginine deiminase pathway (ArcABDC), formate dehydrogenase (FdhF) and pyruvate formate lyase (Pfl), and alternative respiratory proteins, such as arsenate reductases. Notably, haemolytic enzyme encoding genes were induced during anaerobic growth, and enterotoxin genes were induced in high cell density transition and stationary phases of aerobic cultures.

CONCLUSIONS

These data point to induction of stress adaptation and pathogenicity factors and rearrangements of expression of metabolic pathways in response to oxygen limitations in B. cereus.

SIGNIFICANCE AND IMPACT OF THE STUDY

The reported changes in gene expression show that the foodborne pathogen B. cereus can adjust to anaerobic conditions, such as encountered in the human GI-tract.

Fructose and glucose mediates enterotoxin production and anaerobic metabolism of Bacillus cereus ATCC14579(T)

AIMS

To determine the effects of carbohydrates on Bacillus cereus ATCC14579(T) anaerobic metabolism and enterotoxin production in amino acids rich medium.

METHODS AND RESULTS

Bacillus cereus anaerobic growth on different carbohydrates (glucose, fructose, sucrose or glucose-fructose mixture) was examined in synthetic mMOD medium under continuous cultures (mu = 0.2 h(-1)). Fermentation end-products, flux partitioning at each key branch points of the mixed acid pathway and consumption or production of amino acids were determined. On both fructose and sucrose, ATP production was favoured via acetate production from acetyl-CoA. In addition, amino acids present in the growth medium showed significant variations with high consumption of serine and net production of glutamate and alanine on some or all sugars. Enterotoxins Hbl and Nhe production was high during growth on fructose (or mixtures involving a fructose moiety).

CONCLUSIONS

Fructose was identified as a key sugar influencing anaerobic metabolism and toxin production of B. cereus.

SIGNIFICANCE AND IMPACT OF THE STUDY

The physiological differences associated with the fermentation of the various carbohydrates clearly modify toxinogenesis indicating that the risk of foodborne pathogens is to some extent dependent upon the prevailing nutritional environment.

Cereulide synthesis in emetic Bacillus cereus is controlled by the transition state regulator AbrB, but not by the virulence regulator PlcR

Cereulide, a depsipeptide structurally related to the antibiotic valinomycin, is responsible for the emetic type of gastrointestinal disease caused by Bacillus cereus. Recently, it has been shown that cereulide is produced non-ribosomally by the plasmid-encoded peptide synthetase Ces. Using deletion mutants of the emetic reference strain B. cereus F4810/72, the influence of the well-known transcription factors PlcR, Spo0A and AbrB on cereulide production and on the transcription of the cereulide synthetase gene cluster was investigated. Our data demonstrate that cereulide synthesis is independent of the B. cereus specific virulence regulator PlcR but belongs to the Spo0A-AbrB regulon. Although cereulide production turned out to be independent of sporulation, it required the activity of the sporulation factor Spo0A. The σ A-promoted transcription of spo0A was found to be crucial for cereulide production, while the σ H-driven transcription of spo0A did not affect cereulide synthesis. Overexpression of the transition state factor AbrB in B. cereus F4810/72 resulted in a non-toxic phenotype. Moreover, AbrB was shown to bind efficiently to the main promoter region of the ces operon, indicating that AbrB acts as a repressor of cereulide production by negatively affecting ces transcription.

PapR peptide maturation: role of the NprB protease in Bacillus cereus 569 PlcR/PapR global gene regulation

The global transcriptional regulator PlcR controls gene expression in Bacillus cereus and Bacillus thuringiensis. Activity of PlcR is regulated by PapR, the product of an ORF located immediately downstream of plcR. To be active in B. cereus, PapR must be secreted and then processed to the mature peptide by an unknown protease. This peptide is transported by an oligopeptide permease into the cell, where it activates PlcR. In this study, we show that the neutral protease B (NprB) secreted by B. cereus 569 is required for extracellular PapR maturation. Purified recombinant NprB processed the synthetic PapR propeptide to produce a set of peptides derived from the C-terminal domain of PapR. Supplementation of growth media with synthetic PapR-derived C-terminal 5-, 7-, 8- and 27-amino acid (aa) peptides caused activation of intracellular PlcR in a PapR-deficient strain of B. cereus 569 while only the 5- and 7-aa peptides activated PlcR in a nprB mutant. The maximum activity was found for the 7-mer peptide. However, even the 7-mer peptide could not activate PlcR with a C-terminal truncation of as few as 6 aa. This indicates that interactions of the C-terminal regions of both PlcR and PapR are important in transcriptional activation of the B. cereus 569 PlcR regulon.

ORF 2 from the Bacillus cereus linear plasmid pBClin15 encodes a DNA binding protein

AIMS

To isolate and identify DNA-binding protein(s) with affinity for the mobile chromosomal repeat element bcr1 in Bacillus cereus group bacteria.

METHODS AND RESULTS

A biotinylated bcr1 element was immobilized to streptavidin-coated magnetic beads and used to pull out a 20 kDa DNA-binding protein from a whole cell protein extract of B. cereus ATCC 14579. The protein was identified as the product of ORF 2 encoded by the bacteriophage-related autonomously replicating linear genetic element pBClin15 carried by the strain. DNA binding was not bcr1-specific. By Northern blotting ORF 2 was co-transcribed with ORF 1, and also in certain instances with ORF 3 by transcriptional readthrough of the terminator located between ORF 2 and ORF 3.

CONCLUSIONS

ORF 2 from pBClin15 encodes a DNA-binding protein. ORF 2 is co-transcribed with its upstream gene ORF 1, and in a subset of the transcripts also with the downstream gene ORF 3 through alternative transcription termination.

SIGNIFICANCE AND IMPACT OF THE STUDY

The B. cereus group contains bacterial species of medical and economic importance. Bacteriophages or phage-encoded proteins from these bacteria have been suggested as potential therapeutic agents. Understanding the biology of bacteriophage-related genetic elements through functional characterization of their genes is of high relevance.

The YvfTU two-component system is involved in plcR expression in Bacillus cereus

Background

Most extracellular virulence factors produced by Bacillus cereus are regulated by the pleiotropic transcriptional activator PlcR. Among strains belonging to the B. cereus group, the plcR gene is always located in the vicinity of genes encoding the YvfTU two-component system. The putative role of YvfTU in the expression of the PlcR regulon was therefore investigated.

Results

Expression of the plcR gene was monitored using a transcriptional fusion with a lacZ reporter gene in a yvfTU mutant and in its B. cereus ATCC 14579 parental strain. Two hours after the onset of the stationary phase, a stage at which the PlcR regulon is highly expressed, the plcR expression in the yvfTU mutant was only 50% of that of its parental strain. In addition to the reduced plcR expression in the yvfTU mutant, a few members of the PlcR regulon showed a differential expression, as revealed by transcriptomic and proteomic analyses. The virulence of the yvfTU mutant in a Galleria mellonella insect model was slightly lower than that of the parental strain.

Conclusion

The YvfTU two-component system is not required for the expression of most of the virulence factors belonging to the PlcR regulon. However, YvfTU is involved in expression of plcR, a major regulator of virulence in B. cereus.

Erythrophore cell response to food-associated pathogenic bacteria: implications for detection

Cell-based biosensors have been proposed for use as function-based detectors of toxic agents. We report the use of Betta splendens chromatophore cells, specifically erythrophore cells, for detection of food-associated pathogenic bacteria. Evaluation of erythrophore cell response, using Bacillus spp., has revealed that this response can distinguish pathogenic Bacillus cereus from a non-pathogenic B. cereus ΔplcR deletion mutant and a non-pathogenic Bacillus subtilis. Erythrophore cells were exposed to Salmonella enteritidis, Clostridium perfringens and Clostridium botulinum. Each bacterial pathogen elicited a response from erythrophore cells that was distinguished from the corresponding bacterial growth medium, and this observed response was unique for each bacterial pathogen. These findings suggest that erythrophore cell response has potential for use as a biosensor in the detection and toxicity assessment for food-associated pathogenic bacteria.

Molecular basis for group-specific activation of the virulence regulator PlcR by PapR heptapeptides

The transcriptional regulator PlcR and its cognate cell–cell signalling peptide PapR form a quorum-sensing system that controls the expression of extra-cellular virulence factors in various species of the Bacillus cereus group. PlcR and PapR alleles are clustered into four groups defining four pherotypes. However, the molecular basis for group specificity remains elusive, largely because the biologically relevant PapR form is not known. Here, we show that the in vivo active form of PapR is the C-terminal heptapeptide of the precursor, and not the pentapeptide, as previously suggested. Combining genetic complementation, anisotropy assays and structural analysis we provide a detailed functional and structural explanation for the group specificity of the PlcR–PapR quorum-sensing system. We further show that the C-terminal helix of the PlcR regulatory domain, specifically the 278 residue, in conjunction with the N-terminal residues of the PapR heptapeptide determines this system specificity. Variability in the specificity-encoding regions of plcR and papR genes suggests that selection and evolution of quorum-sensing systems play a major role in adaptation and ecology of Bacilli.

ApoFnr binds as a monomer to promoters regulating the expression of enterotoxin genes of Bacillus cereus

Bacillus cereus Fnr is a member of the Crp/Fnr (cyclic AMP-binding protein/fumarate nitrate reduction regulatory protein) family of helix-turn-helix transcriptional regulators. It is essential for the expression of hbl and nhe enterotoxin genes independently of the oxygen tension in the environment. We studied aerobic Fnr binding to target sites in promoters regulating the expression of enterotoxin genes. B. cereus Fnr was overexpressed and purified as either a C-terminal His-tagged (Fnr(His)) fusion protein or an N-terminal fusion protein tagged with the Strep-tag (IBA BioTAGnology) ((Strep)Fnr). Both recombinant Fnr proteins were produced as apoforms (clusterless) and occurred as mixtures of monomers and oligomers in solution. However, apoFnr(His) was mainly monomeric, while apo(Strep)Fnr was mainly oligomeric, suggesting that the His-tagged C-terminal extremity may interfere with oligomerization. The oligomeric state of apo(Strep)Fnr was dithiothreitol sensitive, underlining the importance of a disulfide bridge for apoFnr oligomerization. Electrophoretic mobility shift assays showed that monomeric apoFnr, but not oligomeric apoFnr, bound to specific sequences located in the promoter regions of the enterotoxin regulators fnr, resDE, and plcR and the structural genes hbl and nhe. The question of whether apoFnr binding is regulated in vivo by redox-dependent oligomerization is discussed.

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.

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.

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.

The redox regulator Fnr is required for fermentative growth and enterotoxin synthesis in Bacillus cereus F4430/73

Glucose-grown cells of Bacillus cereus respond to anaerobiosis and low extracellular oxidoreduction potentials (ORP), notably by enhancing enterotoxin production. This response involves the ResDE two-component system. We searched the B. cereus genome for other redox response regulators potentially involved in this adaptive process, and we identified one gene encoding a protein predicted to have an amino acid sequence 58% identical (80% similar) to that of the Bacillus subtilis Fnr redox regulator. The fnr gene of the food-borne pathogen B. cereus F4430/73 has been cloned and partially characterized. We showed that fnr was up-regulated during anaerobic fermentation, especially when fermentation occurred at low ORP (under highly reducing conditions). The expression of fnr was down-regulated in the presence of O(2) and nitrate which, unlike fumarate, stimulated the respiratory pathways. The inactivation of B. cereus fnr abolished fermentative growth but only moderately affected aerobic and anaerobic nitrate respiratory growth. Analyses of glucose by-products and the transcription profiles of key catabolic genes confirmed the strong regulatory impact of Fnr on B. cereus fermentative pathways. More importantly, the fnr mutation strongly decreased the expression of PlcR-dependent hbl and nhe genes, leading to the absence of hemolysin BL (Hbl) and nonhemolytic enterotoxin (Nhe) secretion by the mutant. These data indicate that fnr is essential for both fermentation and toxinogenesis. The results also suggest that both Fnr and the ResDE two-component system belong to a redox regulatory pathway that functions at least partially independently of the pleiotropic virulence gene regulator PlcR to regulate enterotoxin gene expression.

Control of enterotoxin gene expression in Bacillus cereus F4430/73 involves the redox-sensitive ResDE signal transduction system

In contrast to Bacillus subtilis, the role of the two-component regulatory system ResDE has not yet been investigated in the facultative anaerobe Bacillus cereus. We examined the role of ResDE in the food-borne pathogen B. cereus F4430/73 by constructing resDE and resE mutants. Growth performances, glucose metabolism, and expression of hemolysin BL (Hbl) and nonhemolytic enterotoxin (Nhe) were analyzed in the three strains under distinct oxygenation and extracellular oxidoreduction potential (ORP) conditions. We show that growth and glucose metabolism were only moderately perturbed in both resDE and resE mutants under aerobiosis, microaerobiosis, and anaerobiosis generated under N(2) atmosphere (initial ORP = +45 mV). The major effects of resDE and resE mutations were observed under low-ORP anaerobic conditions generated under hydrogen atmosphere (iORP = -148 mV). These conditions normally favor enterotoxin production in the wild type. The resE mutation was more deleterious to the cells than the resDE mutation, causing growth limitation and strong deregulation of key catabolic genes. More importantly, the resE mutation abolished the production of enterotoxins under all of the conditions examined. The resDE mutation only decreased enterotoxin expression under anaerobiosis, with a more pronounced effect under low-ORP conditions. Thus, the ResDE system was found to exert major control on both fermentative growth and enterotoxin expression, and it is concluded that the ResDE system of B. cereus should be considered an anaerobic redox regulator. The data presented also provide evidence that the ResDE-dependent regulation of enterotoxins might function at least partially independently of the pleiotropic virulence gene regulator PlcR.

Characterization of Bacillus anthracis-like bacteria isolated from wild great apes from Cote d'Ivoire and Cameroon

We present the microbiological and molecular characterization of bacteria isolated from four chimpanzees and one gorilla thought to have died of an anthrax-like disease in Côte d'Ivoire and Cameroon. These isolates differed significantly from classic Bacillus anthracis by the following criteria: motility, resistance to the gamma phage, and, for isolates from Cameroon, resistance to penicillin G. A capsule was expressed not only after induction by CO(2) and bicarbonate but also under normal growth conditions. Subcultivation resulted in beta-hemolytic activity and gamma phage susceptibility in some subclones, suggesting differences in gene regulation compared to classic B. anthracis. The isolates from Côte d'Ivoire and Cameroon showed slight differences in their biochemical characteristics and MICs of different antibiotics but were identical in all molecular features and sequences analyzed. PCR and Southern blot analyses confirmed the presence of both the toxin and the capsule plasmid, with sizes corresponding to the B. anthracis virulence plasmids pXO1 and pXO2. Protective antigen was expressed and secreted into the culture supernatant. The isolates possessed variants of the Ba813 marker and the SG-749 fragment differing from that of classic B. anthracis strains. Multilocus sequence typing revealed a close relationship of our atypical isolates with both classic B. anthracis strains and two uncommonly virulent Bacillus cereus and Bacillus thuringiensis isolates. We propose that the newly discovered atypical B. anthracis strains share a common ancestor with classic B. anthracis or that they emerged recently by transfer of the B. anthracis plasmids to a strain of the B. cereus group.

Differential proteomic analysis of the Bacillus anthracis secretome: distinct plasmid and chromosome CO2-dependent cross talk mechanisms modulate extracellular proteolytic activities

The secretomes of a virulent Bacillus anthracis strain and of avirulent strains (cured of the virulence plasmids pXO1 and pXO2), cultured in rich and minimal media, were studied by a comparative proteomic approach. More than 400 protein spots, representing the products of 64 genes, were identified, and a unique pattern of protein relative abundance with respect to the presence of the virulence plasmids was revealed. In minimal medium under high CO(2) tension, conditions considered to simulate those encountered in the host, the presence of the plasmids leads to enhanced expression of 12 chromosome-carried genes (10 of which could not be detected in the absence of the plasmids) in addition to expression of 5 pXO1-encoded proteins. Furthermore, under these conditions, the presence of the pXO1 and pXO2 plasmids leads to the repression of 14 chromosomal genes. On the other hand, in minimal aerobic medium not supplemented with CO(2), the virulent and avirulent B. anthracis strains manifest very similar protein signatures, and most strikingly, two proteins (the metalloproteases InhA1 and NprB, orthologs of gene products attributed to the Bacillus cereus group PlcR regulon) represent over 90% of the total secretome. Interestingly, of the 64 identified gene products, at least 31 harbor features characteristic of virulence determinants (such as toxins, proteases, nucleotidases, sulfatases, transporters, and detoxification factors), 22 of which are differentially regulated in a plasmid-dependent manner. The nature and the expression patterns of proteins in the various secretomes suggest that distinct CO(2)-responsive chromosome- and plasmid-encoded regulatory factors modulate the secretion of potential novel virulence factors, most of which are associated with extracellular proteolytic activities.

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.

Toxin-producing ability among Bacillus spp. outside the Bacillus cereus group

A total of 333 Bacillus spp. isolated from foods, water, and food plants were examined for the production of possible enterotoxins and emetic toxins using a cytotoxicity assay on Vero cells, the boar spermatozoa motility assay, and a liquid chromatography-mass spectrometry method. Eight strains produced detectable toxins; six strains were cytotoxic, three strains produced putative emetic toxins (different in size from cereulide), and one strain produced both cytotoxin(s) and putative emetic toxin(s). The toxin-producing strains could be assigned to four different species, B. subtilis, B. mojavensis, B. pumilus, or B. fusiformis, by using a polyphasic approach including biochemical, chemotaxonomic, and DNA-based analyses. Four of the strains produced cytotoxins that were concentrated by ammonium sulfate followed by dialysis, and two strains produced cytotoxins that were not concentrated by such a treatment. Two cultures maintained full cytotoxic activity, two cultures reduced their activity, and two cultures lost their activity after boiling. The two most cytotoxic strains (both B. mojavensis) were tested for toxin production at different temperatures. One of these strains produced cytotoxin at growth temperatures ranging from 25 to 42 degrees C, and no reduction in activity was observed even after 24 h of growth at 42 degrees C. The strains that produced putative emetic toxins were tested for the influence of time and temperature on the toxin production. It was shown that they produced putative emetic toxin faster or just as fast at 30 as at 22 degrees C. None of the cytotoxic strains produced B. cereus-like enterotoxins as tested by PCR or by immunological methods.

Specificity and polymorphism of the PlcR-PapR quorum-sensing system in the Bacillus cereus group

The expression of extracellular virulence factors in various species of the Bacillus cereus group is controlled by the plcR and papR genes, which encode a transcriptional regulator and a cell-cell signaling peptide, respectively. A processed form of PapR, presumably a pentapeptide, specifically interacts with PlcR to facilitate its binding to its DNA targets. This activating mechanism is strain specific, with this specificity being determined by the first residue of the pentapeptide. We carried out in vivo complementation assays and compared the PlcR-PapR sequences of 29 strains from the B. cereus group. Our findings suggested that the fifth amino acid of the pentapeptide is also involved in the specificity of activation. We identified four classes of PlcR-PapR pairs, defining four distinct pherotypes in the B. cereus group. We used these findings to look at the evolution of the PlcR-PapR quorum-sensing system with regard to the phylogeny of the species forming the B. cereus group.

Bacillus cereus food poisoning and its toxins

The genus Bacillus includes members that demonstrate a wide range of diversity from physiology and ecological niche to DNA sequence and gene regulation. The species of most interest tend to be known for their pathogenicity and are closely linked genetically. Bacillus anthracis causes anthrax, and Bacillus thuringiensis is widely used for its insecticidal properties but has also been associated with foodborne disease. Bacillus cereus causes two types of food poisoning, the emetic and diarrheal syndromes, and a variety of local and systemic infections. Although in this review we provide information on the genus and a variety of species, the primary focus is on the B. cereus strains and toxins that are involved in foodborne illness. B. cereus produces a large number of potential virulence factors, but for the majority of these factors their roles in specific infections have not been established. To date, only cereulide and the tripartite hemolysin BL have been identified specifically as emetic and diarrheal toxins, respectively. Nonhemolytic enterotoxin, a homolog of hemolysin BL, also has been associated with the diarrheal syndrome. Recent findings regarding these and other putative enterotoxins are discussed.

The bcr1 DNA repeat element is specific to the Bacillus cereus group and exhibits mobile element characteristics

Bacillus cereus strains ATCC 10987 and ATCC 14579 harbor an approximately 155-bp repeated element, bcr1, which is conserved in B. cereus, B. anthracis, B. thuringiensis, and B. mycoides but not in B. subtilis and B. licheniformis. In this study, we show by Southern blot hybridizations that bcr1 is present in all 54 B. cereus group strains tested but absent in 11 Bacillus strains outside the group, suggesting that bcr1 may be specific and ubiquitous to the B. cereus group. By comparative analysis of the complete genome sequences of B. cereus ATCC 10987, B. cereus ATCC 14579, and B. anthracis Ames, we show that bcr1 is exclusively present in the chromosome but absent from large plasmids carried by these strains and that the numbers of full-length bcr1 repeats for these strains are 79, 54, and 12, respectively. Numerous copies of partial bcr1 elements are also present in the three genomes (91, 128, and 53, respectively). Furthermore, the genomic localization of bcr1 is not conserved between strains with respect to chromosomal position or organization of gene neighbors, as only six full-length bcr1 loci are common to at least two of the three strains. However, the intergenic sequence surrounding a specific bcr1 repeat in one of the three strains is generally strongly conserved in the other two, even in loci where bcr1 is found exclusively in one strain. This finding indicates that bcr1 either has evolved by differential deletion from a very high number of repeats in a common ancestor to the B. cereus group or is moving around the chromosome. The identification of bcr1 repeats interrupting genes in B. cereus ATCC 10987 and ATCC 14579 and the presence of a flanking TTTAT motif in each end show that bcr1 exhibits features characteristic of a mobile element.