Identification of genes involved in the activation of the Bacillus thuringiensis inhA metalloprotease gene at the onset of sporulation

The sprT Gene of Bacillus velezensis FZB42 Is Involved in Biofilm Formation and Bacilysin Production

Bacillus velezensis FZB42, a representative strain of plant-growth-promoting rhizobacteria (PGPR), can form robust biofilm and produce multiple antibiotics against a wild range of phytopathogens. In this study, we observed different biofilm morphology of the mutant Y4, derived from a TnYLB-1 transposon insertion library of B. velezensis FZB42. We identified that the transposon was inserted into the sprT gene in Y4. Our bioinformatics analysis revealed that the SprT protein is an unstable hydrophilic protein located in the cytoplasm. It is highly conserved in Bacillus species and predicted to function as a metalloprotease by binding zinc ions. We also demonstrated that ΔsprT significantly reduced the swarming ability of FZB42 by ~5-fold and sporulation capacity by ~25-fold. In addition, the antagonistic experiments showed that, compared to the wild type, the ΔsprT strain exhibited significantly reduced inhibition against Staphylococcus aureus ATCC-9144 and Phytophthora sojae, indicating that the inactivation of sprT led to decreased production of the antibiotic bacilysin. The HPLC-MS analysis confirmed that bacilysin was indeed decreased in the ΔsprT strain, and qPCR analysis revealed that ΔsprT down-regulated the expression of the genes for bacilysin biosynthesis. Our results suggest that the sprT gene plays a regulatory role in multiple characteristics of B. velezensis FZB42, including biofilm formation, swarming, sporulation, and antibiotic production.

Cationic protein 8 plays multiple roles in Galleria mellonella immunity

Galleria mellonella cationic protein 8 (GmCP8) is a hemolymph protein previously identified as an opsonin and an inhibitor of fungal proteases. In this work, we showed its bactericidal activity toward Pseudomonas entomophila, Pseudomonas aeruginosa, Bacillus thuringiensis, Staphylococcus aureus, and Escherichia coli and against yeast-like fungi Candida albicans. The activity against E. coli was correlated with bacterial membrane permeabilization. In turn, in the case of P. entomophila, B. thuringiensis, and C. albicans, the atomic force microscopy analysis of the microbial surface showed changes in the topography of cells and changes in their nanomechanical properties. GmCP8 also showed the inhibitory activity toward the serine protease trypsin and the metalloproteinase thermolysin. The expression of the gene encoding the GmCP8 protein did not increase either in the gut or in the fat body of G. mellonella after oral infection with P. entomophila. Similarly, the amount of GmCP8 in the hemolymph of G. mellonella did not change in immune-challenged insects. However, when GmCP8 was injected into the G. mellonella hemocel, a change in the survival curve was observed in the infected larvae. Our results shed new light on the function of GmCP8 protein in insect immunity, indicating its role in humoral defence mechanisms.

The Fate of Bacteria of the Bacillus cereus Group in the Amoeba Environment

The Bacillus cereus sensu lato group consists of several closely related species, including B. anthracis, B. cereus sensu stricto, and B. thuringiensis. Spores of these pathogenic bacteria are commonly found in the soil but evidence suggests that they are unable to grow in such a natural environment in the absence of nutrient input. Amoebas have been reported to be an amplifier for several species of pathogenic bacteria and their potential involvement to explain the large amount of B. thuringiensis and B. cereus spores in soil has been frequently proposed. Here, we studied the fate of Bacillus and amoebas when cultured together. We show that the virulence factors produced by B. thuringiensis and B. cereus do not affect the amoeba Acanthamoeba castellanii, which, on the contrary, can phagocytose and effectively digest vegetative Bacillus cells to grow and prevent the formation of cysts. Bacterial spores can germinate in the amoeba environment and the vegetative cells can then form chains or aggregates that appear to be less efficiently phagocyted by the amoeba. The use of transcriptional fusions between fluorescent reporter genes and stationary phase- and sporulation-specific promoters showed that the sporulation process occurs more efficiently in the presence of amoebas than in their absence. Moreover, our results showed the amoeba environment to promote spore germination and allow the bacteria to complete their developmental cycle. Overall, this study suggests that the amoeba-Bacillus interaction creates a virtuous circle in which each protagonist helps the other to develop.

Tandem Mass Tag-Based Quantitative Proteomics and Virulence Phenotype of Hemolymph-Treated Bacillus thuringiensis kurstaki Cells Reveal New Insights on Bacterial Pathogenesis in Insects

The spore-forming bacterium Bacillus thuringiensis (Bt) of the Bacillus cereus group uses toxin-opened breaches at the insect midgut epithelium to infest the hemolymph, where it can rapidly propagate despite antimicrobial host defenses and induce host death by acute septicemia. The response of Bt to host hemolymph and the latter's role in bacterial pathogenesis is an area that needs clarification. Here, we report a proteomic analysis of the Bt kurstaki strain HD73 (Btk) hemolymph stimulon showing significant changes in 60 (34 up- and 26 downregulated) differentially accumulated proteins (DAPs). Gene ontology (GO) enrichment analysis revealed that DAPs were mainly related to glutamate metabolism, transketolase activity, and ATP-dependent transmembrane transport. KEGG analysis disclosed that DAPs were highly enriched in the biosynthesis of bacterial secondary metabolites, ansamycins. Interestingly, about 30% of all DAPs were in silico predicted as putative virulence factors. Further characterization of hemolymph effects on Btk showed enhanced autoaggregation in liquid cultures and biofilm formation in microtiter polystyrene plates. Hemolymph-exposed Btk cells were less immunogenic in mice, suggesting epitope masking of selected surface proteins. Bioassays with intrahemocoelically infected Bombyx mori larvae showed that hemolymph preexposure significantly increased Btk toxicity and reproduction within the insect (spore count per cadaver) at low inoculum doses, possibly due to 'virulence priming'. Collectively, our findings suggest that the Btk hemolymph stimulon could be partially responsible for bacterial survival and propagation within the hemolymph of infected insects, contributing to its remarkable success as an entomopathogen. All mass spectrometry data are available via ProteomeXchange with identifier PXD021830. IMPORTANCE After ingestion by a susceptible insect and damaging its midgut epithelium, the bacterium Bacillus thuringiensis (Bt) reaches the insect blood (hemolymph), where it propagates despite the host's antimicrobial defenses and induces insect death by acute septicemia. Although the hemolymph stage of the Bt toxic pathway is determinant for the infested insects' fate, the response of Bt to hemolymph and the latter's role in bacterial pathogenesis has been poorly explored. In this study, we identified the bacterial proteins differentially expressed by Bt after hemolymph exposure. We found that about 30% of hemolymph-regulated Bt proteins were potential virulence factors, including manganese superoxide dismutase, a described inhibitor of hemocyte respiratory burst. Additionally, contact with hemolymph enhanced Bt virulence phenotypes, such as cell aggregation and biofilm formation, altered bacterial immunogenicity, and increased Bt toxicity to intrahemocoelically injected insects.

Transition Phase Regulator AbrB Positively Regulates the sip1Ab1 Gene Expression in Bacillus thuringiensis

Bacillus thuringiensis secreted insecticidal proteins (Sip) are a secretion that is toxic to coleopteran pests. However, the transcriptional mechanism of sip genes is still unknown. The transcriptional regulation of the sip1Ab1 gene and the expression of the Sip1Ab1 protein were investigated in this study. The results demonstrated that the secretion of the Sip1Ab1 protein in HD73 was almost the same as that in the original QZL38 strain during the transition phase. Analysis of the β-galactosidase activities of sip1Ab1-lacZ in both the HD73 and abrB mutant strains indicated that the transcription of sip1Ab1 is dependent on AbrB. Electrophoretic mobility shift assays showed that AbrB could bind with the sip1Ab1 promoter, and two binding sites of AbrB in the region of the promoter of sip1Ab1 were determined by DNase I footprinting assays. All of the above-described results proved that AbrB positively regulates the sip1Ab1 gene.

IMPORTANCEBacillus thuringiensis Sip proteins are secreted insecticidal toxins that are toxic to coleopteran pests. In this study, we investigated the transcriptional mechanism of the sip gene and showed strong evidence that Sip1Ab1 is secreted in the transition phase and that AbrB, a transition phase regulator that is usually a repressor, positively and directly regulates sip1Ab1. Reports of AbrB positive regulation are rare, even in Bacillus subtilis. To the best of our knowledge, no toxic gene has been reported to be positively regulated by AbrB in Bacillus species.

Immune Inhibitor A Metalloproteases Contribute to Virulence in Bacillus Endophthalmitis

Endophthalmitis is a devastating infection that can cause blindness. Over half of Bacillus endophthalmitis cases result in significant loss of useful vision. Bacillus produces many virulence factors that may contribute to retinal damage and robust inflammation. We analyzed Bacillus immune inhibitor A (InhA) metalloproteases in the context of this disease, hypothesizing that InhAs contribute to Bacillus intraocular virulence and inflammation. We analyzed phenotypes and infectivity of wild type (WT), InhA1-deficient (ΔinhA1), InhA2-deficient (ΔinhA2), or InhA1, A2, and A3-deficient (ΔinhA1-3) Bacillus thuringiensis. In vitro analysis of growth, proteolysis, and cytotoxicity were compared. WT and InhA mutants were similarly cytotoxic to retinal cells. Mutants ΔinhA1 and ΔinhA2 entered log phase growth earlier than WT. Proteolysis by the ΔinhA1-3 mutant was decreased, but this strain grew similar to WT in vitro. Experimental endophthalmitis was initiated by intravitreally infecting C57BL/6J mice with 200 CFU of B. thuringiensis WT or InhA mutants. Eyes were analyzed for intraocular Bacillus and myeloperoxidase concentrations, retinal function loss, and gross histological changes. Eyes infected with ΔinhA1 or ΔinhA2 strains contained greater numbers of bacteria than eyes infected with WT throughout the infection course. Eyes infected with single mutants had inflammation and retinal function loss similar to eyes infected with WT strain. Eyes infected with ΔinhA1-3 cleared the infection. RT-PCR results suggested that there may be compensatory expression of the other InhAs in the single InhA mutant. These results indicate that together, the InhA metalloproteases contribute to the severity of infection and inflammation in Bacillus endophthalmitis.

Rap-Phr Systems from Plasmids pAW63 and pHT8-1 Act Together To Regulate Sporulation in the Bacillus thuringiensis Serovar kurstaki HD73 Strain

Bacillus thuringiensis is a Gram-positive spore-forming bacterium pathogenic to various insect species. This property is due to the Cry toxins encoded by plasmid genes and mostly produced during sporulation. B. thuringiensis contains a remarkable number of extrachromosomal DNA molecules and a great number of plasmid rap-phr genes. Rap-Phr quorum-sensing systems regulate different bacterial processes, notably the commitment to sporulation in Bacillus species. Rap proteins are quorum sensors acting as phosphatases on Spo0F, an intermediate of the sporulation phosphorelay, and are inhibited by Phr peptides that function as signaling molecules. In this study, we characterize the Rap63-Phr63 system encoded by the pAW63 plasmid from the B. thuringiensis serovar kurstaki HD73 strain. Rap63 has moderate activity on sporulation and is inhibited by the Phr63 peptide. The rap63-phr63 genes are cotranscribed, and the phr63 gene is also transcribed from a σ^H-specific promoter. We show that Rap63-Phr63 regulates sporulation together with the Rap8-Phr8 system harbored by plasmid pHT8_1 of the HD73 strain. Interestingly, the deletion of both phr63 and phr8 genes in the same strain has a greater negative effect on sporulation than the sum of the loss of each phr gene. Despite the similarities in the Phr8 and Phr63 sequences, there is no cross talk between the two systems. Our results suggest a synergism of these two Rap-Phr systems in the regulation of the sporulation of B. thuringiensis at the end of the infectious cycle in insects, thus pointing out the roles of the plasmids in the fitness of the bacterium.IMPORTANCE The life cycle of Bacillus thuringiensis in insect larvae is regulated by quorum-sensing systems of the RNPP family. After the toxemia caused by Cry insecticidal toxins, the sequential activation of these systems allows the bacterium to trigger first a state of virulence (regulated by PlcR-PapR) and then a necrotrophic lifestyle (regulated by NprR-NprX); ultimately, sporulation is controlled by the Rap-Phr systems. Our study describes a new rap-phr operon carried by a B. thuringiensis plasmid and shows that the Rap protein has a moderate effect on sporulation. However, this system, in combination with another plasmidic rap-phr operon, provides effective control of sporulation when the bacteria develop in the cadavers of infected insect larvae. Overall, this study highlights the important adaptive role of the plasmid Rap-Phr systems in the developmental fate of B. thuringiensis and its survival within its ecological niche.

Rap Protein Paralogs of Bacillus thuringiensis: a Multifunctional and Redundant Regulatory Repertoire for the Control of Collective Functions

Quorum sensing (QS) is a mechanism of synthesis and detection of signaling molecules to regulate gene expression and coordinate behaviors in bacterial populations. In Bacillus subtilis, multiple paralog Rap-Phr QS systems (receptor-signaling peptides) are highly redundant and multifunctional, interconnecting the regulation of differentiation processes such as sporulation and competence. However, their functions in the Bacillus cereus group are largely unknown. We evaluated the functions of Rap proteins in Bacillus thuringiensis Bt8741, which codes for eight Rap-Phr systems; these were individually overexpressed to study their participation in sporulation, biofilm formation, spreading, and extracellular proteolytic activity. Our results show that four Rap-Phr systems (RapC, RapK, RapF, and RapLike) inhibit sporulation, two of which (RapK and RapF) probably dephosphorylate Spo0F from the Spo0A phosphorelay; these two Rap proteins also inhibit biofilm formation. Four systems (RapC, RacF1, RacF2, and RapLike) participate in spreading inhibition; finally, six systems (RapC, -F, -F2, -I, and -I1 and RapLike) decrease extracellular proteolytic activity. We foresee that functions performed by Rap proteins of Bt8741 could also be carried out by Rap homologs in other strains within the B. cereus group. These results indicate that Rap-Phr systems constitute a highly multifunctional and redundant regulatory repertoire that enables B. thuringiensis and other species from the B. cereus group to efficiently regulate collective functions during their life cycle in the face of changing environments.IMPORTANCE The Bacillus cereus group of bacteria includes species of high economic, clinical, biological warfare, and biotechnological interest, e.g., B. anthracis in bioterrorism, B. cereus in food intoxications, and B. thuringiensis in biocontrol. Knowledge about the ecology of these bacteria is hindered by our limited understanding of the regulatory circuits that control differentiation and specialization processes. Here, we uncover the participation of eight Rap quorum-sensing receptors in collective functions of B. thuringiensis These proteins are highly multifunctional and redundant in their functions, linking ecologically relevant processes such as sporulation, biofilm formation, spreading, extracellular proteolytic activity, and probably other functions in species from the B. cereus group.

Diversity of the Rap-Phr quorum-sensing systems in the Bacillus cereus group

Bacteria of the Bacillus cereus group colonize several ecological niches and infect different hosts. Bacillus cereus, a ubiquitous species causing food poisoning, Bacillus thuringiensis, an entomopathogen, and Bacillus anthracis, which is highly pathogenic to mammals, are the most important species of this group. These species are closely related genetically, and their specific toxins are encoded by plasmids. The infectious cycle of B. thuringiensis in its insect host is regulated by quorum-sensing systems from the RNPP family. Among them, the Rap-Phr systems, which are well-described in Bacillus subtilis, regulate essential processes, such as sporulation. Given the importance of these systems, we performed a global in silico analysis to investigate their prevalence, distribution, diversity and their role in sporulation in B. cereus group species. The rap-phr genes were identified in all selected strains with 30% located on plasmids, predominantly in B. thuringiensis. Despite a high variability in their sequences, there is a remarkable association between closely related strains and their Rap-Phr profile. Based on the key residues involved in RapH phosphatase activity, we predicted that 32% of the Rap proteins could regulate sporulation by preventing the phosphorylation of Spo0F. These Rap are preferentially located on plasmids and mostly related to B. thuringiensis. The predictions were partially validated by in vivo sporulation experiments suggesting that the residues linked to the phosphatase function are necessary but not sufficient to predict this activity. The wide distribution and diversity of Rap-Phr systems could strictly control the commitment to sporulation and then improve the adaptation capacities of the bacteria to environmental changes.

The Bacillus cereus Group: Bacillus Species with Pathogenic Potential

The Bacillus cereus group includes several Bacillus species with closely related phylogeny. The most well-studied members of the group, B. anthracis, B. cereus, and B. thuringiensis, are known for their pathogenic potential. Here, we present the historical rationale for speciation and discuss shared and unique features of these bacteria. Aspects of cell morphology and physiology, and genome sequence similarity and gene synteny support close evolutionary relationships for these three species. For many strains, distinct differences in virulence factor synthesis provide facile means for species assignment. B. anthracis is the causative agent of anthrax. Some B. cereus strains are commonly recognized as food poisoning agents, but strains can also cause localized wound and eye infections as well as systemic disease. Certain B. thuringiensis strains are entomopathogens and have been commercialized for use as biopesticides, while some strains have been reported to cause infection in immunocompromised individuals. In this article we compare and contrast B. anthracis, B. cereus, and B. thuringiensis, including ecology, cell structure and development, virulence attributes, gene regulation and genetic exchange systems, and experimental models of disease.

InhA1-Mediated Cleavage of the Metalloprotease NprA Allows Bacillus cereus to Escape From Macrophages

Bacillus cereus is a Gram-positive spore-forming bacterium causing food poisoning and serious opportunistic infections. These infections are characterized by bacterial accumulation in the host despite the induction of inflammation. To circumvent inflammation, bacteria must resist the bactericidal activity of professional phagocytes, which constitute a first line of host defense against pathogens. Interactions between phagocytic cells and B. cereus are still poorly characterized and the mechanism of resistance to the host immune system is not known yet. We have previously shown that the spores are phagocytosed by macrophages but survive and escape from these cells. The metalloprotease InhA1 is a key effector involved in these processes. inhA1-deficient spores are retained intracellularly, in contrast to the wild type strain spores. NprA is also a B. cereus metalloprotease able to cleave tissue components such as fibronectin, laminin, and collagen. Here, we show that NprA, concomitantly secreted with InhA1 in the B. cereus secretome, is essential to promote bacterial escape from macrophages. We show that InhA1 cleaves NprA at specific sites. This cleavage allows liberation of the mature form of the NprA protein in the supernatant of the wild type strain. This mature form of NprA is actually the principal effector allowing bacterial escape from host macrophages.

Analysis of abrB Expression during the Infectious Cycle of Bacillus thuringiensis Reveals Population Heterogeneity

Using the model host/pathogen pair Galleria mellonella/Bacillus thuringiensis, we have shown that these bacteria could kill their insect host, survive in its cadaver and form spores by sequentially activating virulence, necrotrophism and sporulation genes. However, the population isolated from the cadavers was heterogeneous, including non-sporulating cells in an unknown physiological state. To characterize these bacteria, we used a transcriptional fusion between the promoter of a gene expressed during early exponential growth (abrB) and a reporter gene encoding a destabilized version of GFP, in combination with a fluorescent reporter of the necrotrophic state. The composition of the bacterial population during infection was then analyzed by flow cytometry. We showed that the PabrB promoter was activated in the population that had turned on the necrotrophic reporter, suggesting a re-entry into vegetative growth. Strikingly, the cells that did not go through the necrotrophic state did not activate the PabrB promoter and appear as a dormant subpopulation. We propose a new model describing the B. thuringiensis cell types during infection.

Changes in the hemolymph protein profiles in Galleria mellonella infected with Bacillus thuringiensis involve apolipophorin III. The effect of heat shock

This report concerns the effect of heat shock on host-pathogen interaction in Galleria mellonella infected with Bacillus thuringiensis. We show enhanced activity against Gram-positive bacteria in the hemolymph of larvae pre-exposed to heat shock before infection with B. thuringiensis. Heat shock influenced the protein pattern in the hemolymph of infected larvae: more peptides with a molecular weight below 10 kDa were detected in comparison with nonshocked animals. Additionally, we noticed that the amount of apolipophorin III (apoLp-III) in the hemolymph decreased transiently following infection, which was considerably higher in larvae pre-exposed to heat shock. On the other hand, its expression in the fat body showed a consequent infection-induced decline, observed equally in shocked and nonshocked animals. This suggests that the amount of apoLp-III in the hemolymph of G. mellonella larvae is regulated at multiple levels. We also report that this protein is more resistant to degradation in the hemolymph of larvae pre-exposed to heat shock in comparison to nonshocked larvae. Two-dimensional analysis revealed the presence of three isoforms of apoLp-III, all susceptible to proteolytic degradation. However, one of them was the most abundant, both in the protease-treated and untreated hemolymph. Taking into consideration that, in general, apoLp-III has a stimulative effect on different immune-related hemolymph proteins and peptides, the reported findings bring us closer to understanding the effect of heat shock on the resistance of G. mellonella to infection.

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.

Heat shock affects host-pathogen interaction in Galleria mellonella infected with Bacillus thuringiensis

We report that Galleria mellonella larvae exposed to heat shock was more resistant to infection with entomopathogenic bacteria Bacillus thuringiensis. The insects were exposed to a temperature of 40°C for 30 min directly before injection of vegetative bacterial cells. It appeared that the kinetics of the immune response was affected in heat-shocked animals. The infection-induced antimicrobial activity of larval hemolymph was stronger in shocked animals in comparison to the non-shocked ones. Hemolymph proteins of molecular weight below 10 kDa, corresponding to the size of antimicrobial peptides, were responsible for this activity. Furthermore, the transcription level of genes encoding antimicrobial peptides: cecropin, gallerimycin, and galiomycin was increased in the fat bodies of insects exposed to heat shock before infection. On the contrary, the heat-shock treatment did not enhance expression of the metalloproteinase inhibitor-IMPI in the infected animals. The difference in the amount of antimicrobial peptides and, consequently, in the defense activity of insect hemolymph, persisted after the action of bacterial metalloproteinases, which are well-known virulence factors. Furthermore, peptides with antimicrobial activity in the hemolymph of infected larvae pre-exposed to heat shock appeared to be more resistant to proteolytic degradation both in vitro and in vivo. Our results point to the mechanism of cross-protection of thermal stress toward innate immune response.

The red flour beetle as a model for bacterial oral infections

Experimental infection systems are important for studying antagonistic interactions and coevolution between hosts and their pathogens. The red flour beetle Tribolium castaneum and the spore-forming bacterial insect pathogen Bacillus thuringiensis (Bt) are widely used and tractable model organisms. However, they have not been employed yet as an efficient experimental system to study host-pathogen interactions. We used a high throughput oral infection protocol to infect T. castaneum insects with coleopteran specific B. thuringiensis bv. tenebrionis (Btt) bacteria. We found that larval mortality depends on the dietary spore concentration and on the duration of exposure to the spores. Furthermore, differential susceptibility of larvae from different T. castaneum populations indicates that the host genetic background influences infection success. The recovery of high numbers of infectious spores from the cadavers indicates successful replication of bacteria in the host and suggests that Btt could establish infectious cycles in T. castaneum in nature. We were able to transfer plasmids from Btt to a non-pathogenic but genetically well-characterised Bt strain, which was thereafter able to successfully infect T. castaneum, suggesting that factors residing on the plasmids are important for the virulence of Btt. The availability of a genetically accessible strain will provide an ideal model for more in-depth analyses of pathogenicity factors during oral infections. Combined with the availability of the full genome sequence of T. castaneum, this system will enable analyses of host responses during infection, as well as addressing basic questions concerning host-parasite coevolution.

Activity of the Bacillus thuringiensis NprR-NprX cell-cell communication system is co-ordinated to the physiological stage through a complex transcriptional regulation

NprR is a quorum sensor of the RNPP family found in bacteria of the Bacillus cereus group. In association with its cognate peptide NprX, NprR controls the expression of genes essential for survival and sporulation of Bacillus thuringiensis during its necrotrophic development in insects. Here, we report that the nprR-nprX genes are not autoregulated and are co-transcribed from a σ(A) -dependent promoter (PA ) located upstream from nprR. The transcription from PA starts at the onset of the stationary phase and is controlled by two transcriptional regulators: CodY and PlcR. The nutritional repressor CodY represses nprR-nprX transcription during the exponential growth phase and the quorum sensor PlcR activates nprR-nprX transcription at the onset of stationary phase. We show that nprX is also transcribed independently of nprR from two promoters, PH and PE , dependent on the sporulation-specific sigma factors, σ(H) and σ(E) respectively. Both promoters ensure nprX transcription during late stationary phase while transcription from PA has decreased. These results show that the activity of the NprR-NprX quorum sensing system is tightly co-ordinated to the physiological stage throughout the developmental process of the Bacillus.

CodY orchestrates the expression of virulence determinants in emetic Bacillus cereus by impacting key regulatory circuits

Bacillus cereus causes gastrointestinal diseases and local and systemic infections elicited by the depsipeptide cereulide, enterotoxins, phospholipases, cytolysins and proteases. The PlcR-PapR quorum sensing system activates the expression of several virulence factors, whereas the Spo0A-AbrB regulatory circuit partially controls the plasmid-borne cereulide synthetase (ces) operon. Here, we show that CodY, a nutrient-responsive regulator of Gram-positive bacteria, has a profound effect on both regulatory systems, which have been assumed to operate independently of each other. Deletion of codY resulted in downregulation of virulence genes belonging to the PlcR regulon and a concomitant upregulation of the ces genes. CodY was found to be a repressor of the ces operon, but did not interact with the promoter regions of PlcR-dependent virulence genes in vitro, suggesting an indirect regulation of the latter. Furthermore, CodY binds to the promoter of the immune inhibitor metalloprotease InhA1, demonstrating that CodY directly links B. cereus metabolism to virulence. In vivo studies using a Galleria mellonella infection model, showed that the codY mutant was substantially attenuated, highlighting the importance of CodY as a key regulator of pathogenicity. Our results demonstrate that CodY profoundly modulates the virulence of B. cereus, possibly controlling the development of pathogenic traits in suitable host environments.

How the insect pathogen bacteria Bacillus thuringiensis and Xenorhabdus/Photorhabdus occupy their hosts

Insects are the largest group of animals on earth. Like mammals, virus, fungi, bacteria and parasites infect them. Several tissue barriers and defense mechanisms are common for vertebrates and invertebrates. Therefore some insects, notably the fly Drosophila and the caterpillar Galleria mellonella, have been used as models to study host-pathogen interactions for several insect and mammal pathogens. They are excellent tools to identify pathogen determinants and host tissue cell responses. We focus here on the comparison of effectors used by two different groups of bacterial insect pathogens to accomplish the infection process in their lepidopteran larval host: Bacillus thuringiensis and the nematode-associated bacteria, Photorhabdus and Xenorhabdus. The comparison reveals similarities in function and expression profiles for some genes, which suggest that such factors are conserved during evolution in order to attack the tissue encountered during the infection process.

Proteomic analysis of Bacillus thuringiensis at different growth phases by using an automated online two-dimensional liquid chromatography-tandem mass spectrometry strategy

The proteome of a new Bacillus thuringiensis subsp. kurstaki strain, 4.0718, from the middle vegetative (T(1)), early sporulation (T(2)), and late sporulation (T(3)) phases was analyzed using an integrated liquid chromatography (LC)-based protein identification system. The system comprised two-dimensional (2D) LC coupled with nanoscale electrospray ionization (ESI) tandem mass spectrometry (MS/MS) on a high-resolution hybrid mass spectrometer with an automated data analysis system. After deletion of redundant proteins from the different batches and B. thuringiensis subspecies, 918, 703, and 778 proteins were identified in the respective three phases. Their molecular masses ranged from 4.6 Da to 477.4 Da, and their isoelectric points ranged from 4.01 to 11.84. Function clustering revealed that most of the proteins in the three phases were functional metabolic proteins, followed by proteins participating in cell processes. Small molecular and macromolecular metabolic proteins were further classified according to the Kyoto Encyclopedia of Genes and Genome and BioCyc metabolic pathway database. Three protoxins (Cry2Aa, Cry1Aa, and Cry1Ac) as well as a series of potential intracellular active factors were detected. Many significant proteins related to spore and crystal formation, including sporulation proteins, help proteins, chaperones, and so on, were identified. The expression patterns of two identified proteins, CotJc and glutamine synthetase, were validated by Western blot analysis, which further confirmed the MS results. This study is the first to use shotgun technology to research the proteome of B. thuringiensis. Valuable experimental data are provided regarding the methodology of analyzing the B. thuringiensis proteome (which can be used to produce insecticidal crystal proteins) and have been added to the related protein database.

Bacillus anthracis protease InhA regulates BslA-mediated adhesion in human endothelial cells

To achieve widespread dissemination in the host, Bacillus anthracis cells regulate their attachment to host endothelium during infection. Previous studies identified BslA (Bacillus anthracis S-layer Protein A), a virulence factor of B. anthracis, as necessary and sufficient for adhesion of vegetative cells to human endothelial cells. While some factors have been identified, bacteria-specific contributions to BslA mediated adhesion remain unclear. Using the attenuated vaccine Sterne 7702 strain of B. anthracis, we tested the hypothesis that InhA (immune inhibitor A), a B. anthracis protease, regulates BslA levels affecting the bacteria's ability to bind to endothelium. To test this, a combination of inhA mutant and complementation analysis in adhesion and invasion assays, Western blot and InhA inhibitor assays were employed. Results show InhA downregulates BslA activity reducing B. anthracis adhesion and invasion in human brain endothelial cells. BslA protein levels in ΔinhA bacteria were significantly higher than wild-type and complemented strains showing InhA levels and BslA expression are inversely related. BslA was sensitive to purified InhA degradation in a concentration- and time-dependent manner. Taken together these data support the role of InhA regulation of BslA-mediated vegetative cell adhesion and invasion.

Haemolysin II is a Bacillus cereus virulence factor that induces apoptosis of macrophages

Bacillus cereus is a Gram-positive spore-forming bacterium causing food poisoning and serious opportunistic infections. These infections are characterized by bacterial accumulation despite the recruitment of phagocytic cells. The precise mechanisms and the bacterial factors allowing B. cereus to circumvent host immune responses remain to be elucidated. We have previously shown that B. cereus induces macrophage cell death by an unknown mechanism. Here we identified the toxic component from the B. cereus supernatant. We report that Haemolysin II (HlyII) provokes macrophage cell death by apoptosis through its pore-forming activity. The HlyII-induced apoptotic pathway is caspase 3 and 8 dependent, thus most likely mediated by the death receptor pathway. Using insects and mice as in vivo models, we show that deletion of hlyII strongly reduces virulence. In addition, we show that after infection of Bombyx mori larvae, the immune cells are apoptotic, demonstrating that HlyII induces apoptosis of phagocytic cells in vivo. Altogether, our results clearly unravel HlyII as a novel virulence protein that induces apoptosis in phagocytic cells in vitro and in vivo.

Transcription of gene in an acrystalliferous strain of Bacillus thuringiensis XBU001 positively regulated by the metalloprotease camelysin gene at the onset of stationary phase

The calY gene, encoding metalloprotease camelysin in the Bacillus thuringiensis acrystalliferous strain XBU001, was amplified and sequenced. The camelysin from the calY sequence was 199 amino acids in size (c. 22 000 Da). The temperature-sensitive plasmid pKESX was used to construct a metalloprotease camelysin-deficient strain of B. thuringiensis. The calY gene was replaced by an erythromycin-resistant gene in KCTF. Sodium dodecyl sulfate polyacrylamide gel electrophoresis and MS analysis showed that the metalloprotease InhA was not expressed after knocking out the gene calY. The temperature-sensitive plasmid pKPC was used to construct a metalloprotease camelysin complementation strain KCTFC. The InhA protein was found in KCTFC. Analysis of the expression of InhA in the wild-type strain KCTF12, camelysin-deficient and complementation strains indicated that inhA expression depended on camelysin. Although camelysin did not directly regulate the expression of the InhA through binding to the promoter of the inhA, the results suggest that camelysin can positively regulate the expression of the InhA protein.

Bacillus anthracis sin locus and regulation of secreted proteases

Bacillus anthracis shares many regulatory loci with the nonpathogenic Bacillus species Bacillus subtilis. One such locus is sinIR, which in B. subtilis controls sporulation, biofilm formation, motility, and competency. As B. anthracis is not known to be motile, to be naturally competent, or to readily form biofilms, we hypothesized that the B. anthracis sinIR regulon is distinct from that of B. subtilis. A genome-wide expression microarray analysis of B. anthracis parental and sinR mutant strains indicated limited convergence of the B. anthracis and B. subtilis SinR regulons. The B. anthracis regulon includes homologues of some B. subtilis SinR-regulated genes, including the signal peptidase gene sipW near the sinIR locus and the sporulation gene spoIIE. The B. anthracis SinR protein also negatively regulates transcription of genes adjacent to the sinIR locus that are unique to the Bacillus cereus group species. These include calY and inhA1, structural genes for the metalloproteases camelysin and immune inhibitor A1 (InhA1), which have been suggested to be associated with virulence in B. cereus and B. anthracis, respectively. Electrophoretic mobility shift assays revealed direct binding of B. anthracis SinR to promoter DNA from strongly regulated genes, such as calY and sipW, but not to the weakly regulated inhA1 gene. Assessment of camelysin and InhA1 levels in culture supernates from sinR-, inhA1-, and calY-null mutants showed that the concentration of InhA1 in the culture supernatant is inversely proportional to the concentration of camelysin. Our data are consistent with a model in which InhA1 protease levels are controlled at the transcriptional level by SinR and at the posttranslational level by camelysin.

InhA1, NprA, and HlyII as candidates for markers to differentiate pathogenic from nonpathogenic Bacillus cereus strains

Bacillus cereus is found in food, soil, and plants, and the ability to cause food-borne diseases and opportunistic infection presumably varies among strains. Therefore, measuring harmful toxin production, in addition to the detection of the bacterium itself, may be key for food and hospital safety purposes. All previous studies have focused on the main known virulence factors, cereulide, Hbl, Nhe, and CytK. We examined whether other virulence factors may be specific to pathogenic strains. InhA1, NprA, and HlyII have been described as possibly contributing to B. cereus pathogenicity. We report the prevalence and expression profiles of these three new virulence factor genes among 57 B. cereus strains isolated from various sources, including isolates associated with gastrointestinal and nongastrointestinal diseases. Using PCR, quantitative reverse transcriptase PCR, and virulence in vivo assays, we unraveled these factors as potential markers to differentiate pathogenic from nonpathogenic strains. We show that the hlyII gene is carried only by strains with a pathogenic potential and that the expression levels of inhA1 and nprA are higher in the pathogenic than in the nonpathogenic group of strains studied. These data deliver useful information about the pathogenicity of various B. cereus strains.

The InhA metalloproteases of Bacillus cereus contribute concomitantly to virulence

The virulence of Bacillus cereus requires that bacteria have the capacity to colonize their host, degrade specific tissues, and circumvent the host immune system. To study this aspect of pathogenesis, we focused on three metalloproteases, InhA1, InhA2, and InhA3, which share more than 66% identity. The expression of these metalloprotease genes was assessed by transcriptional fusions with a lacZ reporter gene. The expression profiles suggest a complementary time course of InhA production. Indeed, the genes are simultaneously expressed but are oppositely controlled during stationary phase. We constructed single and multiple inhA mutants and assessed the bacterial locations of the proteins as well as their individual or additive roles in macrophage escape and toxicity, antibacterial-peptide cleavage, and virulence. InhA1, a major component of the spore exosporium, is the only InhA metalloprotease involved in bacterial escape from macrophages. A mutant lacking inhA1, inhA2, and inhA3 shows a strong decrease in the level of virulence for insects. Taken together, these results show that the InhA metalloproteases of B. cereus are important virulence factors that may allow the bacteria to counteract the host immune system.

Metalloproteases and the degradome

Metalloproteases comprise a heterogeneous group of proteolytic enzymes whose main characteristic is the utilization of a metal ion to polarize a water molecule and perform hydrolytic reactions. These enzymes represent the most densely populated catalytic class of proteases in many organisms and play essential roles in multiple biological processes. In this chapter, we will first present a general description of the complexity of metalloproteases in the context of the degradome, which is defined as the complete set of protease genes encoded by the genome of a certain organism. We will also discuss the functional relevance of these enzymes in a large variety of biological and pathological conditions. Finally, we will analyze in more detail three families of metalloproteases: ADAMs (a disintegrin and metalloproteinase), ADAMTSs (ADAMs with thrombospondin domains), and MMPs (matrix metalloproteinases) which have a growing relevance in a number of human pathologies including cancer, arthritis, neurodegenerative disorders, and cardiovascular diseases.

The dlt operon of Bacillus cereus is required for resistance to cationic antimicrobial peptides and for virulence in insects

The dlt operon encodes proteins that alanylate teichoic acids, the major components of cell walls of gram-positive bacteria. This generates a net positive charge on bacterial cell walls, repulsing positively charged molecules and conferring resistance to animal and human cationic antimicrobial peptides (AMPs) in gram-positive pathogenic bacteria. AMPs damage the bacterial membrane and are the most effective components of the humoral immune response against bacteria. We investigated the role of the dlt operon in insect virulence by inactivating this operon in Bacillus cereus, which is both an opportunistic human pathogen and an insect pathogen. The Delta dlt(Bc) mutant displayed several morphological alterations but grew at a rate similar to that for the wild-type strain. This mutant was less resistant to protamine and several bacterial cationic AMPs, such as nisin, polymyxin B, and colistin, in vitro. It was also less resistant to molecules from the insect humoral immune system, lysozyme, and cationic AMP cecropin B from Spodoptera frugiperda. Delta dlt(Bc) was as pathogenic as the wild-type strain in oral infections of Galleria mellonella but much less virulent when injected into the hemocoels of G. mellonella and Spodoptera littoralis. We detected the dlt operon in three gram-negative genera: Erwinia (Erwinia carotovora), Bordetella (Bordetella pertussis, Bordetella parapertussis, and Bordetella bronchiseptica), and Photorhabdus (the entomopathogenic bacterium Photorhabdus luminescens TT01, the dlt operon of which did not restore cationic AMP resistance in Delta dlt(Bc)). We suggest that the dlt operon protects B. cereus against insect humoral immune mediators, including hemolymph cationic AMPs, and may be critical for the establishment of lethal septicemia in insects and in nosocomial infections in humans.

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.

NMR structure of AbhN and comparison with AbrBN: FIRST insights into the DNA binding promiscuity and specificity of AbrB-like transition state regulator proteins

Understanding the molecular mechanisms of transition state regulator proteins is critical, since they play a pivotal role in the ability of bacteria to cope with changing environments. Although much effort has focused on their genetic characterization, little is known about their structural and functional conservation. Here we present the high resolution NMR solution structure of the N-terminal domain of the Bacillus subtilis transition state regulator Abh (AbhN), only the second such structure to date. We then compare AbhN to the N-terminal DNA-binding domain of B. subtilis AbrB (AbrBN). This is the first such comparison between two AbrB-like transition state regulators. AbhN and AbrBN are very similar, suggesting a common structural basis for their DNA binding. However, we also note subtle variances between the AbhN and AbrBN structures, which may play important roles in DNA target specificity. The results of accompanying in vitro DNA-binding studies serve to highlight binding differences between the two proteins.

The InhA1 metalloprotease allows spores of the B. cereus group to escape macrophages

Bacteria of the Bacillus cereus group are resistant to the immune systems of various hosts and establish potent infections, implying that bacteria circumvent the bactericidal activity of host phagocytic cells. We investigated the fate of Bacillus spores after their internalization by macrophages. We found that these spores survive and escape from macrophages, and that the bacterial metalloprotease InhA1, the major component of the exosporium, is essential for efficient spore release from macrophages. InhA1 from Bacillus thuringiensis also enables Bacillus subtilis to escape from macrophages. Analysis of membrane permeability showed that the bacteria cause alterations in the macrophage membranes and that InhA1 is involved in these processes. Thus, InhA1 contributes to protect the bacteria against the host immune system. These findings provide further insight into the pathogenicity of B. cereus group members.

A comparative study ofBacillus cereus,Bacillus thuringiensis andBacillus anthracis extracellular proteomes

Bacillus cereus, Bacillus thuringiensis and Bacillus anthracis are closely related species that share a similar genetic background but occupy different ecological niches. Virulence plasmids bearing genes coding for toxins, may explain, at least partly, this specialization. We have compared by 2-DE in the early stationary phase of growth the extracellular proteomes of three strains of these species that have lost their virulence plasmids. Proteins expected to be secreted or to belong to the cell wall or to the cytosol were found in the three proteomes. For the cell wall and cytosolic proteins located in the extracellular space, the three proteomes were similar. Cytosolic proteins included enolase, GroEL, PdhB, PdhD, SodA and others. Cell surface proteins were mainly autolysins, proteases, nucleotidases and OppAs. In contrast, the secreted proteins profiles of B. cereus and B. thuringiensis were quite different from that of B. anthracis. B. cereus and B. thuringiensis extracellular proteomes both contained large amounts of secreted degradative enzymes and toxins, including nine proteases, three phospholipases, two haemolysins and several enterotoxins. Most of the genes encoding these enzymes and toxins are controlled by the transcriptional activator PlcR. The extracellular proteome of the pXO1-, pXO2- B. anthracis 9131 strain contained only one secreted protein: the metalloprotease InhA1, also found in the proteomes of the two other strains and possibly involved in antibacterial peptide degradation.

Proteomics reveals that proteins expressed during the early stage of Bacillus anthracis infection are potential targets for the development of vaccines and drugs

In this review, we advance a new concept in developing vaccines and/or drugs to target specific proteins expressed during the early stage of Bacillus anthracis (anthrax) infection and address existing challenges to this concept. Three proteins (immune inhibitor A, GPR-like spore protease, and alanine racemase) initially identified by proteomics in our laboratory were found to have differential expressions during anthrax spore germination and early outgrowth. Other studies of different bacillus strains indicate that these three proteins are involved in either germination or cytotoxicity of spores, suggesting that they may serve as potential targets for the design of anti-anthrax vaccines and drugs.

Plasmid-encoded regulator of extracellular proteases in Bacillus anthracis

Bacillus anthracis Sterne cured of the pXO1 plasmid had enhanced secreted protease activity during the postexponential phase but no change in hemolytic or lecithinase activities. A zymogen profile revealed at least six proteases, including serine, metal, and perhaps cysteine types. There were similar amounts of protease secreted by the closely related species Bacillus cereus and Bacillus thuringiensis, but the patterns differed. Among the pXO1 plasmid-encoded proteins, there is a tetratricopeptide protein designated Cot43 that is related to the Rap proteins of Bacillus subtilis and the PlcR pleiotropic regulator of secreted enzymes and toxins in B. thuringiensis. A disruption of the cot43 gene resulted in overproduction of several proteases to a somewhat greater extent than in the plasmid-cured strain. Transformation of either of these strains with a clone of the cot43 gene resulted in the inhibition of accumulation of some of the proteases and induction of at least one. On the basis of lacZ fusions, transcription of the cot43 gene increased in late exponential cells at the time of protease accumulation. The expression of lacZ fusions to the upstream regions of two B. anthracis extracellular protease genes was greater in the strain with the disruption of cot43 than in the Sterne strain, indicating regulation at the level of transcription. In B. anthracis, a pXO1 plasmid-encoded protein directly modulates or indirectly regulates the transcription of genes for several chromosomally encoded extracellular proteases.

Characterization of two Bacillus thuringiensis genes identified by in vivo screening of virulence factors

Bacillus thuringiensis vegetative cells are known to be highly pathogenic when injected into the hemocoel of susceptible insect larvae. This pathogenicity is due to the capacity of B. thuringiensis to cause septicemia in the host. We screened a B. thuringiensis mini-Tn10 insertion library for loss of virulence against Bombyx mori larvae on injection into the hemocoel. Three clones with attenuated virulence were isolated, corresponding to two different mini-Tn10 insertions mapping to the yqgB/yqfZ locus. Single disruptions of the yqgB and yqfZ genes did not affect virulence against B. mori. In contrast, the inactivation of both genes simultaneously reproduced the effect of the mini-Tn10 insertion and resulted in a significant delay to infection. The double DeltayqgB DeltayqfZ mutant was also nonmotile, and its growth was affected at 25 degrees C. We analyzed lacZ transcriptional fusions and detected promoter activity upstream from yqgB at 25 and 37 degrees C. Overall, our findings suggest that the yqgB and yqfZ genes encode adaptive factors that may act in synergy, enabling the bacteria to cope with the physical environment in vivo, facilitating colonization of the host.

Micropatterns of Spores Displaying Heterologous Proteins

Micropatterns of Bacillus thuringiensis spores were generated by a combination of surface chemistry, microcontact printing, and spore surface display technique. The outermost layers of B. thuringiensis spores were engineered to present enhanced green fluorescent protein (EGFP), and the biospecific interaction between biotin and streptavidin was utilized to spatially direct the EGFP-presenting spores onto the micropatterned surfaces. The viability of the micropatterned spores was confirmed by the pattern generation of vegetative cells after the germination.

Evidence for multiple levels of regulation of Oenococcus oeni clpP-clpL locus expression in response to stress

A locus containing the clpP and clpL genes in the lactic acid bacterium Oenococcus oeni was studied. Real-time reverse transcription-PCR analysis revealed different induction factors involved in expression of these genes during stress. According to the conditions, clpP and clpL genes could be transcripted as two distinct transcripts or cotranscripted. The clpP promoter depended on the CtsR regulator, but surprisingly the clpL promoter did not. The amount of the clpL transcript depended on mRNA stability. This clp ATPase gene is at least controlled at the posttranscriptional level.

Structural aspects of the metzincin clan of metalloendopeptidases

Metalloendopeptidases are present across all kingdoms of living organisms; they are ubiquitous and widely involved in metabolism regulation through their ability either to extensively degrade proteins or to selectively hydrolyze specific peptide bonds. They must be subjected to exquisite spatial and temporal control to prevent this vast potential from becoming destructive. These enzymes are mostly zinc-dependent and the majority of them, named zincins, possess a short consensus sequence, HEXXH, with the two histidines acting as ligands of the catalytic zinc and the glutamate as the general base. A subclass of the zincins is characterized by a C-terminally elongated motif, HEXXHXXGXXH/D, with an additional strictly conserved glycine and a third zinc-binding histidine or aspartate. Currently, representative three-dimensional structures of six different proteinase families bearing this motif show, despite low sequence similarity, comparable overall topology. This includes a substrate-binding crevice, which subdivides the enzyme moiety into an upper and a lower subdomain. A common five-stranded beta-sheet and two alpha-helices are always found in the upper subdomain. The second of these helices encompasses the first half of the elongated consensus sequence and is therefore termed the active-site helix. Other shared characteristics are an invariant methionine-containing Met-turn beneath the catalytic metal and a further C-terminal helix in the lower subdomain. All these structural features identify the metzincin clan of metalloendopeptidases. This clan is reviewed from a structural point of view, based on the reported structures of representative members of the astacins, adamalysins, serralysins, matrixins, snapalysins, and leishmanolysins, and of inhibited forms, either by specific endogenous protein inhibitors or by zymogenic pro-domains. Moreover, newly available genomic sequences have unveiled novel putative metzincin families and new hypothetical members of existing ones.

The Bacillus thuringiensis PlcR-regulated gene inhA2 is necessary, but not sufficient, for virulence

We previously reported that Bacillus thuringiensis strain 407 Cry 32(-) secretes a zinc-requiring metalloprotease, InhA2, that is essential for virulence in orally infected insects. Analysis of the inhA2-lacZ transcriptional fusion showed that inhA2 expression is repressed in a PlcR(-) background. Using DNase I footprinting experiments, we demonstrated that PlcR activates inhA2 transcription directly by binding to a DNA sequence showing a one-residue mismatch with the previously reported PlcR box. It was previously reported that PlcR is essential for B. thuringiensis virulence in oral infection by contributing to the synergistic properties of the spores on the insecticidal activity of the Cry1C protein. We used complementation experiments to investigate whether the PlcR(-) phenotype was due to the absence of InhA2. The results indicated that overexpression of inhA2 in the (Delta)plcR strain did not restore the wild-type phenotype. However, virulence was fully restored in the (Delta)inhA2 complemented mutant. Thus, inhA2 is the first example of a PlcR-regulated gene found to be directly involved in virulence. However, it is not sufficient for pathogenicity when the other members of the PlcR regulon are lacking. This suggests that InhA2 may act in concert with other PlcR-regulated gene products to provide virulence.

Two-dimensional electrophoresis analysis of the extracellular proteome of Bacillus cereus reveals the importance of the PlcR regulon

Many virulence factors are secreted by the gram-positive, spore forming bacterium Bacillus cereus. Most of them are regulated by the transcriptional activator, PlcR, which is maximally expressed at the beginning of the stationary phase. We used a proteomic approach to study the impact of the PlcR regulon on the secreted proteins of B. cereus, by comparing the extracellular proteomes of strains ATCC 14579 and ATCC 14579 Delta plcR, in which plcR has been disrupted. Our study indicated that, quantitatively, most of the proteins secreted at the onset of the stationary phase are putative virulence factors, all of which are regulated, directly or indirectly, by PlcR. The inactivation of plcR abolished the secretion of some of these virulence factors, and strongly decreased that of others. The genes encoding proteins that are not secreted in the DeltaplcR mutant possessed a regulatory sequence, the PlcR box, upstream from their coding sequence. These proteins include collagenase, phospholipases, haemolysins, proteases and enterotoxins. Proteins for which the secretion was strongly decreased, but not abolished, in the DeltaplcR mutant did not display the PlcR box upstream from their genes. These proteins include flagellins and InhA2. InhA2 is a homologue of InhA, a Bacillus thuringiensis metalloprotease that specifically degrades antibacterial peptides. The mechanism by which PlcR affects the production of flagellins and InhA2 is not known.

The InhA2 metalloprotease of Bacillus thuringiensis strain 407 is required for pathogenicity in insects infected via the oral route

The entomopathogenic bacterium Bacillus thuringiensis is known to secrete a zinc metalloprotease (InhA) that specifically cleaves antibacterial peptides produced by insect hosts. We identified a second copy of the inhA gene, named inhA2, in B. thuringiensis strain 407 Cry(-). The inhA2 gene encodes a putative polypeptide showing 66.2% overall identity with the InhA protein and harboring the zinc-binding domain (HEXXH), which is characteristic of the zinc-requiring metalloproteases. We used a transcriptional inhA2'-lacZ fusion to show that inhA2 expression is induced at the onset of the stationary phase and is overexpressed in a Spo0A minus background. The presence of a reverse Spo0A box in the promoter region of inhA2 suggests that Spo0A directly regulates the transcription of inhA2. To determine the role of the InhA and InhA2 metalloproteases in pathogenesis, we used allelic exchange to isolate single and double mutant strains for the two genes. Spores and vegetative cells of the mutant strains were as virulent as those of the parental strain in immunized Bombyx mori larvae infected by the intrahemocoelic route. Exponential phase cells of all the strains displayed the same in vitro potential for colonizing the vaccinated hemocoel. We investigated the synergistic effect of the mutant strain spores on the toxicity of Cry1C proteins against Galleria mellonella larvae infected via the oral pathway. The spores of DeltainhA2 mutant strain were ineffective in providing synergism whereas those of the DeltainhA mutant strain were not. These results indicate that the B. thuringiensis InhA2 zinc metalloprotease has a vital role in virulence when the host is infected via the oral route.