plcR papR-independent expression of anthrolysin O by Bacillus anthracis

Direct Regulons of AtxA, the Master Virulence Regulator of Bacillus anthracis

AtxA, the master virulence regulator of Bacillus anthracis, regulates the expression of three toxins and genes for capsule formation that are required for the pathogenicity of B. anthracis. Recent transcriptome analyses showed that AtxA affects a large number of genes on the chromosome and plasmids, suggesting a role as a global regulator. However, information on genes directly regulated by AtxA is scarce. In this work, we conducted genome-wide analyses and cataloged the binding sites of AtxA in vivo and transcription start sites on the B. anthracis genome. By integrating these results, we detected eight genes as direct regulons of AtxA. These consisted of five protein-coding genes, including two of the three toxin genes, and three genes encoding the small RNAs XrrA and XrrB and a newly discovered 95-nucleotide small RNA, XrrC. Transcriptomes from single-knockout mutants of these small RNAs revealed changes in the transcription levels of genes related to the aerobic electron transport chain, heme biosynthesis, and amino acid metabolism, suggesting their function for the control of cell physiology. These results reveal the first layer of the gene regulatory network for the pathogenicity of B. anthracis and provide a data set for the further study of the genomics and genetics of B. anthracis. IMPORTANCE Bacillus anthracis is the Gram-positive bacterial species that causes anthrax. Anthrax is still prevalent in countries mainly in Asia and Africa, where it causes economic damage and remains a public health issue. The mechanism of pathogenicity is mainly explained by the three toxin proteins expressed from the pXO1 plasmid and by proteins involved in capsule formation expressed from the pXO2 plasmid. AtxA is a protein expressed from the pXO1 plasmid that is known to upregulate genes involved in toxin production and capsule formation and is thus considered the master virulence regulator of B. anthracis. Therefore, understanding the detailed mechanism of gene regulation is important for the control of anthrax. The significance of this work lies in the identification of genes that are directly regulated by AtxA via genome-wide analyses. The results reveal the first layer of the gene regulatory network for the pathogenicity of B. anthracis and provide useful resources for a further understanding of B. anthracis.

The Streptococcus pyogenes signaling peptide SpoV regulates streptolysin O and enhances survival in murine blood

Streptococcus pyogenes (Group A Streptococcus, GAS) is a human pathogen that causes a wide range of diseases. For successful colonization within a variety of host niches, GAS must sense and respond to environmental changes. Intercellular communication mediated by peptides is one way GAS coordinates gene expression in response to diverse environmental stressors, which enhances bacterial survival and contributes to virulence. Using peptidomics we identified SpoV (Streptococcal peptide controlling virulence) in culture supernatant fluids. SpoV is a secreted peptide encoded near the gene encoding the extracellular cholesterol-dependent cytolysin streptolysin O (slo) The addition of synthetic SpoV peptide derivatives, but not control peptides, increased slo transcript abundance in an M49 isolate but not in an M3 isolate. Deletion of spoV decreased slo transcript abundance, extracellular SLO protein levels, and SLO-specific hemolytic activity. Complementation of the spoV mutant increased slo transcript abundance. Lastly, a spoV mutant was deficient in the ability to survive in murine blood compared to the parental strain. Moreover, pre-incubation of the spoV mutant with synthetic SpoV peptide derivatives increased GAS survival. Our findings show that slo expression is regulated, in part, by the GAS-specific signaling peptide SpoV.IMPORTANCEGAS secretes signaling peptides that can alter gene expression and impact virulence. We used peptidomics to identify a signaling peptide designated SpoV. Further, we showed that SpoV altered the expression of the cholesterol-dependent cytolysin SLO. Peptide signaling plays an important regulatory role during disease progression among several bacterial pathogens, including GAS. The therapeutic potential of manipulating peptide-controlled regulatory networks is an attractive option for the development of novel therapeutic strategies that disrupt virulence gene expression.

PlcRa, a new quorum-sensing regulator from Bacillus cereus, plays a role in oxidative stress responses and cysteine metabolism in stationary phase

We characterized a new quorum-sensing regulator, PlcRa, which is present in various members of the B. cereus group and identified a signaling heptapeptide for PlcRa activity: PapRa₇. We demonstrated that PlcRa is a 3D structural paralog of PlcR using sequence analysis and homology modeling. A comparison of the transcriptomes at the onset of stationary phase of a ΔplcRa mutant and the wild-type B. cereus ATCC 14579 strain showed that 68 genes were upregulated and 49 genes were downregulated in the ΔplcRa mutant strain (>3-fold change). Genes involved in the cysteine metabolism (putative CymR regulon) were downregulated in the ΔplcRa mutant strain. We focused on the gene with the largest difference in expression level between the two conditions, which encoded -AbrB2- a new regulator of the AbrB family. We demonstrated that purified PlcRa bound specifically to the abrB2 promoter in the presence of synthetic PapRa₇, in an electrophoretic mobility shift assay. We further showed that the AbrB2 regulator controlled the expression of the yrrT operon involved in methionine to cysteine conversion. We found that the ΔplcRa mutant strain was more sensitive to hydrogen peroxide- and disulfide-induced stresses than the wild type. When cystine was added to the culture of the ΔplcRa mutant, challenged with hydrogen peroxide, growth inhibition was abolished. In conclusion, we identified a new RNPP transcriptional regulator in B. cereus that activated the oxidative stress response and cysteine metabolism in transition state cells.

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.

The role of anthrolysin O in gut epithelial barrier disruption during Bacillus anthracis infection

Gastrointestinal (GI) anthrax, caused by the bacterial infection of Bacillus anthracis, posts a significant bioterrorism threat by its relatively high mortality rate in humans. Different from inhalational anthrax by the route of infection, accumulating evidence indicates the bypass of vegetative bacteria across GI epithelium is required to initiate GI anthrax. Previously, we reported that purified anthrolysin O (ALO), instead of tripartite anthrax edema and lethal toxins, is capable of disrupting gut epithelial tight junctions and barrier function in cultured cells. Here, we show that ALO can disrupt intestinal tissue barrier function in an ex vivo mouse model. To explore the effects of ALO in a cell culture model of B. anthracis infection, we showed that anthrax bacteria can effectively reduce the monolayer integrity of human Caco-2 brush-border expressor (C2BBE) cells based on the reduced transepithelial resistance and the increased leakage of fluorescent dye. This disruption is likely caused by tight junction dysfunction observed by the reorganization of the tight junction protein occludin. Consequently, we observe significant passage of vegetative anthrax bacteria across C2BBE cells. This barrier disruption and bacterial crossover requires ALO since ALO-deficient B. anthracis strains fail to induce monolayer dysfunction and allow the passage of anthrax bacteria. Together these findings point to a pivotal role for ALO within the establishment of GI anthrax infection and the initial bypass of the epithelial barrier.

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.

Novel and unique diagnostic biomarkers for Bacillus anthracis infection

A search for bacterium-specific biomarkers in peripheral blood following infection with Bacillus anthracis was carried out with rabbits, using a battery of specific antibodies generated by DNA vaccination against 10 preselected highly immunogenic bacterial antigens which were identified previously by a genomic/proteomic/serologic screen of the B. anthracis secretome. Detection of infection biomarkers in the circulation of infected rabbits could be achieved only after removal of highly abundant serum proteins by chromatography using a random-ligand affinity column. Besides the toxin component protective antigen, the following three secreted proteins were detected in the circulation of infected animals: the chaperone and protease HtrA (BA3660), an NlpC/P60 endopeptidase (BA1952), and a protein of unknown function harboring two SH3 (Src homology 3) domains (BA0796). The three proteins could be detected in plasma samples from infected animals exhibiting 10(3) to 10(5) CFU/ml blood and also in standard blood cultures at 3 to 6 h post-bacterial inoculation at a bacteremic level as low as 10(3) CFU/ml. Furthermore, the three biomarkers appear to be present only in the secretome of B. anthracis, not in those of the related pathogens B. thuringiensis and B. cereus. To the best of our knowledge, this is the first report of direct detection of B. anthracis-specific proteins, other than the toxin components, in the circulation of infected animals.

An extracytoplasmic function sigma factor controls beta-lactamase gene expression in Bacillus anthracis and other Bacillus cereus group species

The susceptibility of most Bacillus anthracis strains to beta-lactam antibiotics is intriguing considering that the closely related species Bacillus cereus and Bacillus thuringiensis typically produce beta-lactamases and the B. anthracis genome harbors two beta-lactamase genes, bla1 and bla2. We show that beta-lactamase activity associated with B. anthracis is affected by two genes, sigP (BA2502) and rsiP (BA2503), predicted to encode an extracytoplasmic function sigma factor and an anti-sigma factor, respectively. Deletion of the sigP-rsiP locus abolished beta-lactamase activity in a naturally occurring penicillin-resistant strain and had no effect on beta-lactamase activity in a prototypical penicillin-susceptible strain. Complementation with sigP and rsiP from the penicillin-resistant strain, but not with sigP and rsiP from the penicillin-susceptible strain, conferred constitutive beta-lactamase activity in both mutants. These results are attributed to a nucleotide deletion near the 5' end of rsiP in the penicillin-resistant strain that is predicted to result in a nonfunctional protein. B. cereus and B. thuringiensis sigP and rsiP homologues are required for inducible penicillin resistance in these species. Expression of the B. cereus or B. thuringiensis sigP and rsiP genes in a B. anthracis sigP-rsiP-null mutant confers inducible production of beta-lactamase activity, suggesting that while B. anthracis contains the genes necessary for sensing beta-lactam antibiotics, the B. anthracis sigP and rsiP gene products are not sufficient for bla induction.

Cellular functions and X-ray structure of anthrolysin O, a cholesterol-dependent cytolysin secreted by Bacillus anthracis

Anthrolysin O (ALO) is a pore-forming, cholesterol-dependent cytolysin (CDC) secreted by Bacillus anthracis, the etiologic agent for anthrax. Growing evidence suggests the involvement of ALO in anthrax pathogenesis. Here, we show that the apical application of ALO decreases the barrier function of human polarized epithelial cells as well as increases intracellular calcium and the internalization of the tight junction protein occludin. Using pharmacological agents, we also found that barrier function disruption requires increased intracellular calcium and protein degradation. We also report a crystal structure of the soluble state of ALO. Based on our analytical ultracentrifugation and light scattering studies, ALO exists as a monomer. Our ALO structure provides the molecular basis as to how ALO is locked in a monomeric state, in contrast to other CDCs that undergo antiparallel dimerization or higher order oligomerization in solution. ALO has four domains and is globally similar to perfringolysin O (PFO) and intermedilysin (ILY), yet the highly conserved undecapeptide region in domain 4 (D4) adopts a completely different conformation in all three CDCs. Consistent with the differences within D4 and at the D2-D4 interface, we found that ALO D4 plays a key role in affecting the barrier function of C2BBE cells, whereas PFO domain 4 cannot substitute for this role. Novel structural elements and unique cellular functions of ALO revealed by our studies provide new insight into the molecular basis for the diverse nature of the CDC family.

Characterization of a small PlcR-regulated gene co-expressed with cereolysin O

Background

In the human pathogen Bacillus cereus, the expression of most extracellular virulence factors is controlled by the transcriptional activator PlcR. Among these virulence factors, cereolysin O (Clo) is an haemolysin belonging to the cholesterol-dependant cytolysins, a protein family extensively studied in Gram-positive bacteria.

Results

In the genomes of bacteria belonging to the B. cereus group, including Bacillus anthracis and Bacillus thuringiensis, a small gene encoding a 26-amino acid peptide was present in multicopy. One copy was always found upstream from the gene encoding Clo. In B. cereus ATCC 14579, the small gene and the clo gene are co-transcribed. Transcriptional fusions showed that the three paralogues identified in this strain were expressed in a PlcR-dependent manner. We propose to name these peptides Spp for small PlcR-regulated peptides. We show that a synthetic peptide corresponding to the deduced product of the spp genes displayed antibacterial activity.

Conclusion

The co-expression of spp, a small PlcR-regulated multicopy gene with clo suggests a yet unidentified relationship between Spp and the cholesterol-dependent cytolysin in bacteria belonging to the B.cereus group.

Identification of in vivo-expressed immunogenic proteins by serological proteome analysis of the Bacillus anthracis secretome

In a previous comparative proteomic study of Bacillus anthracis examining the influence of the virulence plasmids and of various growth conditions on the composition of the bacterial secretome, we identified 64 abundantly expressed proteins (T. Chitlaru, O. Gat, Y. Gozlan, N. Ariel, and A. Shafferman, J. Bacteriol. 188:3551-3571, 2006). Using a battery of sera from B. anthracis-infected animals, in the present study we demonstrated that 49 of these proteins are immunogenic. Thirty-eight B. anthracis immunogens are documented in this study for the first time. The relative immunogenicities of the 49 secreted proteins appear to span a >10,000-fold range. The proteins eliciting the highest humoral response in the course of infection include, in addition to the well-established immunogens protective antigen (PA), Sap, and EA1, GroEL (BA0267), AhpC (BA0345), MntA (BA3189), HtrA (BA3660), 2,3-cyclic nucleotide diesterase (BA4346), collagen adhesin (BAS5205), an alanine amidase (BA0898), and an endopeptidase (BA1952), as well as three proteins having unknown functions (BA0796, BA0799, and BA0307). Of these 14 highly potent secreted immunogens, 11 are known to be associated with virulence and pathogenicity in B. anthracis or in other bacterial pathogens. Combining the results reported here with the results of a similar study of the membranal proteome of B. anthracis (T. Chitlaru, N. Ariel, A. Zvi, M. Lion, B. Velan, A. Shafferman, and E. Elhanany, Proteomics 4:677-691, 2004) and the results obtained in a functional genomic search for immunogens (O. Gat, H. Grosfeld, N. Ariel, I. Inbar, G. Zaide, Y. Broder, A. Zvi, T. Chitlaru, Z. Altboum, D. Stein, S. Cohen, and A. Shafferman, Infect. Immun. 74:3987-4001, 2006), we generated a list of 84 in vivo-expressed immunogens for future evaluation for vaccine development, diagnostics, and/or therapeutic intervention. In a preliminary study, the efficacies of eight immunogens following DNA immunization of guinea pigs were compared to the efficacy of a PA DNA vaccine. All eight immunogens induced specific high antibody titers comparable to the titers elicited by PA; however, unlike PA, none of them provided protection against a lethal challenge (50 50% lethal doses) of virulent B. anthracis strain Vollum spores.