Virulence gene expression is independent of ResDE-regulated respiration control in Bacillus anthracis

Adaptation in Bacillus cereus: From Stress to Disease

Bacillus cereus is a food-borne pathogen that causes diarrheal disease in humans. After ingestion, B. cereus experiences in the human gastro-intestinal tract abiotic physical variables encountered in food, such as acidic pH in the stomach and changing oxygen conditions in the human intestine. B. cereus responds to environmental changing conditions (stress) by reversibly adjusting its physiology to maximize resource utilization while maintaining structural and genetic integrity by repairing and minimizing damage to cellular infrastructure. As reviewed in this article, B. cereus adapts to acidic pH and changing oxygen conditions through diverse regulatory mechanisms and then exploits its metabolic flexibility to grow and produce enterotoxins. We then focus on the intricate link between metabolism, redox homeostasis, and enterotoxins, which are recognized as important contributors of food-borne disease.

Loss of σI affects heat-shock response and virulence gene expression in Bacillus anthracis

The pathogenesis of Bacillus anthracis depends on several virulence factors, including the anthrax toxin. Loss of the alternative sigma factor σI results in a coordinate decrease in expression of all three toxin subunits. Our observations suggest that loss of σI alters the activity of the master virulence regulator AtxA, but atxA transcription is unaffected by loss of σI. σI-containing RNA polymerase does not appear to directly transcribe either atxA or the toxin gene pagA. As in Bacillus subtilis, loss of σI in B. anthracis results in increased sensitivity to heat shock and transcription of sigI, encoding σI, is induced by elevated temperature. Encoded immediately downstream of and part of a bicistronic message with sigI is an anti-sigma factor, RsgI, which controls σI activity. Loss of RsgI has no direct effect on virulence gene expression. sigI appears to be expressed from both the σI and σA promoters, and transcription from the σA promoter is likely more significant to virulence regulation. We propose a model in which σI can be induced in response to heat shock, whilst, independently, σI is produced under non-heat-shock, toxin-inducing conditions to indirectly regulate virulence gene expression.

Loss of Homogentisate 1,2-Dioxygenase Activity in Bacillus anthracis Results in Accumulation of Protective Pigment

Melanin production is important to the pathogenicity and survival of some bacterial pathogens. In Bacillus anthracis, loss of hmgA, encoding homogentisate 1,2-dioxygenase, results in accumulation of a melanin-like pigment called pyomelanin. Pyomelanin is produced in the mutant as a byproduct of disrupted catabolism of L-tyrosine and L-phenylalanine. Accumulation of pyomelanin protects B. anthracis cells from UV damage but not from oxidative damage. Neither loss of hmgA nor accumulation of pyomelanin alter virulence gene expression, sporulation or germination. This is the first investigation of homogentisate 1,2-dioxygenase activity in the Gram-positive bacteria, and these results provide insight into a conserved aspect of bacterial physiology.

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

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

The two CcdA proteins of Bacillus anthracis differentially affect virulence gene expression and sporulation

The cytochrome c maturation system influences the expression of virulence factors in Bacillus anthracis. B. anthracis carries two copies of the ccdA gene, encoding predicted thiol-disulfide oxidoreductases that contribute to cytochrome c maturation, while the closely related organism Bacillus subtilis carries only one copy of ccdA. To investigate the roles of the two ccdA gene copies in B. anthracis, strains were constructed without each ccdA gene, and one strain was constructed without both copies simultaneously. Loss of both ccdA genes results in a reduction of cytochrome c production, an increase in virulence factor expression, and a reduction in sporulation efficiency. Complementation and expression analyses indicate that ccdA2 encodes the primary CcdA in B. anthracis, active in all three pathways. While CcdA1 retains activity in cytochrome c maturation and virulence control, it has completely lost its activity in the sporulation pathway. In support of this finding, expression of ccdA1 is strongly reduced when cells are grown under sporulation-inducing conditions. When the activities of CcdA1 and CcdA2 were analyzed in B. subtilis, neither protein retained activity in cytochrome c maturation, but CcdA2 could still function in sporulation. These observations reveal the complexities of thiol-disulfide oxidoreductase function in pathways relevant to virulence and physiology.

From transcriptional landscapes to the identification of biomarkers for robustness

The ability of microorganisms to adapt to changing environments and gain cell robustness, challenges the prediction of their history-dependent behaviour. Using our model organism Bacillus cereus, a notorious Gram-positive food spoilage and pathogenic spore-forming bacterium, a strategy will be described that allows for identification of biomarkers for robustness. First an overview will be presented of its two-component systems that generally include a transmembrane sensor histidine kinase and its cognate response regulator, allowing rapid and robust responses to fluctuations in the environment. The role of the multisensor hybrid kinase RsbK and the PP2C-type phosphatase RsbY system in activation of the general stress sigma factor σB is highlighted. An extensive comparative analysis of transcriptional landscapes derived from B. cereus exposed to mild stress conditions such as heat, acid, salt and oxidative stress, revealed that, amongst others σB regulated genes were induced in most conditions tested. The information derived from the transcriptome data was subsequently implemented in a framework for identifying and selecting cellular biomarkers at their mRNA, protein and/or activity level, for mild stressinduced microbial robustness towards lethal stresses. Exposure of unstressed and mild stress-adapted cells to subsequent lethal stress conditions (heat, acid and oxidative stress) allowed for quantification of the robustness advantage provided by mild stress pretreatment using the plate-count method. The induction levels of the selected candidate-biomarkers, σB protein, catalase activity and transcripts of certain proteases upon mild stress treatment, were significantly correlated to mild stress-induced enhanced robustness towards lethal thermal, oxidative and acid stresses, and were therefore suitable to predict these adaptive traits. Cellular biomarkers that are quantitatively correlated to adaptive behavior will facilitate our ability to predict the impact of adaptive behavior on cell robustness and will allow to control and/or exploit these adaptive traits. Extrapolation to other species and genera is discussed such as avenues towards mechanism-based design of microbial fitness and robustness.

Autoregulatory characteristics of a Bacillus anthracis serine/threonine kinase

BA-Stk1 is a serine/threonine kinase (STK) expressed by Bacillus anthracis. In previous studies, we found that BA-Stk1 activity is modulated through dephosphorylation by a partner phosphatase, BA-Stp1. In this study, we identified critical phosphorylation regions of BA-Stk1 and determined the contributions of these phosphodomains to autophosphorylation and substrate phosphorylation. The data indicate that BA-Stk1 undergoes trans-autophosphorylation within a regulatory domain, referred to as the activation loop, which carries eight putative regulatory serine and threonine residues. We identified activation loop mutants that impacted kinase activity in three different manners: regulation of autophosphorylation (T162), regulation of substrate phosphorylation (T159 and S169), and regulation of overall kinase activity (T163). Tandem mass spectrometry (MS/MS) analysis of the phosphorylation profile of each mutant revealed a second site of phosphorylation on the kinase that was influenced by the phosphorylation status of the activation loop. This second region of the kinase contained a single phosphorylation residue, S214. Previous work has shown S214 to be necessary for downstream substrate phosphorylation, and we have shown that this residue is subject to dephosphorylation by BA-Stp1. These findings indicate a connection between the phosphorylation status of the activation loop and phosphorylation of S214, and this suggests a previously undescribed model for how a bacterial STK shifts from a state of autophosphorylation to targeting downstream substrates.

At the crossroads of bacterial metabolism and virulence factor synthesis in Staphylococci

Bacteria live in environments that are subject to rapid changes in the availability of the nutrients that are necessary to provide energy and biosynthetic intermediates for the synthesis of macromolecules. Consequently, bacterial survival depends on the ability of bacteria to regulate the expression of genes coding for enzymes required for growth in the altered environment. In pathogenic bacteria, adaptation to an altered environment often includes activating the transcription of virulence genes; hence, many virulence genes are regulated by environmental and nutritional signals. Consistent with this observation, the regulation of most, if not all, virulence determinants in staphylococci is mediated by environmental and nutritional signals. Some of these external signals can be directly transduced into a regulatory response by two-component regulators such as SrrAB; however, other external signals require transduction into intracellular signals. Many of the external environmental and nutritional signals that regulate virulence determinant expression can also alter bacterial metabolic status (e.g., iron limitation). Altering the metabolic status results in the transduction of external signals into intracellular metabolic signals that can be "sensed" by regulatory proteins (e.g., CodY, Rex, and GlnR). This review uses information derived primarily using Bacillus subtilis and Escherichia coli to articulate how gram-positive pathogens, with emphasis on Staphylococcus aureus and Staphylococcus epidermidis, regulate virulence determinant expression in response to a changing environment.

ResDE-dependent regulation of enterotoxin gene expression in Bacillus cereus: evidence for multiple modes of binding for ResD and interaction with Fnr

In the food-borne pathogen Bacillus cereus F4430/73, the production of major virulence factors hemolysin BL (Hbl) and nonhemolytic enterotoxin (Nhe) is regulated through complex mechanisms. The two-component regulatory system ResDE is involved in the activation of hbl and nhe transcription. Here, the response regulator ResD and the sensor kinase ResE were overexpressed and purified, and autophosphorylation of ResE and transphosphorylation of ResD by ResE were demonstrated in vitro. ResD is mainly monomeric in solution, regardless of its phosphorylation state. ResD was shown to interact directly with promoter regions (p) of the enterotoxin regulator genes resDE, fnr, and plcR and the enterotoxin structural genes nhe and hbl, but with different affinities. Binding of ResD to pplcR, pnhe, and phbl was not dependent on the ResD phosphorylation status. In contrast, ResD phosphorylation significantly increased interactions between ResD and presDE and pfnr. Taken together, these results showed that phosphorylation of ResD results in a different target expression pattern. Furthermore, ResD and the redox activator Fnr were found to physically interact and simultaneously bind their target DNAs. We propose that unphosphorylated ResD acts as an antiactivator of Fnr, while phosphorylated ResD acts as a coactivator of Fnr. Finally, our findings represent the first molecular evidence of the role of ResDE as a sentinel system capable of sensing redox changes and coordinating a response that modulates B. cereus virulence.

Two small c-type cytochromes affect virulence gene expression in Bacillus anthracis

Regulated expression of the genes for anthrax toxin proteins is essential for the virulence of the pathogenic bacterium Bacillus anthracis. Induction of toxin gene expression depends on several factors, including temperature, bicarbonate levels, and metabolic state of the cell. To identify factors that regulate toxin expression, transposon mutagenesis was performed under non-inducing conditions and mutants were isolated that untimely expressed high levels of toxin. A number of these mutations clustered in the haem biosynthetic and cytochrome c maturation pathways. Genetic analysis revealed that two haem-dependent, small c-type cytochromes, CccA and CccB, located on the extracellular surface of the cytoplasmic membrane, regulate toxin gene expression by affecting the expression of the master virulence regulator AtxA. Deregulated AtxA expression in early exponential phase resulted in increased expression of toxin genes in response to loss of the CccA-CccB signalling pathway. This is the first function identified for these two small c-type cytochromes of Bacillus species. Extension of the transposon screen identified a previously uncharacterized protein, BAS3568, highly conserved across many bacterial and archeal species, as involved in cytochrome c activity and virulence regulation. These findings are significant not only to virulence regulation in B. anthracis, but also to analysis of virulence regulation in many pathogenic bacteria and to the study of cytochrome c activity in Gram-positive bacteria.

The bicarbonate transporter is essential for Bacillus anthracis lethality

In the pathogenic bacterium Bacillus anthracis, virulence requires induced expression of the anthrax toxin and capsule genes. Elevated CO2/bicarbonate levels, an indicator of the host environment, provide a signal ex vivo to increase expression of virulence factors, but the mechanism underlying induction and its relevance in vivo are unknown. We identified a previously uncharacterized ABC transporter (BAS2714-12) similar to bicarbonate transporters in photosynthetic cyanobacteria, which is essential to the bicarbonate induction of virulence gene expression. Deletion of the genes for the transporter abolished induction of toxin gene expression and strongly decreased the rate of bicarbonate uptake ex vivo, demonstrating that the BAS2714-12 locus encodes a bicarbonate ABC transporter. The bicarbonate transporter deletion strain was avirulent in the A/J mouse model of infection. Carbonic anhydrase inhibitors, which prevent the interconversion of CO2 and bicarbonate, significantly affected toxin expression only in the absence of bicarbonate or the bicarbonate transporter, suggesting that carbonic anhydrase activity is not essential to virulence factor induction and that bicarbonate, and not CO2, is the signal essential for virulence induction. The identification of this novel bicarbonate transporter essential to virulence of B. anthracis may be of relevance to other pathogens, such as Streptococcus pyogenes, Escherichia coli, Borrelia burgdorferi, and Vibrio cholera that regulate virulence factor expression in response to CO2/bicarbonate, and suggests it may be a target for antibacterial intervention.