The three Bacillus anthracis toxin genes are coordinately regulated by bicarbonate and temperature

Quantifying in vitro B. anthracis growth and PA production and decay: a mathematical modelling approach

Protective antigen (PA) is a protein produced by Bacillus anthracis. It forms part of the anthrax toxin and is a key immunogen in US and UK anthrax vaccines. In this study, we have conducted experiments to quantify PA in the supernatants of cultures of B. anthracis Sterne strain, which is the strain used in the manufacture of the UK anthrax vaccine. Then, for the first time, we quantify PA production and degradation via mathematical modelling and Bayesian statistical techniques, making use of this new experimental data as well as two other independent published data sets. We propose a single mathematical model, in terms of delay differential equations (DDEs), which can explain the in vitro dynamics of all three data sets. Since we did not heat activate the B. anthracis spores prior to inoculation, germination occurred much slower in our experiments, allowing us to calibrate two additional parameters with respect to the other data sets. Our model is able to distinguish between natural PA decay and that triggered by bacteria via proteases. There is promising consistency between the different independent data sets for most of the parameter estimates. The quantitative characterisation of B. anthracis PA production and degradation obtained here will contribute towards the ambition to include a realistic description of toxin dynamics, the host immune response, and anti-toxin treatments in future mechanistic models of anthrax infection.

Bacillus anthracis, "la maladie du charbon", Toxins, and Institut Pasteur

Institut Pasteur and Bacillus anthracis have enjoyed a relationship lasting almost 120 years, starting from its foundation and the pioneering work of Louis Pasteur in the nascent fields of microbiology and vaccination, and blooming after 1986 following the molecular biology/genetic revolution. This contribution will give a historical overview of these two research eras, taking advantage of the archives conserved at Institut Pasteur. The first era mainly focused on the production, characterisation, surveillance and improvement of veterinary anthrax vaccines; the concepts and technologies with which to reach a deep understanding of this research field were not yet available. The second period saw a new era of B. anthracis research at Institut Pasteur, with the anthrax laboratory developing a multi-disciplinary approach, ranging from structural analysis, biochemistry, genetic expression, and regulation to bacterial-host cell interactions, in vivo pathogenicity, and therapy development; this led to the comprehensive unravelling of many facets of this toxi-infection. B. anthracis may exemplify some general points on how science is performed in a given society at a given time and how a scientific research domain evolves. A striking illustration can be seen in the additive layers of regulations that were implemented from the beginning of the 21st century and their impact on B. anthracis research. B. anthracis and anthrax are complex systems that raise many valuable questions regarding basic research. One may hope that B. anthracis research will be re-initiated under favourable circumstances later at Institut Pasteur.

Role of serine/threonine protein phosphatase PrpN in the life cycle of Bacillus anthracis

Reversible protein phosphorylation at serine/threonine residues is one of the most common protein modifications, widely observed in all kingdoms of life. The catalysts controlling this modification are specific serine/threonine kinases and phosphatases that modulate various cellular pathways ranging from growth to cellular death. Genome sequencing and various omics studies have led to the identification of numerous serine/threonine kinases and cognate phosphatases, yet the physiological relevance of many of these proteins remain enigmatic. In Bacillus anthracis, only one ser/thr phosphatase, PrpC, has been functionally characterized; it was reported to be non-essential for bacterial growth and survival. In the present study, we characterized another ser/thr phosphatase (PrpN) of B. anthracis by various structural and functional approaches. To examine its physiological relevance in B. anthracis, a null mutant strain of prpN was generated and shown to have defects in sporulation and reduced synthesis of toxins (PA and LF) and the toxin activator protein AtxA. We also identified CodY, a global transcriptional regulator, as a target of PrpN and ser/thr kinase PrkC. CodY phosphorylation strongly controlled its binding to the promoter region of atxA, as shown using phosphomimetic and phosphoablative mutants. In nutshell, the present study reports phosphorylation-mediated regulation of CodY activity in the context of anthrax toxin synthesis in B. anthracis by a previously uncharacterized ser/thr protein phosphatase–PrpN.

Reversible protein phosphorylation at specific ser/thr residues causes conformational changes in the protein structure, thereby modulating its cellular activity. In B. anthracis, though the role of ser/thr phosphorylation is implicated in various cellular pathways including pathogenesis, till date only one STP (PrpC) has been functionally characterized. This manuscript reports functional characterization of another STP (PrpN) in B. anthracis and with the aid of a null mutant strain (BAS ΔprpN) we provide important insight regarding the role of PrpN in the life cycle of B. anthracis. We have also identified the global transcriptional regulator, CodY as a target of PrpN and PrkC, and for the first time showed the physiological relevance of CodY phosphorylation status in the regulation of anthrax toxin synthesis.

Neurochemical and Neurobiological Weapons

Nerve agents and neurobiological weapons are among the most devastating and lethal of weapons. Acetylcholinesterase inhibitors act by increasing the amount of acetylcholine in the neuromuscular junction, resulting in flaccid paralysis. Tabun, VX, soman, and sarin are the major agents in this category. Exposure to nerve agents can be inhalational or through dermal contact. Neurotoxins may have peripheral and central effects on the nervous system. Atropine is an effective antidote to nerve agents. Neurobiological weapons entail using whole organisms or organism-synthesized toxins as agents. Some organisms that can be used as biological weapons include smallpox virus.

A Unique Isolation of a Lytic Bacteriophage Infected Bacillus anthracis Isolate from Pafuri, South Africa

Bacillus anthracis is a soil-borne, Gram-positive endospore-forming bacterium and the causative agent of anthrax. It is enzootic in Pafuri, Kruger National Park in South Africa. The bacterium is amplified in a wild ungulate host, which then becomes a source of infection to the next host upon its death. The exact mechanisms involving the onset (index case) and termination of an outbreak are poorly understood, in part due to a paucity of information about the soil-based component of the bacterium’s lifecycle. In this study, we present the unique isolation of a dsDNA bacteriophage from a wildebeest carcass site suspected of having succumbed to anthrax. The aggressively lytic bacteriophage hampered the initial isolation of B. anthracis from samples collected at the carcass site. Classic bacteriologic methods were used to test the isolated phage on B. anthracis under different conditions to simulate deteriorating carcass conditions. Whole genome sequencing was employed to determine the relationship between the bacterium isolated on site and the bacteriophage-dubbed Bacillus phage Crookii. The 154,012 bp phage belongs to Myoviridae and groups closely with another African anthrax carcass-associated Bacillus phage WPh. Bacillus phage Crookii was lytic against B. cereus sensu lato group members but demonstrated a greater affinity for encapsulated B. anthracis at lower concentrations (<1 × 10⁸ pfu) of bacteriophage. The unusual isolation of this bacteriophage demonstrates the phage’s role in decreasing the inoculum in the environment and impact on the life cycle of B. anthracis at a carcass site.

DABs are inorganic carbon pumps found throughout prokaryotic phyla

Bacterial autotrophs often rely on CO₂ concentrating mechanisms (CCMs) to assimilate carbon. Although many CCM proteins have been identified, a systematic screen of the components of CCMs is lacking. Here, we performed a genome-wide barcoded transposon screen to identify essential and CCM-related genes in the γ-proteobacterium Halothiobacillus neapolitanus. Screening revealed that the CCM comprises at least 17 and probably no more than 25 genes, most of which are encoded in 3 operons. Two of these operons (DAB1 and DAB2) contain a two-gene locus that encodes a domain of unknown function (Pfam: PF10070) and a putative cation transporter (Pfam: PF00361). Physiological and biochemical assays demonstrated that these proteins-which we name DabA and DabB, for DABs accumulate bicarbonate-assemble into a heterodimeric complex, which contains a putative β-carbonic anhydrase-like active site and functions as an energy-coupled inorganic carbon (C_i) pump. Interestingly, DAB operons are found in a diverse range of bacteria and archaea. We demonstrate that functional DABs are present in the human pathogens Bacillus anthracis and Vibrio cholerae. On the basis of these results, we propose that DABs constitute a class of energized C_i pumps and play a critical role in the metabolism of C_i throughout prokaryotic phyla.

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.

Zeptomole per milliliter detection and quantification of edema factor in plasma by LC-MS/MS yields insights into toxemia and the progression of inhalation anthrax

Inhalation of Bacillus anthracis spores can cause a rapidly progressing fatal infection. B. anthracis secretes three protein toxins: lethal factor (LF), edema factor (EF), and protective antigen (PA). EF and LF may circulate as free or PA-bound forms. Both free EF (EF) and PA-bound-EF (ETx) have adenylyl cyclase activity converting ATP to cAMP. We developed an adenylyl cyclase activity-based method for detecting and quantifying total EF (EF+ETx) in plasma. The three-step method includes magnetic immunocapture with monoclonal antibodies, reaction with ATP generating cAMP, and quantification of cAMP by isotope-dilution HPLC-MS/MS. Total EF was quantified from 5PL regression of cAMP vs ETx concentration. The detection limit was 20 fg/mL (225 zeptomoles/mL for the 89 kDa protein). Relative standard deviations for controls with 0.3, 6.0, and 90 pg/mL were 11.7-16.6% with 91.2-99.5% accuracy. The method demonstrated 100% specificity in 238 human serum/plasma samples collected from unexposed healthy individuals, and 100% sensitivity in samples from 3 human and 5 rhesus macaques with inhalation anthrax. Analysis of EF in the rhesus macaques showed that it was detected earlier post-exposure than B. anthracis by culture and PCR. Similar to LF, the kinetics of EF over the course of infection were triphasic, with an initial rise (phase-1), decline (phase-2), and final rapid rise (phase-3). EF levels were ~ 2-4 orders of magnitude lower than LF during phase-1 and phase-2 and only ~ 6-fold lower at death/euthanasia. Analysis of EF improves early diagnosis and adds to our understanding of anthrax toxemia throughout infection. The LF/EF ratio may also indicate the stage of infection and need for advanced treatments.

Targeting Metalloenzymes for Therapeutic Intervention

Metalloenzymes are central to a wide range of essential biological activities, including nucleic acid modification, protein degradation, and many others. The role of metalloenzymes in these processes also makes them central for the progression of many diseases and, as such, makes metalloenzymes attractive targets for therapeutic intervention. Increasing awareness of the role metalloenzymes play in disease and their importance as a class of targets has amplified interest in the development of new strategies to develop inhibitors and ultimately useful drugs. In this Review, we provide a broad overview of several drug discovery efforts focused on metalloenzymes and attempt to map out the current landscape of high-value metalloenzyme targets.

Regulons and protein-protein interactions of PRD-containing Bacillus anthracis virulence regulators reveal overlapping but distinct functions

Bacillus anthracis produces three regulators, AtxA, AcpA and AcpB, which control virulence gene transcription and belong to an emerging class of regulators termed 'PCVRs' (Phosphoenolpyruvate-dependent phosphotransferase regulation Domain-Containing Virulence Regulators). AtxA, named for its control of toxin gene expression, is the master virulence regulator and archetype PCVR. AcpA and AcpB are less well studied. Reports of PCVR activity suggest overlapping function. AcpA and AcpB independently positively control transcription of the capsule biosynthetic operon capBCADE, and culture conditions that enhance AtxA level or activity result in capBCADE transcription in strains lacking acpA and acpB. We used RNA-Seq to assess the regulons of the paralogous regulators in strains constructed to express individual PCVRs at native levels. Plasmid and chromosome-borne genes were PCVR controlled, with AtxA, AcpA and AcpB having a ≥ 4-fold effect on transcript levels of 145, 130 and 49 genes respectively. Several genes were coregulated by two or three PCVRs. We determined that AcpA and AcpB form homomultimers, as shown previously for AtxA, and we detected AtxA-AcpA heteromultimers. In co-expression experiments, AcpA activity was reduced by increased levels of AtxA. Our data show that the PCVRs have specific and overlapping activity and that PCVR stoichiometry and potential heteromultimerization can influence target gene expression.

A Dual Role for the Bacillus anthracis Master Virulence Regulator AtxA: Control of Sporulation and Anthrax Toxin Production

Bacillus anthracis is an endemic soil bacterium that exhibits two different lifestyles. In the soil environment, B. anthracis undergoes a cycle of saprophytic growth, sporulation, and germination. In mammalian hosts, the pathogenic lifestyle of B. anthracis is spore germination followed by vegetative cell replication, but cells do not sporulate. During infection, and in specific culture conditions, transcription of the structural genes for the anthrax toxin proteins and the biosynthetic operon for capsule synthesis is positively controlled by the regulatory protein AtxA. A critical role for the atxA gene in B. anthracis virulence has been established. Here we report an inverse relationship between toxin production and sporulation that is linked to AtxA levels. During culture in conditions favoring sporulation, B. anthracis produces little to no AtxA. When B. anthracis is cultured in conditions favoring toxin gene expression, AtxA is expressed at relatively high levels and sporulation rate and efficiency are reduced. We found that a mutation within the atxA promoter region resulting in AtxA over-expression leads to a marked sporulation defect. The sporulation phenotype of the mutant is dependent upon pXO2-0075, an atxA-regulated open reading frame located on virulence plasmid pXO2. The predicted amino acid sequence of the pXO2-0075 protein has similarity to the sensor domain of sporulation sensor histidine kinases. It was shown previously that pXO2-0075 overexpression suppresses sporulation. We have designated pXO2-0075 “skiA” for “sporulation kinase inhibitor.” Our results indicate that in addition to serving as a positive regulator of virulence gene expression, AtxA modulates B. anthracis development.

A Single Nucleotide Polymorphism Is Involved in Regulation of Growth and Spore Formation of Bacillus anthracis Pasteur II Strain

Anthrax toxins and capsules, which are encoded by genes located on pXO1 and pXO2, respectively, are major virulence factors of Bacillus anthracis. Our previous studies demonstrated that exposure to high-temperatures is unable to abolish the pXO1 plasmid of the Pasteur II strain, but the growth of the strain was obviously slower than that of the Sterne strain and wild-type virulent strain. To elucidate a potential regulatory mechanism of slowing growth, we employed comparative genome and bioinformatic analysis and revealed a unique SNP (G to T) at the 143135 bp position in pXO1 that is possibly involved in the mediation of growth of Pasteur II. However, the T to G mutation in groR did not result in any change of the amino acid sequence. A predominant nucleotide G existed at the 143135 bp in pXO1 of 100 wild-type B. anthracis isolates and 9 isolates documented in GenBank, whereas T replaced G in pXO1 of the Pasteur II strain. Further analysis indicate that the SNP is located in a gene between 143042 and 143173 bp, and that it encodes a small protein of 43 amino acids and is termed as a growth regulator (GroR). Site-directed mutagenesis and gene deletion demonstrates that groR regulates the growth and spore formation of B. anthracis. Our results indicate that the pXO1 plasmid is involved in the regulation of growth and spore formation in B. anthracis.

The pag Gene of pXO1 Is Involved in Capsule Biosynthesis of Bacillus anthracis Pasteur II Strain

The poly-γ-D-glutamic acid capsule and anthrax toxins are major virulence factors of Bacillus anthracis. Genes responsible for capsule biosynthesis are located on pXO2, whereas genes encoding the toxins, which are composed of edema factors, lethal factors, and protective antigens (PA), are located on pXO1. In this study, we found that the pag null mutation not only eliminated the production of the protective antigen, it also eliminated the ability of the B. anthracis Pasteur II strain to form capsules. qPCR analysis revealed that the deletion of pag decreased the transcription levels of the capABCD operon and its regulatory genes acpA and acpB. The introduction of the acpA or acpB plasmid complemented the effect of the pag null mutation on capsule formation. Taken together, the above results suggest that PA probably affects capsule biosynthesis by altering the expression of acpA and acpB. In addition, we found that the deletion mutation of pag remarkably attenuated bacterial pathogenicity in a mouse model of infection. Our results indicate that besides encoding the protective antigen, the pag gene of pXO1 is also involved in the modulation of capsule biosynthesis. Our findings provide new insight into the regulation mechanisms of capsule formation in B. anthracis Pasteur II strain.

Changes in Bacillus anthracis CodY regulation under host-specific environmental factor deprived conditions

Background

Host-specific environmental factors induce changes in Bacillus anthracis gene transcription during infection. A global transcription regulator, CodY, plays a pivotal role in regulating central metabolism, biosynthesis, and virulence in B. anthracis. In this study, we utilized RNA-sequencing to assess changes in the transcriptional patterns of CodY-regulated B. anthracis genes in response to three conditions of environmental starvation: iron, CO₂, or glucose deprivation. In addition, we performed chromatin immunoprecipitation on newly identified CodY-mediated genes.

Results

Environmental deprivation induced transcriptional changes in CodY-regulated genes in both wild-type and codY null strains, and both CodY-specific and environment-specific patterns were observed. In the iron-depleted condition, overexpression of iron homeostasis genes was observed independent of codY deletion; however, transcription of siderophore and amino acid biosynthesis genes was CodY dependent. Although CodY has a significant regulatory role in central metabolism and the carbon overflow pathway, metabolism-associated genes exhibited CodY-independent expression patterns under glucose starvation. Genes that were differentially expressed in response to CO₂ availability showed CodY-dependent regulation, though their maximal expression did require a supply of CO₂/bicarbonate.

Conclusions

We speculate that CodY regulates the expression of environmental-responsive genes in a hierarchical manner and is likely associated with other transcription regulators that are specific for a particular environmental change.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3004-8) contains supplementary material, which is available to authorized users.

In vivo dynamics of active edema and lethal factors during anthrax

Lethal and edema toxins are critical virulence factors of Bacillus anthracis. However, little is known about their in vivo dynamics of production during anthrax. In this study, we unraveled for the first time the in vivo kinetics of production of the toxin components EF (edema factor) and LF (lethal factor) during cutaneous infection with a wild-type toxinogenic encapsulated strain in immuno-competent mice. We stratified the asynchronous infection process into defined stages through bioluminescence imaging (BLI), while exploiting sensitive quantitative methods by measuring the enzymatic activity of LF and EF. LF was produced in high amounts, while EF amounts steadily increased during the infectious process. This led to high LF/EF ratios throughout the infection, with variations between 50 to a few thousands. In the bloodstream, the early detection of active LF and EF despite the absence of bacteria suggests that they may exert long distance effects. Infection with a strain deficient in the protective antigen toxin component enabled to address its role in the diffusion of LF and EF within the host. Our data provide a picture of the in vivo complexity of the infectious process.

Quantitative Determination of Lethal Toxin Proteins in Culture Supernatant of Human Live Anthrax Vaccine Bacillus anthracis A16R

Bacillus anthracis (B. anthracis) is the etiological agent of anthrax affecting both humans and animals. Anthrax toxin (AT) plays a major role in pathogenesis. It includes lethal toxin (LT) and edema toxin (ET), which are formed by the combination of protective antigen (PA) and lethal factor (LF) or edema factor (EF), respectively. The currently used human anthrax vaccine in China utilizes live-attenuated B. anthracis spores (A16R; pXO1+, pXO2−) that produce anthrax toxin but cannot produce the capsule. Anthrax toxins, especially LT, have key effects on both the immunogenicity and toxicity of human anthrax vaccines. Thus, determining quantities and biological activities of LT proteins expressed by the A16R strain is meaningful. Here, we explored LT expression patterns of the A16R strain in culture conditions using another vaccine strain Sterne as a control. We developed a sandwich ELISA and cytotoxicity-based method for quantitative detection of PA and LF. Expression and degradation of LT proteins were observed in culture supernatants over time. Additionally, LT proteins expressed by the A16R and Sterne strains were found to be monomeric and showed cytotoxic activity, which may be the main reason for side effects of live anthrax vaccines. Our work facilitates the characterization of anthrax vaccines components and establishment of a quality control standard for vaccine production which may ultimately help to ensure the efficacy and safety of the human anthrax vaccine A16R.

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.

Thermal control of virulence factors in bacteria: a hot topic

Pathogenic bacteria sense environmental cues, including the local temperature, to control the production of key virulence factors. Thermal regulation can be achieved at the level of DNA, RNA or protein and although many virulence factors are subject to thermal regulation, the exact mechanisms of control are yet to be elucidated in many instances. Understanding how virulence factors are regulated by temperature presents a significant challenge, as gene expression and protein production are often influenced by complex regulatory networks involving multiple transcription factors in bacteria. Here we highlight some recent insights into thermal regulation of virulence in pathogenic bacteria. We focus on bacteria which cause disease in mammalian hosts, which are at a significantly higher temperature than the outside environment. We outline the mechanisms of thermal regulation and how understanding this fundamental aspect of the biology of bacteria has implications for pathogenesis and human health.

Anthrax Pathogenesis

Anthrax is caused by the spore-forming, gram-positive bacterium Bacillus anthracis. The bacterium's major virulence factors are (a) the anthrax toxins and (b) an antiphagocytic polyglutamic capsule. These are encoded by two large plasmids, the former by pXO1 and the latter by pXO2. The expression of both is controlled by the bicarbonate-responsive transcriptional regulator, AtxA. The anthrax toxins are three polypeptides-protective antigen (PA), lethal factor (LF), and edema factor (EF)-that come together in binary combinations to form lethal toxin and edema toxin. PA binds to cellular receptors to translocate LF (a protease) and EF (an adenylate cyclase) into cells. The toxins alter cell signaling pathways in the host to interfere with innate immune responses in early stages of infection and to induce vascular collapse at late stages. This review focuses on the role of anthrax toxins in pathogenesis. Other virulence determinants, as well as vaccines and therapeutics, are briefly discussed.

Crystal structure of Bacillus anthracis virulence regulator AtxA and effects of phosphorylated histidines on multimerization and activity

The Bacillus anthracis virulence regulator AtxA controls transcription of the anthrax toxin genes and capsule biosynthetic operon. AtxA activity is elevated during growth in media containing glucose and CO(2)/bicarbonate, and there is a positive correlation between the CO(2)/bicarbonate signal, AtxA activity and homomultimerization. AtxA activity is also affected by phosphorylation at specific histidines. We show that AtxA crystallizes as a dimer. Distinct folds associated with predicted DNA-binding domains (HTH1 and HTH2) and phosphoenolpyruvate: carbohydrate phosphotransferase system-regulated domains (PRD1 and PRD2) are apparent. We tested AtxA variants containing single and double phosphomimetic (His→Asp) and phosphoablative (His→Ala) amino acid changes for activity in B. anthracis cultures and for protein-protein interactions in cell lysates. Reduced activity of AtxA H199A, lack of multimerization and activity of AtxAH379D variants, and predicted structural changes associated with phosphorylation support a model for control of AtxA function. We propose that (i) in the AtxA dimer, phosphorylation of H199 in PRD1 affects HTH2 positioning, influencing DNA-binding; and (ii) phosphorylation of H379 in PRD2 disrupts dimer formation. The AtxA structure is the first reported high-resolution full-length structure of a PRD-containing regulator, and can serve as a model for proteins of this family, especially those that link virulence to bacterial metabolism.

N-acetylglucosamine deacetylases modulate the anchoring of the gamma-glutamyl capsule to the cell wall of Bacillus anthracis

Bacillus anthracis has a complex cell wall structure composed of a peptidoglycan (PG) layer to which major structures are anchored such as a neutral polysaccharide, an S-layer, and a poly-γ-D-glutamate (PDGA) capsule. Many of these structures have central roles in the biology of B. anthracis, particularly, in virulence. However, little attention has been devoted to structurally study the PG and how it is modified in the presence of these secondary cell wall components. We present here the fine structure of the PG of the encapsulated RPG1 strain harboring both pXO1 and pXO2 virulence plasmids. We show that B. anthracis has a high degree of cross-linking and its GlcNAc residues are highly modified by N-deacetylation. The PG composition is not dependent on the presence of either LPXTG proteins or the capsule. Using NMR analysis of the PG-PDGA complex, we provide evidence for the anchoring of the PDGA to the glucosamine residues. We show that anchoring of the PDGA capsule is impaired in two PG N-deacetylase mutants, Ba1961 and Ba3679. Thus, these multiple N-deactylase activities would constitute excellent drug targets in B. anthracis by simultaneously affecting its resistance to lysozyme and to phagocytosis impairing B. anthracis survival in the host.

Transcriptome analysis identifies Bacillus anthracis genes that respond to CO2 through an AtxA-dependent mechanism

Background

Upon infection of a mammalian host, Bacillus anthracis responds to host cues, and particularly to elevated temperature (37°C) and bicarbonate/CO₂ concentrations, with increased expression of virulence factors that include the anthrax toxins and extracellular capsular layer. This response requires the presence of the pXO1 virulence plasmid-encoded pleiotropic regulator AtxA. To better understand the genetic basis of this response, we utilized a controlled in vitro system and Next Generation sequencing to determine and compare RNA expression profiles of the parental strain and an isogenic AtxA-deficient strain in a 2 × 2 factorial design with growth environments containing or lacking carbon dioxide.

Results

We found 15 pXO1-encoded genes and 3 chromosomal genes that were strongly regulated by the separate or synergistic actions of AtxA and carbon dioxide. The majority of the regulated genes responded to both AtxA and carbon dioxide rather than to just one of these factors. Interestingly, we identified two previously unrecognized small RNAs that are highly expressed under physiological carbon dioxide concentrations in an AtxA-dependent manner. Expression levels of the two small RNAs were found to be higher than that of any other gene differentially expressed in response to these conditions. Secondary structure and small RNA-mRNA binding predictions for the two small RNAs suggest that they may perform important functions in regulating B. anthracis virulence.

Conclusions

A majority of genes on the virulence plasmid pXO1 that are regulated by the presence of either CO₂ or AtxA separately are also regulated synergistically in the presence of both. These results also elucidate novel pXO1-encoded small RNAs that are associated with virulence conditions.

Designing inhibitors of anthrax toxin

INTRODUCTION

Present-day rational drug design approaches are based on exploiting unique features of the target biomolecules, small- or macromolecule drug candidates and physical forces that govern their interactions. The 2013 Nobel Prize in chemistry awarded 'for the development of multiscale models for complex chemical systems' once again demonstrated the importance of the tailored drug discovery that reduces the role of the trial-and-error approach to a minimum. The intentional dissemination of Bacillus anthracis spores in 2001 via the so-called anthrax letters has led to increased efforts, politically and scientifically, to develop medical countermeasures that will protect people from the threat of anthrax bioterrorism.

AREAS COVERED

This article provides an overview of the recent rational drug design approaches for discovering inhibitors of anthrax toxin. The review also directs the readers to the vast literature on the recognized advances and future possibilities in the field.

EXPERT OPINION

Existing options to combat anthrax toxin lethality are limited. With the only anthrax toxin inhibiting therapy (protective antigen-targeting with a monoclonal antibody, raxibacumab) approved to treat inhalational anthrax, the situation, in our view, is still insecure. Further, the FDA's animal rule for drug approval, which clears compounds without validated efficacy studies on humans, creates a high level of uncertainty, especially when a well-characterized animal model does not exist. Better identification and validation of anthrax toxin therapeutic targets at the molecular level as well as elucidation of the parameters determining the corresponding therapeutic windows are still necessary for more effective therapeutic options.

Delineating the effect of host environmental signals on a fully virulent strain of Bacillus anthracis using an integrated transcriptomics and proteomics approach

Pathogenic bacteria sense the host environment and regulate expression of virulence-related genes. Environmental signals like temperature, bicarbonate/CO2 and glucose induce toxin production in Bacillus anthracis, but the mechanisms by which these signals contribute to virulence and overall physiological adaptation remains elusive. An integrated, systems level investigation using transcriptomics and iTRAQ-based proteomics was done to assess the effect of temperature, bicarbonate/CO2 and glucose on B. anthracis. Significant changes observed in amino acid, carbohydrate, energy and nucleotide metabolism indicates events of metabolic readjustments by environmental factors. Directed induction of genes involved in polyamine biosynthesis and iron metabolism revealed the redirection of cellular metabolite pool towards iron uptake. Protein levels of glycolytic enzymes, ptsH and Ldh along with transcripts involved in immune evasion (mprF, bNOS, Phospholipases and asnA), cell surface remodeling (rfbABCD, antABCD, and cls) and utilization of lactate (lutABC) and inositol showed constant repression under environmental perturbations. Discrepancies observed in mRNA/protein level of genes involved in glycolysis, protein synthesis, stress response and nucleotide metabolism hinted at the existence of additional regulatory layers and illustrated the utility of an integrated approach. The above findings might assist in the identification of novel adaptive strategies of B. anthracis during host associated survival and pathogenesis.

BIOLOGICAL SIGNIFICANCE

In this study, the changes observed at both transcript and protein level were quantified and integrated to understand the effect of host environmental factors (host temperature, bicarbonate and glucose) in shaping the physiology and adaptive strategies of a fully virulent strain of B. anthracis for efficient survival and virulence in its host. Perturbations affecting toxin production were found to concordantly affect vital metabolic pathways and several known as well as novel virulence factors. These changes act as a valuable asset for generating testable hypotheses that can be further verified by detailed molecular and mutant studies to identify novel adaptive strategies of B. anthracis during infection. Adaptation of an integrated transcriptomics and proteomics approach also led to the identification of discrepancies between mRNA/protein levels among genes across major functional categories. Few of these discrepancies have been previously reported in literature for model organisms. However their existence in B. anthracis and that too as a result of growth perturbations have not been reported till date. These findings demonstrate a substantial role of regulatory processes post mRNA synthesis via post transcriptional, translational or protein degradation mechanisms. This article is part of a Special Issue entitled: Proteomics of non-model organisms.

Key tissue targets responsible for anthrax-toxin-induced lethality

Bacillus anthracis, the causative agent of anthrax disease, is lethal owing to the actions of two exotoxins: anthrax lethal toxin (LT) and oedema toxin (ET). The key tissue targets responsible for the lethal effects of these toxins are unknown. Here we generated cell-type-specific anthrax toxin receptor capillary morphogenesis protein-2 (CMG2)-null mice and cell-type-specific CMG2-expressing mice and challenged them with the toxins. Our results show that lethality induced by LT and ET occurs through damage to distinct cell types; whereas targeting cardiomyocytes and vascular smooth muscle cells is required for LT-induced mortality, ET-induced lethality occurs mainly through its action in hepatocytes. Notably, and in contradiction to what has been previously postulated, targeting of endothelial cells by either toxin does not seem to contribute significantly to lethality. Our findings demonstrate that B. anthracis has evolved to use LT and ET to induce host lethality by coordinately damaging two distinct vital systems.

Effects of experimental exclusion of scavengers from carcasses of anthrax-infected herbivores on Bacillus anthracis sporulation, survival, and distribution

Scavenging of anthrax carcasses has long been hypothesized to play a critical role in the production of the infectious spore stage of Bacillus anthracis after host death, though empirical studies assessing this are lacking. We compared B. anthracis spore production, distribution, and survival at naturally occurring anthrax herbivore carcasses that were either experimentally caged to exclude vertebrate scavengers or left unmanipulated. We found no significant effect of scavengers on soil spore density (P > 0.05). Soil stained with terminally hemorrhaged blood and with nonhemorrhagic fluids exhibited high levels of B. anthracis spore contamination (ranging from 10(3) to 10(8) spores/g), even in the absence of vertebrate scavengers. At most of the carcass sites, we also found that spore density in samples taken from hemorrhagic-fluid-stained soil continued to increase for >4 days after host death. We conclude that scavenging by vertebrates is not a critical factor in the life cycle of B. anthracis and that anthrax control measures relying on deterrence or exclusion of vertebrate scavengers to prevent sporulation are unlikely to be effective.

Identification of CodY targets in Bacillus anthracis by genome-wide in vitro binding analysis

In Gram-positive bacteria, CodY is an important regulator of genes whose expression changes under conditions of nutrient limitation. Bacillus anthracis CodY represses or activates directly or indirectly approximately 500 genes. Affinity purification of CodY-DNA complexes was used to identify the direct targets of CodY. Of the 389 DNA binding sites that were copurified with CodY, 132 sites were in or near the regulatory regions governing the expression of 197 CodY-controlled genes, indicating that CodY controls many other genes indirectly. CodY-binding specificity was verified using electrophoretic mobility shift and DNase I footprinting assays for three CodY targets. Analysis of the bound sequences led to the identification of a B. anthracis CodY-binding consensus motif that was found in 366 of the 389 affinity-purified DNA regions. Regulation of the expression of the two genes directly controlled by CodY, sap and eag, encoding the two surface layer (S-layer) proteins, was analyzed further by monitoring the expression of transcriptional lacZ reporter fusions in parental and codY mutant strains. CodY proved to be a direct repressor of both sap and eag expression. Since the expression of the S-layer genes is under the control of both CodY and PagR (a regulator that responds to bicarbonate), their expression levels respond to both metabolic and environmental cues.

Identification of a CO2 responsive regulon in Bordetella

Sensing the environment allows pathogenic bacteria to coordinately regulate gene expression to maximize survival within or outside of a host. Here we show that Bordetella species regulate virulence factor expression in response to carbon dioxide levels that mimic in vivo conditions within the respiratory tract. We found strains of Bordetella bronchiseptica that did not produce adenylate cyclase toxin (ACT) when grown in liquid or solid media with ambient air aeration, but produced ACT and additional antigens when grown in air supplemented to 5% CO(2). Transcriptome analysis and quantitative real time-PCR analysis revealed that strain 761, as well as strain RB50, increased transcription of genes encoding ACT, filamentous hemagglutinin (FHA), pertactin, fimbriae and the type III secretion system in 5% CO(2) conditions, relative to ambient air. Furthermore, transcription of cyaA and fhaB in response to 5% CO(2) was increased even in the absence of BvgS. In vitro analysis also revealed increases in cytotoxicity and adherence when strains were grown in 5% CO(2). The human pathogens B. pertussis and B. parapertussis also increased transcription of several virulence factors when grown in 5% CO(2), indicating that this response is conserved among the classical bordetellae. Together, our data indicate that Bordetella species can sense and respond to physiologically relevant changes in CO(2) concentrations by regulating virulence factors important for colonization, persistence and evasion of the host immune response.

Debridement increases survival in a mouse model of subcutaneous anthrax

Anthrax is caused by infection with Bacillus anthracis, a spore-forming gram-positive bacterium. A major virulence factor for B. anthracis is an immunomodulatory tripartite exotoxin that has been reported to alter immune cell chemotaxis and activation. It has been proposed that B. anthracis infections initiate through entry of spores into the regional draining lymph nodes where they germinate, grow, and disseminate systemically via the efferent lymphatics. If this model holds true, it would be predicted that surgical removal of infected tissues, debridement, would have little effect on the systemic dissemination of bacteria. This model was tested through the development of a mouse debridement model. It was found that removal of the site of subcutaneous infection in the ear increased the likelihood of survival and reduced the quantity of spores in the draining cervical lymph nodes (cLN). At the time of debridement 12 hours post-injection measurable levels of exotoxins were present in the ear, cLN, and serum, yet leukocytes within the cLN were activated; countering the concept that exotoxins inhibit the early inflammatory response to promote bacterial growth. We conclude that the initial entry of spores into the draining lymph node of cutaneous infections alone is not sufficient to cause systemic disease and that debridement should be considered as an adjunct to antibiotic therapy.

Bacillus anthracis virulence regulator AtxA: oligomeric state, function and CO(2) -signalling

AtxA, a unique regulatory protein of unknown molecular function, positively controls expression of the major virulence genes of Bacillus anthracis. The 475 amino acid sequence of AtxA reveals DNA binding motifs and regions similar to proteins associated with the phosphoenolpyruvate: carbohydrate phosphotransferase system (PTS). We used strains producing native and functional epitope-tagged AtxA proteins to examine protein-protein interactions in cell lysates and in solutions of purified protein. Co-affinity purification, non-denaturing polyacrylamide gel electrophoresis and bis(maleimido)hexane (BMH) cross-linking experiments revealed AtxA homo-multimers. Dimers were the most abundant species. BMH cross-links available cysteines within 13 Å. To localize interaction sites, six AtxA mutants containing distinct Cys→Ser substitutions were tested for multimerization and cross-linking. All mutants multimerized, but one mutation, C402S, prevented cross-linking. Thus, BMH uses C402 to make the inter-molecular bond between AtxA proteins, but C402 is not required for protein-protein interaction. C402 is in a region bearing amino acid similarity to Enzyme IIB proteins of the PTS. The AtxA EIIB motif may function in protein oligomerization. Finally, cultures grown with elevated CO(2) /bicarbonate exhibited increased AtxA dimer/monomer ratios and increased AtxA activity, relative to cultures grown without added CO(2) /bicarbonate, suggesting that this host-associated signal enhances AtxA function by shifting the dimer/monomer equilibrium towards the dimeric state.

CodY regulation is required for full virulence and heme iron acquisition in Bacillus anthracis

Capsule and toxin are the major virulence factors of Bacillus anthracis. The B. anthracis pleiotropic regulator CodY activates toxin gene expression by post-translationally regulating the accumulation of the global regulator AtxA. However, the role of CodY on B. anthracis capsulation and virulence of encapsulated strains has been unknown. The role of CodY in B. anthracis virulence was studied in mouse and guinea pig models. Spore outgrowth and dissemination of the vegetative cells was followed in mice by bioluminescent imaging. We also determined the state of capsulation and the iron requirement for growth of the codY mutant. In all models tested, the codY mutant strain was strongly attenuated compared to the wild-type strain and, in mice, also compared to the atxA strain. The disruption of codY did not affect either ex vivo or in vivo capsulation, whereas atxA deletion affected ex vivo capsulation only. The disruption of codY led to a delayed initiation of dissemination but similar kinetics of subsequent spread of the bacilli. The codY mutant cannot grow on heme iron as sole iron source, whereas the parental and complemented strains can. The lack of CodY-mediated transcription weakens virulence by controlling iron acquisition and synthesis of toxin, but without modifying capsulation.

Use of microwells to investigate the effect of quorum sensing on growth and antigen production in Bacillus anthracis Sterne 34F2

AIM

The aim of this study was to investigate the role of quorum sensing in Bacillus anthracis growth and toxin production.

METHODS AND RESULTS

A microwell plate culture method was developed to simulate the normal UK-licensed anthrax vaccine production run. Once established, sterile supernatant additions from a previous B. anthracis culture were made, and reductions in lag phase and early stimulation of the anthrax toxin component protective antigen (PA) were monitored using ELISA. The addition of the quorum-sensing inhibitor, fur-1, prolonged the lag phase and impeded PA production. Spin filters of various sizes were used to identify the molecular weight fraction of the sterile supernatant responsible for the autoinducer effect. A weight fraction between 5 and 10 kDa was responsible for the autoinducer effect; however, further identification using mass spectroscopy proved inconclusive.

CONCLUSIONS

Quorum sensing mediated by the autoinducer two molecule plays a significant role in both B. anthracis growth and toxin production.

SIGNIFICANCE AND IMPACT OF THE STUDY

While genomic analysis has eluded to the importance of LuxS and quorum sensing in anthrax, this is the first analysis using a production strain of B. anthracis and a quorum-sensing inhibitor to monitor the effect on growth and toxin production. This gives insights into anthrax pathogenicity and vaccine manufacture.

The CodY pleiotropic repressor controls virulence in gram-positive pathogens

CodY is involved in the adaptive response to starvation in at least 30 different low G+C gram-positive bacteria. After dimerization and activation by cofactor binding, CodY binds to a consensus palindromic DNA sequence, leading to the repression of approximately 5% of the genome. CodY represses the transcription of target genes when bound to DNA by competition with the RNA polymerase for promoter binding, or by interference with transcriptional elongation as a roadblock. CodY displays enhanced affinity for its DNA target when bound to GTP and/or branched chain amino acids (BCAA). When nutrients become limiting in the postexponential growth phase, a decrease of intracellular levels of GTP and BCAA causes a deactivation of CodY and decreases its affinity for DNA, leading to the induction of its regulon. CodY-regulated genes trigger adaptation of the bacteria to starvation by highly diverse mechanisms, such as secretion of proteases coupled to expression of amino acid transporters, and promotion of survival strategies like sporulation or biofilm formation. Additionally, in pathogenic bacteria, several virulence factors are regulated by CodY. As a function of their access to nutrients, pathogenic gram-positive bacteria express virulence factors in a codY-dependant manner. This is true for the anthrax toxins of Bacillus anthracis and the haemolysins of Staphylococcus aureus. The purpose of this review is to illustrate CodY-regulated mechanisms on virulence in major gram-positive pathogens.

Role of luxS in Bacillus anthracis growth and virulence factor expression

Quorum-sensing (QS), the regulation of bacterial gene expression in response to changes in cell density, involves pathways that synthesize signaling molecules (auto-inducers). The luxS/AI-2-mediated QS system has been identified in both gram-positive and gram-negative bacteria. Bacillus anthracis, the etiological agent of anthrax, possesses genes involved in luxS/AI-2-mediated QS, and deletion of luxS in B. anthracis Sterne strain 34F2 results in inhibition of AI-2 synthesis and a growth defect. In the present study, we created a ΔluxS B. anthracis strain complemented in trans by insertion of a cassette, including luxS and a gene encoding erythromycin resistance, into the truncated plcR regulator locus. The complemented ΔluxS strain has restored AI-2 synthesis and wild-type growth. A B. anthracis microarray study revealed consistent differential gene expression between the wild-type and ΔluxS strain, including downregulation of the B. anthracis S-layer protein gene EA1 and pXO1 virulence genes. These data indicate that B. anthracis may use luxS/AI-2-mediated QS to regulate growth, density-dependent gene expression and virulence factor expression.

Bistability in myo-inositol utilization by Salmonella enterica serovar Typhimurium

The capability of Salmonella enterica serovar Typhimurium strain 14028 (S. Typhimurium 14028) to utilize myo-inositol (MI) is determined by the genomic island GEI4417/4436 carrying the iol genes that encode enzymes, transporters, and a repressor responsible for the MI catabolic pathway. In contrast to all bacteria investigated thus far, S. Typhimurium 14028 growing on MI as the sole carbon source is characterized by a remarkable long lag phase of 40 to 60 h. We report here that on solid medium with MI as the sole carbon source, this human pathogen exhibits a bistable phenotype characterized by a dissection into large colonies and a slow-growing bacterial background. This heterogeneity is reversible and therefore not caused by mutation, and it is not observed in the absence of the iol gene repressor IolR nor in the presence of at least 0.55% CO(2). Bistability is correlated with the activity of the iolE promoter (P(iolE)), but not of P(iolC) or P(iolD), as shown by promoter-gfp fusions. On the single-cell level, fluorescence microscopy and flow cytometry analysis revealed a gradual switch of P(iolE) from the "off" to the "on" status during the late lag phase and the transition to the log phase. Deletion of iolR or the addition of 0.1% NaHCO(3) induced an early growth start of S. Typhimurium 14028 in minimal medium with MI. The addition of ethoxyzolamide, an inhibitor of carboanhydrases, elongated the lag phase in the presence of bicarbonate. The positive-feedback loop via repressor release and positive induction by bicarbonate-CO(2) might allow S. Typhimurium 14028 to adapt to rapidly changing environments. The phenomenon described here is a novel example of bistability in substrate degradation, and, to our knowledge, is the first demonstration of gene regulation by bicarbonate-CO(2) in Salmonella.

Comparative proteome analysis of Bacillus anthracis with pXO1 plasmid content

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

AtxA, a Bacillus anthracis global virulence regulator

Fifteen years ago, AtxA was isolated as a toxin gene activator and five years later it was shown to be a Bacillus anthracis master regulator. AtxA controls the expression of more than a hundred genes belonging to all genetic elements, the chromosome and both virulence plasmids, including those encoding the major virulence factors. AtxA can activate or repress gene expression. The mechanism by which AtxA exerts its control is unknown; it is indirect on some genes but may be direct on others. The expression of many AtxA-controlled genes is induced by the presence of bicarbonate/CO(2). AtxA links the metabolic state and virulence gene expression.

A multiplex real-time PCR for identifying and differentiating B. anthracis virulent types

Bacillus anthracis is closely related to the endospore forming bacteria Bacillus cereus and Bacillus thuringiensis. For accurate detection of the life threatening pathogen B. anthracis, it is essential to distinguish between these three species. Here we present a novel multiplex real-time PCR for simultaneous specific identification of B. anthracis and discrimination of different B. anthracis virulence types. Specific B. anthracis markers were selected by whole genome comparison and different sets of primers and probes with optimal characteristic for multiplex detection of the B. anthracis chromosome, the B. anthracis pXO1 and pXO2 plasmids and an internal control (IC) were designed. The primer sets were evaluated using a panel of B. anthracis strains and exclusivity was tested using genetically closely related B. cereus strains. The robustness of final primer design was evaluated by laboratories in three different countries using five different real-time PCR thermocyclers. Testing of a panel of more than 20 anthrax strains originating from different locations around the globe, including the recent Swedish anthrax outbreak strain, showed that all strains were detected correctly.

Targeted proteomics approach to species-level identification of Bacillus thuringiensis spores by AP-MALDI-MS

Anthrax infections progress at a rapid pace, making rapid detection methods of utmost importance. MALDI-MS proteomics methods focused on Bacillus anthracis detection have targeted chromosomally encoded proteins, which are highly conserved between closely related species, hindering species identification. Presented here is an AP-MALDI-MS method targeting plasmid-borne proteins from Bacillus spores for species-level identification. A bioinformatics analysis revealed that 60.3% and 75.4% of tryptic peptides from plasmid-borne proteins of B. anthracis and B. thuringiensis were species-specific, respectively. Reported here is a method in which plasmid-borne delta-endotoxins were extracted directly from B. thuringiensis spores in 100 mM KOH. The pH was then adjusted to 8 and a 5-min trypsin digestion was performed on the extracted proteins. The resulting tryptic peptides were analyzed by AP-MALDI-MS/MS, which produced a definitive identification the B. thuringiensis species-specific Cry1Ab protein with a MASCOT score of 278 and expect value of 7.5 x 10(-23). This method has demonstrated the detection and identification of B. thuringiensis spores at the species level following a 5-min trypsin digestion. The challenges in applying a similar approach to the detection of plasmid-borne protein toxins from B. anthracis are also discussed.

Role of the protective antigen octamer in the molecular mechanism of anthrax lethal toxin stabilization in plasma

Anthrax is caused by strains of Bacillus anthracis that produce two key virulence factors, anthrax toxin (Atx) and a poly-gamma-D-glutamic acid capsule. Atx is comprised of three proteins: protective antigen (PA) and two enzymes, lethal factor (LF) and edema factor (EF). To disrupt cell function, these components must assemble into holotoxin complexes, which contain either a ring-shaped homooctameric or homoheptameric PA oligomer bound to multiple copies of LF and/or EF, producing lethal toxin (LT), edema toxin, or mixtures thereof. Once a host cell endocytoses these complexes, PA converts into a membrane-inserted channel that translocates LF and EF into the cytosol. LT can assemble on host cell surfaces or extracellularly in plasma. We show that, under physiological conditions in bovine plasma, LT complexes containing heptameric PA aggregate and inactivate more readily than LT complexes containing octameric PA. LT complexes containing octameric PA possess enhanced stability, channel-forming activity, and macrophage cytotoxicity relative to those containing heptameric PA. Under physiological conditions, multiple biophysical probes reveal that heptameric PA can prematurely adopt the channel conformation, but octameric PA complexes remain in their soluble prechannel configuration, which allows them to resist aggregation and inactivation. We conclude that PA may form an octameric oligomeric state as a means to produce a more stable and active LT complex that could circulate freely in the blood.

Full expression of Bacillus anthracis toxin gene in the presence of bicarbonate requires a 2.7-kb-long atxA mRNA that contains a terminator structure

Bacillus anthracis toxin gene expression requires AtxA, a virulence regulator that also activates capsule gene transcription and controls expression of more than a hundred genes. Here we report that atxA mRNA is 2.7-kb-long and ends, after a 500 nt-long 3' untranslated region, with a stem loop structure followed by a run of U's. The presence of this structure stabilizes atxA mRNA and is necessary for AtxA maximal accumulation, full expression of the PA toxin gene, pagA and optimal PA accumulation. This structure displays terminator activity independently of its orientation when cloned between an inducible promoter and a reporter gene. The 3.6-kb-long DNA fragment carrying both AtxA promoters and the terminator is sufficient for full expression of pagA in the presence of bicarbonate. No pXO1-encoded element other than the DNA fragment encompassing the 2.7 kb atxA transcript and the pagA promoter is required for bicarbonate induction of pagA transcription.

Temporal production of the two Bacillus anthracis siderophores, petrobactin and bacillibactin

Bacillus anthracis secretes two siderophores, petrobactin (PB) and bacillibactin (BB). These siderophores were temporally produced during germination and outgrowth of spores (the usual infectious form of B. anthracis) in low-iron medium. The siderophore PB was made first while BB secretion began several hours later. Spore outgrowth early in an infection may require PB, whereas delayed BB production suggests a role for BB in the later stages of the infection. Incubation of cultures (inoculated as vegetative cells) at 37 degrees C, as compared to 2 degrees C, increased PB production and decreased secretion of BB, suggesting that the production of PB and BB responded to the host temperature signal. The dual siderophores of B. anthracis may fulfill independent roles in the life cycle of B. anthracis.

Comparative analysis of the virulence of invertebrate and mammalian pathogenic bacteria in the oral insect infection model Galleria mellonella

Infection of Galleria mellonella by feeding a mixture of Bacillus thuringiensis spores or vegetative bacteria in association with the toxin Cry1C results in high levels of larval mortality. Under these conditions the toxin or bacteria have minimal effects on the larva when inoculated separately. In order to evaluate whether G. mellonella can function as an oral infection model for human and entomo-bacterial pathogens, we tested strains of Bacillus cereus, Bacillus anthracis, Enterococcus faecalis, Listeria monocytogenes, Pseudomonas aeruginosa and a Drosophila targeting Pseudomonas entomophila strain. Six B. cereus strains (5 diarrheal, 1 environmental isolate) were first screened in 2nd instar G. mellonella larvae by free ingestion and four of them were analyzed by force-feeding 5th instar larvae. The virulence of these B. cereus strains did not differ from the B. thuringiensis virulent reference strain 407Cry(-) with the exception of strain D19 (NVH391/98) that showed a lower virulence. Following force-feeding, 5th instar G. mellonella larvae survived infection with B. anthracis, L. monocytogenes, E. faecalis and P. aeruginosa strains in contrast to the P. entomophila strain which led to high mortality even without Cry1C toxin co-ingestion. Thus, specific virulence factors adapted to the insect intestine might exist in B. thuringiensis/B. cereus and P. entomophila. This suggests a co-evolution between host and pathogens and supports the close links between B. thuringiensis and B. cereus and more distant links to their relative B. anthracis.

The global regulator CodY regulates toxin gene expression in Bacillus anthracis and is required for full virulence

In gram-positive bacteria, CodY is an important regulator of genes whose expression changes upon nutrient limitation and acts as a repressor of virulence gene expression in some pathogenic species. Here, we report the role of CodY in Bacillus anthracis, the etiologic agent of anthrax. Disruption of codY completely abolished virulence in a toxinogenic, noncapsulated strain, indicating that the activity of CodY is required for full virulence of B. anthracis. Global transcriptome analysis of a codY mutant and the parental strain revealed extensive differences. These differences could reflect direct control for some genes, as suggested by the presence of CodY binding sequences in their promoter regions, or indirect effects via the CodY-dependent control of other regulatory proteins or metabolic rearrangements in the codY mutant strain. The differences included reduced expression of the anthrax toxin genes in the mutant strain, which was confirmed by lacZ reporter fusions and immunoblotting. The accumulation of the global virulence regulator AtxA protein was strongly reduced in the mutant strain. However, in agreement with the microarray data, expression of atxA, as measured using an atxA-lacZ transcriptional fusion and by assaying atxA mRNA, was not significantly affected in the codY mutant. An atxA-lacZ translational fusion was also unaffected. Overexpression of atxA restored toxin component synthesis in the codY mutant strain. These results suggest that CodY controls toxin gene expression by regulating AtxA accumulation posttranslationally.

Bacillus anthracis physiology and genetics

Bacillus anthracis is a member of the Bacillus cereus group species (also known as the "group 1 bacilli"), a collection of Gram-positive spore-forming soil bacteria that are non-fastidious facultative anaerobes with very similar growth characteristics and natural genetic exchange systems. Despite their close physiology and genetics, the B. cereus group species exhibit certain species-specific phenotypes, some of which are related to pathogenicity. B. anthracis is the etiologic agent of anthrax. Vegetative cells of B. anthracis produce anthrax toxin proteins and a poly-d-glutamic acid capsule during infection of mammalian hosts and when cultured in conditions considered to mimic the host environment. The genes associated with toxin and capsule synthesis are located on the B. anthracis plasmids, pXO1 and pXO2, respectively. Although plasmid content is considered a defining feature of the species, pXO1- and pXO2-like plasmids have been identified in strains that more closely resemble other members of the B. cereus group. The developmental nature of B. anthracis and its pathogenic (mammalian host) and environmental (soil) lifestyles of make it an interesting model for study of niche-specific bacterial gene expression and physiology.

Cellular and systemic effects of anthrax lethal toxin and edema toxin

Anthrax lethal toxin (LT) and edema toxin (ET) are the major virulence factors of anthrax and can replicate the lethality and symptoms associated with the disease. This review provides an overview of our current understanding of anthrax toxin effects in animal models and the cytotoxicity (necrosis and apoptosis) induced by LT in different cells. A brief reexamination of early historic findings on toxin in vivo effects in the context of our current knowledge is also presented.

The surface of Bacillus anthracis

Bacillus anthracis is a Gram positive organism possessing a complex parietal structure. An S-layer, a bi-dimensional crystalline layer, and a peptidic capsule surround the thick peptidoglycan of bacilli harvested during infection. A review of the current literature indicates that elements from each of these three structures, as well as membrane components, have been studied. So-called cell-wall secondary polymers, be they attached to the cell-wall or to the membrane play important functions, either per se or because they permit the anchoring of proteins. Some surface proteins, whichever compartment they are attached to, play, as had been hypothesized, key roles in virulence. Others, of yet unknown function, are nevertheless expressed in vivo. This review will focus on well-studied polymers or proteins and indicate, when appropriate, the mechanisms by which they are targeted to their respective locations.

Comparative transcriptional profiling of Bacillus cereus sensu lato strains during growth in CO2-bicarbonate and aerobic atmospheres

Background

Bacillus species are spore-forming bacteria that are ubiquitous in the environment and display a range of virulent and avirulent phenotypes. This range is particularly evident in the Bacillus cereus sensu lato group; where closely related strains cause anthrax, food-borne illnesses, and pneumonia, but can also be non-pathogenic. Although much of this phenotypic range can be attributed to the presence or absence of a few key virulence factors, there are other virulence-associated loci that are conserved throughout the B. cereus group, and we hypothesized that these genes may be regulated differently in pathogenic and non-pathogenic strains.

Methodology/Principal Findings

Here we report transcriptional profiles of three closely related but phenotypically unique members of the Bacillus cereus group—a pneumonia-causing B. cereus strain (G9241), an attenuated strain of B. anthracis (Sterne 34F₂), and an avirulent B. cereus strain (10987)—during exponential growth in two distinct atmospheric environments: 14% CO₂/bicarbonate and ambient air. We show that the disease-causing Bacillus strains undergo more distinctive transcriptional changes between the two environments, and that the expression of plasmid-encoded virulence genes was increased exclusively in the CO₂ environment. We observed a core of conserved metabolic genes that were differentially expressed in all three strains in both conditions. Additionally, the expression profiles of putative virulence genes in G9241 suggest that this strain, unlike Bacillus anthracis, may regulate gene expression with both PlcR and AtxA transcriptional regulators, each acting in a different environment.

Conclusions/Significance

We have shown that homologous and even identical genes within the genomes of three closely related members of the B. cereus sensu lato group are in some instances regulated very differently, and that these differences can have important implications for virulence. This study provides insights into the evolution of the B. cereus group, and highlights the importance of looking beyond differences in gene content in comparative genomics studies.