Germination of Bacillus anthracis spores within alveolar macrophages

Mechanisms of NK cell-macrophage Bacillus anthracis crosstalk: a balance between stimulation by spores and differential disruption by toxins

NK cells are important immune effectors for preventing microbial invasion and dissemination, through natural cytotoxicity and cytokine secretion. Bacillus anthracis spores can efficiently drive IFN-γ production by NK cells. The present study provides insights into the mechanisms of cytokine and cellular signaling that underlie the process of NK-cell activation by B. anthracis and the bacterial strategies to subvert and evade this response. Infection with non-toxigenic encapsulated B. anthracis induced recruitment of NK cells and macrophages into the mouse draining lymph node. Production of edema (ET) or lethal (LT) toxin during infection impaired this cellular recruitment. NK cell depletion led to accelerated systemic bacterial dissemination. IFN-γ production by NK cells in response to B. anthracis spores was: i) contact-dependent through RAE-1-NKG2D interaction with macrophages; ii) IL-12, IL-18, and IL-15-dependent, where IL-12 played a key role and regulated both NK cell and macrophage activation; and iii) required IL-18 for only an initial short time window. B. anthracis toxins subverted both NK cell essential functions. ET and LT disrupted IFN-γ production through different mechanisms. LT acted both on macrophages and NK cells, whereas ET mainly affected macrophages and did not alter NK cell capacity of IFN-γ secretion. In contrast, ET and LT inhibited the natural cytotoxicity function of NK cells, both in vitro and in vivo. The subverting action of ET thus led to dissociation in NK cell function and blocked natural cytotoxicity without affecting IFN-γ secretion. The high efficiency of this process stresses the impact that this toxin may exert in anthrax pathogenesis, and highlights a potential usefulness for controlling excessive cytotoxic responses in immunopathological diseases. Our findings therefore exemplify the delicate balance between bacterial stimulation and evasion strategies. This highlights the potential implication of the crosstalk between host innate defences and B. anthracis in initial anthrax control mechanisms.

NK cells are important immune effectors that perform a surveillance task and react to transformed, stressed, and virally infected cells. They represent a first-line defence against cancer and pathogen invasion. Different pathogens trigger distinct NK-cell activation pathways. The Bacillus anthracis spore is the highly resistant form that enters the host and provokes anthrax. This microbe kills through a combination of acute bacterial infection and devastating toxemia. In the present study, we characterise the crosstalk between NK cells and spores, as well as the strategies used by B. anthracis to evade initial control mechanisms and impact anthrax pathogenesis. Our findings exemplify the spores' property to efficiently drive a high production of IFN-γ by NK cells, as well as the complex pathways used for activation which require both cytokine and cellular signaling. B. anthracis subverts this response through its toxins by paralysing essential NK cell functions. Furthermore, edema toxin from B. anthracis blocks natural cytotoxicity without affecting IFN-γ secretion. The CyaA toxin of Bordetella pertussis possesses the same enzymatic activity and has a similar effect. The high efficiency of these toxins in blocking cytotoxicity in vivo implies possible exploitation of their subverting activity to modulate excessive cytotoxic responses in immunopathological diseases.

Cellular iron distribution in Bacillus anthracis

Although successful iron acquisition by pathogens within a host is a prerequisite for the establishment of infection, surprisingly little is known about the intracellular distribution of iron within bacterial pathogens. We have used a combination of anaerobic native liquid chromatography, inductively coupled plasma mass spectrometry, principal-component analysis, and peptide mass fingerprinting to investigate the cytosolic iron distribution in the pathogen Bacillus anthracis. Our studies identified three of the major iron pools as being associated with the electron transfer protein ferredoxin, the miniferritin Dps2, and the superoxide dismutase (SOD) enzymes SodA1 and SodA2. Although both SOD isozymes were predicted to utilize manganese cofactors, quantification of the metal ions associated with SodA1 and SodA2 in cell extracts established that SodA1 is associated with both manganese and iron, whereas SodA2 is bound exclusively to iron in vivo. These data were confirmed by in vitro assays using recombinant protein preparations, showing that SodA2 is active with an iron cofactor, while SodA1 is cambialistic, i.e., active with manganese or iron. Furthermore, we observe that B. anthracis cells exposed to superoxide stress increase their total iron content more than 2-fold over 60 min, while the manganese and zinc contents are unaffected. Notably, the acquired iron is not localized to the three identified cytosolic iron pools.

The five near-iron transporter (NEAT) domain anthrax hemophore, IsdX2, scavenges heme from hemoglobin and transfers heme to the surface protein IsdC

Pathogenic bacteria require iron to replicate inside mammalian hosts. Recent studies indicate that heme acquisition in Gram-positive bacteria is mediated by proteins containing one or more near-iron transporter (NEAT) domains. Bacillus anthracis is a spore-forming, Gram-positive pathogen and the causative agent of anthrax disease. The rapid, extensive, and efficient replication of B. anthracis in host tissues makes this pathogen an excellent model organism for the study of bacterial heme acquisition. B. anthracis secretes two NEAT hemophores, IsdX1 and IsdX2. IsdX1 contains a single NEAT domain, whereas IsdX2 has five, a novel property among hemophores. To understand the functional significance of harboring multiple, non-identical NEAT domains, we purified each individual NEAT domain of IsdX2 as a GST fusion and analyzed the specific function of each domain as it relates to heme acquisition and transport. NEAT domains 1, 3, 4, and 5 all bind heme, with domain 5 having the highest affinity. All NEATs associate with hemoglobin, but only NEAT1 and -5 can extract heme from hemoglobin, seemingly by a specific and active process. NEAT1, -3, and -4 transfer heme to IsdC, a cell wall-anchored anthrax NEAT protein. These results indicate that IsdX2 has all the features required to acquire heme from the host and transport heme to the bacterial cell wall. Additionally, these results suggest that IsdX2 may accelerate iron import rates by acting as a "heme sponge" that enhances B. anthracis replication in iron-starved environments.

Mechanism of inhibition of Bacillus anthracis spore outgrowth by the lantibiotic nisin

The lantibiotic nisin inhibits growth of vegetative Gram-positive bacteria by binding to lipid II, which disrupts cell wall biosynthesis and facilitates pore formation. Nisin also inhibits the outgrowth of bacterial spores, including spores of Bacillus anthracis, whose structural and biochemical properties are fundamentally different from those of vegetative bacteria. The molecular basis of nisin inhibition of spore outgrowth had not been identified, as previous studies suggested that inhibition of spore outgrowth involved either covalent binding to a spore target or loss of membrane integrity; disruption of cell wall biosynthesis via binding to lipid II had not been investigated. To provide insights into the latter possibility, the effects of nisin were compared with those of vancomycin, another lipid II binding antibiotic that inhibits cell wall biosynthesis but does not form pores. Nisin and vancomycin both inhibited the replication of vegetative cells, but only nisin inhibited the transition from a germinated spore to a vegetative cell. Moreover, vancomycin prevented nisin’s activity in competition studies, suggesting that the nisin-lipid II interaction is important for inhibition of spore outgrowth. In experiments with fluorescently labeled nisin, no evidence was found for a covalent mechanism for inhibition of spore outgrowth. Interestingly, mutants in the hinge region (N20P/M21P and M21P/K22P) that still bind lipid II but cannot form pores had potent antimicrobial activity against vegetative B. anthracis cells but did not inhibit spore outgrowth. Therefore, pore formation is essential for the latter activity but not the former. Collectively, these studies suggest that nisin utilizes lipid II as the germinated spore target during outgrowth inhibition and that nisin-mediated membrane disruption is essential to inhibit spore development into vegetative cells.

A plant based protective antigen [PA(dIV)] vaccine expressed in chloroplasts demonstrates protective immunity in mice against anthrax

The currently available anthrax vaccines are limited by being incompletely characterized, potentially reactogenic and have an expanded dosage schedule. Plant based vaccines offer safe alternative for vaccine production. In the present study, we expressed domain IV of Bacillus anthracis protective antigen gene [PA(dIV)] in planta (by nuclear agrobacterium and chloroplast transformation) and E. coli [rPA(dIV)]. The presence of transgene and the expression of PA(dIV) in planta was confirmed by molecular analysis. Expression levels up to 5.3% of total soluble protein (TSP) were obtained with AT rich (71.8% AT content) PA(dIV) gene in transplastomic plants while 0.8% of TSP was obtained in nuclear transformants. Further, we investigated the protective response of plant and E. coli derived PA(dIV) in mice by intraperitoneal (i.p.) and oral immunizations with or without adjuvant. Antibody titers of >10(4) were induced upon i.p. and oral immunizations with plant derived PA(dIV) and oral immunization with E. coli derived PA(dIV). Intraperitoneal injections with adjuvanted E. coli derived PA(dIV), generated highest antibody titers of >10(5). All the immunized groups demonstrated predominant IgG1 titers over IgG2a indicating a polarized Th2 type response. We also evaluated the mucosal antibody response in orally immunized groups. When fecal extracts were analyzed, low sIgA titer was demonstrated in adjuvanted plant and E. coli derived PA(dIV) groups. Further, PA(dIV) antisera enhanced B. anthracis spore uptake by macrophages in vitro and also demonstrated an anti-germinating effect suggesting a potent role at mucosal surfaces. The antibodies from various groups were efficient in neutralizing the lethal toxin in vitro. When mice were challenged with B. anthracis, mice immunized with adjuvanted plant PA(dIV) imparted 60% and 40% protection while E. coli derived PA(dIV) conferred 100% and 80% protection upon i.p. and oral immunizations. Thus, our study is the first attempt in highlighting the efficacy of plant expressed PA(dIV) by oral immunization in murine model.

The early humoral immune response to Bacillus anthracis toxins in patients infected with cutaneous anthrax

Bacillus anthracis, the causative agent of anthrax, produces a tripartite toxin composed of two enzymatically active subunits, lethal factor (LF) and edema factor (EF), which, when associated with a cell-binding component, protective antigen (PA), form lethal toxin and edema toxin, respectively. In this preliminary study, we characterized the toxin-specific antibody responses observed in 17 individuals infected with cutaneous anthrax. The majority of the toxin-specific antibody responses observed following infection were directed against LF, with immunoglobulin G (IgG) detected as early as 4 days after the onset of symptoms in contrast to the later and lower EF- and PA-specific IgG responses. Unlike the case with infection, the predominant toxin-specific antibody response of those immunized with the US anthrax vaccine absorbed and UK anthrax vaccine precipitated licensed anthrax vaccines was directed against PA. We observed that the LF-specific human antibodies were, like anti-PA antibodies, able to neutralize toxin activity, suggesting the possibility that they may contribute to protection. We conclude that an antibody response to LF might be a more sensitive diagnostic marker of anthrax than to PA. The ability of human LF-specific antibodies to neutralize toxin activity supports the possible inclusion of LF in future anthrax vaccines.

Bacillus anthracis interacts with plasmin(ogen) to evade C3b-dependent innate immunity

The causative agent of anthrax, Bacillus anthracis, is capable of circumventing the humoral and innate immune defense of the host and modulating the blood chemistry in circulation to initiate a productive infection. It has been shown that the pathogen employs a number of strategies against immune cells using secreted pathogenic factors such as toxins. However, interference of B. anthracis with the innate immune system through specific interaction of the spore surface with host proteins such as the complement system has heretofore attracted little attention. In order to assess the mechanisms by which B. anthracis evades the defense system, we employed a proteomic analysis to identify human serum proteins interacting with B. anthracis spores, and found that plasminogen (PLG) is a major surface-bound protein. PLG efficiently bound to spores in a lysine- and exosporium-dependent manner. We identified α-enolase and elongation factor tu as PLG receptors. PLG-bound spores were capable of exhibiting anti-opsonic properties by cleaving C3b molecules in vitro and in rabbit bronchoalveolar lavage fluid, resulting in a decrease in macrophage phagocytosis. Our findings represent a step forward in understanding the mechanisms involved in the evasion of innate immunity by B. anthracis through recruitment of PLG resulting in the enhancement of anti-complement and anti-opsonization properties of the pathogen.

Antibody responses to a spore carbohydrate antigen as a marker of nonfatal inhalation anthrax in rhesus macaques

The Bacillus anthracis exosporium protein BclA contains an O-linked antigenic tetrasaccharide whose terminal sugar is known as anthrose (J. M. Daubenspeck et al., J. Biol. Chem. 279:30945-30953, 2004). We hypothesized that serologic responses to anthrose may have diagnostic value in confirming exposure to aerosolized B. anthracis. We evaluated the serologic responses to a synthetic anthrose-containing trisaccharide (ATS) in a group of five rhesus macaques that survived inhalation anthrax following exposure to B. anthracis Ames spores. Two of five animals (RM2 and RM3) were treated with ciprofloxacin starting at 48 hours postexposure and two (RM4 and RM5) at 72 h postexposure; one animal (RM1) was untreated. Infection was confirmed by blood culture and detection of anthrax toxin lethal factor (LF) in plasma. Anti-ATS IgG responses were determined at 14, 21, 28, and 35 days postexposure, with preexposure serum as a control. All animals, irrespective of ciprofloxacin treatment, mounted a specific, measurable anti-ATS IgG response. The earliest detectable responses were on days 14 (RM1, RM2, and RM5), 21 (RM4), and 28 (RM3). Specificity of the anti-ATS responses was demonstrated by competitive-inhibition enzyme immunoassay (CIEIA), in which a 2-fold (wt/wt) excess of carbohydrate in a bovine serum albumin (BSA) conjugate of the oligosaccharide (ATS-BSA) effected >94% inhibition, whereas a structural analog lacking the 3-hydroxy-3-methyl-butyryl moiety at the C-4" of the anthrosyl residue had no inhibition activity. These data suggest that anti-ATS antibody responses may be used to identify aerosol exposure to B. anthracis spores. The anti-ATS antibody responses were detectable during administration of ciprofloxacin.

Bacillus anthracis spore interactions with mammalian cells: relationship between germination state and the outcome of in vitro

Background

During inhalational anthrax, internalization of Bacillus anthracis spores by host cells within the lung is believed to be a key step for initiating the transition from the localized to disseminated stages of infection. Despite compelling in vivo evidence that spores remain dormant within the bronchioalveolar spaces of the lungs, and germinate only after uptake into host cells, most in vitro studies of infection have been conducted under conditions that promote rapid germination of spores within the culture medium.

Results

Using an in vitro model of infection, we evaluated the influence of the germination state of B. anthracis spores, as controlled by defined culture conditions, on the outcome of infection. Spores prepared from B. anthracis Sterne 7702 germinated in a variety of common cell culture media supplemented with fetal bovine serum (FBS) while, in the absence of FBS, germination was strictly dependent on medium composition. RAW264.7 macrophage-like cells internalized spores to the same extent in either germinating or non-germinating media. However, significantly more viable, intracellular B. anthracis were recovered from cells infected under non-germinating conditions compared to germinating conditions. At the same time, RAW264.7 cells demonstrated a significant loss in viability when infected under non-germinating conditions.

Conclusions

These results suggest that the outcome of host cell infection is sensitive to the germination state of spores at the time of uptake. Moreover, this study demonstrates the efficacy of studying B. anthracis spore infection of host cells within a defined, non-germinating, in vitro environment.

Resolving the infection process reveals striking differences in the contribution of environment, genetics and phylogeny to host-parasite interactions

Background

Infection processes consist of a sequence of steps, each critical for the interaction between host and parasite. Studies of host-parasite interactions rarely take into account the fact that different steps might be influenced by different factors and might, therefore, make different contributions to shaping coevolution. We designed a new method using the Daphnia magna - Pasteuria ramosa system, one of the rare examples where coevolution has been documented, in order to resolve the steps of the infection and analyse the factors that influence each of them.

Results

Using the transparent Daphnia hosts and fluorescently-labelled spores of the bacterium P. ramosa, we identified a sequence of infection steps: encounter between parasite and host; activation of parasite dormant spores; attachment of spores to the host; and parasite proliferation inside the host. The chances of encounter had been shown to depend on host genotype and environment. We tested the role of genetic and environmental factors in the newly described activation and attachment steps. Hosts of different genotypes, gender and species were all able to activate endospores of all parasite clones tested in different environments; suggesting that the activation cue is phylogenetically conserved. We next established that parasite attachment occurs onto the host oesophagus independently of host species, gender and environmental conditions. In contrast to spore activation, attachment depended strongly on the combination of host and parasite genotypes.

Conclusions

Our results show that different steps are influenced by different factors. Host-type-independent spore activation suggests that this step can be ruled out as a major factor in Daphnia-Pasteuria coevolution. On the other hand, we show that the attachment step is crucial for the pronounced genetic specificities of this system. We suggest that this one step can explain host population structure and could be a key force behind coevolutionary cycles. We discuss how different steps can explain different aspects of the coevolutionary dynamics of the system: the properties of the attachment step, explaining the rapid evolution of infectivity and the properties of later parasite proliferation explaining the evolution of virulence. Our study underlines the importance of resolving the infection process in order to better understand host-parasite interactions.

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.

Modeling the host response to inhalation anthrax

Inhalation anthrax, an often fatal infection, is initiated by endospores of the bacterium Bacillus anthracis, which are introduced into the lung. To better understand the pathogenesis of an inhalation anthrax infection, we propose a two-compartment mathematical model that takes into account the documented early events of such an infection. Anthrax spores, once inhaled, are readily taken up by alveolar phagocytes, which then migrate rather quickly out of the lung and into the thoracic/mediastinal lymph nodes. En route, these spores germinate to become vegetative bacteria. In the lymph nodes, the bacteria kill the host cells and are released into the extracellular environment where they can be disseminated into the blood stream and grow to a very high level, often resulting in the death of the infected person. Using this framework as the basis of our model, we explore the probability of survival of an infected individual. This is dependent on several factors, such as the rate of migration and germination events and treatment with antibiotics.

Mechanisms of iron import in anthrax

During an infection, bacterial pathogens must acquire iron from the host to survive. However, free iron is sequestered in host proteins, which presents a barrier to iron-dependent bacterial replication. In response, pathogens have developed mechanisms to acquire iron from the host during infection. Interestingly, a significant portion of the iron pool is sequestered within heme, which is further bound to host proteins such as hemoglobin. The copious amount of heme-iron makes hemoglobin an ideal molecule for targeted iron uptake during infection. While the study of heme acquisition is well represented in Gram-negative bacteria, the systems and mechanism of heme uptake in Gram-positive bacteria has only recently been investigated. Bacillus anthracis, the causative agent of anthrax disease, represents an excellent model organism to study iron acquisition processes owing to a multifaceted lifecycle consisting of intra- and extracellular phases and a tremendous replicative potential upon infection. This review provides an in depth description of the current knowledge of B. anthracis iron acquisition and applies these findings to a general understanding of how pathogenic Gram-positive bacteria transport this critical nutrient during infection.

Interferon-inducible CXC chemokines directly contribute to host defense against inhalational anthrax in a murine model of infection

Chemokines have been found to exert direct, defensin-like antimicrobial activity in vitro, suggesting that, in addition to orchestrating cellular accumulation and activation, chemokines may contribute directly to the innate host response against infection. No observations have been made, however, demonstrating direct chemokine-mediated promotion of host defense in vivo. Here, we show that the murine interferon-inducible CXC chemokines CXCL9, CXCL10, and CXCL11 each exert direct antimicrobial effects in vitro against Bacillus anthracis Sterne strain spores and bacilli including disruptions in spore germination and marked reductions in spore and bacilli viability as assessed using CFU determination and a fluorometric assay of metabolic activity. Similar chemokine-mediated antimicrobial activity was also observed against fully virulent Ames strain spores and encapsulated bacilli. Moreover, antibody-mediated neutralization of these CXC chemokines in vivo was found to significantly increase host susceptibility to pulmonary B. anthracis infection in a murine model of inhalational anthrax with disease progression characterized by systemic bacterial dissemination, toxemia, and host death. Neutralization of the shared chemokine receptor CXCR3, responsible for mediating cellular recruitment in response to CXCL9, CXCL10, and CXCL11, was not found to increase host susceptibility to inhalational anthrax. Taken together, our data demonstrate a novel, receptor-independent antimicrobial role for the interferon-inducible CXC chemokines in pulmonary innate immunity in vivo. These data also support an immunomodulatory approach for effectively treating and/or preventing pulmonary B. anthracis infection, as well as infections caused by pathogenic and potentially, multi-drug resistant bacteria including other spore-forming organisms.

Innate immunity is critical to host defense and plays a central role in protecting the lungs from respiratory pathogens. Among the mediators important in the innate host response to pulmonary infection are chemokines, proteins originally described for their ability to regulate immune cell trafficking during an inflammatory response. More recently, chemokines have been found to exert direct antimicrobial activity against a broad range of bacteria and fungi in vitro. While these observations suggest chemokines may contribute to host defense by killing microorganisms at local sites of infection through activities not associated with cellular chemokine receptors, the biological relevance of direct chemokine-mediated antimicrobial activity in vivo has not been established. Here we show that the murine chemokines CXCL9, CXCL10, and CXCL11 exert direct antimicrobial effects against B. anthracis in vitro and that neutralization of these CXC chemokines, but not their shared receptor CXCR3, increases host susceptibility to pulmonary B. anthracis infection in vivo. These data provide unique insight into the host mediators important in host-pathogen interaction and pathogenesis of disease and support the emerging concept that host chemokines mediate efficient, pleiotropic roles that include receptor-independent promotion of host defense in vivo.

Testing nucleoside analogues as inhibitors of Bacillus anthracis spore germination in vitro and in macrophage cell culture

Bacillus anthracis, the etiological agent of anthrax, has a dormant stage in its life cycle known as the endospore. When conditions become favorable, spores germinate and transform into vegetative bacteria. In inhalational anthrax, the most fatal manifestation of the disease, spores enter the organism through the respiratory tract and germinate in phagosomes of alveolar macrophages. Germinated cells can then produce toxins and establish infection. Thus, germination is a crucial step for the initiation of pathogenesis. B. anthracis spore germination is activated by a wide variety of amino acids and purine nucleosides. Inosine and l-alanine are the two most potent nutrient germinants in vitro. Recent studies have shown that germination can be hindered by isomers or structural analogues of germinants. 6-Thioguanosine (6-TG), a guanosine analogue, is able to inhibit germination and prevent B. anthracis toxin-mediated necrosis in murine macrophages. In this study, we screened 46 different nucleoside analogues as activators or inhibitors of B. anthracis spore germination in vitro. These compounds were also tested for their ability to protect the macrophage cell line J774a.1 from B. anthracis cytotoxicity. Structure-activity relationship analysis of activators and inhibitors clarified the binding mechanisms of nucleosides to B. anthracis spores. In contrast, no structure-activity relationships were apparent for compounds that protected macrophages from B. anthracis-mediated killing. However, multiple inhibitors additively protected macrophages from B. anthracis.

Pathology of inhalational anthrax animal models

Anthrax is a lethal disease caused by the bacterium Bacillus anthracis. There are three principal forms of the disease in humans-cutaneous, gastrointestinal, and inhalational-depending on the route of exposure. Of these, inhalational anthrax is the most dangerous; it is rapidly fatal; and it has been used as a deadly biological warfare agent in the last decade. Suitable animal models of inhalational anthrax have been utilized to study pathogenesis of disease, investigate bacterial characteristics such as virulence, and test effectiveness of vaccines and therapeutics. To date, mice, guinea pigs, rabbits, and nonhuman primates are the principal animal species used to study inhalational anthrax. Mice are valuable in studying early pathogenesis and bacterial characteristics. Few pathologic changes occur in the mouse models but may include marked bacteremia and lymphocyte destruction in the spleen and mediastinal lymph nodes. Rabbits and guinea pigs rapidly develop fulminate systemic disease, and pathologic findings often include necrotizing lymphadenitis; splenitis; pneumonia; vasculitis; and hemorrhage, congestion, and edema in multiple tissues. Nonhuman primates consistently develop the full range of classic lesions of human inhalational anthrax, including meningitis; lymphadenitis; splenitis; mediastinitis; pneumonia; vasculitis; and hemorrhage, congestion, and edema in multiple tissues. This review focuses on basic characteristics of the bacterium and its products, key aspects of pathogenesis, and the pathologic changes commonly observed in each animal model species.

Spore-forming Bacilli and Clostridia in human disease

Many Gram-positive spore-forming bacteria in the Firmicute phylum are important members of the human commensal microbiota, which, in rare cases, cause opportunistic infections. Other spore-formers, however, have evolved to become dedicated pathogens that can cause a striking variety of diseases. Despite variations in disease presentation, the etiologic agent is often the spore, with bacterially produced toxins playing a central role in the pathophysiology of infection. This review will focus on the specific diseases caused by spores of the Clostridia and Bacilli.

The pathogenesis of acute pulmonary viral and bacterial infections: investigations in animal models

Acute viral and bacterial infections in the lower respiratory tract are major causes of morbidity and mortality worldwide. The proper study of pulmonary infections requires interdisciplinary collaboration among physicians and biomedical scientists to develop rational hypotheses based on clinical studies and to test these hypotheses in relevant animal models. Animal models for common lung infections are essential to understand pathogenic mechanisms and to clarify general mechanisms for host protection in pulmonary infections, as well as to develop vaccines and therapeutics. Animal models for uncommon pulmonary infections, such as those that can be caused by category A biothreat agents, are also very important because the infrequency of these infections in humans limits in-depth clinical studies. This review summarizes our understanding of innate and adaptive immune mechanisms in the lower respiratory tract and discusses how animal models for selected pulmonary pathogens can contribute to our understanding of the pathogenesis of lung infections and to the search for new vaccines and therapies.

Transport of Bacillus anthracis from the lungs to the draining lymph nodes is a rapid process facilitated by CD11c+ cells

Inhalational anthrax is established after inhaled Bacillus anthracis spores are transported to the lung associated lymph nodes. Dendritic cells (CD11c+ cells) located in the lungs are phagocytes that maintain many capabilities consistent with transport. This study investigates the role of dendritic cells as conduits of spores from the lung to the draining lymph nodes. The intratracheally spore-challenged mouse model of inhalational anthrax was utilized to investigate in vivo activities of CD11c+ cells. FITC labeled spores were delivered to the lungs of mice. Subsequently lung associated lymph nodes were isolated after infection and CD11c+ cells were found in association with the labeled spores. Further investigation of CD11c+ cells in early anthrax events was facilitated by use of the CD11c-diphtheria toxin (DT) receptor-green fluorescent protein transgenic mice in which CD11c+ cells can be transiently depleted by treatment with DT. We found that the presence of CD11c+ cells was necessary for efficient traffic of the spore to lung associated lymph nodes at early times after infection. Cultured dendritic cells were used to determine that these cells are capable of B. anthracis spore phagocytosis, and support germination and outgrowth. This data demonstrates that CD11c+ cells are likely carriers of B. anthracis spores from the point of inhalation in the lung to the lung associated lymph nodes. The cultured dendritic cell allows for spore germination and outgrowth supporting the concept that the CD11c+ cell responsible for this function can be a dendritic cell.

Genetic analysis of petrobactin transport in Bacillus anthracis

Iron acquisition mechanisms play an important role in the pathogenesis of many infectious microbes. In Bacillus anthracis, the siderophore petrobactin is required for both growth in iron-depleted conditions and for full virulence of the bacterium. Here we demonstrate the roles of two putative petrobactin binding proteins FatB and FpuA (encoded by GBAA5330 and GBAA4766 respectively) in B. anthracis iron acquisition and pathogenesis. Markerless deletion mutants were created using allelic exchange. The Delta fatB strain was capable of wild-type levels of growth in iron-depleted conditions, indicating that FatB does not play an essential role in petrobactin uptake. In contrast, Delta fpuA bacteria exhibited a significant decrease in growth under low-iron conditions when compared with wild-type bacteria. This mutant could not be rescued by the addition of exogenous purified petrobactin. Further examination of this strain demonstrated increased levels of petrobactin accumulation in the culture supernatants, suggesting no defect in siderophore synthesis or export but, instead, an inability of Delta fpuA to import this siderophore. Delta fpuA spores were also significantly attenuated in a murine model of inhalational anthrax. These results provide the first genetic evidence demonstrating the role of FpuA in petrobactin uptake.

BslA, the S-layer adhesin of B. anthracis, is a virulence factor for anthrax pathogenesis

Microbial pathogens use adhesive surface proteins to bind to and interact with host tissues, events that are universal for the pathogenesis of infectious diseases. A surface adhesin of Bacillus anthracis, the causative agent of anthrax, required to mediate these steps has not been discovered. Previous work identified BslA, an S-layer protein, to be necessary and sufficient for adhesion of the anthrax vaccine strain, Bacillus anthracis Sterne, to host cells. Here we asked whether encapsulated bacilli require BslA for anthrax pathogenesis in guinea pigs. Compared with the highly virulent parent strain B. anthracis Ames, bslA mutants displayed a dramatic increase in the lethal dose and in mean time-to-death. Whereas all tissues of animals infected with B. anthracis Ames contained high numbers of bacilli, only few vegetative forms could be recovered from internal organs of animals infected with the bslA mutant. Surface display of BslA occurred at the poles of encapsulated bacilli and enabled the binding of vegetative forms to host cells. Together these results suggest that BslA functions as the surface adhesin of the anthrax pathogen B. anthracis strain Ames.

The role of Bacillus anthracis germinant receptors in germination and virulence

Nutrient-dependent germination of Bacillus anthracis spores is stimulated when receptors located in the inner membrane detect combinations of amino acid and purine nucleoside germinants. B. anthracis produces five distinct germinant receptors, GerH, GerK, GerL, GerS and GerX. Otherwise isogenic mutant strains expressing only one of these receptors were created and tested for germination and virulence. The GerH receptor was necessary and sufficient for wild-type levels of germination with inosine-containing germinants in the absence of other receptors. GerK and GerL were sufficient for germination in 50 mM L-alanine. When mutants were inoculated intratracheally, any receptor, except for GerX, was sufficient to allow for a fully virulent infection. In contrast, when inoculated subcutaneously only the GerH receptor was able to facilitate a fully virulent infection. These results suggest that route of infection determines germinant receptor requirements. A mutant lacking all five germinant receptors was also attenuated and exhibited a severe germination defect in vitro. Together, these data give us a greater understanding of the earliest moments of germination, and provide a more detailed picture of the signals required to stimulate this process.

Regulatory interactions of a virulence-associated serine/threonine phosphatase-kinase pair in Bacillus anthracis

In the current study, we examined the regulatory interactions of a serine/threonine phosphatase (BA-Stp1), serine/threonine kinase (BA-Stk1) pair in Bacillus anthracis. B. anthracis STPK101, a null mutant lacking BA-Stp1 and BA-Stk1, was impaired in its ability to survive within macrophages, and this correlated with an observed reduction in virulence in a mouse model of pulmonary anthrax. Biochemical analyses confirmed that BA-Stp1 is a PP2C phosphatase and dephosphorylates phosphoserine and phosphothreonine residues. Treatment of BA-Stk1 with BA-Stp1 altered BA-Stk1 kinase activity, indicating that the enzymatic function of BA-Stk1 can be influenced by BA-Stp1 dephosphorylation. Using a combination of mass spectrometry and mutagenesis approaches, three phosphorylated residues, T165, S173, and S214, in BA-Stk1 were identified as putative regulatory targets of BA-Stp1. Further analysis found that T165 and S173 were necessary for optimal substrate phosphorylation, while S214 was necessary for complete ATP hydrolysis, autophosphorylation, and substrate phosphorylation. These findings provide insight into a previously undescribed Stp/Stk pair in B. anthracis.

Penetration of the blood-brain barrier by Bacillus anthracis requires the pXO1-encoded BslA protein

Anthrax is a zoonotic disease caused by the gram-positive spore-forming bacterium Bacillus anthracis. Human infection occurs after the ingestion, inhalation, or cutaneous inoculation of B. anthracis spores. The subsequent progression of the disease is largely mediated by two native virulence plasmids, pXO1 and pXO2, and is characterized by septicemia, toxemia, and meningitis. In order to produce meningitis, blood-borne bacteria must interact with and breach the blood-brain barrier (BBB) that is composed of a specialized layer of brain microvascular endothelial cells (BMEC). We have recently shown that B. anthracis Sterne is capable of penetrating the BBB in vitro and in vivo, establishing the classic signs of meningitis; however, the molecular mechanisms underlying the central nervous system (CNS) tropism are not known. Here, we show that attachment to and invasion of human BMEC by B. anthracis Sterne is mediated by the pXO1 plasmid and an encoded envelope factor, BslA. The results of studies using complementation analysis, recombinant BslA protein, and heterologous expression demonstrate that BslA is both necessary and sufficient to promote adherence to brain endothelium. Furthermore, mice injected with the BslA-deficient strain exhibited a significant decrease in the frequency of brain infection compared to mice injected with the parental strain. In addition, BslA contributed to BBB breakdown by disrupting tight junction protein ZO-1. Our results identify the pXO1-encoded BslA adhesin as a critical mediator of CNS entry and offer new insights into the pathogenesis of anthrax meningitis.

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.

Gene expression profiling of human alveolar macrophages infected by B. anthracis spores demonstrates TNF-alpha and NF-kappab are key components of the innate immune response to the pathogen

BACKGROUND

Bacillus anthracis, the etiologic agent of anthrax, has recently been used as an agent of bioterrorism. The innate immune system initially appears to contain the pathogen at the site of entry. Because the human alveolar macrophage (HAM) plays a key role in lung innate immune responses, studying the HAM response to B. anthracis is important in understanding the pathogenesis of the pulmonary form of this disease.

METHODS

In this paper, the transcriptional profile of B. anthracis spore-treated HAM was compared with that of mock-infected cells, and differentially expressed genes were identified by Affymetrix microarray analysis. A portion of the results were verified by Luminex protein analysis.

RESULTS

The majority of genes modulated by spores were upregulated, and a lesser number were downregulated. The differentially expressed genes were subjected to Ingenuity Pathway analysis, the Database for Annotation, Visualization and Integrated Discovery (DAVID) analysis, the Promoter Analysis and Interaction Network Toolset (PAINT) and Oncomine analysis. Among the upregulated genes, we identified a group of chemokine ligand, apoptosis, and, interestingly, keratin filament genes. Central hubs regulating the activated genes were TNF-alpha, NF-kappaB and their ligands/receptors. In addition to TNF-alpha, a broad range of cytokines was induced, and this was confirmed at the level of translation by Luminex multiplex protein analysis. PAINT analysis revealed that many of the genes affected by spores contain the binding site for c-Rel, a member of the NF-kappaB family of transcription factors. Other transcription regulatory elements contained in many of the upregulated genes were c-Myb, CP2, Barbie Box, E2F and CRE-BP1. However, many of the genes are poorly annotated, indicating that they represent novel functions. Four of the genes most highly regulated by spores have only previously been associated with head and neck and lung carcinomas.

CONCLUSION

The results demonstrate not only that TNF-alpha and NF-kappab are key components of the innate immune response to the pathogen, but also that a large part of the mechanisms by which the alveolar macrophage responds to B. anthracis are still unknown as many of the genes involved are poorly annotated.

The germination-specific lytic enzymes SleB, CwlJ1, and CwlJ2 each contribute to Bacillus anthracis spore germination and virulence

The bacterial spore cortex is critical for spore stability and dormancy and must be hydrolyzed by germination-specific lytic enzymes (GSLEs), which allows complete germination and vegetative cell outgrowth. We created in-frame deletions of three genes that encode GSLEs that have been shown to be active in Bacillus anthracis germination: sleB, cwlJ1, and cwlJ2. Phenotypic analysis of individual null mutations showed that the removal of any one of these genes was not sufficient to disrupt spore germination in nutrient-rich media. This finding indicates that these genes have partially redundant functions. Double and triple deletions of these genes resulted in more significant defects. Although a small subset of DeltasleB DeltacwlJ1 spores germinate with wild-type kinetics, for the overall population there is a 3-order-of-magnitude decrease in the colony-forming efficiency compared with wild-type spores. DeltasleB DeltacwlJ1 DeltacwlJ2 spores are unable to complete germination in nutrient-rich conditions in vitro. Both DeltasleB DeltacwlJ1 and DeltasleB DeltacwlJ1 DeltacwlJ2 spores are significantly attenuated, but are not completely devoid of virulence, in a mouse model of inhalation anthrax. Although unable to germinate in standard nutrient-rich media, spores lacking SleB, CwlJ1, and CwlJ2 are able to germinate in whole blood and serum in vitro, which may explain the persistent low levels of virulence observed in mouse infections. This work contributes to our understanding of GSLE activation and function during germination. This information may result in identification of useful therapeutic targets for the disease anthrax, as well as provide insights into ways to induce the breakdown of the protective cortex layer, facilitating easier decontamination of resistant spores.

Efficacy of recombinant anthrax vaccine against Bacillus anthracis aerosol spore challenge: preclinical evaluation in rabbits and Rhesus monkeys

This report describes the immunogenicity and protective efficacy of Escherichia coli-expressed recombinant protective antigen (rPA) in New Zealand White rabbits and Rhesus Macaques against an aerosol challenge with Bacillus anthracis spores (IVRI strain, tox+cap+). A dose-ranging study was performed in which it became evident that the level of anti-PA IgG and toxin-neutralizing antibody titer was directly proportional to the dose of rPA administered. However, the onset time of primary and secondary immune response was not dependent on the dosage. Revaccination of primed animals with the same threshold dose yielded a robust and rapid secondary response. Quantitative differences in peak titers were obtained for both the animal models, in addition to qualitative differences in the immune kinetics. In spite of a weak priming response, the secondary response in rabbits peaked earlier than that in macaques once the booster dose was administered. However, evaluation of the post-challenge quantitative anti-rPA ELISA titer measurements indicated higher titers for non-human primates as compared to the lagomorphs. Importantly, 100% protection was seen for the dosage groups that received > or = 25 microg rPA, following a challenge against a target dose of 1000 LD(50) of aerosolized spores of Bacillus anthracis.

Animal models of human anthrax: the Quest for the Holy Grail

Anthrax is rare among humans, few data can be collected from infected individuals and they provide a fragmentary view of the dynamics of infection and human host-pathogen interactions. Therefore, the development of animal models is necessary. Anthrax has the particularity of being a toxi-infection, a combination of infection and toxemia. The ideal animal model would explore these two different facets and mimic human disease as much as possible. In the past decades, the main effort has been focused on modelling of inhalational anthrax and the perception of specific aspects of the infection has evolved in recent years. In this review, we consider criteria which can lead to the most appropriate choice of a given animal species for modelling human anthrax. We will highlight the positive input and limitations of different models and show that they are not mutually exclusive. On the contrary, their contribution to anthrax research can be more rewarding when taken in synergy. We will also present a reappraisal of inhalational anthrax and propose reflections on key points, such as portal of entry, connections between mediastinal lymph nodes, pleura and lymphatic drainage.

Gamma-phage lysin PlyG sequence-based synthetic peptides coupled with Qdot-nanocrystals are useful for developing detection methods for Bacillus anthracis by using its surrogates, B. anthracis-Sterne and B. cereus-4342

BACKGROUND

Previous reports of site-directed deletion analysis on gamma (gamma)-phage lysin protein (PlyG) have demonstrated that removal of a short amino acid sequence in the C-terminal region encompassing a 10-amino acid motif (190LKMTADFILQ199) abrogates its binding activity specific to the cell wall of Bacillus anthracis. Whether short synthetic peptides representing the10-amino acid PlyG putative binding motif flanked by surrounding N- and C-terminal residues also selectively bind to the bacterial cell wall has not been evaluated. If such peptides do demonstrate selective binding to the cell wall, they could serve as bio-probes towards developing detection technologies for B. anthracis. Furthermore, by using B. anthracis (Sterne, 34F2), an animal vaccine and B. cereus-4342, a gamma-phage susceptible rare strain as surrogates of B. anthracis, development of proof-of-concepts for B. anthracis are feasible.

RESULTS

Using four different methods, we evaluated six synthetic peptides representing the putative binding motif including flanking sequences (PlyG-P1 through P6) for the bacterial cell wall binding capacity. Our analysis identified PlyG-P1, PlyG-P3 and PlyG-P5 to have binding capability to both B. anthracis (Sterne, 34F2) and B. cereus-4342. The peptides however did not bind to B. cereus-11778, B. thuringiensis, and B. cereus-10876 suggesting their specificity for B. anthracis-Sterne and B. cereus-4342. PlyG-P3 in combination with fluorescent light microscopy detected even a single bacterium in plasma spiked with the bacteria.

CONCLUSION

Overall, these studies illustrate that the short 10-amino acid sequence 'LKMTADFILQ' in fact is a stand-alone bacterial cell wall-binding motif of PlyG. In principle, synthetic peptides PlyG-P1, PlyG-P3 and PlyG-P5, especially PlyG-P3 coupled with Qdot-nanocrystals are useful as high-sensitivity bio-probes in developing detection technologies for B. anthracis.

Effects of altering the germination potential of Bacillus anthracis spores by exogenous means in a mouse model

Inhalational anthrax is the most severe form of anthrax. It has been shown in small-animal and non-human primate models that relatively large pools of ungerminated Bacillus anthracis spores can remain within the alveolar spaces for days to weeks post-inhalation or until transported to areas more favourable for germination and bacillary outgrowth. In this study, spores of the Ames strain that were exposed to germination-inducing media prior to intranasal delivery were significantly less infectious than spores delivered in either water or germination-inhibitory medium. The effect of manipulating the germination potential of these spores within the lungs of infected mice by exogenous germination-altering media was examined. The data suggested that neither inducing germination nor inhibiting germination of spores within the lungs protected mice from the ensuing infection. Germination-altering strategies could, instead, significantly increase the severity of disease in a mouse model of inhalational anthrax when implemented in vivo. It was shown that germination-altering strategies, in this study, were not beneficial to the infected host and are impractical as in vivo countermeasures.

Involvement of TLR2 in innate response to Bacillus anthracis infection

The involvement of TLR2 receptor in the innate response to infection with Bacillus anthracis was investigated. We studied the response to virulent or attenuated Vollum strains in either in vitro assays using macrophage cultures, or in an in vivo model comparing the sensitivity of Syrian hamster cells (expressing normal TLR2) to Chinese hamster cells (lacking functional TLR2) to infection by the various B. anthracis strains. Phagocytosis experiments with murine cell cultures or primary macrophages from both hamster strains, using virulent or attenuated Tox+Cap -, Tox-Cap+ or Tox-Cap- spores indicated that the secretion of TNF-α was induced by all the bacterial spores and purified spore antigens. In contrast, capsular antigens induce secretion of TNF-α only by Syrian hamster macrophages indicating the involvement of a functional TLR2 in macrophage activation. Challenge experiments with both hamster strains by intranasal spore inoculation, indicated that, while both strains are equally sensitive to infection with the virulent strain, the Chinese hamster demonstrated a higher sensitivity to infection with the toxinogenic or encapsulated strains. In conclusion, our findings imply that TLR2 has an important role in the attempt of the innate immunity to control B. anthracis infection, although TNF-α secretion was found to be mediated by both TLR2-dependent and TLR2-independent pathways.

Pathogenic Bacillus anthracis in the progressive gene losses and gains in adaptive evolution

Background

Sequence mutations represent a driving force of adaptive evolution in bacterial pathogens. It is especially evident in reductive genome evolution where bacteria underwent lifestyles shifting from a free-living to a strictly intracellular or host-depending life. It resulted in loss-of-function mutations and/or the acquisition of virulence gene clusters. Bacillus anthracis shares a common soil bacterial ancestor with its closely related bacillus species but is the only obligate, causative agent of inhalation anthrax within the genus Bacillus. The anthrax-causing Bacillus anthracis experienced the similar lifestyle changes. We thus hypothesized that the bacterial pathogen would follow a compatible evolution path.

Results

In this study, a cluster-based evolution scheme was devised to analyze genes that are gained by or lost from B. anthracis. The study detected gene losses/gains at two separate evolutionary stages. The stage I is when B. anthracis and its sister species within the Bacillus cereus group diverged from other species in genus Bacillus. The stage II is when B. anthracis differentiated from its two closest relatives: B. cereus and B. thuringiensis. Many genes gained at these stages are homologues of known pathogenic factors such those for internalin, B. anthracis-specific toxins and large groups of surface proteins and lipoproteins.

Conclusion

The analysis presented here allowed us to portray a progressive evolutionary process during the lifestyle shift of B. anthracis, thus providing new insights into how B. anthracis had evolved and bore a promise of finding drug and vaccine targets for this strategically important pathogen.

Antimicrobial effects of interferon-inducible CXC chemokines against Bacillus anthracis spores and bacilli

Based on previous studies showing that host chemokines exert antimicrobial activities against bacteria, we sought to determine whether the interferon-inducible Glu-Leu-Arg-negative CXC chemokines CXCL9, CXCL10, and CXCL11 exhibit antimicrobial activities against Bacillus anthracis. In vitro analysis demonstrated that all three CXC chemokines exerted direct antimicrobial effects against B. anthracis spores and bacilli including marked reductions in spore and bacillus viability as determined using a fluorometric assay of bacterial viability and CFU determinations. Electron microscopy studies revealed that CXCL10-treated spores failed to undergo germination as judged by an absence of cytological changes in spore structure that occur during the process of germination. Immunogold labeling of CXCL10-treated spores demonstrated that the chemokine was located internal to the exosporium in association primarily with the spore coat and its interface with the cortex. To begin examining the potential biological relevance of chemokine-mediated antimicrobial activity, we used a murine model of inhalational anthrax. Upon spore challenge, the lungs of C57BL/6 mice (resistant to inhalational B. anthracis infection) had significantly higher levels of CXCL9, CXCL10, and CXCL11 than did the lungs of A/J mice (highly susceptible to infection). Increased CXC chemokine levels were associated with significantly reduced levels of spore germination within the lungs as determined by in vivo imaging. Taken together, our data demonstrate a novel antimicrobial role for host chemokines against B. anthracis that provides unique insight into host defense against inhalational anthrax; these data also support the notion for an innovative approach in treating B. anthracis infection as well as infections caused by other spore-forming organisms.

Role of visible light-activated photocatalyst on the reduction of anthrax spore-induced mortality in mice

Background

Photocatalysis of titanium dioxide (TiO₂) substrates is primarily induced by ultraviolet light irradiation. Anion-doped TiO₂ substrates were shown to exhibit photocatalytic activities under visible-light illumination, relative environmentally-friendly materials. Their anti-spore activity against Bacillus anthracis, however, remains to be investigated. We evaluated these visible-light activated photocatalysts on the reduction of anthrax spore-induced pathogenesis.

Methodology/Principal Findings

Standard plating method was used to determine the inactivation of anthrax spore by visible light-induced photocatalysis. Mouse models were further employed to investigate the suppressive effects of the photocatalysis on anthrax toxin- and spore-mediated mortality. We found that anti-spore activities of visible light illuminated nitrogen- or carbon-doped titania thin films significantly reduced viability of anthrax spores. Even though the spore-killing efficiency is only approximately 25%, our data indicate that spores from photocatalyzed groups but not untreated groups have a less survival rate after macrophage clearance. In addition, the photocatalysis could directly inactivate lethal toxin, the major virulence factor of B. anthracis. In agreement with these results, we found that the photocatalyzed spores have tenfold less potency to induce mortality in mice. These data suggest that the photocatalysis might injury the spores through inactivating spore components.

Conclusion/Significance

Photocatalysis induced injuries of the spores might be more important than direct killing of spores to reduce pathogenicity in the host.

Efficacy of a vaccine based on protective antigen and killed spores against experimental inhalational anthrax

Protective antigen (PA)-based anthrax vaccines acting on toxins are less effective than live attenuated vaccines, suggesting that additional antigens may contribute to protective immunity. Several reports indicate that capsule or spore-associated antigens may enhance the protection afforded by PA. Addition of formaldehyde-inactivated spores (FIS) to PA (PA-FIS) elicits total protection against cutaneous anthrax. Nevertheless, vaccines that are effective against cutaneous anthrax may not be so against inhalational anthrax. The aim of this work was to optimize immunization with PA-FIS and to assess vaccine efficacy against inhalational anthrax. We assessed the immune response to recombinant anthrax PA from Bacillus anthracis (rPA)-FIS administered by various immunization protocols and the protection provided to mice and guinea pigs infected through the respiratory route with spores of a virulent strain of B. anthracis. Combined subcutaneous plus intranasal immunization of mice yielded a mucosal immunoglobulin G response to rPA that was more than 20 times higher than that in lung mucosal secretions after subcutaneous vaccination. The titers of toxin-neutralizing antibody and antispore antibody were also significantly higher: nine and eight times higher, respectively. The optimized immunization elicited total protection of mice intranasally infected with the virulent B. anthracis strain 17JB. Guinea pigs were fully protected, both against an intranasal challenge with 100 50% lethal doses (LD(50)) and against an aerosol with 75 LD(50) of spores of the highly virulent strain 9602. Conversely, immunization with PA alone did not elicit protection. These results demonstrate that the association of PA and spores is very much more effective than PA alone against experimental inhalational anthrax.

The Bacillus anthracis protein MprF is required for synthesis of lysylphosphatidylglycerols and for resistance to cationic antimicrobial peptides

During inhalational anthrax, Bacillus anthracis survives and replicates in alveolar macrophages, followed by rapid invasion into the host's bloodstream, where it multiplies to cause heavy bacteremia. B. anthracis must therefore defend itself from host immune functions encountered during both the intracellular and the extracellular stages of anthrax infection. In both of these niches, cationic antimicrobial peptides are an essential component of the host's innate immune response that targets B. anthracis. However, the genetic determinants of B. anthracis contributing to resistance to these peptides are largely unknown. Here we generated Tn917 transposon mutants in the DeltaANR strain (pXO1- pXO2-) of B. anthracis and screened them for altered protamine susceptibility. A protamine-sensitive mutant identified carried the transposon inserted in the BA1486 gene encoding a putative membrane protein homologous to MprF known in several gram-positive pathogens. A mutant strain with the BAS1375 gene (the orthologue of BA1486) deleted in the Sterne 34F2 strain (pXO1+ pXO2-) of B. anthracis exhibited hypersusceptibility not only to protamine but also to alpha-helical cathelicidin LL-37 and beta-sheet defensin human neutrophil peptide 1 compared to the wild-type Sterne strain. Analysis of membrane lipids using isotopic labeling demonstrated that the BAS1375 deletion mutant is unable to synthesize lysinylated phosphatidylglycerols, and this defect is rescued by genetic complementation. Further, we determined the structures of these lysylphosphatidylglycerols by using various mass spectrometric analyses. These results demonstrate that in B. anthracis a functional MprF is required for the biosynthesis of lysylphosphatidylglycerols, which is critical for resistance to cationic antimicrobial peptides.

Role of anthrax toxins in dissemination, disease progression, and induction of protective adaptive immunity in the mouse aerosol challenge model

Anthrax toxins significantly contribute to anthrax disease pathogenesis, and mechanisms by which the toxins affect host cellular responses have been identified with purified toxins. However, the contribution of anthrax toxin proteins to dissemination, disease progression, and subsequent immunity after aerosol infection with spores has not been clearly elucidated. To better understand the role of anthrax toxins in pathogenesis in vivo and to investigate the contribution of antibody to toxin proteins in protection, we completed a series of in vivo experiments using a murine aerosol challenge model and a collection of in-frame deletion mutants lacking toxin components. Our data show that after aerosol exposure to Bacillus anthracis spores, anthrax lethal toxin was required for outgrowth of bacilli in the draining lymph nodes and subsequent progression of infection beyond the lymph nodes to establish disseminated disease. After pulmonary exposure to anthrax spores, toxin expression was required for the development of protective immunity to a subsequent lethal challenge. However, immunoglobulin (immunoglobulin G) titers to toxin proteins, prior to secondary challenge, did not correlate with the protection observed upon secondary challenge with wild-type spores. A correlation was observed between survival after secondary challenge and rapid anamnestic responses directed against toxin proteins. Taken together, these studies indicate that anthrax toxins are required for dissemination of bacteria beyond the draining lymphoid tissue, leading to full virulence in the mouse aerosol challenge model, and that primary and anamnestic immune responses to toxin proteins provide protection against subsequent lethal challenge. These results provide support for the utility of the mouse aerosol challenge model for the study of inhalational anthrax.

Four superoxide dismutases contribute to Bacillus anthracis virulence and provide spores with redundant protection from oxidative stress

The Bacillus anthracis genome encodes four superoxide dismutases (SODs), enzymes capable of detoxifying oxygen radicals. That two of these SODs, SOD15 and SODA1, are present in the outermost layers of the B. anthracis spore is indicated by previous proteomic analyses of the exosporium. Given the requirement that spores must survive interactions with reactive oxygen species generated by cells such as macrophages during infection, we hypothesized that SOD15 and SODA1 protect the spore from oxidative stress and contribute to the pathogenicity of B. anthracis. To test these theories, we constructed a double-knockout (Delta sod15 Delta sodA1) mutant of B. anthracis Sterne strain 34F2 and assessed its lethality in an A/J mouse intranasal infection model. The 50% lethal dose of the Delta sod15 Delta sodA1 strain was similar to that of the wild type (34F2), but surprisingly, measurable whole-spore SOD activity was greater than that in 34F2. A quadruple-knockout strain (Delta sod15 Delta sodA1 Delta sodC Delta sodA2) was then generated, and as anticipated, spore-associated SOD activity was diminished. Moreover, the quadruple-knockout strain, compared to the wild type, was attenuated more than 40-fold upon intranasal challenge of mice. Spore resistance to exogenously generated oxidative stress and to macrophage-mediated killing correlated with virulence in A/J mice. Allelic exchange that restored sod15 and sodA1 to their wild-type state restored wild-type characteristics. We conclude that SOD molecules within the spore afford B. anthracis protection against oxidative stress and enhance the pathogenicity of B. anthracis in the lung. We also surmise that the presence of four SOD alleles within the genome provides functional redundancy for this key enzyme.

The mechanism of Bacillus anthracis intracellular germination requires multiple and highly diverse genetic loci

In an effort to better understand the mechanisms by which Bacillus anthracis establishes disease, experiments were undertaken to identify the genes essential for intracellular germination. Eighteen diverse genetic loci were identified via an enrichment protocol using a transposon-mutated library of B. anthracis spores, which was screened for mutants delayed in intracellular germination. Fourteen transposon mutants were identified in genes not previously associated with B. anthracis germination and included disruption of factors involved in membrane transport, transcriptional regulation, and intracellular signaling. Four mutants contained transposon insertions in gerHA, gerHB, gerHC, and pagA, respectively, each of which has been previously associated with germination or survival of B. anthracis within macrophages. Strain MIGD101 (named for macrophage intracellular germination defective 101) was of particular interest, since this mutant contained a transposon insertion in an intergenic region between BAs2807 and BAs2808, and was the most highly represented mutant in the enrichment. Analysis of B. anthracis MIGD101 by confocal microscopy and differential heat sensitivity following macrophage infection revealed ungerminated spores within the cell. Moreover, B. anthracis MIGD101 was attenuated in cell killing relative to the parent strain. Further experimental analysis found that B. anthracis MIGD101 was defective in five known B. anthracis germination pathways, supporting a mechanism wherein the intergenic region between BAs2807 and BAs2808 has a global affect on germination of this pathogen. Collectively, these findings provide insight into the mechanisms supporting B. anthracis germination within host cells.