Molecular pathogenesis of Bacillus anthracis infection

Analysis of anti-protective antigen IgG subclass distribution in recipients of anthrax vaccine adsorbed (AVA) and patients with cutaneous and inhalation anthrax

The anti-PA IgG1, IgG2, IgG3, and IgG4 subclass responses to clinical anthrax and to different numbers of anthrax vaccine adsorbed (AVA, BioThrax) injections were determined in a cross-sectional study of sera from 63 vaccinees and 13 clinical anthrax patients. The data show that both vaccination with three AVA injections and clinical anthrax elicit anti-PA IgG1, IgG2, and IgG3 subclass responses. An anti-PA IgG4 response was detected in AVA recipients after the fourth injection. The anthrax lethal toxin (LTx) neutralization efficacy of sera from recipients who received 4 to > or =10 AVA injections did not vary significantly in relation to changes in distribution of anti-PA IgG1 and IgG4 subclasses.

Bacterial evasion of antimicrobial peptides by biofilm formation

Biofilm formation is a main virulence determinant in many bacterial infections. It significantly increases bacterial resistance to antibiotics and innate host defense. In general, the specific physiology of biofilms and the barrier function of the extracellular biofilm matrix determine resistance to antibacterials. However, resistance to antimicrobial peptides appears to be mainly based on the interaction with biofilm and capsule exopolymers. These polymers may work by electrostatic repulsion and/or sequestration of antibacterial substances. As biofilm polymers play an eminent role in biofilm structuring and resistance, their destruction by dedicated enzymes is a promising attempt to prevent colonization and develop treatment for biofilm-associated infections.

Expression and purification of functional human anthrax toxin receptor (ATR/TEM8) binding domain from Escherichia coli

Anthrax is caused by the gram-positive, spore-forming bacterium, Bacillus anthracis. Anthrax receptors play a crucial role in the pathogenesis of the anthrax disease. Anthrax toxin receptor ATR/TEM8 VWA domain is responsible for the binding of protective antigen (PA) of B. anthracis, and thus an attractive target for structure-based drug therapies. However, the production of soluble and functional ATR/TEM8 VWA domain currently requires the use of mammalian expression systems. In this work, we expressed the ATR/TEM8 VWA domain as a fusion protein in Escherichia coli. Recombinant ATR/TEM8 VWA domain has been purified to homogeneity, and its identity has been verified by both N-terminal protein microsequencing and mass spectrometry. The purified ATR/TEM8 VWA domain exhibits very high affinity to PA based on BIAcore assay. Moreover, like the domain expressed in mammalian system, the bacterially expressed ATR/TEM8 VWA domain can block cytotoxicity induced by anthrax toxins, suggesting that the bacterially expressed ATR/TEM8 VWA domain is properly folded and fully functional.

Past, imminent and future human medical countermeasures for anthrax

AIM

Anthrax is caused by the bacterium Bacillus anthracis. Although primarily a disease of animals, it can also infect man, sometimes with fatal consequences. As a result of concerns over the illicit use of this organism, considerable effort is focussed on the development of therapies capable of conferring protection against anthrax. This brief review will describe the efforts being made to address these issues.

METHODS AND RESULTS

A review of the literature and the proceedings of the sixth international conference on anthrax, held in Santa Fe, USA in 2005 shows intense activity, but there has been as yet no real progress. While effective antibiotics, antitoxins and vaccines are available, concerns over their toxicity and the emergence of resistant strains have driven the development of second-generation products. The principal target for vaccine development is Protective Antigen (PA), the nontoxic cell-binding component of anthrax lethal toxin. While the recombinant products currently undergoing human clinical trials will offer considerable advantages in terms of reduced side effects and ease of production, they would still require multiple, needle-based dosing, and the inclusion of the adjuvant alum makes them expensive to administer and stockpile. To address these issues, researchers are developing vaccine formulations, which stimulate rapid protection following needle-free injection (nasal, oral or transcutaneous), and are stable at room temperature to facilitate stockpiling and mass vaccination programs.

CONCLUSIONS

An array of medical countermeasures targeting B. anthracis will become available over the next 5-10 years.

SIGNIFICANCE AND IMPACT OF THE STUDY

The huge investment of research dollars is expected to dramatically expand the knowledge base. A better understanding of basic issues, such as survival in nature and pathogenesis in humans, will facilitate the development of new modalities to eliminate the threat posed by this organism.

Bacillus anthracis multiplication, persistence, and genetic exchange in the rhizosphere of grass plants

Bacillus anthracis, the causative agent of anthrax, is known for its rapid proliferation and dissemination in mammalian hosts. In contrast, little information exists regarding the lifestyle of this important pathogen outside of the host. Considering that Bacillus species, including close relatives of B. anthracis, are saprophytic soil organisms, we investigated the capacity of B. anthracis spores to germinate in the rhizosphere and to establish populations of vegetative cells that could support horizontal gene transfer in the soil. Using a simple grass plant-soil model system, we show that B. anthracis strains germinate on and around roots, growing in characteristic long filaments. From 2 to 4 days postinoculation, approximately one-half of the B. anthracis CFU recovered from soil containing grass seedlings arose from heat-sensitive organisms, while B. anthracis CFU retrieved from soil without plants consisted of primarily heat-resistant spores. Co-inoculation of the plant-soil system with spores of a fertile B. anthracis strain carrying the tetracycline resistance plasmid pBC16 and a selectable B. anthracis recipient strain resulted in transfer of pBC16 from the donor to the recipient as early as 3 days postinoculation. Our findings demonstrate that B. anthracis can survive as a saprophyte outside of the host. The data suggest that horizontal gene transfer in the rhizosphere of grass plants may play a role in the evolution of the Bacillus cereus group species.

Regulation of Apoptosis by Gram-Positive Bacteria: Mechanistic Diversity and Consequences for Immunity

Apoptosis, or programmed cell death (PCD), is an important physiological mechanism, through which the human immune system regulates homeostasis and responds to diverse forms of cellular damage. PCD may also be involved in immune counteraction to microbial infection. Over the past decade, the amount of research on bacteria-induced PCD has grown tremendously, and the implications of this mechanism on immunity are being elucidated. Some pathogenic bacteria actively trigger the suicide response in critical lineages of leukocytes that orchestrate both the innate and adaptive immune responses; other bacteria proactively prevent PCD to benefit their own survival and persistence. Currently, the microbial virulence factors, which represent the keys to unlocking the suicide response in host cells, are a primary focus of this field. In this review, we discuss these bacterial "apoptosis regulatory molecules" and the apoptotic events they either trigger or prevent, the host target cells of this regulatory activity, and the possible ramifications for immunity to infection. Gram-positive pathogens including Staphylococcus, Streptococcus, Bacillus, Listeria, and Clostridia species are discussed as important agents of human infection that modulate PCD pathways in eukaryotic cells.

Proteomics study of anthrax lethal toxin-treated murine macrophages

The anthrax lethal toxin (LeTx) is composed of two proteins, protective antigen and lethal factor, which bind and enter the cell through a host receptor termed the anthrax toxin receptor (ATR). In the cell, LeTx targets p38, part of the MAP kinase signaling pathway. The toxin appears to initiate an apoptotic pathway in infected cells, indicating additional downstream targets of the toxin. We have applied a proteomics approach to investigate these downstream targets and the affected processes. In this study we have used an improved strategy for fractionation based on protein pI, off-gel electrophoresis, employed in conjunction with relative quantitation using the mass labeling approach. In our survey, 67 proteins were observed and quantified from the cytosol of RAW 264.7 cells with respect to control versus toxin-treated cells. Many of these proteins are involved in the oxidative stress response, as well as apoptosis, and thus likely to be relevant to the effects of anthrax in infected cells. Our results indicate that the tumor necrosis factor-alpha-mediated pathway is compromised in intoxicated cells. The knowledge of such changes and the pathways leading to the changes should be of great value in understanding and combating this disease.

Real-time monitoring of the membrane-binding and insertion properties of the cholesterol-dependent cytolysin anthrolysin O fromBacillus anthracis

Bacillus anthracis has recently been shown to secrete a potently hemolytic/cytolytic protein that has been designated anthrolysin O (ALO). In this work, we initiated a study of this potential anthrax virulence factor in an effort to understand the membrane-binding properties of this protein. Recombinant anthrolysin O (rALO35-512) and two N-terminally truncated versions of ALO (rALO390-512 and rALO403-512) from B. anthracis were overproduced in Escherichia coli and purified to homogeneity. The role of cholesterol in the cytolytic activity of ALO was probed in cellular cholesterol depletion assays using mouse and human macrophage-like lines, and also Drosophila Schneider 2 cells. Challenging the macrophage cells with rALO35-512, but not rALO390-512 or rALO403-512, resulted in cell death by lysis, with this cytolysis being abolished by depletion of the membrane cholesterol. Drosophila cells, which contain ergosterol as their major membrane sterol, were resistant to rALO-mediated cytolysis. In order to determine the molecular mechanism of this resistance, the interaction of rALO with model membranes comprised of POPC alone, or with a variety of structurally similar sterols including ergosterol, was probed using Biacore. Both rALO35-512 and rALO403-512 demonstrated robust binding to model membranes composed of POPC and cholesterol, with amount of protein bound proportional to the cholesterol content. Ergosterol supported greatly reduced binding of both rALO35-512 and rALO403-512, whereas other sterols tested did not support binding. The rALO403-512--membrane interaction demonstrated an equilibrium dissociation constant (KD) in the low nanomolar range, whereas rALO35-512 exhibited complex kinetics likely due to the multiple events involved in pore formation. These results establish the pivotal role of cholesterol in the action of rALO. The biosensor method developed to measure ALO recognition of cholesterol in a membrane environment could be extended to provide a platform for the screening of inhibitors of other membrane-binding proteins and peptides.

Immune evasion by staphylococci

Staphylococcus aureus can cause superficial skin infections and, occasionally, deep-seated infections that entail spread through the blood stream. The organism expresses several factors that compromise the effectiveness of neutrophils and macrophages, the first line of defence against infection. S. aureus secretes proteins that inhibit complement activation and neutrophil chemotaxis or that lyse neutrophils, neutralizes antimicrobial defensin peptides, and its cell surface is modified to reduce their effectiveness. The organism can survive in phagosomes, express polysaccharides and proteins that inhibit opsonization by antibody and complement, and its cell wall is resistant to lysozyme. Furthermore, S. aureus expresses several types of superantigen that corrupt the normal humoral immune response, resulting in anergy and immunosuppression. In contrast, Staphylococcus epidermidis must rely primarily on cell-surface polymers and the ability to form a biolfilm to survive in the host.

Induction of protective immunity to anthrax lethal toxin with a nonhuman primate adenovirus-based vaccine in the presence of preexisting anti-human adenovirus immunity

Prevention or therapy for bioterrorism-associated anthrax infections requires rapidly acting effective vaccines. We recently demonstrated (Y. Tan, N. R. Hackett, J. L. Boyer, and R. G. Crystal, Hum. Gene Ther. 14:1673-1682, 2003) that a single administration of a recombinant serotype 5 adenovirus (Ad) vector expressing anthrax protective antigen (PA) provides rapid protection against anthrax lethal toxin challenge. However, approximately 35 to 50% of humans have preexisting neutralizing antibodies against Ad5. This study assesses the hypothesis that a recombinant adenovirus vaccine based on the nonhuman primate-derived serotype AdC7, against which humans do not have immunity, expressing PA (AdC7PA) will protect against anthrax lethal toxin even in the presence of preexisting anti-Ad5 immunity. Naive and Ad5-immunized BALB/c mice received (intramuscularly) 10(8) to 10(11) particle units (PU) of AdC7PA, Ad5PA (a human serotype Ad5-based vector expressing a secreted form of PA), or AdNull (an Ad5 vector with no transgene). Robust anti-PA immunoglobulin G and neutralizing antibodies were detected by 2 to 4 weeks following administration of AdC7PA to naive or Ad5 preimmunized mice, whereas low anti-PA titers were detected in Ad5-preimmunized mice following administration of Ad5PA. To assess protection in vivo, naive or mice previously immunized against Ad5 were immunized with AdC7PA or Ad5PA and then challenged with a lethal intravenous dose of Bacillus anthracis lethal toxin. Whereas Ad5PA protected naive mice against challenge with B. anthracis lethal toxin, Ad5PA was ineffective in mice that were previously immunized against Ad5. In contrast, AdC7PA functioned effectively not only to protect naive mice but also to protect Ad5-preimmunized mice, with 100% survival after lethal toxin challenge. These data suggest the nonhuman-based vector AdC7PA is an effective vaccine for the development of protective immunity against B. anthracis and importantly functions as a "sero-switch" base for an adenovirus vaccine to function in the context of preexisting anti-Ad immunity.

Adenovirus-based genetic vaccines for biodefense

The robust host responses elicited against transgenes encoded by (E1-)(E3-) adenovirus (Ad) gene transfer vectors can be used to develop Ad-based vectors as platform technologies for vaccines against potential bioterror pathogens. This review focuses on pathogens of major concern as bioterror agents and why Ad vectors are ideal as anti-bioterror vaccine platforms, providing examples from our laboratories of using Ad vectors as vaccines against potential bioterror pathogens and how Ad vectors can be developed to enhance vaccine efficacy in the bioterror war.

Mass value assignment of total and subclass immunoglobulin G in a human standard anthrax reference serum

An anti-Anthrax Vaccine Adsorbed (anti-AVA) standard human reference serum pool, AVR414, has been prepared, and the total and protective antigen (PA)-specific immunoglobulin G (IgG) were quantified. AVR414 was prepared by plasmapheresis of healthy adults who had received a minimum of four subcutaneous injections of AVA. Mass values (in milligrams per milliliter) for total IgG and IgG subclasses 1 to 4 were determined by radial immunodiffusion. Anti-PA-specific IgG assignment (in micrograms per milliliter) was done by consensus of two complementary approaches: homologous enzyme-linked immunosorbent assay (ELISA) with affinity-purified anti-PA IgG as a calibrator and summation of mean PA-specific IgG subclass concentrations determined by IgG subclass-specific ELISA using the United States National Reference Preparation for Human Serum Proteins as a standard. The total IgG concentration assigned to AVR414 reference serum was 8.33 mg/ml. IgG subclass concentrations were the following: for IgG1, 4.48 mg/ml; for IgG2, 3.35 mg/ml; for IgG3, 0.37 mg/ml; and for IgG4, 0.30 mg/ml. The assigned mass value for total anti-PA-specific IgG was 141.2 microg/ml. Anti-PA-specific IgG subclass concentrations were the following: for IgG1, 79.6 microg/ml; for IgG2, 35.3 microg/ml; for IgG3, 3.2 microg/ml; and for IgG4, 25.3 microg/ml. Human reference serum pool AVR414 will have direct application in the standardization of anthrax serological assays, in reagent qualification, and as a standard for quantification of PA-specific IgG in humans who have been vaccinated with or otherwise exposed to Bacillus anthracis PA.

Anthrax vaccine design: strategies to achieve comprehensive protection against spore, bacillus, and toxin

The successful use of Bacillus anthracis as a lethal biological weapon has prompted renewed research interest in the development of more effective vaccines against anthrax. The disease consists of three critical components: spore, bacillus, and toxin, elimination of any of which confers at least partial protection against anthrax. Current remedies rely on postexposure antibiotics to eliminate bacilli and pre- and postexposure vaccination to target primarily toxins. Vaccines effective against toxin have been licensed for human use, but need improvement. Vaccines against bacilli have recently been developed by us and others. Whether effective vaccines will be developed against spores is still an open question. An ideal vaccine would confer simultaneous protection against spores, bacilli, and toxins. One step towards this goal is our dually active vaccine, designed to destroy both bacilli and toxin. Existing and potential strategies towards potent and effective anthrax vaccines are discussed in this review.

CpG oligodeoxynucleotides adsorbed onto polylactide-co-glycolide microparticles improve the immunogenicity and protective activity of the licensed anthrax vaccine

To reduce the biothreat posed by anthrax, efforts are under way to improve the protection afforded by vaccination. This work examines the ability of immunostimulatory CpG oligodeoxynucleotides (ODN) adsorbed onto cationic polylactide-co-glycolide (PLG) microparticles (CpG ODN-PLG) to accelerate and boost the protective immunity elicited by Anthrax Vaccine Adsorbed (AVA, the licensed human anthrax vaccine). The results indicate that coadministering CpG ODN-PLG with AVA induces a stronger and faster immunoglobulin G response against the protective antigen of anthrax than AVA alone. Immunized mice were protected from lethal anthrax challenge within 1 week of vaccination with CpG ODN-PLG plus AVA, with the level of protection correlating with serum immunoglobulin G anti-protective antigen titers.

Key role of poly-gamma-DL-glutamic acid in immune evasion and virulence of Staphylococcus epidermidis

Coagulase-negative staphylococci, with the leading species Staphylococcus epidermidis, are the predominant cause of hospital-acquired infections. Treatment is especially difficult owing to biofilm formation and frequent antibiotic resistance. However, virulence mechanisms of these important opportunistic pathogens have remained poorly characterized. Here we demonstrate that S. epidermidis secretes poly-gamma-DL-glutamic acid (PGA) to facilitate growth and survival in the human host. Importantly, PGA efficiently sheltered S. epidermidis from key components of innate host defense, namely antimicrobial peptides and neutrophil phagocytosis, and was indispensable for persistence during device-related infection. Furthermore, PGA protected S. epidermidis from high salt concentration, a key feature of its natural environment, the human skin. Notably, PGA was synthesized by all tested strains of S. epidermidis and a series of closely related coagulase-negative staphylococci, most of which are opportunistic pathogens. Our study presents important novel biological functions for PGA and indicates that PGA represents an excellent target for therapeutic maneuvers aimed at treating disease caused by S. epidermidis and related staphylococci.

Evaluation of an anti-rPA IgG ELISA for measuring the antibody response in mice

A recombinant protective antigen (rPA)-based enzyme-linked immunosorbent assay (ELISA) was developed to measure the serological response of female A/J mice after inoculation with the new rPA-based anthrax vaccine. Several fundamental parameters of the ELISA were evaluated: specificity, precision, accuracy, linearity, and stability. Experimental results suggested that the quantitative anti-rPA IgG ELISA could be used to measure antibody levels in female A/J mice and may be useful as a potency assay to monitor consistency of manufacture of a rPA-based vaccine for planned clinical trials.

Passive Immunotherapy for Anthrax Toxin Mediated by an Adenovirus Expressing an Anti-Protective Antigen Single-Chain Antibody

In the 2001 U.S. bioterror attacks, 33,000 individuals required postexposure prophylaxis, 18 subjects contracted anthrax (11 inhalation, 7 cutaneous), and despite optimal medical therapy, 5 deaths resulted. Rapid protection against anthrax is required in a bioterrorism scenario; this study describes an in vivo gene transfer-based therapy that uses a human adenovirus (Ad)-based vector (AdalphaPAscAb) encoding a single-chain antibody directed against protective antigen (PA), a critical component of Bacillus anthracis lethal toxin. Following AdalphaPAscAb administration to mice, anti-PA single-chain antibody and anti-PA neutralizing activity were detected in serum over a 2-week period. Substantial survival advantage from anthrax lethal toxin was conferred by AdalphaPAscAb following administration from 1 to 14 days prior to toxin challenge, compared to no survival associated with an Ad vector expressing a control single-chain antibody. Passive immunotherapy with an Ad-based vector may be a rapid, convenient approach for protecting a susceptible population against anthrax, including use as an adjunct to antibiotic therapy.

Binary bacterial toxins: biochemistry, biology, and applications of common Clostridium and Bacillus proteins

Certain pathogenic species of Bacillus and Clostridium have developed unique methods for intoxicating cells that employ the classic enzymatic "A-B" paradigm for protein toxins. The binary toxins produced by B. anthracis, B. cereus, C. botulinum, C. difficile, C. perfringens, and C. spiroforme consist of components not physically associated in solution that are linked to various diseases in humans, animals, or insects. The "B" components are synthesized as precursors that are subsequently activated by serine-type proteases on the targeted cell surface and/or in solution. Following release of a 20-kDa N-terminal peptide, the activated "B" components form homoheptameric rings that subsequently dock with an "A" component(s) on the cell surface. By following an acidified endosomal route and translocation into the cytosol, "A" molecules disable a cell (and host organism) via disruption of the actin cytoskeleton, increasing intracellular levels of cyclic AMP, or inactivation of signaling pathways linked to mitogen-activated protein kinase kinases. Recently, B. anthracis has gleaned much notoriety as a biowarfare/bioterrorism agent, and of primary interest has been the edema and lethal toxins, their role in anthrax, as well as the development of efficacious vaccines and therapeutics targeting these virulence factors and ultimately B. anthracis. This review comprehensively surveys the literature and discusses the similarities, as well as distinct differences, between each Clostridium and Bacillus binary toxin in terms of their biochemistry, biology, genetics, structure, and applications in science and medicine. The information may foster future studies that aid novel vaccine and drug development, as well as a better understanding of a conserved intoxication process utilized by various gram-positive, spore-forming bacteria.

CpG oligonucleotides improve the protective immune response induced by the anthrax vaccination of rhesus macaques

Synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG motifs act as immune adjuvants, improving the immune response elicited by co-administered vaccines. Combining CpG ODN with anthrax vaccine adsorbed (AVA, the licensed human vaccine) increased the speed, magnitude and avidity of the resultant anti-anthrax response. The protective activity of these Abs was established by passive transfer to anthrax-challenged mice. The ability of CpG ODN to accelerate and magnify the immune response to AVA suggests this strategy may contribute to the development of prophylactic and therapeutic vaccines against biothreat pathogens.

Murine model of pulmonary anthrax: kinetics of dissemination, histopathology, and mouse strain susceptibility

Bioweapons are most often designed for delivery to the lung, although this route is not the usual portal of entry for many of the pathogens in the natural environment. Vaccines and therapeutics that are efficacious for natural routes of infection may not be effective against the pulmonary route. Pulmonary models are needed to investigate the importance of specific bacterial genes in virulence, to identify components of the host immune system that are important in providing innate and acquired protection, and for testing diagnostic and therapeutic strategies. This report describes the characteristics of host and Bacillus anthracis interactions in a murine pulmonary-infection model. The infective dose varied depending on the route and method of inoculation. The germination process in the lung began within 1 h of inoculation into the lung, although growth within the lung was limited. B. anthracis was found in the lung-associated lymph nodes approximately 5 h after infection. Minimal pneumonitis was associated with the lung infection, but significant systemic pathology was noted after dissemination. Infected mice typically succumbed to infection approximately 3 to 4 days after inoculation. The 50% lethal doses differed among inbred strains of mice, but within a given mouse strain, neither the age nor the sex of the mice influenced susceptibility to B. anthracis.

Genetic immunization against anthrax

The objective of this study was to determine whether a DNA prime-protein boost immunization against the Bacillus anthracis protective antigen (PA) and lethal factor (LF) antigens could induce a protective immune response against significant aerosol challenge in the rabbit model. Rabbits were vaccinated with different regimens of DNA vaccines (Table 1) and aerosol challenged with B. anthracis spores, Ames strain, with an average dose of 50 LD(50s) with a range from 18 to 169 LD(50s.) Of the five vaccinated rabbits that survived, two were immunized intramuscularly (i.m.) with DNA followed with a protein boost and three were immunized subcutaneous (s.q.) with recombinant protein. A major factor predicting survival was the ability of the animal to mount a lasting antibody response to PA. Rabbit sera were collected prior to and following aerosol challenge and titrated for PA antibodies by indirect ELISA. The results of this study indicate that DNA-based immunization against PA and LF induces significant protective immunity against aerosol challenge in the rabbit model and compares favorably with protein-based immunization.

Impairment of dendritic cells and adaptive immunity by anthrax lethal toxin

Anthrax poses a clear and present danger as an agent of biological terrorism. Infection with Bacillus anthracis, the causative agent of anthrax, if untreated can result in rampant bacteraemia, multisystem dysfunction and death. Anthrax lethal toxin (LT) is a critical virulence factor of B. anthracis, which occurs as a complex of protective antigen and lethal factor. Here we demonstrate that LT severely impairs the function of dendritic cells--which are pivotal to the establishment of immunity against pathogens--and host immune responses by disrupting the mitogen-activated protein (MAP) kinase intracellular signalling network. Dendritic cells exposed to LT and then stimulated with lipopolysaccharide do not upregulate co-stimulatory molecules, secrete greatly diminished amounts of proinflammatory cytokines, and do not effectively stimulate antigen-specific T cells in vivo. Furthermore, injections of LT induce a profound impairment of antigen-specific T- and B-cell immunity. These data suggest a role for LT in suppressing host immunity during B. anthracis infections, and represent an immune evasion strategy, where a microbe targets MAP kinases in dendritic cells to disarm the immune response.

Bacillus anthracis: current knowledge in relation to contamination of food

In this article, information related to anthrax and its etiologic agent, Bacillus anthracis, in food is reviewed. The major topics discussed include the taxonomic relationship of B. anthracis to other Bacillus species, methods used for the recovery of the organism from surfaces and foods, routes of infection, the pathogenesis of the organism, the microbial ecology of the vegetative cell and spore in foods and the environment, chemical and physical treatments for spore inactivation, and the control of the disease in animals.

Bacillus anthracis

The events of 11 September 2001 and the subsequent anthrax outbreaks have shown that the West needs to be prepared for an increasing number of terrorist attacks, which may include the use of biological warfare. Bacillus anthracis has long been considered a potential biological warfare agent, and this review will discuss the history of its use as such. It will also cover the biology of this organism and the clinical features of the three disease forms that it can produce: cutaneous, gastrointestinal, and inhalation anthrax. In addition, treatment and vaccination strategies will be reviewed.

Bacillus anthracis virulence in Guinea pigs vaccinated with anthrax vaccine adsorbed is linked to plasmid quantities and clonality

Bacillus anthracis is a bacterial pathogen of great importance, both historically and in the present. This study presents data collected from several investigations and indicates that B. anthracis virulence is associated with the clonality and virulence of plasmids pXO1 and pXO2. Guinea pigs vaccinated with Anthrax Vaccine Adsorbed were challenged with 20 B. anthracis isolates representative of worldwide genetic diversity. These same isolates were characterized with respect to plasmid copy number by using a novel method of quantitative PCR developed for rapid and efficient detection of B. anthracis from environmental samples. We found that the copy numbers for both pXO1 and pXO2 differed from those in previously published reports. By combining the data on survival, plasmid copy numbers, and clonality, we developed a model predicting virulence. This model was validated by using a randomly chosen set of 12 additional B. anthracis isolates. Results from this study will be helpful in future efforts to elucidate the basis for variation in the virulence of this important pathogen.

Introduction: anthrax history, disease and ecology

The familiarity with the ancient disease anthrax from the second millennium B.C. through the second millennium A.D. is reviewed, providing the backdrop to the modern understanding of this disease as covered in the remainder of the volume. By means of an overview of the aetiology, ecology, epidemiology, clinical manifestations, pathology and bacteriology of the naturally acquired disease, this opening chapter also lays down the groundwork for the subsequent state-of-the-art chapters.

Anthrax vaccines

The only impetus for the development of new anthrax vaccines is to protect humans against the intentional use of Bacillus anthracis as a bioterrorist or warfare agent. Live attenuated vaccines against anthrax in domesticated animals were among the very first vaccines developed. This was followed by the development of nonliving component vaccines leading to the eventual licensure of protein-based vaccines for human use in the 1970s. This chapter will review the recent advances in developing protein, live attenuated, and genetic vaccines against anthrax.

Search for potential vaccine candidate open reading frames in the Bacillus anthracis virulence plasmid pXO1: in silico and in vitro screening

A genomic analysis of the Bacillus anthracis virulence plasmid pXO1, aimed at identifying potential vaccine candidates and virulence-related genes, was carried out. The 143 previously defined open reading frames (ORFs) (R. T. Okinaka, K. Cloud, O. Hampton, A. R. Hoffmaster, K. K. Hill, P. Keim, T. M. Koehler, G. Lamke, S. Kumano, J. Mahillon, D. Manter, Y. Martinez, D. Ricke, R. Svensson, and P. J. Jackson, J. Bacteriol. 181:6509-6515, 1999) were subjected to extensive sequence similarity searches (with the nonredundant and unfinished microbial genome databases), as well as motif, cellular location, and domain analyses. A comparative genomics analysis was conducted with the related genomes of Bacillus subtilis, Bacillus halodurans, and Bacillus cereus and the pBtoxis plasmid of Bacillus thuringiensis var. israeliensis. As a result, the percentage of ORFs with clues about their functions increased from approximately 30% (as previously reported) to more than 60%. The bioinformatics analysis permitted identification of novel genes with putative relevance for pathogenesis and virulence. Based on our analyses, 11 putative proteins were chosen as targets for functional genomics studies. A rapid and efficient functional screening method was developed, in which PCR-amplified full-length linear DNA products of the selected ORFs were transcribed and directly translated in vitro and their immunogenicities were assessed on the basis of their reactivities with hyperimmune anti-B. anthracis antisera. Of the 11 ORFs selected for analysis, 9 were successfully expressed as full-length polypeptides, and 3 of these were found to be antigenic and to have immunogenic potential. The latter ORFs are currently being evaluated to determine their vaccine potential.

Anthrax toxin: a tripartite lethal combination

Anthrax is a severe bacterial infection that occurs when Bacillus anthracis spores gain access into the body and germinate in macrophages, causing septicemia and toxemia. Anthrax toxin is a binary A-B toxin composed of protective antigen (PA), lethal factor (LF), and edema factor (EF). PA mediates the entry of either LF or EF into the cytosol of host cells. LF is a zinc metalloprotease that inactivates mitogen-activated protein kinase kinase inducing cell death, and EF is an adenylyl cyclase impairing host defences. Inhibitors targeting different steps of toxin activity have recently been developed. Anthrax toxin has also been exploited as a therapeutic agent against cancer.

2001: a year of major advances in anthrax toxin research

Anthrax is caused when spores of Bacillus anthracis enter a host and germinate. The bacteria multiply and secrete a tripartite toxin causing local edema and, in systemic infection, death. In nature, anthrax is primarily observed in cattle and other herbivores; humans are susceptible but rarely affected. In 2001, anthrax spores were used effectively for the first time in bioterrorist attacks, resulting in 11 confirmed cases of human disease and five deaths. These events have underscored the need for improved prophylaxis, therapeutics and a molecular understanding of the toxin. The good news about anthrax is that several decisive discoveries regarding the toxin have been reported recently. Most notably, the toxin receptor was identified, the 3-D structures of two of the toxin subunits were solved and potent in vivo inhibitors were designed. These findings have improved our understanding of the intoxication mechanism and are stimulating the design of strategies to fight disease in the future.

Human antibodies against spores of the genus Bacillus: a model study for detection of and protection against anthrax and the bioterrorist threat

A naive, human single-chain Fv (scFv) phage-display library was used in bio-panning against live, native spores of Bacillus subtilis IFO 3336 suspended in solution. A direct in vitro panning and enzyme-linked immunosorbent assay-based selection afforded a panel of nine scFv-phage clones of which two, 5B and 7E, were chosen for further study. These two clones differed in their relative specificity and affinity for spores of B. subtilis IFO 3336 vs. a panel of spores from 11 other Bacillus species/strains. A variety of enzyme-linked immunosorbent assay protocols indicated these scFv-phage clones recognized different spore epitopes. Notably, some spore epitopes markedly changed between the free and microtiter-plate immobilized state as revealed by antibody-phage binding. An additional library selection procedure also was examined by constructing a Fab chain-shuffled sublibrary from the nine positive clones and by using a subtractive panning strategy to remove crossreactivity with B. licheniformis 5A24. The Fab-phage clone 52 was improved compared with 5B and was comparable to 7E in binding B. subtilis IFO 3336 vs. B. licheniformis 5A24, yet showed a distinctive crossreactivity pattern with other spores. We also developed a method to directly detect individual spores by using fluorescently labeled antibody-phage. Finally, a variety of "powders" that might be used in deploying spores of B. anthracis were examined for antibody-phage binding. The strategies described provide a foundation to discover human antibodies specific for native spores of B. anthracis that can be developed as diagnostic and therapeutic reagents.

Bacillus anthracis pXO1 plasmid sequence conservation among closely related bacterial species

The complete sequencing and annotation of the 181.7-kb Bacillus anthracis virulence plasmid pXO1 predicted 143 genes but could only assign putative functions to 45. Hybridization assays, PCR amplification, and DNA sequencing were used to determine whether pXO1 open reading frame (ORF) sequences were present in other bacilli and more distantly related bacterial genera. Eighteen Bacillus species isolates and four other bacterial species were tested for the presence of 106 pXO1 ORFs. Three ORFs were conserved in most of the bacteria tested. Many of the pXO1 ORFs were detected in closely related Bacillus species, and some were detected only in B. anthracis isolates. Three isolates, Bacillus cereus D-17, B. cereus 43881, and Bacillus thuringiensis 33679, contained sequences that were similar to more than one-half of the pXO1 ORF sequences examined. The majority of the DNA fragments that were amplified by PCR from these organisms had DNA sequences between 80 and 98% similar to that of pXO1. Pulsed-field gel electrophoresis revealed large potential plasmids present in both B. cereus 43881 (341 kb) and B. thuringiensis ATCC 33679 (327 kb) that hybridized with a DNA probe composed of six pXO1 ORFs.

Protection against anthrax lethal toxin challenge by genetic immunization with a plasmid encoding the lethal factor protein

The ability of genetic vaccination to protect against a lethal challenge of anthrax toxin was evaluated. BALB/c mice were immunized via gene gun inoculation with eucaryotic expression vector plasmids encoding either a fragment of the protective antigen (PA) or a fragment of lethal factor (LF). Plasmid pCLF4 contains the N-terminal region (amino acids [aa] 10 to 254) of Bacillus anthracis LF cloned into the pCI expression plasmid. Plasmid pCPA contains a biologically active portion (aa 175 to 764) of B. anthracis PA cloned into the pCI expression vector. One-micrometer-diameter gold particles were coated with plasmid pCLF4 or pCPA or a 1:1 mixture of both and injected into mice via gene gun (1 microg of plasmid DNA/injection) three times at 2-week intervals. Sera were collected and analyzed for antibody titer as well as antibody isotype. Significantly, titers of antibody to both PA and LF from mice immunized with the combination of pCPA and pCLF4 were four to five times greater than titers from mice immunized with either gene alone. Two weeks following the third and final plasmid DNA boost, all mice were challenged with 5 50% lethal doses of lethal toxin (PA plus LF) injected intravenously into the tail vein. All mice immunized with pCLF4, pCPA, or the combination of both survived the challenge, whereas all unimmunized mice did not survive. These results demonstrate that DNA-based immunization alone can provide protection against a lethal toxin challenge and that DNA immunization against the LF antigen alone provides complete protection.

Genetic, physical, and transcript map of the Ltxs1 region of mouse chromosome 11

Lethal factor (LF) is a toxin secreted by Bacillus anthracis that plays an important role in the pathogenesis of anthrax. Intoxication with LF results in a macrophage-specific cytolysis that is not well understood. Interestingly, inbred mouse strains exhibit dramatic differences in the susceptibility of their cultured macrophages to killing by LF, and a gene that influences this phenotype, called Ltxs1, has been mapped to mouse chromosome 11. Here we report a high-resolution genetic map that confines the Ltxs1 region to a 0.51-cM interval between D11Mit90 and D11Die37/D11Die38. We have also constructed a complete physical map of YAC and BAC clones covering the Ltxs1 region. In conjunction with synteny homology searching, BLAST searches of sequences obtained from the clones in the physical map have revealed 14 known genes and five ESTs that reside in the critical interval. Additionally, a region of 100 kb or more is deleted in the Ltxs1 interval of some strains. Our genetic, physical, and transcript map provides an important resource for the molecular cloning of Ltxs1.

Bacillus anthracis, a bug with attitude!

The sequencing of the Bacillus anthracis genome and virulence plasmids represents the greatest advance in anthrax research in the past 100 years. The data will provide the foundation of all future work on this organism and will be invaluable to researchers in their battle to understand the basis of the host-microbe interaction.