Understanding Bacillus anthracis pathogenesis

Persistent enrichment of multidrug-resistant Klebsiella in oral and nasal communities during long-term starvation

BACKGROUND

The human oral and nasal cavities can act as reservoirs for opportunistic pathogens capable of causing acute infection. These microbes asymptomatically colonize the human oral and nasal cavities which facilitates transmission within human populations via the environment, and they routinely possess clinically significant antibiotic resistance genes. Among these opportunistic pathogens, the Klebsiella genus stands out as a notable example, with its members frequently linked to nosocomial infections and multidrug resistance. As with many colonizing opportunistic pathogens, the essential transmission factors influencing the spread of Klebsiella species among both healthy and diseased individuals remain unclear.

RESULTS

Here, we explored a possible explanation by investigating the ability of oral and nasal Klebsiella species to outcompete their native microbial community members under in vitro starvation conditions, which could be analogous to external hospital environments or the microenvironment of mechanical ventilators. When K. pneumoniae and K. aerogenes were present within a healthy human oral or nasal sample, the bacterial community composition shifted dramatically under starvation conditions and typically became enriched in Klebsiella species. Furthermore, introducing K. pneumoniae exogenously into a native microbial community lacking K. pneumoniae, even at low inoculum, led to repeated enrichment under starvation. Precise monitoring of K. pneumoniae within these communities undergoing starvation indicated rapid initial growth and prolonged viability compared to other members of the microbiome. K. pneumoniae strains isolated from healthy individuals' oral and nasal cavities also exhibited resistance to multiple classes of antibiotics and were genetically similar to clinical and gut isolates. In addition, we found that in the absence of Klebsiella species, other understudied opportunistic pathogens, such as Peptostreptococcus, increased in relative abundance under starvation conditions.

CONCLUSIONS

Our findings establish an environmental and microbiome community circumstance that allows for the enrichment of Klebsiella species and other opportunistic pathogens. Klebsiella's enrichment may hinge on its ability to quickly outgrow other members of the microbiome. The ability to outcompete other commensal bacteria and to persist under harsh environmental conditions could be an important factor that contributes to enhanced transmission in both commensal and pathogenic contexts. Video Abstract.

Persistent enrichment of multidrug resistant Klebsiella in oral and nasal communities during long-term starvation

The human oral and nasal cavities can act as reservoirs for opportunistic pathogens capable of causing acute infection. These microbes asymptomatically colonize the human oral and nasal cavities which facilitates transmission within human populations via the environment, and they routinely possess a clinically-significant antibiotic-resistance genes. Among these opportunistic pathogens, the Klebsiella genus stands out as a notable example, with its members frequently linked to nosocomial infections and multidrug resistance. As with many colonizing opportunistic pathogens, how Klebsiella transitions from an asymptomatic colonizer to a pathogen remains unclear. Here, we explored a possible explanation by investigating the ability of oral and nasal Klebsiella to outcompete their native microbial community members under in vitro starvation conditions, which could be analogous to external hospital environments. When Klebsiella was present within a healthy human oral or nasal sample, the bacterial community composition shifted dramatically under starvation conditions and typically became dominated by Klebsiella. Furthermore, introducing K. pneumoniae exogenously into a native microbial community lacking K. pneumoniae, even at low inoculum, led to repeated dominance under starvation. K.pneumoniae strains isolated from healthy individuals' oral and nasal cavities also exhibited resistance to multiple classes of antibiotics and were genetically similar to clinical and gut isolates. In addition, we found that in the absence of Klebsiella, other understudied opportunistic pathogens, such as Peptostreptococcus, dominate under starvation conditions. Our findings establish an environmental circumstance that allows for the outgrowth of Klebsiella and other opportunistic pathogens. The ability to outcompete other commensal bacteria and to persist under harsh environmental conditions may contribute to the colonization-to-infection transition of these opportunistic pathogens.

A design of experiments screen reveals that Clostridium novyi-NT spore germinant sensing is stereoflexible for valine and its analogs

Although Clostridium novyi-NT is an anti-cancer bacterial therapeutic which germinates within hypoxic tumors to kill cancer cells, the actual germination triggers for C. novyi-NT are still unknown. In this study, we screen candidate germinants using combinatorial experimental designs and discover by serendipity that D-valine is a potent germinant, inducing 50% spore germination at 4.2 mM concentration. Further investigation revealed that five D-valine analogs are also germinants and four of these analogs are enantiomeric pairs. This stereoflexible effect of L- and D-amino acids shows that spore germination is a complex process where enantiomeric interactions can be confounders. This study also identifies L-cysteine as a germinant, and hypoxanthine and inosine as co-germinants. Several other amino acids promote (L-valine, L-histidine, L-threonine and L-alanine) or inhibit (L-arginine, L-glycine, L-lysine, L-tryptophan) germination in an interaction-dependent manner. D-alanine inhibits all germination, even in complex growth media. This work lays the foundation for improving the germination efficacy of C. novyi-NT spores in tumors.

Cutaneous Anthrax in a Farmer Man: A Case Report

Introduction:

Anthrax is an acute infection caused by Bacillus anthracis, an anaerobic, spore-forming, Gram-positive bacterium. Anthrax disease is a common disease among herbivores and humans that is transmitted to humans through direct contact with farm animals that are sick or have died of anthrax.

Case Summary:

A 43-year-old man having livestock as a profession had a cutaneous anthrax infection on the dorsal surface of his left thumb. The patient was infected with anthrax by slaughtering a sheep infected with Bacillus anthracis. He was treated with penicillin after an early diagnosis of anthrax bacillus.

Conclusion:

Anthrax infection is a highly contagious disease, and early detection is very important. There is a need to strengthen the health care system for farm animals and educate farmers on how not to consume the meat of diseased carcasses and how to dispose them of hygienically, especially in high-risk areas.

Rapid Presumptive Identification of Bacillus anthracis Isolates Using the Tetracore RedLine Alert™ Test

A comprehensive laboratory evaluation of the Tetracore RedLine Alert test, a lateral flow immunoassay (LFA) for the rapid presumptive identification of Bacillus anthracis, was conducted at 2 different test sites. The study evaluated the sensitivity of this assay using 16 diverse strains of B. anthracis grown on sheep blood agar (SBA) plates. In addition, 83 clinically relevant microorganisms were tested to assess the specificity of the RedLine Alert test. The results indicated that the RedLine Alert test for the presumptive identification of B. anthracis is highly robust, specific, and sensitive. RedLine Alert is a rapid test that has applicability for use in a clinical setting for ruling-in or ruling-out nonhemolytic colonies of Bacillus spp. grown on SBA medium as presumptive isolates of B. anthracis.

Characterization of Bacillus Subtilis Viruses vB_BsuM-Goe2 and vB_BsuM-Goe3

The Spounavirinae viruses are ubiquitous in nature and have an obligatory virulent lifestyle. They infect Firmicutes, a bacterial phylum containing an array of environmental non-pathogenic and pathogenic organisms. To expand the knowledge of this viral subfamily, new strains were isolated and investigated in this study. Here we present two new viruses, vB_BsuM-Goe2 and vB_BsuM-Goe3, isolated from raw sewage and infecting Bacillus species. Both were morphologically classified via transmission electron microscopy (TEM) as members of the Spounavirinae subfamily belonging to the Myoviridae family. Genomic sequencing and analyses allowed further affiliation of vB_BsuM-Goe2 to the SPO1-like virus group and vB_BsuM-Goe3 to the Bastille-like virus group. Experimentally determined adsorption constant, latency period, burst size and host range for both viruses revealed different survival strategies. Thus vB_BsuM-Goe2 seemed to rely on fewer host species compared to vB_BsuM-Goe3, but efficiently recruits those. Stability tests pointed out that both viruses are best preserved in LB-medium or TMK-buffer at 4 or 21 °C, whereas cryopreservation strongly reduced viability.

Comprehensive Laboratory Evaluation of a Highly Specific Lateral Flow Assay for the Presumptive Identification of Bacillus anthracis Spores in Suspicious White Powders and Environmental Samples

We conducted a comprehensive, multiphase laboratory evaluation of the Anthrax BioThreat Alert(®) test strip, a lateral flow immunoassay (LFA) for the rapid detection of Bacillus anthracis spores. The study, conducted at 2 sites, evaluated this assay for the detection of spores from the Ames and Sterne strains of B. anthracis, as well as those from an additional 22 strains. Phylogenetic near neighbors, environmental background organisms, white powders, and environmental samples were also tested. The Anthrax LFA demonstrated a limit of detection of about 10(6) spores/mL (ca. 1.5 × 10(5) spores/assay). In this study, overall sensitivity of the LFA was 99.3%, and the specificity was 98.6%. The results indicated that the specificity, sensitivity, limit of detection, dynamic range, and repeatability of the assay support its use in the field for the purpose of qualitatively evaluating suspicious white powders and environmental samples for the presumptive presence of B. anthracis spores.

Ser/Thr protein kinase PrkC-mediated regulation of GroEL is critical for biofilm formation in Bacillus anthracis

PrkC is a conserved Ser/Thr protein kinase encoded in Bacillus anthracis genome. PrkC is shown to be important for B. anthracis pathogenesis, but little is known about its other functions and phosphorylated substrates. Systemic analyses indicate the compelling role of PrkC in phosphorylating multiple substrates, including the essential chaperone GroEL. Through mass spectrometry, we identified that PrkC phosphorylates GroEL on six threonine residues that are distributed in three canonical regions. Phosphorylation facilitates the oligomerization of GroEL to the physiologically active tetradecameric state and increases its affinity toward the co-chaperone GroES. Deletion of prkC in B. anthracis abrogates its ability to form biofilm. Overexpression of native GroEL recovers the biofilm-forming ability of prkC deletion strain. Similar overexpression of GroEL phosphorylation site mutants (Thr to Ala) does not augment biofilm formation. Further analyses indicate the phosphorylation of GroEL in diverse bacterial species. Thus, our results suggest that PrkC regulates biofilm formation by modulating the GroEL activity in a phosphorylation-dependent manner. The study deciphers the molecular signaling events that are important for biofilm formation in B. anthracis.

An enzyme that adds phosphate groups to other proteins, PrkC, mediates molecular signaling events that allow anthrax bacteria to form biofilms. Bacillus anthracis is widely used as a model to explore the formation of biofilms that allows many bacterial infections to resist immune defenses. An international research team led by Yogendra Singh and Andaleeb Sajid at the CSIR-Institute of Genomics and Integrative Biology in Delhi, India, studied the bacterial protein kinase PrkC. The researchers found that PrkC phosphorylates a “chaperone” protein that assist the assembly and disassembly of other protein-based structures. This signaling protein and the chaperone help in biofilm formation. Establishing this link in the signaling chain leading to biofilms will guide future research to combat the role of biofilms in disease.

Bacterial Toxins as Pathogen Weapons Against Phagocytes

Bacterial toxins are virulence factors that manipulate host cell functions and take over the control of vital processes of living organisms to favor microbial infection. Some toxins directly target innate immune cells, thereby annihilating a major branch of the host immune response. In this review we will focus on bacterial toxins that act from the extracellular milieu and hinder the function of macrophages and neutrophils. In particular, we will concentrate on toxins from Gram-positive and Gram-negative bacteria that manipulate cell signaling or induce cell death by either imposing direct damage to the host cells cytoplasmic membrane or enzymatically modifying key eukaryotic targets. Outcomes regarding pathogen dissemination, host damage and disease progression will be discussed.

Development of a Zealand white rabbit deposition model to study inhalation anthrax

Despite using rabbits in several inhalation exposure experiments to study diseases such as anthrax, there is a lack of understanding regarding deposition characteristics and fate of inhaled particles (bio-aerosols and viruses) in the respiratory tracts of rabbits. Such information allows dosimetric extrapolation to humans to inform human outcomes. The lung geometry of the New Zealand white rabbit (referred to simply as rabbits throughout the article) was constructed using recently acquired scanned images of the conducting airways of rabbits and available information on its acinar region. In addition, functional relationships were developed for the lung and breathing parameters of rabbits as a function of body weight. The lung geometry and breathing parameters were used to extend the existing deposition model for humans and several other species to rabbits. Evaluation of the deposition model for rabbits was made by comparing predictions with available measurements in the literature. Deposition predictions in the lungs of rabbits indicated smaller deposition fractions compared to those found in humans across various particle diameter ranges. The application of the deposition model for rabbits was demonstrated by extrapolating deposition predictions in rabbits to find equivalent human exposure concentrations assuming the same dose-response relationship between the two species. Human equivalent exposure concentration levels were found to be much smaller than those for rabbits.

Gastrointestinal helminths may affect host susceptibility to anthrax through seasonal immune trade-offs

Background

Most vertebrates experience coinfections, and many pathogen-pathogen interactions occur indirectly through the host immune system. These interactions are particularly strong in mixed micro-macroparasite infections because of immunomodulatory effects of helminth parasites. While these trade-offs have been examined extensively in laboratory animals, few studies have examined them in natural systems. Additionally, many wildlife pathogens fluctuate seasonally, at least partly due to seasonal host immune changes. We therefore examined seasonality of immune resource allocation, pathogen abundance and exposure, and interactions between infections and immunity in plains zebra (Equus quagga) in Etosha National Park (ENP), Namibia, a system with strongly seasonal patterns of gastrointestinal (GI) helminth infection intensity and concurrent anthrax outbreaks. Both pathogens are environmentally transmitted, and helminth seasonality is driven by environmental pressures on free living life stages. The reasons behind anthrax seasonality are currently not understood, though anthrax is less likely directly driven by environmental factors.

Results

We measured a complex, interacting set of variables and found evidence that GI helminth infection intensities, eosinophil counts, IgE and IgGb antibody titers, and possibly IL-4 cytokine signaling were increased in wetter seasons, and that ectoparasite infestations and possibly IFN-γ cytokine signaling were increased in drier seasons. Monocyte counts and anti-anthrax antibody titers were negatively associated with wet season eosinophilia, and monocytes were negatively correlated with IgGb and IgE titers. Taken together, this supports the hypothesis that ENP wet seasons are characterized by immune resource allocation toward Th-2 type responses, while Th1-type immunity may prevail in drier seasons, and that hosts may experience Th1-Th2 trade-offs. We found evidence that this Th2-type resource allocation is likely driven by GI parasite infections, and that these trade-offs may render hosts less capable of concurrently mounting effective Th1-type immune responses against anthrax.

Conclusions

This study is one of the first to examine laboratory-demonstrated Th1-Th2 trade-offs in a natural system. It provides evidence that seasonally bound pathogens may affect, through immunology, transmission dynamics of pathogens that might otherwise not be seasonally distributed. It suggests that, by manipulating the internal host ecosystem, GI parasites may influence the external ecosystem by affecting the dynamics of another environmentally transmitted pathogen.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-014-0027-3) contains supplementary material, which is available to authorized users.

spxA2, encoding a regulator of stress resistance in Bacillus anthracis, is controlled by SaiR, a new member of the Rrf2 protein family

Spx, a member of the ArsC (arsenate reductase) protein family, is conserved in Gram-positive bacteria, and interacts with RNA polymerase to activate transcription in response to toxic oxidants. In Bacillus anthracis str. Sterne, resistance to oxidative stress requires the activity of two paralogues, SpxA1 and SpxA2. Suppressor mutations were identified in spxA1 mutant cells that conferred resistance to hydrogen peroxide. The mutations generated null alleles of the saiR gene and resulted in elevated spxA2 transcription. The saiR gene resides in the spxA2 operon and encodes a member of the Rrf2 family of transcriptional repressors. Derepression of spxA2 in a saiR mutant required SpxA2, indicating an autoregulatory mechanism of spxA2 control. Reconstruction of SaiR-dependent control of spxA2 was accomplished in Bacillus subtilis, where deletion analysis uncovered two cis-elements within the spxA2 regulatory region that are required for repression. Mutations to one of the sequences of dyad symmetry substantially reduced SaiR binding and SaiR-dependent repression of transcription from the spxA2 promoter in vitro. Previous studies have shown that spxA2 is one of the most highly induced genes in a macrophage infected with B. anthracis. The work reported herein uncovered a key regulator, SaiR, of the Spx system of stress response control.

Centers for disease control and prevention expert panel meetings on prevention and treatment of anthrax in adults

The Centers for Disease Control and Prevention convened panels of anthrax experts to review and update guidelines for anthrax postexposure prophylaxis and treatment. The panels included civilian and military anthrax experts and clinicians with experience treating anthrax patients. Specialties represented included internal medicine, pediatrics, obstetrics, infectious disease, emergency medicine, critical care, pulmonology, hematology, and nephrology. Panelists discussed recent patients with systemic anthrax; reviews of published, unpublished, and proprietary data regarding antimicrobial drugs and anthrax antitoxins; and critical care measures of potential benefit to patients with anthrax. This article updates antimicrobial postexposure prophylaxis and antimicrobial and antitoxin treatment options and describes potentially beneficial critical care measures for persons with anthrax, including clinical procedures for infected nonpregnant adults. Changes from previous guidelines include an expanded discussion of critical care and clinical procedures and additional antimicrobial choices, including preferred antimicrobial drug treatment for possible anthrax meningitis.

NMR conformational properties of an Anthrax Lethal Factor domain studied by multiple amino acid-selective labeling

NMR-based structural biology urgently needs cost- and time-effective methods to assist both in the process of acquiring high-resolution NMR spectra and their subsequent analysis. Especially for bigger proteins (>20 kDa) selective labeling is a frequently used means of sequence-specific assignment. In this work we present the successful overexpression of a polypeptide of 233 residues, corresponding to the structured part of the N-terminal domain of Anthrax Lethal Factor, using Escherichia coli expression system. The polypeptide was subsequently isolated in pure, soluble form and analyzed structurally by solution NMR spectroscopy. Due to the non-satisfying quality and resolution of the spectra of this 27 kDa protein, an almost complete backbone assignment became feasible only by the combination of uniform and novel amino acid-selective labeling schemes. Moreover, amino acid-type selective triple-resonance NMR experiments proved to be very helpful.

Frequent and seasonally variable sublethal anthrax infections are accompanied by short-lived immunity in an endemic system

Few studies have examined host-pathogen interactions in wildlife from an immunological perspective, particularly in the context of seasonal and longitudinal dynamics. In addition, though most ecological immunology studies employ serological antibody assays, endpoint titre determination is usually based on subjective criteria and needs to be made more objective. Despite the fact that anthrax is an ancient and emerging zoonotic infectious disease found world-wide, its natural ecology is not well understood. In particular, little is known about the adaptive immune responses of wild herbivore hosts against Bacillus anthracis. Working in the natural anthrax system of Etosha National Park, Namibia, we collected 154 serum samples from plains zebra (Equus quagga), 21 from springbok (Antidorcas marsupialis) and 45 from African elephants (Loxodonta africana) over 2-3 years, resampling individuals when possible for seasonal and longitudinal comparisons. We used enzyme-linked immunosorbent assays to measure anti-anthrax antibody titres and developed three increasingly conservative models to determine endpoint titres with more rigourous, objective mensuration. Between 52 and 87% of zebra, 0-15% of springbok and 3-52% of elephants had measurable anti-anthrax antibody titres, depending on the model used. While the ability of elephants and springbok to mount anti-anthrax adaptive immune responses is still equivocal, our results indicate that zebra in ENP often survive sublethal anthrax infections, encounter most B. anthracis in the wet season and can partially booster their immunity to B. anthracis. Thus, rather than being solely a lethal disease, anthrax often occurs as a sublethal infection in some susceptible hosts. Though we found that adaptive immunity to anthrax wanes rapidly, subsequent and frequent sublethal B. anthracis infections cause maturation of anti-anthrax immunity. By triggering host immune responses, these common sublethal infections may act as immunomodulators and affect population dynamics through indirect immunological and co-infection effects. In addition, with our three endpoint titre models, we introduce more mensuration rigour into serological antibody assays, even under the often-restrictive conditions that come with adapting laboratory immunology methods to wild systems. With these methods, we identified significantly more zebras responding immunologically to anthrax than have previous studies using less comprehensive titre analyses.

Potential use of inhibitors of bacteria spore germination in the prophylactic treatment of anthrax andClostridium difficile-associated disease

Spore germination is the first step in establishing Bacillus and Clostridium infections. Germination is triggered by the binding of small molecules by the resting spore. Subsequently, the activated spore secretes dipicolinic acid and calcium, the spore core is rehydrated and spore structures are degraded. Inhibition of any of the germination-related events will prevent development to the vegetative stage. Inhibition of spore germination has been studied intensively in the prevention of food spoilage. In this perspective, we propose that similar approaches could be used in the prophylactic control of Bacillus anthracis and Clostridium difficile infections. Inhibition of B. anthracis spore germination could protect military and first-line emergency personnel at high risk for anthrax exposure. Inhibition of C. difficile could prevent human C. difficile-associated disease during antibiotic treatment of immunocompromised patients.

Germination and amplification of anthrax spores by soil-dwelling amoebas

While anthrax is typically associated with bioterrorism, in many parts of the world the anthrax bacillus (Bacillus anthracis) is endemic in soils, where it causes sporadic disease in livestock. These soils are typically rich in organic matter and calcium that promote survival of resilient B. anthracis spores. Outbreaks of anthrax tend to occur in warm weather following rains that are believed to concentrate spores in low-lying areas where runoff collects. It has been concluded that elevated spore concentrations are not the result of vegetative growth as B. anthracis competes poorly against indigenous bacteria. Here, we test an alternative hypothesis in which amoebas, common in moist soils and pools of standing water, serve as amplifiers of B. anthracis spores by enabling germination and intracellular multiplication. Under simulated environmental conditions, we show that B. anthracis germinates and multiplies within Acanthamoeba castellanii. The growth kinetics of a fully virulent B. anthracis Ames strain (containing both the pX01 and pX02 virulence plasmids) and vaccine strain Sterne (containing only pX01) inoculated as spores in coculture with A. castellanii showed a nearly 50-fold increase in spore numbers after 72 h. In contrast, the plasmidless strain 9131 showed little growth, demonstrating that plasmid pX01 is essential for growth within A. castellanii. Electron and time-lapse fluorescence microscopy revealed that spores germinate within amoebal phagosomes, vegetative bacilli undergo multiplication, and, following demise of the amoebas, bacilli sporulate in the extracellular milieu. This analysis supports our hypothesis that amoebas contribute to the persistence and amplification of B. anthracis in natural environments.

Risk factors associated with anthrax in cattle on smallholdings

SUMMARY Unprecedented high rates of anthrax outbreaks have been observed recently in cattle and humans in Bangladesh, with 607 human cases in 2010. By enrolling 15 case and 15 control cattle smallholdings in the spatial zone in July-September 2010, we conducted a case-control study, data of which were analysed by matched-pair analysis and multivariable conditional logistic regression. Feeding animals with uprooted and unwashed grass [odds ratio (OR) 41·2, 95% confidence interval (CI) 3·7-458·8, P=0·003], and feeding water hyacinth (Eichhornia crassipes) (OR 22·2, 95% CI 1·2-418·7, P=0·039) were independent risk factors for anthrax in cattle.

Multigenic control and sex bias in host susceptibility to spore-induced pulmonary anthrax in mice

Mechanisms underlying susceptibility to anthrax infection are unknown. Using a phylogenetically diverse panel of inbred mice and spores of Bacillus anthracis Ames, we investigated host susceptibility to pulmonary anthrax. Susceptibility profiles for survival time and organ pathogen load differed across strains, indicating distinct genetic controls. Tissue infection kinetics analysis showed greater systemic dissemination in susceptible DBA/2J (D) mice but a higher terminal bacterial load in resistant BALB/cJ (C) mice. Interestingly, the most resistant strains, C and C57BL/6J (B), demonstrated a sex bias for susceptibility. For example, BALB/cJ females had a significantly higher survival time and required 4-fold more spores for 100% mortality compared to BALB/cJ males. To identify genetic regions associated with differential susceptibility, survival time and extent of organ infection were assessed using mice derived from two susceptibility models: (i) BXD advanced recombinant inbred strains and (ii) F2 offspring generated from polar responding C and D strains. Genome-wide analysis of BXD strain survival identified linkage on chromosomes 5, 6, 9, 11, and 14. Quantitative trait locus (QTL) analysis of the C×DF2 population revealed a significant QTL (designated Rpai1 for resistance to pulmonary anthrax infection, locus 1) for survival time on chromosome 17 and also identified a chromosome 11 locus for lung pathogen burden. The striking difference between genome-wide linkage profiles for these two mouse models of anthrax susceptibility supports our hypothesis that these are multigenic traits. Our data provide the first evidence for a differential sex response to anthrax resistance and further highlight the unlikelihood of a single common genetic contribution for this response across strains.

Role of luxS in Bacillus anthracis growth and virulence factor expression

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

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.

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.

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.

Purification and biophysical characterization of the core protease domain of anthrax lethal factor

Anthrax lethal toxin (LeTx) stands for the major virulence factor of the anthrax disease. It comprises a 90kDa highly specific metalloprotease, the anthrax lethal factor (LF). LF possesses a catalytic Zn(2+) binding site and is highly specific against MAPK kinases, thus representing the most potent native biomolecule to alter and inactivate MKK [MAPK (mitogen-activated protein kinase) kinases] signalling pathways. Given the importance of the interaction between LF and substrate for the development of anti-anthrax agents as well as the potential treatment of nascent tumours, the analysis of the structure and dynamic properties of the LF catalytic site are essential to elucidate its enzymatic properties. Here we report the recombinant expression and purification of a C-terminal part of LF (LF(672-776)) that harbours the enzyme's core protease domain. The biophysical characterization and backbone assignments ((1)H, (13)C, (15)N) of the polypeptide revealed a stable, well folded structure even in the absence of Zn(2+), suitable for high resolution structural analysis by NMR.

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.

Insights into the anthrax lethal factor-substrate interaction and selectivity using docking and molecular dynamics simulations

The anthrax toxin of the bacterium Bacillus anthracis consists of three distinct proteins, one of which is the anthrax lethal factor (LF). LF is a gluzincin Zn-dependent, highly specific metalloprotease with a molecular mass of approximately 90 kDa that cleaves most isoforms of the family of mitogen-activated protein kinase kinases (MEKs/MKKs) close to their amino termini, resulting in the inhibition of one or more signaling pathways. Previous studies on the crystal structures of uncomplexed LF and LF complexed with the substrate MEK2 or a MKK-based synthetic peptide provided structure-activity correlations and the basis for the rational design of efficient inhibitors. However, in the crystallographic structures, the substrate peptide was not properly oriented in the active site because of the absence of the catalytic zinc atom. In the current study, docking and molecular dynamics calculations were employed to examine the LF-MEK/MKK interaction along the catalytic channel up to a distance of 20 A from the zinc atom. This residue-specific view of the enzyme-substrate interaction provides valuable information about: (i) the substrate selectivity of LF and its inactivation of MEKs/MKKs (an issue highly important not only to anthrax infection but also to the pathogenesis of cancer), and (ii) the discovery of new, previously unexploited, hot-spots of the LF catalytic channel that are important in the enzyme/substrate binding and interaction.

Pulmonary deposition of aerosolized Bacillus atrophaeus in a Swine model due to exposure from a simulated anthrax letter incident

Dry anthrax spore powder is readily disseminated as an aerosol and it is possible that passive dispersion when opening a letter containing anthrax spores may result in lethal doses to humans. The specific aim of this study was to quantify the respirable aerosol hazard associated with opening an envelope/letter contaminated with a dry spore powder of the biological pathogen anthrax in a typical office environment. An envelope containing a letter contaminated with 1.0 g of dry Bacillus atrophaeus (BG) spores (pathogen simulant) was opened in the presence of an unrestrained swine model. Aerosolized spores were detected in the room in seconds and peak concentrations occurred by three minutes. The swine, located approximately 1.5 m from the source, was exposed to the aerosol for 28 min following the letter opening event and then moved to a clean room for 30 min. A necropsy was completed to determine the extent of in vivo spore deposition in the lungs. The median number of viable colony forming units (CFU) measured in the combined right and left lung was 21,200: the average mass of both lungs was 283 g. In excess of 100 CFU per gram of lung tissue was found at sites within the anterior, intermediate and posterior lobes. The results of this study confirmed that opening an envelope containing spores generated an aerosol spanning the respirable particle size range of 1-10 microm, and that normal respiration of swine led to spore deposition throughout the lungs. The observed deposition of spores in the lungs of the swine is within the LD(50) range of 2,500-55,000 estimated for humans for inhaled anthrax. Thus, there would appear to be a significant health risk to those individuals exposed to anthrax spores when opening a contaminated envelope.

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.

Roles of germination-specific lytic enzymes CwlJ and SleB in Bacillus anthracis

The structural characteristics of a spore enable it to withstand stresses that typically kill a vegetative cell. Spores remain dormant until small molecule signals induce them to germinate into vegetative bacilli. Germination requires degradation of the thick cortical peptidoglycan by germination-specific lytic enzymes (GSLEs). Bacillus anthracis has four putative GSLEs, based upon sequence similarities with enzymes in other species: SleB, CwlJ1, CwlJ2, and SleL. In this study, the roles of SleB, CwlJ1, and CwlJ2 were examined. The expression levels of all three genes peak 3.5 h into sporulation. Genetic analysis revealed that, similar to other known GSLEs, none of these gene products are individually required for growth, sporulation, or triggering of germination. However, later germination events are affected in spores lacking CwlJ1 or SleB. Compared to the wild type, germinating spores without CwlJ1 suffer a delay in optical density loss and cortex peptidoglycan release. The absence of SleB also causes a delay in cortex fragment release. A double mutant lacking both SleB and CwlJ1 is completely blocked in cortex hydrolysis and progresses through outgrowth to produce colonies at a frequency 1,000-fold lower than that of the wild-type strain. A null mutation eliminating CwlJ2 has no effect on germination. High-performance liquid chromatography and mass spectroscopy analysis revealed that SleB is required for lytic transglycosylase activity. CwlJ1 also clearly participates in cortex hydrolysis, but its specific mode of action remains unclear. Understanding the lytic germination activities that naturally diminish spore resistance can lead to methods for prematurely inducing them, thus simplifying the process of treating contaminated sites.

Risk factors associated with anthrax outbreak in animals in North Dakota, 2005: a retrospective case-control study

OBJECTIVE

We identified the risk factors associated with the anthrax outbreak Of 2005 in animals in North Dakota.

METHODS

Medical records of the 2005 anthrax outbreak were obtained from the Veterinary Diagnostic Laboratory at North Dakota State University. Additional data were obtained from the North Dakota state veterinarian's office, and supplemental questionnaires were administered to producers. The data obtained included ecological and environmental factors, animal health factors, and management factors.

RESULTS

Anthrax occurred from July 1 to October 12, 2005. The cases were located in eastern North Dakota around the Red River Basin. Ransom, LaMoure, and Barnes counties reported most cases (71%). Species affected included cattle, bison, horses, sheep, elk, deer, pigs, and llamas. The predominant symptom was sudden death (38%) followed by bleeding from orifices (17%). Chi-square analysis indicated significant differences between case and control premises on the following variables: death reported on neighboring pasture, vaccination period, dry conditions, wet conditions, antibiotic use, multiple vaccination, and type of predator (coyote). Factors that significantly (p<0.05) predicted anthrax occurrences on the final logistic regression model were vaccination, use of antibiotics during an outbreak, and period of vaccine administration (before or during the outbreak).

CONCLUSIONS

The characteristics of the anthrax outbreak regarding time and place of occurrence, animals affected, clinical signs reported, and mortality rate were consistent with previous reports of natural anthrax outbreaks in animals. A number of factors that significantly predicted anthrax occurrence in animals in the 2005 outbreak in North Dakota were identified. This information is important in planning appropriate control and prevention measures for anthrax, including recommending the right vaccination and treatment regimens in managing future anthrax outbreaks.

Progressive and destructive hair follicle infections in a murine cutaneous anthrax model

Hair follicles may allow pathogen entry because they represent potential barrier defects and because there is immunological privilege within actively growing follicles. Experimental cutaneous Bacillus anthracis infections in mice have previously shown prominent organism invasion and proliferation within hair follicles. For the present study, C57BL/6 mice were inoculated with B. anthracis (Sterne) spores onto abraded skin with either anagen (actively growing) or telogen (inactive) hair follicles; skin samples were evaluated by histologic methods and electron microscopy. The infections were found to progress similarly in either anagen or telogen hair follicles, with bacilli occasionally invading deeper sites in anagen hair follicles. The infections progressed from the surface inward, rather than growing outward from within the follicles. Infecting bacilli destroyed the hair follicle keratinocytes and were initially not contacted by inflammatory cells within the follicles. However, at 3-4 days after inoculation, inflammatory cells did contact and disperse the massed follicle bacilli and led to apparent resolution of the follicle infections. Therefore, in this model system B. anthracis initially attacks superficial sites in active or inactive hair follicles and then progresses inward, producing destructive infections of the hair follicles; these infections clear when the massed bacilli are eventually contacted and dispersed by inflammatory cells.

Superficial exudates of neutrophils prevent invasion of Bacillus anthracis bacilli into abraded skin of resistant mice

Skin window procedures in humans have shown rapid accumulation of neutrophils into the exuded fluids above abraded skin. The present study was undertaken to determine if similar epicutaneous neutrophil accumulation might explain the extreme resistance of HRS/J mice, both hairless (hr/hr) and haired (hr/+), to experimental cutaneous Bacillus anthracis Sterne infections on abraded skin. In this study, very early (6 h) biopsies demonstrated a lack of bacilli in skin from the HRS/J hr/hr mice, indicating that the organisms never did invade in these animals as opposed to early skin entry and then efficient clearance by host responses in the tissues. Touch preparations of either the inoculation filter or the skin surface revealed more inflammatory cells, fewer bacilli, and a higher percentage of cell-associated bacilli in the HRS/J hr/hr mice than in comparator strains. In the HRS/J mice, cyclophosphamide treatment or separation of inoculated spores from the inflammatory infiltrates by a second filter below both produced marked increases in the number of bacilli observed. Examination of inoculation filter specimens demonstrated ingestion of spores and bacilli by neutrophils inside the filter at 6 h after inoculation. These findings suggest that an early and vigorous inflammatory cell infiltrate in HRS/J mice attacks the inoculated organisms above the skin surface and does not allow them to invade the tissues below.

A review of management practices for the control of anthrax in animals: the 2005 anthrax epizootic in North Dakota--case study

Outbreaks of anthrax have diverse consequences on society. Establishing the appropriate control strategies is very important and crucial in reducing the socio-economic impact of the disease. Control measures are aimed at breaking the cycle of infection, and their implementation must be adhered to rigorously. The objectives of this paper were: (i) to review the control strategies currently used in management of anthrax in animals and (ii) to describe management strategies used by producers in North Dakota during the 2005 anthrax outbreak in livestock. Anthrax control strategies were divided in to strategies that apply before, during, and after an anthrax outbreak. This paper also highlights the problems or constraints faced by North Dakota producers in controlling anthrax during the outbreak of 2005.

Early interactions between fully virulent Bacillus anthracis and macrophages that influence the balance between spore clearance and development of a lethal infection

The role of macrophages in the pathogenesis of anthrax is unresolved. Macrophages are believed to support the initiation of infection by Bacillus anthracis spores, yet are also sporicidal. Furthermore, it is believed that the anthrax toxins suppress normal macrophage function. However, the significance of toxin effects on macrophages has not been addressed in an in vivo infection model. We used mutant derivatives of murine macrophage RAW264.7 cells that are toxin receptor-negative (R3D) to test the role of toxin-targeting of macrophages during a challenge with spores of the Ames strain of B. anthracis in both in vivo and in vitro models. We found that the R3D cells were able to control challenge with Ames when mice were inoculated with the cells prior to spore challenge. These findings were confirmed in vitro by high dose spore infection of macrophages. Interestingly, whereas the R3D cells provided a significantly greater survival advantage against spores than did the wild type RAW264.7 cells or R3D-complemented cells, the protection afforded the mutant and wild type cells was equivalent against a bacillus challenge. The findings appear to be the first specific test of the role of toxin targeting of macrophages during infection with B. anthracis spores.

The integrin Mac-1 (CR3) mediates internalization and directs Bacillus anthracis spores into professional phagocytes

Anthrax, a disease caused by Bacillus anthracis, affects animals and humans. Because the inert spore is the infectious form of the organism that first contacts the potential host, the interaction between the host and spore exosporium is vital to the initiation of disease. Here, we demonstrate that the integrin Mac-1 is essential for the recognition of the major exosporium protein BclA by phagocytic cells. Expression of Mac-1, but not p150/95, in CHO cells markedly enhanced infection with Sterne strain of B. anthracis spores (WT spores). Conversely, CD11b(-/-) macrophages demonstrated a significant decrease in spore uptake when compared with macrophages from normal C57BL/6 mice. However, when CD11b(-/-) macrophages were infected with DeltabclA spores, spore ingestion was no different from their C57BL/6 counterparts. DeltabclA spores were also efficiently internalized by all CHO cell lines tested, independently of Mac-1 expression. Taken together, these results show that there is an alternative Mac-1-independent pathway involved in spore uptake that is unmasked only in the absence of BclA. Survival studies, using C57BL/6 and CD11b(-/-) mice, revealed that CD11b(-/-) mice are more resistant to infection with WT but not DeltabclA spores. Our experiments also show that DeltabclA spores are more virulent than WT spores in C57BL/6 and A/J mice. Overall, our data indicate that the Mac-1/BclA interaction may play a major role in B. anthracis pathogenesis by promoting spore uptake by professional phagocytes and subsequent access to a favorable niche for transport, germination, and outgrowth in lymphoid tissues.

Potential dissemination of Bacillus anthracis utilizing human lung epithelial cells

Dissemination of Bacillus anthracis spores from the lung is a critical early event in the establishment of inhalational anthrax. We recently reported that B. anthracis could adhere to and be internalized by cultured intestinal epithelial and fibroblast cells. Here, using gentamicin protection assays and/or electron microscopy, we found that Sterne strain 7702 spores were able to adhere to and subsequently be internalized by polarized A549 cells and primary human small airway epithelial cells. We showed for the first time that internalized spores were able to survive and that spores could translocate across an A549 cell barrier from the apical side to the basolateral side without disrupting the barrier integrity, suggesting a transcellular route. In addition, dormant spores of fully virulent Ames and UT500 strains were able to adhere to A549 cells at a frequency similar to that of 7702, whereas the capsule in germinated Ames and UT500 spores prevented adherence. Fluorescence microscopy also revealed that dormant Ames spores were internalized at a frequency similar to that of 7702. These findings highlight the possibility of a novel route of dissemination in which B. anthracis utilizes epithelial cells of the lung. The implications of these results to B. anthracis pathogenesis are discussed.

Effects of endogenous D-alanine synthesis and autoinhibition of Bacillus anthracis germination on in vitro and in vivo infections

Bacillus anthracis transitions from a dormant spore to a vegetative bacillus through a series of structural and biochemical changes collectively referred to as germination. The timing of germination is important during early steps in infection and may determine if B. anthracis survives or succumbs to responsive macrophages. In the current study experiments determined the contribution of endogenous D-alanine production to the efficiency and timing of B. anthracis spore germination under in vitro and in vivo conditions. Racemase-mediated production of endogenous D-alanine by B. anthracis altered the kinetics for initiation of germination over a range of spore densities and exhibited a threshold effect wherein small changes in spore number resulted in major changes in germination efficiency. This threshold effect correlated with D-alanine production, was prevented by an alanine racemase inhibitor, and required L-alanine. Interestingly, endogenous production of inhibitory levels of D-alanine was detected under experimental conditions that did not support germination and in a germination-deficient mutant of B. anthracis. Racemase-dependent production of D-alanine enhanced survival of B. anthracis during interaction with murine macrophages, suggesting a role for inhibition of germination during interaction with these cells. Finally, in vivo experiments revealed an approximately twofold decrease in the 50% lethal dose of B. anthracis spores administered in the presence of D-alanine, indicating that rates of germination may be directly influenced by the levels of this amino acid during early stages of disease.

Proteasomes control caspase-1 activation in anthrax lethal toxin-mediated cell killing

Activation of caspase-1 through the inflammasome protein Nalp1b controls anthrax lethal toxin (LT)-induced necrosis in murine macrophages. In this study we analyzed physiological changes controlled by caspase-1 in LT-treated murine macrophages. The caspase-1 inhibitor Boc-D-cmk blocked caspase-1 activity and membrane impairment in LT-treated cells. To determine the relationship between caspase-1 activation and membrane integrity, we added Boc-D-cmk to J774A.1 macrophages at different time points following LT exposure. Remarkably, Boc-D-cmk rescued LT-treated macrophages, even when added at the peak of caspase-1 activation. Late addition of the caspase-1 inhibitor reversed the losses of plasma membrane integrity and metabolic activity in these cells. Similar results were obtained with the proteasome inhibitor MG132, one of the most potent inhibitors of LT toxicity. LT-treated macrophages displaying evidence of membrane impairment recovered upon the addition of MG132, mirroring the Boc-D-cmk response. Strikingly, late addition of proteasome inhibitors also abrogated caspase-1 activity in LT-treated macrophages. Proteasomal control of caspase-1 activity and membrane impairment, however, was restricted to LT-induced cytolysis, because proteasome inhibitors did not block caspase-1 activation and cell death triggered by lipopolysaccharide and nigericin. Our findings indicate that proteasome inhibitors do not target caspase-1 directly but instead control an upstream event in LT-treated macrophages leading to caspase-1 activation. Taken together, caspase-1-mediated necrosis appears to be tightly controlled and differentially regulated by proteasomes depending on the source of caspase-1 induction.

Protective role of Bacillus anthracis exosporium in macrophage-mediated killing by nitric oxide

The ability of the endospore-forming, gram-positive bacterium Bacillus anthracis to survive in activated macrophages is key to its germination and survival. In a previous publication, we discovered that exposure of primary murine macrophages to B. anthracis endospores upregulated NOS 2 concomitant with an .NO-dependent bactericidal response. Since NOS 2 also generates O(2).(-), experiments were designed to determine whether NOS 2 formed peroxynitrite (ONOO(-)) from the reaction of .NO with O(2).(-) and if so, was ONOO(-) microbicidal toward B. anthracis. Our findings suggest that ONOO(-) was formed upon macrophage infection by B. anthracis endospores; however, ONOO(-) does not appear to exhibit microbicidal activity toward this bacterium. In contrast, the exosporium of B. anthracis, which exhibits arginase activity, protected B. anthracis from macrophage-mediated killing by decreasing .NO levels in the macrophage. Thus, the ability of B. anthracis to subvert .NO production has important implications in the control of B. anthracis-induced infection.

Electrical detection of germination of viable model Bacillus anthracis spores in microfluidic biochips

In this paper, we present a new impedance-based method to detect viable spores by electrically detecting their germination in real time within microfluidic biochips. We used Bacillus anthracis Sterne spores as the model organism. During germination, the spores release polar and ionic chemicals, such as dipicolinic acid (DPA), calcium ions, phosphate ions, and amino acids, which correspondingly increase the electrical conductivity of the medium in which the spores are suspended. We first present macro-scale measurements demonstrating that the germination of spores can be electrically detected at a concentration of 10(9) spores ml(-1) in sample volumes of 5 ml, by monitoring changes in the solution conductivity. Germination was induced by introducing an optimized germinant solution consisting of 10 mM L-alanine and 2 mM inosine. We then translated these results to a micro-fluidic biochip, which was a three-layer device: one layer of polydimethylsiloxane (PDMS) with valves, a second layer of PDMS with micro-fluidic channels and chambers, and the third layer with metal electrodes deposited on a pyrex substrate. Dielectrophoresis (DEP) was used to trap and concentrate the spores at the electrodes with greater than 90% efficiency, at a solution flow rate of 0.2 microl min(-1) with concentration factors between 107-109 spores ml(-1), from sample volumes of 1-5 microl. The spores were captured by DEP in deionized water within 1 min (total volume used ranged from 0.02 microl to 0.2 microl), and then germinant solution was introduced to the flow stream. The detection sensitivity was demonstrated to be as low as about a hundred spores in 0.1 nl, which is equivalent to a macroscale detection limit of approximately 10(9) spores ml(-1). We believe that this is the first demonstration of this application in microfluidic and BioMEMS devices.

Identification of an in vivo inhibitor of Bacillus anthracis spore germination

Germination of Bacillus anthracis spores into the vegetative form is an essential step in anthrax pathogenicity. This process can be triggered in vitro by the common germinants inosine and alanine. Kinetic analysis of B. anthracis spore germination revealed synergy and a sequential mechanism between inosine and alanine binding to their cognate receptors. Because inosine is a critical germinant in vitro, we screened inosine analogs for the ability to block in vitro germination of B. anthracis spores. Seven analogs efficiently blocked this process in vitro. This led to the identification of 6-thioguanosine, which also efficiently blocked spore germination in macrophages and prevented killing of these cells mediated by B. anthracis spores. 6-Thioguanosine shows potential as an anti-anthrax therapeutic agent.

Selection of human antibodies against cell surface-associated oligomeric anthrax protective antigen

The protective antigen (PA(83)) of Bacillus anthracis is the dominant antigen in natural and vaccine-induced immunity to anthrax infection. Three human single-chain variable fragments (scFvs) against cell bound PA were isolated from an antibody phage display library. Specifically, the antibodies were evaluated for their ability to bind to cell bound heptameric PA and ultimately protect against the cytotoxicity of lethal toxin. In total, all three scFvs possessed neutralizing activity against the cytotoxic effects of lethal toxin in a macrophage lysis assay. The K(d) values of the Fabs were determined, interestingly their protective effects did not parallel their affinities; hence, a simple binding argument alone to PA(63) cannot be used as the distinguishing feature for the prediction of their neutralization abilities. Immunofluorescent microscopy experiments were conducted and provided strong evidence for Fab binding to oligomeric PA on the cell surface and thus a plausible mechanism for the toxin neutralization activity that was observed. The results of this study presented herein suggest that our antibodies compete with LF-PA cell surface interactions, and thus may provide potential application of human antibodies as passive immunization prophylactics in cases of B. anthracis exposure and infection.

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.

Monoclonal antibodies for Bacillus anthracis spore detection and functional analyses of spore germination and outgrowth

All members of the Bacillus genus produce endospores as part of their life cycle; however, it is not possible to determine the identity of spores by casual or morphological examination. The 2001 anthrax attacks demonstrated a need for fast, dependable methods for detecting Bacillus anthracis spores in vitro and in vivo. We have developed a variety of isotypes and specificities of mAbs that were able to distinguish B. anthracis spores from other Bacillus spores. The majority of Abs were directed toward BclA, a major component of the exosporium, although other components were also distinguished. These Abs did not react with vegetative forms. Some Abs distinguished B. anthracis spores from spores of distantly related species in a highly specific manner, whereas others discriminated among strains that are the closest relatives of B. anthracis. These Abs provide a rapid and reliable means of identifying B. anthracis spores, for probing the structure and function of the exosporium, and in the analysis of the life cycle of B. anthracis.

Importance of nitric oxide synthase in the control of infection by Bacillus anthracis

The spore-forming, gram-positive bacterium Bacillus anthracis, the causative agent of anthrax, has achieved notoriety due to its use as a bioterror agent. In the environment, B. anthracis exists as a dormant endospore. Upon infection, germination of endospores occurs during their internalization within the phagocyte, and the ability to survive exposure to antibacterial killing mechanisms, such as O2*-, NO*, and H2O2, is a key initial event in the infective process. Macrophages generate NO* from the oxidative metabolism of L-arginine, using an isoform of nitric oxide synthase (NOS 2). Exposure of murine macrophages (RAW264.7 cells) to B. anthracis endospores up-regulated the expression of NOS 2 12 h after exposure, and production of NO* was comparable to that achieved following other bacterial infections. Spore-killing assays demonstrated a NO*-dependent bactericidal response that was significantly decreased in the presence of the NOS 2 inhibitor L-N6-(1-iminoethyl)lysine and in L-arginine-depleted media. Interestingly, we also found that B. anthracis bacilli and endospores exhibited arginase activity, possibly competing with host NOS 2 for its substrate, L-arginine. As macrophage-generated NO* is an important pathway in microbial killing, the ability of endospores of B. anthracis to regulate production of this free radical has important implications in the control of B. anthracis-mediated infection.

The dltABCD operon of Bacillus anthracis sterne is required for virulence and resistance to peptide, enzymatic, and cellular mediators of innate immunity

In the environment, the gram-positive bacterium Bacillus anthracis persists as a metabolically dormant endospore. Upon inoculation into the host the endospores germinate and outgrow into vegetative bacilli able to cause disease. The dramatic morphogenic changes to the bacterium during germination and outgrowth are numerous and include major rearrangement of and modifications to the bacterial surface. Such modifications occur during a time in the B. anthracis infectious cycle when the bacterium must guard against a multitude of innate immune mediators. The dltABCD locus of B. anthracis encodes a cell wall d-alanine esterification system that is initiated by transcriptional activation during endospore outgrowth. The level of transcription from the dltABCD operon determined B. anthracis resistance to cationic antibacterial peptides during vegetative growth and cationic peptide, enzymatic, and cellular mediators of innate immunity during outgrowth. Mutation of dltABCD was also attenuating in a mouse model of infection. We propose that the dltABCD locus is important for protection of endosporeforming bacteria from environmental assault during outgrowth and that such protection may be critical during the establishment phase of anthrax.