Cloning of the protective antigen gene of Bacillus anthracis

Conjugation across Bacillus cereus and kin: A review

Horizontal gene transfer (HGT) is a major driving force in shaping bacterial communities. Key elements responsible for HGT are conjugation-like events and transmissible plasmids. Conjugative plasmids can promote their own transfer as well as that of co-resident plasmids. Bacillus cereus and relatives harbor a plethora of plasmids, including conjugative plasmids, which are at the heart of the group species differentiation and specification. Since the first report of a conjugation-like event between strains of B. cereus sensu lato (s.l.) 40 years ago, many have studied the potential of plasmid transfer across the group, especially for plasmids encoding major toxins. Over the years, more than 20 plasmids from B. cereus isolates have been reported as conjugative. However, with the increasing number of genomic data available, in silico analyses indicate that more plasmids from B. cereus s.l. genomes present self-transfer potential. B. cereus s.l. bacteria occupy diverse environmental niches, which were mimicked in laboratory conditions to study conjugation-related mechanisms. Laboratory mating conditions remain nonetheless simplistic compared to the complex interactions occurring in natural environments. Given the health, economic and ecological importance of strains of B. cereus s.l., it is of prime importance to consider the impact of conjugation within this bacterial group.

Molecular characterization of B. anthracis isolates from the anthrax outbreak among cattle in Karnataka, India

Background

Anthrax, a zoonotic disease is caused by the Gram positive bacterium Bacillus anthracis. During January 2013, an anthrax outbreak among cattle was reported in Gundlupet Taluk, neighboring Bandipur National Park and tiger reserve, India. The present study aims at the molecular identification and characterization of 12 B. anthracis isolates from this outbreak by 16S rRNA gene sequencing, screening B. anthracis specific prophages and chromosomal markers, protective antigen (pag) gene and canonical single nucleotide polymorphism (canSNP) analysis to subtype the isolates into one of the twelve globally identified clonal sub-lineages of B. anthracis.

Results

These isolates had identical 16S rDNA nucleotide sequences with B. anthracis specific dual peaks showing mixed base pair R (G/A) at position 1139 with visual inspection while the automated basecaller software indicated a G. Alternatively the nucleotide A at 1146 position was indicative of the 16S rDNA type 7. Multiple sequence alignment with additional 170 (16S rDNA) sequences of B. cereus sensu lato group from GenBank database revealed 28 new 16S types in addition to eleven 16S types reported earlier. The twelve B. anthracis isolates were found to harbor the four B. anthracis specific prophages (lambdaBa01, lambdaBa02, lambdaBa03, and lambdaBa04) along with its four specific loci markers (dhp 61.183, dhp 77.002, dhp 73.019, and dhp 73.017). The pag gene sequencing identified the isolates as protective antigen (PA) genotype I with phenylalanine-proline-alanine phenotype (FPA phenotype). However, sequence clustering with additional 34 pag sequences from GenBank revealed two additional missense mutations at nucleotide positions 196 bp and 869 bp of the 2294 bp pag sequence among the 5 B. cereus strains with pXO1 like plasmids. The canSNP analysis showed that the isolates belong to A.Br.Aust94 sub-lineage that is distributed geographically in countries of Asia, Africa, Europe and Australia.

Conclusions

The analysis of 16S rDNA sequences reiterated the earlier findings that visual inspection of electropherogram for position 1139 having nucleotide R could be used for B. anthracis identification and not the consensus sequence from base caller. The canSNP results indicated that the anthrax outbreak among cattle was caused by B. anthracis of A.Br.Aust94 sub-lineage.

Production of recombinant lethal factor of Bacillus anthracis in Bacillus subtilis

Cancer is considered as a disease with high rates of mortality and morbidity. The limitations and side effects of common treatments have prompted the need for innovative cancer therapies. Furthermore, selectivity and targeting of cancer cells are crucial factors to successful treatment of cancer. One of these methods is the use of bacterial toxins including Bacillus anthracis toxin to aid cancer therapy. This toxin is composed of three polypeptides: protective factor (PA), lethal factor (LF), and edema factor (EF). PA can bind to various surface receptors of all types of human cells and it internalizes the lethal factor and edema factor subunits of the toxin in the cytosol. In the present study, we cloned and expressed the lef gene of B. anthracis as the lethal part of the toxin in Bacillus subtilis WB600 by a shuttle expression vector PHT4. The rLF made in B. subtilis is efficiently secreted by the host into the culture medium which facilitates downstream processing. The rLF can be used to study cancer treatment. Abbreviations: EF: edema factor; LF: lethal factor; PA: protective factor; rLF: recombinant lethal factor; rPAm: recombinant protective factor mutants; uPA: urokinase-type plasminogen activator; uPAR: urokinase-type plasminogen activator receptor.

Vaccines against anthrax based on recombinant protective antigen: problems and solutions

Introduction: Anthrax is a dangerous bio-terror agent because Bacillus anthracis spores are highly resilient and can be easily aerosolized and disseminated. There is a threat of deliberate use of anthrax spores aerosol that could lead to serious fatal diseases outbreaks. Existing control measures against inhalation form of the disease are limited. All of this has provided an impetus to the development of new generation vaccines. Areas сovered: This review is devoted to challenges and achievements in the design of vaccines based on the anthrax recombinant protective antigen (rPA). Scientific databases have been searched, focusing on causes of PA instability and solutions to this problem, including new approaches of rPA expression, novel rPA-based vaccines formulations as well as the simultaneous usage of PA with other anthrax antigens. Expert opinion: PA is a central anthrax toxin component, playing a key role in the defense against encapsulated and unencapsulated strains. Subunit rPA-based vaccines have a good safety and protective profile. However, there are problems of PA instability that are greatly enhanced when using aluminum adjuvants. New adjuvant compositions, dry formulations and resistant to proteolysis and deamidation mutant PA forms can help to handle this issue. Devising a modern anthrax vaccine requires huge efforts.

Immunogenicity of anthrax recombinant peptides and killed spores in goats and protective efficacy of immune sera in A/J mouse model

Anthrax is primarily recognized as an affliction of herbivores with incubation period ranging from three to five days post-infection. Currently, the Sterne live-spore vaccine is the only vaccine approved for control of the disease in susceptible animals. While largely effective, the Sterne vaccine has several problems including adverse reactions in sensitive species, ineffectiveness in active outbreaks and incompatibility with antibiotics. These can be surmounted with the advent of recombinant peptides (non-living) next generation vaccines. The candidate vaccine antigens comprised of recombinant protective antigen (PA), spore-specific antigen (bacillus collagen-like protein of anthracis, BclA) and formaldehyde inactivated spores (FIS). Presently, little information exists on the protectivity of these novel vaccine candidates in susceptible ruminants. Thus, this study sought to assess the immunogenicity of these vaccine candidates in goats and evaluate their protectivity using an in vivo mouse model. Goats receiving a combination of PA, BclA and FIS yielded the highest antibody and toxin neutralizing titres compared to recombinant peptides alone. This was also reflected in the passive immunization experiment whereby mice receiving immune sera from goats vaccinated with the antigen combination had higher survival post-challenge. In conclusion, the current data indicate promising potential for further development of non-living anthrax vaccines in ruminants.

Expression and contribution to virulence of each polysaccharide capsule of Bacillus cereus strain G9241

Bacillus cereus strain G9241 was isolated from a patient with pneumonia who had an anthrax-like illness. Like Bacillus anthracis, the virulence of G9241 is dependent on two large plasmids. In G9241 those plasmids are pBCXO1 and pBC210. There is a multi-gene capsule locus on each of these virulence plasmids, and both capsules are produced by G9241 in vitro and in mice. The hasACB operon on pBCXO1 is responsible for production of a hyaluronic acid (HA) capsule. The locus on pBC210 encodes a putative tetrasaccharide (TS) capsule that assembles in a Wzy-dependent manner. We found that the pBC210 capsule locus is transcribed as two operons and identified the promoter regions responsible for transcription. We constructed isogenic mutants to assess the role of genes in the two TS capsule operons in production of the capsule. Spores of strains deficient in production of either the HA or TS capsule were inoculated subcutaneously or intranasally into A/J and C57BL/6 mice to determine the lethal dose 50% of each bacterial mutant by each route of infection. The loss of the HA capsule attenuated G9241 more than the loss of the TS capsule for both infection routes in both mouse strains. Overall, our data further characterize the unique TS capsule on pBC210 and demonstrate that the two capsules do not have the same impact on virulence of G9241.

Assembly and Function of the Bacillus anthracis S-Layer

Bacillus anthracis, the anthrax agent, is a member of the Bacillus cereus sensu lato group, which includes invasive pathogens of mammals or insects as well as nonpathogenic environmental strains. The genes for anthrax pathogenesis are located on two large virulence plasmids. Similar virulence plasmids have been acquired by other B. cereus strains and enable the pathogenesis of anthrax-like diseases. Among the virulence factors of B. anthracis is the S-layer-associated protein BslA, which endows bacilli with invasive attributes for mammalian hosts. BslA surface display and function are dependent on the bacterial S-layer, whose constituents assemble by binding to the secondary cell wall polysaccharide (SCWP) via S-layer homology (SLH) domains. B. anthracis and other pathogenic B. cereus isolates harbor genes for the secretion of S-layer proteins, for S-layer assembly, and for synthesis of the SCWP. We review here recent insights into the assembly and function of the S-layer and the SCWP.

Genome engineering in Bacillus anthracis using tyrosine site-specific recombinases

Tyrosine site-specific recombinases (T-SSR) are polynucleotidyltransferases that catalyze cutting and joining reactions between short specific DNA sequences. We developed three systems for performing genetic modifications in Bacillus anthracis that use T-SSR and their cognate target sequences, namely Escherichia coli bacteriophage P1 Cre-loxP, Saccharomyces cerevisiae Flp-FRT, and a newly discovered IntXO-PSL system from B. anthracis plasmid pXO1. All three tyrosine recombinase systems were used for creation of a B. anthracis sporulation-deficient, plasmid-free strain deleted for ten proteases which had been identified by proteomic analysis as being present in the B. anthracis secretome. This strain was used successfully for production of various recombinant proteins, including several that are candidates for inclusion in improved anthrax vaccines. These genetic tools developed for DNA manipulation in B. anthracis were also used for construction of strains having chromosomal insertions of 1, 2, or 3 adjacent atxA genes. AtxA is a B. anthracis global transcriptional regulator required for the response of B. anthracis virulence factor genes to bicarbonate. We found a positive correlation between the atxA copy number and the expression level of the pagA gene encoding B. anthracis protective antigen, when strains were grown in a carbon dioxide atmosphere. These results demonstrate that the three T-SSR systems described here provide effective tools for B. anthracis genome editing. These T-SSR systems may also be applicable to other prokaryotes and to eukaryotes.

Recombinant anthrax protective antigen: Observation of aggregation phenomena by TEM reveals specific effects of sterols

Negatively stained transmission electron microscope images are presented that depict the aggregation of recombinant anthrax protective antigen (rPA83 monomer and the PA63 prepore oligomer) under varying in vitro biochemical conditions. Heat treatment (50°C) of rPA83 produced clumped fibrils, but following heating the PA63 prepore formed disordered aggregates. Freeze-thaw treatment of the PA63 prepore generated linear flexuous aggregates of the heptameric oligomers. Aqueous suspensions of cholesterol microcrystals were shown to bind small rPA83 aggregates at the edges of the planar bilayers. With PA63 a more discrete binding of the prepores to the crystalline cholesterol bilayer edges occurs. Sodium deoxycholate (NaDOC) treatment of rPA83 produced quasi helical fibrillar aggregate, similar but not identical to that produced by heat treatment. Remarkably, NaDOC treatment of the PA63 prepores induced transformation into pores, with a characteristic extended ß-barrel. The PA63 pores aggregated as dimers, that aggregated further as angular chains and closed structures in higher NaDOC concentrations. The significance of the sterol interaction is discussed in relation to its likely importance for PA action in vivo.

The roles of AtxA orthologs in virulence of anthrax-like Bacillus cereus G9241

AtxA is a critical transcriptional regulator of plasmid-encoded virulence genes in Bacillus anthracis. Bacillus cereus G9241, which caused an anthrax-like infection, has two virulence plasmids, pBCXO1 and pBC210, that each harbor toxin genes and a capsule locus. G9241 also produces two orthologs of AtxA: AtxA1, encoded on pBCXO1, and AtxA2, encoded on pBC210. The amino acid sequence of AtxA1 is identical to that of AtxA from B. anthracis, while the sequences of AtxA1 and AtxA2 are 79% identical and 91% similar to one another. We found by qRT-PCR that AtxA1 and AtxA2 function as positive regulators of toxin (AtxA1) and capsule operon (both) transcription in G9241 and that a ΔatxA1 mutant produced lower levels of the anthrax toxins and no hyaluronic acid capsule. Deletion of atxA1 or atxA2 decreased the virulence of spores administered intranasally or subcutaneously to C57BL/6 mice but not to A/J mice, and deletion of both genes rendered spores avirulent in A/J mice. In addition, unlike AtxA1, AtxA2 did not form stable homomultimers in vitro, although AtxA1 and AtxA2 formed heterodimers. Our data show that AtxA1 is the primary regulator of G9241 virulence factor expression and that AtxA1 and AtxA2 are both required for full virulence.

Cloning, expression and purification of binding domains of lethal factor and protective antigen of Bacillus anthracis in Escherichia coli and evaluation of their related murine antibody

Anthrax is common disease between human and animals caused by Bacillus anthracis. The cell binding domain of protective antigen (PAD4) and the binding domain of lethal factor (LFD1) have high immunogenicity potential and always were considered as a vaccine candidate against anthrax. The aims of this study are cloning and expressing of PAD4 and LFD1 in Escherichia coli, purification of the recombinant proteins and determination of their immunogenicity through evaluating of the relative produced polyclonal antibodies in mice. PAD4 and LFD1 genes were cloned in pET28a(+) vector and expressed in E. coli Bl21(DE3)PlysS. Expression and purification of the two recombinant proteins were confirmed by SDS-PAGE and Western blotting techniques. The PAD4 and LFD1 were purified using Ni(+)-NTA affinity chromatography (95-98 %), yielding 37.5 and 45 mg/l of culture, respectively. The antigens were injected three times into mice and production of relative antibodies was evaluated by ELISA test. The results showed that both PAD4 and LFD1 are immunogenic, but LFD1 has higher potential to stimulate Murine immune system. With regard to the high level of LFD1 and PAD4 expression and also significant increment in produced polyclonal antibodies, these recombinant proteins can be considered as a recombinant vaccine candidate against anthrax.

Bacillus anthracis acetyltransferases PatA1 and PatA2 modify the secondary cell wall polysaccharide and affect the assembly of S-layer proteins

The envelope of Bacillus anthracis encompasses a proteinaceous S-layer with two S-layer proteins (Sap and EA1). Protein assembly in the envelope of B. anthracis requires S-layer homology domains (SLH) within S-layer proteins and S-layer-associated proteins (BSLs), which associate with the secondary cell wall polysaccharide (SCWP), an acetylated carbohydrate that is tethered to peptidoglycan. Here, we investigated the contributions of two putative acetyltransferases, PatA1 and PatA2, on SCWP acetylation and S-layer assembly. We show that mutations in patA1 and patA2 affect the chain lengths of B. anthracis vegetative forms and perturb the deposition of the BslO murein hydrolase at cell division septa. The patA1 and patA2 mutants are defective for the assembly of EA1 in the envelope but retain the ability of S-layer formation with Sap. SCWP isolated from the patA1 patA2 mutant lacked acetyl moieties identified in wild-type polysaccharide and failed to associate with the SLH domains of EA1. A model is discussed whereby patA1- and patA2-mediated acetylation of SCWP enables the deposition of EA1 as well as BslO near the septal region of the B. anthracis envelope.

Vaccine protection against Bacillus cereus-mediated respiratory anthrax-like disease in mice

Bacillus cereus strains harboring a pXO1-like virulence plasmid cause respiratory anthrax-like disease in humans, particularly in welders. We developed mouse models for intraperitoneal as well as aerosol challenge with spores of B. cereus G9241, harboring pBCXO1 and pBC218 virulence plasmids. Compared to wild-type B. cereus G9241, spores with a deletion of the pBCXO1-carried protective antigen gene (pagA1) were severely attenuated, whereas spores with a deletion of the pBC218-carried protective antigen homologue (pagA2) were not. Anthrax vaccine adsorbed (AVA) immunization raised antibodies that bound and neutralized the pagA1-encoded protective antigen (PA1) but not the PA2 orthologue encoded by pagA2. AVA immunization protected mice against a lethal challenge with spores from B. cereus G9241 or B. cereus Elc4, a strain that had been isolated from a fatal case of anthrax-like disease. As the pathogenesis of B. cereus anthrax-like disease in mice is dependent on pagA1 and PA-neutralizing antibodies provide protection, AVA immunization may also protect humans from respiratory anthrax-like death.

Secretion genes as determinants of Bacillus anthracis chain length

Bacillus anthracis grows in chains of rod-shaped cells, a trait that contributes to its escape from phagocytic clearance in host tissues. Using a genetic approach to search for determinants of B. anthracis chain length, we identified mutants with insertional lesions in secA2. All isolated secA2 mutants exhibited an exaggerated chain length, whereas the dimensions of individual cells were not changed. Complementation studies revealed that slaP (S-layer assembly protein), a gene immediately downstream of secA2 on the B. anthracis chromosome, is also a determinant of chain length. Both secA2 and slaP are required for the efficient secretion of Sap and EA1 (Eag), the two S-layer proteins of B. anthracis, but not for the secretion of S-layer-associated proteins or of other secreted products. S-layer assembly via secA2 and slaP contributes to the proper positioning of BslO, the S-layer-associated protein, and murein hydrolase, which cleaves septal peptidoglycan to separate chains of bacilli. SlaP was found to be both soluble in the bacterial cytoplasm and associated with the membrane. The purification of soluble SlaP from B. anthracis-cleared lysates did not reveal a specific ligand, and the membrane association of SlaP was not dependent on SecA2, Sap, or EA1. We propose that SecA2 and SlaP promote the efficient secretion of S-layer proteins by modifying the general secretory pathway of B. anthracis to transport large amounts of Sap and EA1.

An attenuated strain of Bacillus anthracis (CDC 684) has a large chromosomal inversion and altered growth kinetics

BACKGROUND

An isolate originally labeled Bacillus megaterium CDC 684 was found to contain both pXO1 and pXO2, was non-hemolytic, sensitive to gamma-phage, and produced both the protective antigen and the poly-D-glutamic acid capsule. These phenotypes prompted Ezzell et al., (J. Clin. Microbiol. 28:223) to reclassify this isolate to Bacillus anthracis in 1990.

RESULTS

We demonstrate that despite these B. anthracis features, the isolate is severely attenuated in a guinea pig model. This prompted whole genome sequencing and closure. The comparative analysis of CDC 684 to other sequenced B. anthracis isolates and further analysis reveals: a) CDC 684 is a close relative of a virulent strain, Vollum A0488; b) CDC 684 defines a new B. anthracis lineage (at least 51 SNPs) that includes 15 other isolates; c) the genome of CDC 684 contains a large chromosomal inversion that spans 3.3 Mbp; d) this inversion has caused a displacement of the usual spatial orientation of the origin of replication (ori) to the termination of replication (ter) from 180° in wild-type B. anthracis to 120° in CDC 684 and e) this isolate also has altered growth kinetics in liquid media.

CONCLUSIONS

We propose two alternative hypotheses explaining the attenuated phenotype of this isolate. Hypothesis 1 suggests that the skewed ori/ter relationship in CDC 684 has altered its DNA replication and/or transcriptome processes resulting in altered growth kinetics and virulence capacity. Hypothesis 2 suggests that one or more of the single nucleotide polymorphisms in CDC 684 has altered the expression of a regulatory element or other genes necessary for virulence.

Two capsular polysaccharides enable Bacillus cereus G9241 to cause anthrax-like disease

Bacillus cereus G9241 causes an anthrax-like respiratory illness in humans; however, the molecular mechanisms of disease pathogenesis are not known. Genome sequencing identified two putative virulence plasmids proposed to provide for anthrax toxin (pBCXO1) and/or capsule expression (pBC218). We report here that B. cereus G9241 causes anthrax-like disease in immune-competent mice, which is dependent on each of the two virulence plasmids. pBCXO1 encodes pagA1, the homologue of anthrax protective antigen, as well as hasACB, providing for hyaluronic acid capsule formation, two traits that each contribute to disease pathogenesis. pBC218 harbours bpsX-H, B. cereus exo-polysaccharide, which produce a second capsule. During infection, B. cereus G9241 elaborates both hasACB and bpsX-H capsules, which together are essential for the establishment of anthrax-like disease and the resistance of bacilli to phagocytosis. A single nucleotide deletion causes premature termination of hasA translation in Bacillus anthracis, which is known to escape phagocytic killing by its pXO2 encoded poly-d-γ-glutamic acid (PDGA) capsule. Thus, multiple different gene clusters endow pathogenic bacilli with capsular material, provide for escape from innate host immune responses and aid in establishing the pathogenesis of anthrax-like disease.

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

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

Killed but metabolically active Bacillus anthracis vaccines induce broad and protective immunity against anthrax

Bacillus anthracis is the causative agent of anthrax. We have developed a novel whole-bacterial-cell anthrax vaccine utilizing B. anthracis that is killed but metabolically active (KBMA). Vaccine strains that are asporogenic and nucleotide excision repair deficient were engineered by deleting the spoIIE and uvrAB genes, rendering B. anthracis extremely sensitive to photochemical inactivation with S-59 psoralen and UV light. We also introduced point mutations into the lef and cya genes, which allowed inactive but immunogenic toxins to be produced. Photochemically inactivated vaccine strains maintained a high degree of metabolic activity and secreted protective antigen (PA), lethal factor, and edema factor. KBMA B. anthracis vaccines were avirulent in mice and induced less injection site inflammation than recombinant PA adsorbed to aluminum hydroxide gel. KBMA B. anthracis-vaccinated animals produced antibodies against numerous anthrax antigens, including high levels of anti-PA and toxin-neutralizing antibodies. Vaccination with KBMA B. anthracis fully protected mice against challenge with lethal doses of toxinogenic unencapsulated Sterne 7702 spores and rabbits against challenge with lethal pneumonic doses of fully virulent Ames strain spores. Guinea pigs vaccinated with KBMA B. anthracis were partially protected against lethal Ames spore challenge, which was comparable to vaccination with the licensed vaccine anthrax vaccine adsorbed. These data demonstrate that KBMA anthrax vaccines are well tolerated and elicit potent protective immune responses. The use of KBMA vaccines may be broadly applicable to bacterial pathogens, especially those for which the correlates of protective immunity are unknown.

Genetic diversity in a Bacillus anthracis historical collection (1954 to 1988)

Bacillus anthracis, the etiologic agent of anthrax, has been widely described as a genetically monomorphic species. We used both multiple-locus variable-number tandem-repeat analysis (MLVA) and pagA gene sequencing to determine the genetic diversity of a historical collection of B. anthracis isolates collected from the 1950s to the 1980s from various geographic locations and sources. We sequenced the pagA gene of 124 diverse B. anthracis isolates and found all previously identified B. anthracis pagA types except type 4. Sixty-three of the 124 B. anthracis strains were identified as pagA type 6, while 44 were pagA type 5, 12 were pagA type 1, and individual isolates were identified for types 2 and 3, respectively. Two new pagA genotypes were discovered in three environmental isolates within the historical collection. Two isolates had the same new genotype, and an additional isolate produced a second new genotype. MLVA detected 22 previously described genotypes in the historical collection. In addition, 33 new MLVA genotypes were found. For 11 isolates, an MLVA genotype could not be assigned because one or more alleles did not amplify. While only two additional B. anthracis pagA types were identified, in two instances, the use of pagA sequencing discriminated isolates with the same MLVA genotype. MLVA revealed that 39 of the 124 isolates were previously undocumented genotypes and that 1 isolate was found to be in the C cluster when it was subtyped by MLVA.

Significant passive protective effect against anthrax by antibody to Bacillus anthracis inactivated spores that lack two virulence plasmids

The protective-antigen (PA)-based cell-free vaccine is the only vaccine licensed for use against Bacillus anthracis infection in humans. Although the PA shows strong immunogenicity, the capsule or spore-associated somatic antigens may be important as additional vaccine targets for full protection against anthrax. In this study, the protective effect of spore-associated antigens against B. anthracis infection was determined. Rabbits were immunized with formalin-fixed spores of a non-toxigenic unencapsulated B. anthracis strain that lacked the two virulence plasmids pXO1 and pXO2, and the protective effects of the immune antibody were evaluated. Immunostaining and Western blot analysis revealed that the anti-B. anthracis (anti-BA)-spore IgG specifically bound to the surface of spores or endospores of B. anthracis, but not to vegetative cells, or closely related Bacillus species, such as Bacillus cereus, Bacillus subtilis and Bacillus thuringiensis. Passively transferred anti-BA-spore IgG protected mice from intraperitoneal challenge with a lethal dose of fully virulent B. anthracis spores, and increased the survival rate in a dose-dependent manner. Pre-incubation of spores with antibody also reduced their infectivity in a dose-dependent manner. The number of bacteria (c.f.u.) in spleens and livers of infected mice was significantly lower in antibody-treated mice than in untreated mice. Treatment with anti-BA-spore IgG also inhibited the germination of spores in J774.1 macrophages, suggesting that opsonization of spores promotes phagocytosis and subsequent killing by macrophages. These results indicate the usefulness of spore surface antigens as vaccine targets. In combination with major virulence factors such as the PA, spore-associated antigens may offer a safer and more effective multicomponent vaccine for B. anthracis infection.

Targeting proteins to the cell wall of sporulating Bacillus anthracis

Dormant spores of Bacillus anthracis germinate during host infection and their vegetative growth and dissemination precipitate anthrax disease. Upon host death, bacilli engage a developmental programme to generate infectious spores within carcasses. Hallmark of sporulation in Bacillus spp. is the formation of an asymmetric division septum between mother cell and forespore compartments. We show here that sortase C (SrtC) cleaves the LPNTA sorting signal of BasH and BasI, thereby targeting both polypeptides to the cell wall of sporulating bacilli. Sortase substrates are initially produced in different cell compartments and at different developmental stages but penultimately decorate the envelope of the maturing spore. srtC mutants appear to display no defect during the initial stages of infection and precipitate lethal anthrax disease in guinea pigs at a similar rate as wild-type B. anthracis strain Ames. Unlike wild-type bacilli, srtC mutants do not readily form spores in guinea pig tissue or sheep blood unless their vegetative forms are exposed to air.

Recombinant protective antigen 102 (rPA102): profile of a second-generation anthrax vaccine

Recent terrorist attacks involving the use of Bacillus anthracis spores have stimulated interest in the development of new vaccines for anthrax prevention. Studies of the pathogenesis of anthrax and of the immune responses following infection and immunization underscore the pivotal role that antibodies to the protective antigen play in protection. The most promising vaccine candidates contain purified recombinant protective antigen. Clinical trials of one of these, recombinant protective antigen (rPA)102, are underway. Initial results suggest that rPA102 is well tolerated and immunogenic. Additional trials are necessary to identify optimal formulations and immunization regimens for pre- and postexposure prophylaxis. Future licensure of these and other candidate vaccines will depend on their safety and immunogenicity profiles in humans, and their ability to confer protection in animal models of inhalational anthrax.

Toxin-deficient mutants of Bacillus anthracis are lethal in a murine model for pulmonary anthrax

Bacillus anthracis, the etiologic agent of anthrax, produces at least three primary virulence factors: lethal toxin, edema toxin, and a capsule. The capsule is absolutely required for dissemination and lethality in a murine model of inhalation anthrax, yet the roles for the toxins during infection are ill-defined. We show in a murine model that when spores of specific toxin-null mutants are introduced into the lung, dissemination and lethality are comparable to those of the parent strain. Mutants lacking one or more of the structural genes for the toxin proteins, i.e., protective antigen, lethal factor, and edema factor, disseminated from the lung to the spleen at rates similar to that of the virulent parental strain. The 50% lethal dose (LD50) and mean time to death (MTD) of the mutants did not differ significantly from those of the parent. The LD50s or MTDs were also unaffected relative to those of the parent strain when mice were inoculated intravenously with vegetative cells. Nonetheless, histopathological examination of tissues revealed subtle but distinct differences in infections by the parent compared to some toxin mutants, suggesting that the host response is affected by toxin proteins synthesized during infection.

Genome engineering in Bacillus anthracis using Cre recombinase

Genome engineering is a powerful method for the study of bacterial virulence. With the availability of the complete genomic sequence of Bacillus anthracis, it is now possible to inactivate or delete selected genes of interest. However, many current methods for disrupting or deleting more than one gene require use of multiple antibiotic resistance determinants. In this report we used an approach that temporarily inserts an antibiotic resistance marker into a selected region of the genome and subsequently removes it, leaving the target region (a single gene or a larger genomic segment) permanently mutated. For this purpose, a spectinomycin resistance cassette flanked by bacteriophage P1 loxP sites oriented as direct repeats was inserted within a selected gene. After identification of strains having the spectinomycin cassette inserted by a double-crossover event, a thermo-sensitive plasmid expressing Cre recombinase was introduced at the permissive temperature. Cre recombinase action at the loxP sites excised the spectinomycin marker, leaving a single loxP site within the targeted gene or genomic segment. The Cre-expressing plasmid was then removed by growth at the restrictive temperature. The procedure could then be repeated to mutate additional genes. In this way, we sequentially mutated two pairs of genes: pepM and spo0A, and mcrB and mrr. Furthermore, loxP sites introduced at distant genes could be recombined by Cre recombinase to cause deletion of large intervening regions. In this way, we deleted the capBCAD region of the pXO2 plasmid and the entire 30 kb of chromosomal DNA between the mcrB and mrr genes, and in the latter case we found that the 32 intervening open reading frames were not essential to growth.

Cloning, expression, and characterization of recombinant nitric oxide synthase-like protein from Bacillus anthracis

Nitric oxide synthase (NOS) is amongst a family of evolutionarily conserved enzymes, involved in a multi-turnover process that results in NO as a product. The significant role of NO in various pathological and physiological processes has created an interest in this enzyme from several perspectives. This study describes for the first time, cloning and expression of a NOS-like protein, baNOS, from Bacillus anthracis, a pathogenic bacterium responsible for causing anthrax. baNOS was expressed in Escherichia coli as a soluble and catalytically active enzyme. Homology models generated for baNOS indicated that the key structural features that are involved in the substrate and active site interaction have been highly conserved. Further, the behavior of baNOS in terms of heme-substrate interactions and heme-transitions was studied in detail. The optical perturbation spectra of the heme domain demonstrated that the ligands perturb the heme site in a ligand specific manner. baNOS forms a five-coordinate, high-spin complex with l-arginine analogs and a six-coordinate low-spin complex with inhibitor imidazole. Studies indicated that the binding of l-arginine, N(omega)-hydroxy-l-arginine, and imidazole produces various spectroscopic species that closely correspond to the equivalent complexes of mammalian NOS. The values of spectral binding constants further corroborated these results. The overall conservation of the key structural features and the correlation of heme-substrate interactions in baNOS and mammalian NOS, thus, point towards an interesting phenomenon of convergent evolution. Importantly, the NO generated by NOS of mammalian macrophages plays a potent role in antimicrobicidal activity. Because of the existence of high structural and behavioral similarity between mammalian NOS and baNOS, we propose that NO produced by B. anthracis may also have a pivotal pathophysiological role in anthrax infection. Therefore, this first report of characterization of a NOS-like protein from a pathogenic bacterium opens up avenues for further studies in understanding the importance of this protein in pathogenicity.

Expression and secretion of the protective antigen of Bacillus anthracis in Bacillus brevis

We used the Bacillus brevis-pNU212 system to develop a mass production system for the protective antigen (PA) of Bacillus anthracis. A moderately efficient expression-secretion system for PA was constructed by fusing the PA gene from B. anthracis with the B. brevis cell-wall protein signal-peptide encoding region of pNU212, and by introducing the recombinant plasmid, pNU212-mPA, into B. brevis 47-5Q. The clone producing PA secreted about 300 microg of recombinant PA (rPA) per ml of 5PY-erythromycin medium after 4 days incubation at 30 degrees C. The rPA was fractionated from the culture supernatant of B. brevis 47-5Q carrying pNU212-mPA using ammonium sulfate at 70% saturation followed by anion exchange chromatography on a Hitrap Q, a Hiload 16/60 Superdex 200 gel filtration column and a phenyl sepharose hydrophobic interaction column, yielding 70 mg rPA per liter of culture. The N-terminal sequence of the purified rPA was identical to that of native PA from B. anthracis. The purified rPA exhibited cytotoxicity towards J774A.1 cells when combined with lethal factor. The rPA formulated in either Rehydragel HPA or MPL-TDM-CWS adjuvant (Ribi-Trimix) elicited the expression of a large amount of anti-PA and neutralizing antibodies in guinea pigs and completely protected them against a 100 LD50 challenge with fully virulent B. anthracis spores.

Rapid genotypic detection of Bacillus anthracis and the Bacillus cereus group by multiplex real-time PCR melting curve analysis

Bacillus anthracis has four plasmid possible virulence genotypes: pXO1+/pXO2+, pXO1+/pXO2-, pXO1-/pXO2+ or pXO1-/pXO2-. Due to the lack of a specific chromosomal marker for B. anthracis, differentiation of the pXO1-/pXO2- form of B. anthracis from closely related Bacillus cereus group species is difficult. In this study, we evaluate the ability of sspE, pXO1 and pXO2 primers to discriminate individual B. anthracis and the B. cereus group genotypes using multiplex real-time PCR and melting curve analysis. Optimal conditions for successful multiplex assays have been established. Purified DNAs from 38 bacterial strains including 11 strains of B. anthracis and 18 B. cereus group strains were analyzed. Nine of the B. cereus group near-neighbor strains were shown by multilocus sequence typing to be phylogenetically proximate to the B. anthracis clade. We have demonstrated that the four plasmid genotypes of B. anthracis and B. cereus group near-neighbors were differentially and simultaneously discriminated by this assay.

Molecular basis for improved anthrax vaccines

The current vaccine for anthrax has been licensed since 1970 and was developed based on the outcome of human trials conducted in the 1950s. This vaccine, known as anthrax vaccine adsorbed (AVA), consists of a culture filtrate from an attenuated strain of Bacillus anthracis adsorbed to aluminum salts as an adjuvant. This vaccine is considered safe and effective, but is difficult to produce and is associated with complaints about reactogenicity among users of the vaccine. Much of the work in the past decade on generating a second generation vaccine is based on the observation that antibodies to protective antigen (PA) are crucial in the protection against exposure to virulent anthrax spores. Antibodies to PA are thought to prevent binding to its cellular receptor and subsequent binding of lethal factor (LF) and edema factor (EF), which are required events for the action of the two toxins: lethal toxin (LeTx) and edema toxin (EdTx). The bacterial capsule as well as the two toxins are virulence factors of B. anthracis. The levels of antibodies to PA must exceed a certain minimal threshold in order to induce and maintain protective immunity. Immunity can be generated by vaccination with purified PA, as well as spores and DNA plasmids that express PA. Although antibodies to PA address the toxemia component of anthrax disease, antibodies to additional virulence factors, including the capsule or somatic antigens in the spore, may be critical in development of complete, sterilizing immunity to anthrax exposure. The next generation anthrax vaccines will be derived from the thorough understanding of the interaction of virulence factors with human and animal hosts and the role the immune response plays in providing protective immunity.

Virulence signatures: microarray-based approaches to discovery and analysis

Rapid, accurate, and sensitive detection of biothreat agents requires a broad-spectrum assay capable of discriminating between closely related microbial or viral pathogens. Moreover, in cases where a biological agent release has been identified, forensic analysis demands detailed genetic signature data for accurate strain identification and attribution. To date, nucleic acid sequences have provided the most robust and phylogentically illuminating signature information. Nucleic acid signature sequences are not often linked to genomic or extrachromosomal determinants of virulence, a link that would further facilitate discrimination between pathogens and closely related species. Inextricably coupling genetic determinants of virulence with highly informative nucleic acid signatures would provide a robust means of identifying human, livestock, and agricultural pathogens. By means of example, we present here an overview of two general applications of microarray-based methods for: (1) the identification of candidate virulence factors; and (2) the analysis of genetic polymorphisms that are coupled to Bacillus anthracis virulence factors using an accurate, low cost solid-phase mini-sequencing assay. We show that microarray-based analysis of gene expression can identify potential virulence associated genes for use as candidate signature targets, and, further, that microarray-based single nucleotide polymorphism assays provide a robust platform for the detection and identification of signature sequences in a manner independent of the genetic background in which the signature is embedded. We discuss the strategy as a general approach or pipeline for the discovery of virulence-linked nucleic acid signatures for biothreat agents.

Capsule synthesis by Bacillus anthracis is required for dissemination in murine inhalation anthrax

Bacillus anthracis, the agent of anthrax, produces a poly-D-glutamic acid capsule that has been implicated in virulence. Many strains missing pXO2 (96 kb), which harbors the capsule biosynthetic operon capBCAD, but carrying pXO1 (182 kb) that harbors the anthrax toxin genes, are attenuated in animal models. Also, noncapsulated strains are readily phagocytosed by macrophage cell lines, whereas capsulated strains are resistant to phagocytosis. We show that a strain carrying both virulence plasmids but deleted specifically for capBCAD is highly attenuated in a mouse model for inhalation anthrax. The parent strain and capsule mutant initiated germination in the lungs, but the capsule mutant did not disseminate to the spleen. A mutant harboring capBCAD but deleted for the cap regulators acpA and acpB was also significantly attenuated, in agreement with the capsule-negative phenotype during in vitro growth. Surprisingly, an acpB mutant, but not an acpA mutant, displayed an elevated LD(50) and reduced ability to disseminate, indicating that acpA and acpB are not true functional homologs and that acpB may play a larger role in virulence than originally suspected.

In vivo Bacillus anthracis gene expression requires PagR as an intermediate effector of the AtxA signalling cascade

Transcription of the major Bacillus anthracis virulence genes is triggered by CO2, a signal mimicking the host environment. A 182-kb plasmid, pXO1, carries the anthrax toxin genes and the genes responsible for their regulation of transcription, namely atxA and, pagR, the second gene of the pag operon. AtxA has major effects on the physiology of B. anthracis. It coordinates the transcription activation of the toxin genes with that of the capsule biosynthetic enzyme operon, located on the second virulence plasmid, pXO2. In rich medium, B. anthracis synthesises alternatively two S-layer proteins (Sap and EA1). An exponential phase "Sap-layer" is subsequently replaced by a stationary phase "EA1-layer". S-layer gene transcription is controlled by alternative sigma factors and by Sap acting as a transcriptional repressor of eag. Furthermore, in vitro in presence of CO2 and in vivo, AtxA is part of the sap and eag regulatory network. Only eag is significantly expressed in these conditions and this is due to AtxA activating eag and repressing sap transcription. PagR, and not AtxA itself, is the direct effector of this regulation by binding to sap and eag promoter regions. Therefore, PagR mediates the effect of AtxA on eag and sap and is the most downstream element of a signalling cascade initiated by AtxA. Taken together, these results indicate that the B. anthracis transcriptional regulator AtxA is controlling the synthesis of the three toxin components and of the surface elements (capsule and S-layer). Thus, AtxA is a master regulator that coordinates the response to host signals by orchestrating positive and negative controls over genes located on all genetic elements.

atxA controls Bacillus anthracis capsule synthesis via acpA and a newly discovered regulator, acpB

Two regulatory genes, acpA and atxA, have been reported to control expression of the Bacillus anthracis capsule biosynthesis operon capBCAD. The atxA gene is located on the virulence plasmid pXO1, while pXO2 carries acpA and the cap genes. acpA has been viewed as the major regulator of the cap operon because it is essential for capsule gene expression in a pXO1(-) pXO2(+) strain. atxA is essential for toxin gene transcription but has also been implicated in control of the cap genes. The molecular functions of the regulatory proteins are unknown. We examined cap gene expression in a genetically complete pXO1(+) pXO2(+) strain. Our results indicate that another pXO2 gene, acpB (previously called pXO2-53; accession no. NC002146.1:49418-50866), has a role in cap expression. The predicted amino acid sequence of AcpB is 62% similar to that of AcpA and 50% similar to that of AtxA. Assessment of cap gene transcription revealed that cap expression was not affected in a pXO1(+) pXO2(+) acpB-null mutant and was slightly reduced in an isogenic acpA mutant. However, cap gene expression was abolished in an acpA acpB double mutant. Microscopic examination of capsule synthesis by the mutants corroborated these findings. acpA and acpB expression is controlled by atxA; capsule synthesis and transcription of acpA and acpB were markedly reduced in an atxA mutant. The data suggest that, in a strain containing both virulence plasmids, atxA is the major regulator of capsule synthesis and controls capBCAD expression indirectly, via positive regulation of acpA and acpB.

Structure and function of anthrax toxin

Anthrax toxin is a binary A-B toxin comprised of protective antigen (PA) and two enzymatic moieties, edema factor (EF) and lethal factor (LF). In the presence of a host cell-surface receptor, PA can mediate the delivery of EF and LF from the extracellular milieu into the host cell cytosol to effect toxicity. In this delivery, PA undergoes multiple structural changes--from a monomer to a heptameric prepore to a membrane-spanning heptameric pore. The catalytic factors also undergo dramatic structural changes as they unfold to allow for their translocation across the endosomal membrane and refold to preserve their catalytic activity within the cytosol. In addition to these gross structural changes, the intoxication mechanism depends on the ability of PA to form specific interactions with the host cell receptor, EF, and LF. This chapter presents a review of experiments probing these structural interactions and rearrangements in the hopes of gaining a molecular understanding of toxin action.

Bacillus anthracis genetics and virulence gene regulation

The Bacillus anthracis genome consists of an approximately 5.3-Mb chromosome and two plasmids, pXO1 (182 kb) and pXO2 (96 kb). Genetic analysis has focused primarily on the structural genes for the anthrax toxin proteins, pagA, lef, and cya, the biosynthetic genes for capsule synthesis, capB, capC, and capA, and a gene associated with depolymerization of capsule, dep. The three toxin genes are located at distinct loci on pXO1, while the cap and dep genes are arranged in an apparent operon on pXO2. Additional genes that may play a role in B. anthracis virulence include the germination operon gerX and the general stress transcription factor sigB. Host-related signals affecting transcription of the toxin and capsule genes include temperature (37 degrees C) and bicarbonate/CO2. The B. anthracis plasmids carry two regulatory genes that share little sequence similarity with regulators in other bacteria. The pXO1-encoded gene atxA positively controls expression of the toxin and capsule genes, and has been implicated in control of other genes of unknown function. atxA mutants are avirulent in mice, and mice infected with atxA-null strains show a decreased immunological response to the toxin proteins. The pXO2-encoded regulator, acpA, shares sequence similarity with atxA. Yet acpA function appears to be restricted to positive control of capsule gene expression. The chromosomal gene abrB, a homologue of a well-characterized B. subtilis transition state regulator, controls growth phase-specific transcription of the toxin genes. Genetic manipulation of B. anthracis can be achieved by using natural means of DNA transfer and by electroporation of recombinant DNAs into B. anthracis. Genetic exchange can occur between B. anthracis strains and between B. anthracis and closely-related species. Although pXO1 and pXO2 are not self-transmissible, these plasmids and others can be transferred by conjugative plasmids originating in B. thuringiensis. Generalized transducing phage that permit inter-species transfer of chromosomal and plasmid DNA have also been described.

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.

Expression of protective antigen in transgenic plants: a step towards edible vaccine against anthrax

Protective antigen (PA) is the most potent molecule for vaccination against anthrax. In the present study, we have successfully integrated protective antigen gene in nuclear genome of tobacco plants by Agrobacterium mediated leaf-disc transformation method. Expression of protective antigen gene was detected by immunoblot analysis using antisera raised against purified PA. A distinct band of approximately 83kDa lighted up in the protein extracted from transformed plants while there was no such band in untransformed plants. The plant expressed PA showed biological activity just like native PA, which was demonstrated by cytolytic assay on macrophage like cell lines with lethal factor. This study establishes for the first time expression of PA gene in a plant system and thus marks the first milestone towards developing edible vaccine against anthrax.

Anthrax

Bacillus anthracis was shown to be the etiological agent of anthrax by R. Koch and L. Pasteur at the end of the nineteenth century. The concepts on which medical microbiology are based arose from their work on this bacterium. The link between plasmids and major virulence factors of B. anthracis was not discovered until the 1980s. The three toxin components are organized in two A-B type toxins, and the bacilli are covered by an antiphagocytic polyglutamic capsule. Structure-function analysis of the toxins indicated that the common B-domain binds to a ubiquitous cell receptor and forms a heptamer after proteolytic activation. One enzyme moiety is an adenylate cyclase and the other is a Zn(2+) metalloprotease, which is able to cleave MAPKKs. The capsule covers an S-layer sequentially composed of two distinct proteins. Knowledge of the toxins facilitates the design of safer veterinary vaccines. Spore-structure analysis could contribute to the improvement of human nonliving vaccines. The phylogeny of B. anthracis within the Bacillus cereus group is also reviewed.

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.

Toxins of Bacillus anthracis

Bacillus anthracis, a gram positive bacterium, is the causative agent of anthrax. This organism is capsulogen and toxinogenic. It secretes two toxins which are composed of three proteins: the protective antigen (PA), the lethal factor (LF) and the edema factor (EF). The lethal toxin (PA+LF) provokes a subit death in animals, the edema toxin (PA+EF) induces edema. The edema and the lethal factors are internalised into the eukaryotic target cells via the protective antigen. EF and LF exert a calmoduline dependent adenylate cyclase and a metalloprotease activity respectively. Progress in the structure-function relationship of these three proteins, their regulation mechanisms and their roles in pathogenesis and immunoprotection will be exposed.

Rapid purification of recombinant anthrax-protective antigen under nondenaturing conditions

Anthrax-protective antigen is the central moiety of the anthrax toxin complex that mediates the entry of the other two toxin components, lethal factor and edema factor into the cells. It is also the main immunogen of the cell-free vaccine against anthrax. However, in addition to PA, the vaccine contains trace amounts of other culture-derived proteins that contribute to the side effects of the vaccine like pain, edema, erythrema, etc. Thus there is a need to develop high-resolution purification methods to purify PA to homogeneity. In this study we have presented a purification strategy for rapid purification of recombinant protective antigen under nondenaturing conditions, which ensures that not only biological activity but also the conformational integrity of immunological epitopes is well-preserved. The protein was purified to homogeneity in a two-step purification procedure that takes just 6 h for completion. Three milligrams of recombinant protective antigen obtained from 1-liter culture was comparable to B. anthracis protective antigen in terms of functional and biological activity. Moreover, the immunogenicity elicited by the purified protein in mice was also studied. The studies reported here are part of continuing research that aims to provide a safe and efficacious alternative to the current vaccine against anthrax.

Detection of anthrax vaccine virulence factors by polymerase chain reaction

In Italy, an attenuated Bacillus anthracis strain, named 'Carbosap', is used for immunization against ovine and bovine anthrax. Analysis on 'Carbosap', Sterne vaccine strain F34 and Pasteur vaccine strain SS104, were performed using primers specific for the sequences, encoding the toxic factors, located on plasmids pXO1 and pXO2 and primers specific for the chromosome. The results obtained from polymerase chain reaction (PCR) assay revealed the presence of both plasmids pXO1 and pXO2 in 'Carbosap' strain. This study showed that the 'Carbosap' vaccine strain has a different plasmid pattern in comparison to Pasteur vaccine strain SS104 and Sterne vaccine strain F34.

Correlation between plasmid content and infectivity in Borrelia burgdorferi

Infectivity-associated plasmids were identified in Borrelia burgdorferi B31 by using PCR to detect each of the plasmids in a panel of 19 clonal isolates. The clones exhibited high-, low-, and intermediate-infectivity phenotypes based on their frequency of isolation from needle-inoculated C3H/HeN mice. Presence or absence of 21 of the 22 plasmids was determined in each of the clones by using PCR primers specific for regions unique to each plasmid, as identified in the recently available genome sequence. Southern blot hybridization results were used to confirm the PCR results in some cases. Plasmid lp25 exhibited a direct correlation with infectivity in that it was consistently present in all clones of high or intermediate infectivity and was absent in all low-infectivity clones. lp28-1, containing the vmp-like sequence locus, also correlated with infectivity; all clones that lacked lp28-1 but contained lp25 had an intermediate infectivity phenotype, in which infection was primarily restricted to the joints. Plasmids cp9, cp32-3, lp21, lp28-2, lp28-4, and lp56 apparently are not required for infection in this model, because clones lacking these plasmids exhibited a high-infectivity phenotype. Plasmids cp26, cp32-1, cp32-2 and/or cp32-7, cp32-4, cp32-6, cp32-8, cp32-9, lp17, lp28-3, lp36, lp38, and lp54 were consistently present in all clones examined. On the basis of these results, lp25 and lp28-1 appear to encode virulence factors important in the pathogenesis of B. burgdorferi B31.

Protective antigen-mediated antibody response against a heterologous protein produced in vivo by Bacillus anthracis

Bacillus anthracis secretes a lethal toxin composed of two proteins, the lethal factor (LF) and the protective antigen (PA), which interact within the host or in vitro at the surfaces of eukaryotic cells. Immunization with attenuated B. anthracis strains induces an antibody response against PA and LF. The LF-specific response is potentiated by the binding of LF to PA. In this study, we investigated the capacity of PA to increase the antibody response against a foreign antigen. We constructed a chimeric gene encoding the PA-binding part of LF (LF254) fused to the C fragment of tetanus toxin (ToxC). The construct was introduced by allelic exchange into the locus encoding LF. Two recombinant B. anthracis strains secreting the hybrid protein LF254-ToxC were generated, one in a PA-producing background and the other in a PA-deficient background. Mice were immunized with spores of the strains, and the humoral response and protection against tetanus toxin were assessed. The B. anthracis strain producing both PA and LF254-ToxC induced significantly higher antibody titers and provided better protection against a lethal challenge with tetanus toxin than did its PA-deficient counterpart. Thus, PA is able to potentiate protective immunity against a heterologous antigen, demonstrating the potential of B. anthracis recombinant strains for use as live vaccine vehicles.

Characterization of Bacillus anthracis strains used for vaccination

Three Bacillus anthracis strains, formerly used as anti-anthrax vaccine strains in Argentina, were characterized from genetic and pathogenic perspectives. Southern blotting and PCR with pXO1 and pXO2 probes and primers, as well as pathogenicity and protection tests in guinea pigs and mice, were performed. Two of the B. anthracis strains contained both pXO1 and pXO2 plasmids, as did the fully virulent strains, while the third was a Sterne-type strain (pXO1+, pXO2-). The three strains were, however, markedly less pathogenic than a wild-type virulent strain. The methodology applied here may be used to characterize other B. anthracis strains.

Role of toxin functional domains in anthrax pathogenesis

We investigated the role of the functional domains of anthrax toxins during infection. Three proteins produced by Bacillus anthracis, the protective antigen (PA), the lethal factor (LF), and the edema factor (EF), combine in pairs to produce the lethal (PA+LF) and edema (PA+EF) toxins. A genetic strategy was developed to introduce by allelic exchange specific point mutations or in-frame deletions into B. anthracis toxin genes, thereby impairing either LF metalloprotease or EF adenylate cyclase activity or PA functional domains. In vivo effects of toxin mutations were analyzed in an experimental infection of mice. A tight correlation was observed between the properties of anthrax toxins delivered in vivo and their in vitro activities. The synergic effects of the lethal and edema toxins resulted purely from their enzymatic activities, suggesting that in vivo these toxins may act together. The PA-dependent antibody response to LF induced by immunization with live B. anthracis was used to follow the in vivo interaction of LF and PA. We found that the binding of LF to PA in vivo was necessary and sufficient for a strong antibody response against LF, whereas neither LF activity nor binding of lethal toxin complex to the cell surface was required. Mutant PA proteins were cleaved in mice sera. Thus, our data provide evidence that, during anthrax infection, PA may interact with LF before binding to the cell receptor. Immunoprotection studies indicated that the strain producing detoxified LF and EF, isogenic to the current live vaccine Sterne strain, is a safe candidate for use as a vaccine against anthrax.

Optimized production and purification of Bacillus anthracis lethal factor

Bacillus anthracis lethal factor (LF) is a 90-kDa zinc metalloprotease that plays an important role in the virulence of the organism. LF has previously been purified from Escherichia coli and Bacillus anthracis. The yields and purities of these preparations were inadequate for crystal structure determination. In this study, the genes encoding wild-type LF and a mutated, inactive LF (LF-E687C) were placed in an E. coli-Bacillus shuttle vector so that LF was produced with the protective antigen (PA) signal peptide at its N-terminus. The resulting vectors, pSJ115 and pSJ121, express wild-type and mutated LF fusion proteins, respectively. Expression of the LF genes is under the control of the PA promoter and, during secretion, the PA signal peptide is cleaved to release the 90-kDa LF proteins. The wild-type and mutated LF proteins were purified from the culture medium using three chromatographic steps (Phenyl-Sepharose, Q-Sepharose, and hydroxyapatite). The purified proteins were greater than 95% pure and yields (20-30 mg/L) were higher than those obtained in other expression systems (1-5 mg/L). These proteins have been crystallized and are being used to solve the crystal structure of LF. Their potential use in anthrax vaccines is also discussed.