The presence of a pXO1- and/or pXO2-like plasmid(s) in clinical isolates of Bacillus cereus sensu stricto and in strains of the biopesticide Bacillus thuringiensis has been reported recently, and the pXO2-like plasmid pBT9727 and another pXO2-like plasmid, pAW63, were found to be conjugative. In this study, a total of 1,000 B. cereus group isolates were analyzed for the presence of pXO1- and pXO2-like replicons and for the presence of pXO2-related conjugative modules. pXO1- and pXO2-like replicons were present in ca. 6.6% and 7.7% of random environmental samples, respectively, and ca. 1.54% of the strains were positive for pXO2-like transfer module genes. Only the strains harboring a pXO2-like replicon also contained the corresponding transfer genes. For the strains which contained a pXO1- and/or pXO2-like replicon(s), a large plasmid(s) whose size was similar to that of pXO1-like and/or pXO2-like plasmids was also observed, but none of these isolates were found to carry the Bacillus anthracis toxin or capsule virulence genes. Furthermore, 17 of 22 pXO2-like plasmids containing the transfer modules were able to self-transfer and to mobilize small plasmids. No pXO1- or pXO2-like plasmid lacking the cognate transfer modules has been found to have transfer potential. In the strains possessing the putative pXO2-like conjugative apparatus, variations in the presence of the group II introns B.th.I.1 and B.th.I.2 were observed, suggesting that there is important flexibility in the conjugation modules and their regulation. There was no consistent correlation between a pXO2-like repA dendrogram and the presence of the tra region or between a virB4 dendrogram and transfer ability. Discrepancies between pXO2-like repA and virB4 dendrograms were also observed, indicating that the evolution of pXO2 is an active process.
The transfer of plasmids by mating from four Bacillus thuringiensis subspecies to Bacillus anthracis and Bacillus cereus recipients was monitored by selecting transcipients which acquired plasmid pBC16 (Tcr). Transcipients also inherited a specific large plasmid from each B. thuringiensis donor at a high frequency along with a random array of smaller plasmids. The large plasmids (ca. 50 to 120 megadaltons), pXO13, pXO14, pXO15, and pXO16, originating from B. thuringiensis subsp. morrisoni, B. thuringiensis subsp. toumanoffi, B. thuringiensis subsp. alesti, and B. thuringiensis subsp. israelensis, respectively, were demonstrated to be responsible for plasmid mobilization. Transcipients containing any of the above plasmids had donor capability, while B. thuringiensis strains cured of each of them were not fertile, indicating that the plasmids confer conjugation functions. Confirmation that pXO13, pXO14, and pXO16 were self-transmissible was obtained by the isolation of fertile B. anthracis and B. cereus transcipients that contained only pBC16 and one of these plasmids. pXO14 was efficient in mobilizing the toxin and capsule plasmids, pXO1 and pXO2, respectively, from B. anthracis transcipients to plasmid-cured B. anthracis or B. cereus recipients. DNA-DNA hybridization experiments suggested that DNA homology exists among pXO13, pXO14, and the B. thuringiensis subsp. thuringiensis conjugative plasmids pXO11 and pXO12. Matings performed between strains which each contained the same conjugative plasmid demonstrated reduced efficiency of pBC16 transfer. However, in many instances when donor and recipient strains contained different conjugative plasmids, the efficiency of pBC16 transfer appeared to be enhanced.
The self-transmissible plasmid pXO12 (112.5 kilobases [kb]), originally isolated from strain 4042A of Bacillus thuringiensis subsp. thuringiensis, codes for production of the insecticidal crystal protein (Cry+). The mechanism of pXO12-mediated plasmid transfer was investigated by monitoring the cotransfer of the tetracycline resistance plasmid pBC16 (4.2 kb) and the Bacillus anthracis toxin and capsule plasmids, pXO1 (168 kb) and pXO2 (85.6 kb), respectively. In matings of B. anthracis donors with B. anthracis and Bacillus cereus recipients, the number of Tcr transcipients ranged from 4.8 x 10(4) to 3.9 x 10(6)/ml (frequencies ranged from 1.6 x 10(-4) to 7.1 x 10(-2), and 0.3 to 0.4% of them simultaneously inherited pXO1 or pXO2. Physical analysis of the transferred plasmids suggested that pBC16 was transferred by the process of donation and that the large B. anthracis plasmids were transferred by the process of conduction. The transfer of pXO1 and pXO2 involved the transposition of Tn4430 from pXO12 onto these plasmids. DNA-DNA hybridization experiments demonstrated that Tn4430 was located on a 16.0-kb AvaI fragment of pXO12. Examination of Tra- and Cry- derivatives of pXO12 showed that this fragment also harbored information involved in crystal formation and was adjacent to a restriction fragment containing DNA sequences carrying information required for conjugal transfer.
An 8,883-bp mini-pXO1 plasmid containing a replicon from Bacillus anthracis pXO1 (181.6 kb) was identified by making large deletions in the original plasmid using a newly developed Cre-loxP system. Portions of the truncated mini-pXO1 were cloned into an Escherichia coli vector unable to replicate in B. anthracis. A 5.95-kb region encompassing three putative genes was identified as the minimal pXO1 fragment required for replication of the resulting recombinant shuttle plasmid (named pMR) in B. anthracis. Deletion analysis showed that the only genes essential for replication were the pXO1-14 and pXO1-16 genes, which are transcribed in opposite directions and encode predicted proteins of 66.5 and 67.1 kDa, respectively. The ORF14 protein contains a helix-turn-helix motif, while the ORF16 upstream region contains attributes of a theta-replicating plasmid origin of replication (Ori), namely, an exclusively A+T-containing segment, five 9-bp direct repeats, an inverted repeat, and a σA-dependent promoter for the putative replication initiator Rep protein (ORF16). Spontaneous mutations generated in the ORF14, ORF16, and Ori regions of pMR during PCR amplification produced a temperature-sensitive plasmid that is unable to replicate in B. anthracis at 37°C. The efficacy of transformation of plasmid-free B. anthracis Ames and Sterne strains by the original pMR was ∼103 CFU/μg, while Bacillus cereus strains 569 and ATCC 10987 were transformed with efficiencies of 104 and 102 CFU/μg, respectively. Around 95% of B. anthracis cells retained pMR after one round of sporulation and germination.
To facilitate the analysis of genetic determinants carried by large resident plasmids of Bacillus anthracis, a mating system was developed which promotes plasmid transfer among strains of B. anthracis, B. cereus, and B. thuringiensis. Transfer of the selectable tetracycline resistance plasmid pBC16 and other plasmids from B. thuringiensis to B. anthracis and B. cereus recipients occurred during mixed incubation in broth. Two plasmids, pXO11 and pXO12, found in B. thuringiensis were responsible for plasmid mobilization. B. anthracis and B. cereus transcipients inheriting either pXO11 or pXO12 were, in turn, effective donors. Transcipients harboring pXO12 were more efficient donors than those harboring pXO11; transfer frequencies ranged from 10(-4) to 10(-1) and from 10(-8) to 10(-5), respectively. Cell-to-cell contact was necessary for plasmid transfer, and the addition of DNase had no effect. The high frequencies of transfer, along with the fact that cell-free filtrates of donor cultures were ineffective, suggested that transfer was not phage mediated. B. anthracis and B. cereus transcipients which inherited pXO12 also acquired the ability to produce parasporal crystals (Cry+) resembling those produced by B. thuringiensis, indicating that pXO12 carries a gene(s) involved in crystal formation. Transcipients which inherited pXO11 were Cry-. This mating system provides an efficient method for interspecies transfer of a large range of Bacillus plasmids by a conjugation-like process.
The large pXO1 plasmid (181.6 kb) of Bacillus anthracis encodes the anthrax toxin proteins. Previous studies have shown that two separate regions of pXO1 can support replication of pXO1 miniplasmids when introduced into plasmid-less strains of this organism. No information is currently available on the ability of the above two replicons, termed RepX and ORFs 14/16 replicons, to support replication of the full-length pXO1 plasmid. We generated mutants of the full-length pXO1 plasmid in which either the RepX or the ORFs 14/16 replicon was inactivated by TargeTron insertional mutagenesis. Plasmid pXO1 derivatives containing only the RepX or the ORFs 14/16 replicon were able to replicate when introduced into a plasmid-less B. anthracis strain. Plasmid copy number analysis showed that the ORFs 14/16 replicon is more efficient than the RepX replicon. Our studies demonstrate that both the RepX and ORFs 14/16 replicons can independently support the replication of the full-length pXO1 plasmid.
Plasmid pXO1; Replication; Replicon; TargeTron mutagenesis
Bacillus anthracis pXO1 minireplicon (MR) plasmid consisting of open reading frames (ORFs) GBAA_pXO1_0020 to GBAA_pXO1_0023 is not stably maintained in B. anthracis, whereas the full-size parent pXO1 plasmid (having 181,677 bp and 217 ORFs) is extremely stable under the same growth conditions. Two genetic tools developed for DNA manipulation in B. anthracis (Cre-loxP and Flp-FRT systems) were used to identify pXO1 regions important for plasmid stability. We localized a large segment of pXO1 that enables stable plasmid maintenance during vegetative growth. Further genetic analysis identified three genes that are necessary for pXO1 maintenance: amsP (GBAA_pXO1_0069), minP (GBAA_pXO1_0082), and sojP (GBAA_pXO1_0084). Analysis of conserved domains in the corresponding proteins indicated that only AmsP (activator of maintenance system of pXO1) is predicted to bind DNA, due to its strong helix-turn-helix domain. Two conserved domains were found in the MinP protein (Min protein from pXO1): an N-terminal domain having some similarity to the B. anthracis septum site-determining protein MinD and a C-terminal domain that resembles a baculovirus single-stranded-DNA-binding protein. The SojP protein (Soj from pXO1) contains putative Walker box motifs and belongs to the ParA family of ATPases. No sequences encoding other components of type I plasmid partition systems, namely, cis-acting centromere parS and its binding ParB protein, were identified within the pXO1 genome. A model describing the role of the MinP protein in pXO1 distribution between daughter cells is proposed.
Bacillus anthracis, the causative agent of anthrax, secretes numerous proteins into the extracellular environment during infection. A comparative proteomic approach was employed to elucidate the differences among the extracellular proteomes (secretomes) of three isogenic strains of B. anthracis that differed solely in their plasmid contents. The strains utilized were the wild-type virulent B. anthracis RA3 (pXO1+ pXO2+) and its two nonpathogenic derivative strains: the toxigenic, nonencapsulated RA3R (pXO1+ pXO2−) and the totally cured, nontoxigenic, nonencapsulated RA3:00 (pXO1− pXO2−). Comparative proteomics using two-dimensional gel electrophoresis followed by computer-assisted gel image analysis was performed to reveal unique, up-regulated, or down-regulated secretome proteins among the strains. In total, 57 protein spots, representing 26 different proteins encoded on the chromosome or pXO1, were identified by peptide mass fingerprinting. S-layer-derived proteins, such as Sap and EA1, were most frequently observed. Many sporulation-associated enzymes were found to be overexpressed in strains containing pXO1+. This study also provides evidence that pXO2 is necessary for the maximal expression of the pXO1-encoded toxins lethal factor (LF), edema factor (EF), and protective antigen (PA). Several newly identified putative virulence factors were observed; these include enolase, a high-affinity zinc uptake transporter, the peroxide stress-related alkyl hydroperoxide reductase, isocitrate lyase, and the cell surface protein A.
Bacillus anthracis, the etiologic agent of anthrax, is genetically close to and commonly shares a giant gene pool with B. cereus and B. thuringiensis. In view of the human pathogenicity and the long persistence in the environment of B. anthracis, there is growing concern about the effects of genetic exchange with B. anthracis on public health. In this work, we demonstrate that an insecticidal plasmid, pHT73, from B. thuringiensis strain KT0 could be efficiently transferred into two attenuated B. anthracis strains, Ba63002R (pXO1+ pXO2−) and Ba63605R (pXO1− pXO2+), by conjugation in liquid medium in the laboratory, with transfer rates of 2.3 × 10−4 and 1.6 × 10−4 CFU/donor, respectively. The B. anthracis transconjugants containing both pHT73 and pXO1 or pXO2 could produce crystal protein Cry1Ac encoded by plasmid pHT73 and had high toxicity to Helicoverpa armigera larvae. Furthermore, the compatibility and stability of pHT73 with pXO1/pXO2 were demonstrated. The data are informative for further investigation of the safety of B. thuringiensis and closely related strains in food and in the environment.
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.
Fully virulent Bacillus anthracis bacilli are encapsulated and toxinogenic. These bacteria contain two plasmids, pXO1 and pXO2, carrying genes coding for toxins (pag, lef, and cya) and for capsule synthetic enzymes (capB, capC, capA, and dep), respectively. A transcriptional fusion between the capB regulatory region and the lacZ reporter gene was constructed to study the regulation of capsule synthesis. A single copy of this fusion was inserted into the cap region of pXO2. The influence of environmental factors on the capB-lacZ fusion expression was initially analyzed in a pXO1-negative background: bicarbonate but not temperature induced the transcription from the capB promoter. A strain carrying the recombinant pXO2 and (delta)pag pXO1 was constructed for transregulatory studies. The pXO1 plasmid strongly enhanced capsule formation without modifying the bicarbonate-dependent induction level. A (delta)cap pXO2 was transduced into a strain containing pXO1 harboring a pag-lacZ transcriptional fusion (19). pXO2 showed no influence on the toxin gene transcription.
The plasmids of the members of the Bacillus cereus sensu lato group of organisms are essential in defining the phenotypic traits associated with pathogenesis and ecology. For example, Bacillus anthracis contains two plasmids, pXO1 and pXO2, encoding toxin production and encapsulation, respectively, that define this species pathogenic potential, whereas the presence of a Bt toxin-encoding plasmid defines Bacillus thuringiensis isolates. In this study the plasmids from B. cereus isolates that produce emetic toxin or are linked to periodontal disease were sequenced and analyzed. Two periodontal isolates examined contained almost identical ∼272-kb plasmids, named pPER272. The emetic toxin-producing isolate contained one ∼270-kb plasmid, named pCER270, encoding the cereulide biosynthesis gene cluster. Comparative sequence analyses of these B. cereus plasmids revealed a high degree of sequence similarity to the B. anthracis pXO1 plasmid, especially in a putative replication region. These plasmids form a newly defined group of pXO1-like plasmids. However, these novel plasmids do not contain the pXO1 pathogenicity island, which in each instance is replaced by plasmid specific DNA. Plasmids pCER270 and pPER272 share regions that are not found in any other pXO1-like plasmids. Evolutionary studies suggest that these plasmids are more closely related to each other than to other identified B. cereus plasmids. Screening of a population of B. cereus group isolates revealed that pXO1-like plasmids are more often found in association with clinical isolates. This study demonstrates that the pXO1-like plasmids may define pathogenic B. cereus isolates in the same way that pXO1 and pXO2 define the B. anthracis species.
Emerging B. cereus strains that cause anthrax-like disease have been isolated in Cameroon (CA strain) and Côte d’Ivoire (CI strain). These strains are unusual, because their genomic characterisation shows that they belong to the B. cereus species, although they harbour two plasmids, pBCXO1 and pBCXO2, that are highly similar to the pXO1 and pXO2 plasmids of B. anthracis that encode the toxins and the polyglutamate capsule respectively. The virulence factors implicated in the pathogenicity of these B. cereus bv anthracis strains remain to be characterised. We tested their virulence by cutaneous and intranasal delivery in mice and guinea pigs; they were as virulent as wild-type B. anthracis. Unlike as described for pXO2-cured B. anthracis, the CA strain cured of the pBCXO2 plasmid was still highly virulent, showing the existence of other virulence factors. Indeed, these strains concomitantly expressed a hyaluronic acid (HA) capsule and the B. anthracis polyglutamate (PDGA) capsule. The HA capsule was encoded by the hasACB operon on pBCXO1, and its expression was regulated by the global transcription regulator AtxA, which controls anthrax toxins and PDGA capsule in B. anthracis. Thus, the HA and PDGA capsules and toxins were co-regulated by AtxA. We explored the respective effect of the virulence factors on colonisation and dissemination of CA within its host by constructing bioluminescent mutants. Expression of the HA capsule by itself led to local multiplication and, during intranasal infection, to local dissemination to the adjacent brain tissue. Co-expression of either toxins or PDGA capsule with HA capsule enabled systemic dissemination, thus providing a clear evolutionary advantage. Protection against infection by B. cereus bv anthracis required the same vaccination formulation as that used against B. anthracis. Thus, these strains, at the frontier between B. anthracis and B. cereus, provide insight into how the monomorphic B. anthracis may have emerged.
Anthrax is caused by the bacterium Bacillus anthracis that affects all mammals worldwide. It emerged more than 10,000 years ago from a Bacillus cereus precursor. In the past decade, B. cereus bacteria were isolated in the USA from anthrax-like pneumonia cases. They harbour one virulence plasmid very similar to the toxin–encoding plasmid of B. anthracis. Recently, an anthrax-like disease in great apes in Africa was caused by emerging B. cereus strains, named B. cereus biovar anthracis. These strains are atypical as they possess both plasmids coding for toxin and capsule similar to those so far found only in B. anthracis. These unusual pathogenic B. cereus are currently neglected. We explored the virulence of these pathogens and their colonisation and dissemination capacity within the murine host. We found that these toxinogenic strains harbour two capsules, the classical B. anthracis capsule and an additional polysaccharidic capsule. This latter capsule confers virulence alone or in combination with toxins. Both capsules are concomitantly expressed, under the control of a common global regulator and host signals. Our results show that acquisition of new genetic information by these B. cereus clearly gives them a selective advantage, favouring their dissemination within infected hosts and the environment.
Virulent and certain avirulent strains of Bacillus anthracis harbor a plasmid, designated pXO2, which is involved in the synthesis of capsules. Two classes of rough, noncapsulated (Cap-) variants were isolated from the capsule-producing (Cap+) Pasteur vaccine strains ATCC 6602 and ATCC 4229. One class was cured of pXO2, and the other class still carried it. Reversion to Cap+ was demonstrable only in rough variants which had retained pXO2. Proof that pXO2 is involved in capsule synthesis came from experiments in which the plasmid was transferred by CP-51-mediated transduction and by a mating system in which plasmid transfer is mediated by a Bacillus thuringiensis fertility plasmid, pXO12. Cells of Bacillus cereus and a previously noncapsulated (pXO2-) strain of B. anthracis produced capsules after the acquisition of pXO2.
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.
We present the microbiological and molecular characterization of bacteria isolated from four chimpanzees and one gorilla thought to have died of an anthrax-like disease in Côte d'Ivoire and Cameroon. These isolates differed significantly from classic Bacillus anthracis by the following criteria: motility, resistance to the gamma phage, and, for isolates from Cameroon, resistance to penicillin G. A capsule was expressed not only after induction by CO2 and bicarbonate but also under normal growth conditions. Subcultivation resulted in beta-hemolytic activity and gamma phage susceptibility in some subclones, suggesting differences in gene regulation compared to classic B. anthracis. The isolates from Côte d'Ivoire and Cameroon showed slight differences in their biochemical characteristics and MICs of different antibiotics but were identical in all molecular features and sequences analyzed. PCR and Southern blot analyses confirmed the presence of both the toxin and the capsule plasmid, with sizes corresponding to the B. anthracis virulence plasmids pXO1 and pXO2. Protective antigen was expressed and secreted into the culture supernatant. The isolates possessed variants of the Ba813 marker and the SG-749 fragment differing from that of classic B. anthracis strains. Multilocus sequence typing revealed a close relationship of our atypical isolates with both classic B. anthracis strains and two uncommonly virulent Bacillus cereus and Bacillus thuringiensis isolates. We propose that the newly discovered atypical B. anthracis strains share a common ancestor with classic B. anthracis or that they emerged recently by transfer of the B. anthracis plasmids to a strain of the B. cereus group.
Several circulating Bacillus anthracis strains isolated in Italy and belonging to the A1.a cluster, genotype 3 (A1.a-3) are genotypically indistinguishable from Carbosap, a live attenuated vaccine strain, containing both pXO1 and pXO2 plasmids. The genotype was assessed by using eight-locus multilocus variable-number tandem repeat analysis. We describe here the use of a ninth locus able to explore variability among strains that have the same genotype. It is important to be able to genotype the wild isolate of B. anthracis strains from outbreaks of anthrax in areas where Carbosap vaccination of cattle and sheep is common practice. A total of 27 representative field strains isolated in Italy and four vaccinal strains, namely, Carbosap, Sterne, Pasteur I, and Pasteur II, were characterized by a ninth marker, called pXO2-A. Twenty-three field strains were genotype 3 and therefore identical to Carbosap. The marker was in the pXO2 plasmid and is based on the polymorphism of the already-known VX2-3 locus. Detection was obtained by PCR with fluorescence-labeled forward primers in order to produce appropriate fragments for capillary electrophoresis with an ABI 310 genetic analyzer. Genetic relationships showed heterogeneity in all of the examined samples. Interestingly, with respect to genotype 3, samples grouped into eight different subtypes, A to H, and the subtype G, had only two samples indistinguishable from Carbosap. The results of the present study confirm the validity of a hierarchical progressive protocol for discrimination among closely related isolates.
The secretomes of a virulent Bacillus anthracis strain and of avirulent strains (cured of the virulence plasmids pXO1 and pXO2), cultured in rich and minimal media, were studied by a comparative proteomic approach. More than 400 protein spots, representing the products of 64 genes, were identified, and a unique pattern of protein relative abundance with respect to the presence of the virulence plasmids was revealed. In minimal medium under high CO2 tension, conditions considered to simulate those encountered in the host, the presence of the plasmids leads to enhanced expression of 12 chromosome-carried genes (10 of which could not be detected in the absence of the plasmids) in addition to expression of 5 pXO1-encoded proteins. Furthermore, under these conditions, the presence of the pXO1 and pXO2 plasmids leads to the repression of 14 chromosomal genes. On the other hand, in minimal aerobic medium not supplemented with CO2, the virulent and avirulent B. anthracis strains manifest very similar protein signatures, and most strikingly, two proteins (the metalloproteases InhA1 and NprB, orthologs of gene products attributed to the Bacillus cereus group PlcR regulon) represent over 90% of the total secretome. Interestingly, of the 64 identified gene products, at least 31 harbor features characteristic of virulence determinants (such as toxins, proteases, nucleotidases, sulfatases, transporters, and detoxification factors), 22 of which are differentially regulated in a plasmid-dependent manner. The nature and the expression patterns of proteins in the various secretomes suggest that distinct CO2-responsive chromosome- and plasmid-encoded regulatory factors modulate the secretion of potential novel virulence factors, most of which are associated with extracellular proteolytic activities.
Plasmid pXO1 encodes the tripartite anthrax toxin, which is the major virulence factor of Bacillus anthracis. In spite of the important role of pXO1 in anthrax pathogenesis, very little is known about its replication and maintenance in B. anthracis. We cloned a 5-kb region of the pXO1 plasmid into an Escherichia coli vector and showed that this plasmid can replicate when introduced into B. anthracis. Mutational analysis showed that open reading frame 45 (repX) of pXO1 was required for the replication of the miniplasmid in B. anthracis. Interestingly, repX showed limited homology to bacterial FtsZ proteins that are involved in cell division. A mutation in the predicted GTP binding domain of RepX abolished its replication activity. Genes almost identical to repX are contained on several megaplasmids in members of the Bacillus cereus group, including a B. cereus strain that causes an anthrax-like disease. Our results identify a novel group of FtsZ-related initiator proteins that are required for the replication of virulence plasmids in B. anthracis and possibly in related organisms. Such replication proteins may provide novel drug targets for the elimination of plasmids encoding the anthrax toxin and other virulence factors.
Bacillus cereus, Bacillus anthracis and Bacillus thuringiensis belong to the genetically close-knit Bacillus cereus sensu lato group, a family of rod-shaped Gram-positive bacteria. pAW63 is the first conjugative plasmid from the B. cereus group to be completely sequenced.
The 71,777 bp nucleotide sequence of pAW63 reveals a modular structure, including a 42 kb tra region encoding homologs of the Type IV secretion systems components VirB11, VirB4 and VirD4, as well as homologs of Gram-positive conjugation genes from Enterococcus, Lactococcus, Listeria, Streptococcus and Staphylococcus species. It also firmly establishes the existence of a common backbone between pAW63, pXO2 from Bacillus anthracis and pBT9727 from the pathogenic Bacillus thuringiensis serovar konkukian strain 97-27. The alignment of these three plasmids highlights the presence of well conserved segments, in contrast to distinct regions of high sequence plasticity. The study of their specific differences has provided a three-point reference framework that can be exploited to formulate solid hypotheses concerning the functionalities and the molecular evolution of these three closely related plasmids. This has provided insight into the chronology of their divergence, and led to the discovery of two Type II introns on pAW63, matching copies of the mobile element IS231L in different loci of pXO2 and pBT9727, and the identification on pXO2 of a 37 kb pathogenicity island (PAI) containing the anthrax capsule genes.
The complete sequence determination of pAW63 has led to a functional map of the plasmid yielding insights into its conjugative apparatus, which includes T4SS-like components, as well as its resemblance to other large plasmids of Gram-positive bacteria. Of particular interest is the extensive homology shared between pAW63 and pXO2, the second virulence plasmid of B. anthracis, as well as pBT9727 from the pathogenic strain B. thuringiensis serovar konkukian strain 97-27.
Bacillus anthracis is a member of the Bacillus cereus group species (also known as the “group 1 bacilli”), a collection of Gram-positive spore-forming soil bacteria that are non-fastidious facultative anaerobes with very similar growth characteristics and natural genetic exchange systems. Despite their close physiology and genetics, the Bacillus cereus group species exhibit certain species-specific phenotypes, some of which are related to pathogenicity. B. anthracis is the etiologic agent of anthrax. Vegetative cells of B. anthracis produce anthrax toxin proteins and a poly-D-glutamic acid capsule during infection of mammalian hosts and when cultured in conditions considered to mimic the host environment. The genes associated with toxin and capsule synthesis are located on the B. anthracis plasmids, pXO1 and pXO2, respectively. Although plasmid content is considered a defining feature of the species, pXO1- and pXO2-like plasmids have been identified in strains that more closely resemble other members of the B. cereus group. The developmental nature of B. anthracis and its pathogenic (mammalian host) and environmental (soil) lifestyles of make it an interesting model for study of niche-specific bacterial gene expression and physiology.
anthracis; Bacillus cereus group; bacteriophage; plasmid; genetic exchange; virulence gene expression
The pag gene of Bacillus anthracis, located on plasmid pXO1 (185 kb), encodes protective antigen, a component of the anthrax lethal and edema toxins. Synthesis of protective antigen is enhanced during growth of the organism with elevated levels of CO2. The CO2 effect is at the level of transcription, and pXO1-encoded regulatory factors have been implicated in control of pag expression. We used a Tn917-LTV3 insertion mutant of B. anthracis in which the wild-type pag gene on pXO1 was replaced with a pag-lacZ transcriptional fusion to monitor pag promoter activity. Expression of the pag-lacZ fusion is induced five- to eightfold during growth in 5% CO2 compared with growth in air. Growth in 20% CO2 increases transcription up to 19-fold. By monitoring pag-lacZ expression in atmospheres with different O2 and CO2 concentrations, we demonstrated definitively that the CO2 effect is specific and not simply a result of increased anaerobiosis. The results of 5' end mapping of pag transcripts indicate multiple sites of transcript initiation. We have determined two major apparent start sites, designated P1 and P2, located at positions -58 and -26 relative to the translation initiation codon, respectively. Analysis of total RNA from late-log-phase cells shows comparable initiation from P1 and P2 in wild-type strains grown in aerobic conditions. However, initiation from P1 is increased approximately 10-fold in cultures grown with an elevated level (5%) of CO2. We have identified a locus on pXO1, more than 13 kb upstream from the pag gene, which enhances pag transcription. When added in trans, this locus increases the level of transcripts with 5' ends mapping to P1 but has no effect on the level of transcripts with 5' ends mapping to P2. The CO2 effect on P1 is observed only in the presence of the activator locus.
Bacillus anthracis produces the anthrax toxin proteins protective antigen (PA), lethal factor (LF), and edema factor (EF) in a growth phase-dependent manner when cultured in liquid medium. Expression of the toxin genes pagA, lef, and cya peaks in late log phase, and steady-state levels of the toxin proteins are highest during the transition into stationary phase. Here we show that an apparent transition state regulator negatively regulates toxin gene expression. We identified two orthologues of the B. subtilis transition state regulator abrB in the B. anthracis genome: one on the chromosome and one on the 182-kb virulence plasmid pXO1. The orthologue located on the chromosome is predicted to encode a 94-amino-acid protein that is 85% identical to B. subtilis AbrB. The hypothetical protein encoded on pXO1 is 41% identical to B. subtilis AbrB but missing 27 amino acid residues from the amino terminus compared to the B. subtilis protein. Deletion of the pXO1-encoded abrB orthologue did not affect toxin gene expression under the conditions tested. However, a B. anthracis mutant in which the chromosomal abrB gene was deleted expressed pagA earlier and at a higher level than the parent strain. Expression of a transcriptional pagA-lacZ fusion in the abrB mutant was increased up to 20-fold during early exponential growth compared to the parent strain and peaked in mid-exponential rather than late exponential phase. In contrast to the strong effect of abrB on pagA expression, lef-lacZ and cya-lacZ expression during early-log-phase growth was increased only two- to threefold in the abrB null mutant. Western hybridization analysis showed increased PA, LF, and EF synthesis by the mutant. As is true in B. subtilis, the B. anthracis abrB gene is negatively regulated by spo0A. Our findings tie anthrax toxin gene expression to the complex network of postexponential phase adaptive responses that have been well studied in B. subtilis.
The large plasmid pXO1 encoding the anthrax toxin is important for the virulence of Bacillus anthracis. It is essential to cure pXO1 from B. anthracis to evaluate its role in the pathogenesis of anthrax infection. Because conventional methods for curing plasmids (e.g., curing agents or growth at elevated temperatures) can induce mutations in the host chromosomal DNA, we developed a specific and reliable method to eliminate pXO1 from B. anthracis using plasmid incompatibility. Three putative replication origins of pXO1 were inserted into a temperature-sensitive plasmid to generate three incompatible plasmids. One of the three plasmids successfully eliminated the large plasmid pXO1 from B. anthracis vaccine strain A16R and wild type strain A16. These findings provided additional information about the replication/partitioning of pXO1 and demonstrated that introducing a small incompatible plasmid can generate plasmid-cured strains of B. anthracis without inducing spontaneous mutations in the host chromosome.
Bacillus anthracis secretes several virulence factors targeting different host organs and cell types during inhalational anthrax infection. The bacterial expression of a key virulence factor, lethal toxin (LeTx) is closely tied to another factor, edema toxin (EdTx). Both are transcribed on the same virulence plasmid (pXO1) and both have been the subject of much individual study. Their combined effect during virulent anthrax likely modulates both the global transcriptional and the phenotypic response of macrophages and phagocytes. In fact, responses brought about by the toxins may be different than each of their individual effects.
Here we report the transcriptional and apoptotic responses of the macrophage-like phagocytic cell line THP-1 exposed to B. anthracis Sterne (pXO1+) spores, and B. anthracis Δ Sterne (pXO1-) spores. These cells are resistant to LeTx-induced cytolysis, a phenotype seen in macrophages from several mouse strains which are sensitive to toxigenic anthrax infection. Our results indicate that the pXO1-containing strain induces higher pro-inflammatory transcriptional responses during the first 4 hours of interaction with bacterium, evident in the upregulation of several genes relevant to Nf-κB, phosphatases, prostaglandins, and TNF-α, along with decreases in expression levels of genes for mitochondrial components. Both bacterial strains induce apoptosis, but in the toxigenic strain-challenged cells, apoptosis is delayed.
This delay in apoptosis occurs despite the much higher level of TNF-α secretion induced by the toxigenic-strain challenge. Interestingly, CFLAR, an important apoptotic inhibitor which blocks apoptosis induced by large amounts of extracellular TNF-α, is upregulated significantly during toxigenic-strain infection, but not at all during non-toxigenic-strain infection, indicating that it may play a role in blocking or delaying TNF-α-mediated apoptosis. The suppression of apoptosis by the toxigenic anthrax strain is consistent with the notion that apoptosis itself may represent a protective host cell response.