Distribution, Diversity, and Potential Mobility of Extrachromosomal Elements Related to the Bacillus anthracis pXO1 and pXO2 Virulence Plasmids

Abstract

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 Bacillus cereus group includes Bacillus anthracis, B. cereus sensu stricto, Bacillus thuringiensis, Bacillus mycoides, Bacillus pseudomycoides, and Bacillus weihenstephanensis. All members of this group are genetically closely related and could be considered members of a single species or taxon (15,16,24,25). Several of their major phenotypic features, including their virulence spectra, are directly associated with large plasmids. In B. anthracis, the anthrax toxin and capsule genes responsible for anthrax disease are located on the 182-kb pXO1 and 95-kb pXO2 plasmids, respectively (26, 27). These plasmids are not self-transmissible but can be mobilized by conjugative plasmids, such as pXO14 from B. thuringiensis (29). The biopesticide properties that are the main characteristics of B. thuringiensis and distinguish it from B. cereus are due to large plasmids carrying cry genes. These genes are responsible for the production of different Cry toxins that form insecticidal crystal protein inclusions. Some of these large plasmids have been found to have conjugative transfer capability (12,20). For instance, plasmid pHT73 from B. thuringiensis subsp. kurstaki carries cry toxin genes and is conjugative (41). Another ...

Biological Diversity of Prokaryotic Type IV Secretion Systems

“…plasmids that are closely related to Bacillus anthracis pX02. Plasmids of this group include pX02, pAW63, and pBT9727, and a recent survey also identified pX02-like plasmids encoding VirB11 homologs in Bacillus isolates from a variety of environmental niches (137). Many of these plasmids encode functional T4SS, as shown by their capacity to transfer small mobilizable plasmids (137).…”

Identification and Validation of Specific Markers of Bacillus anthracis Spores by Proteomics and Genomics Approaches

“…Indeed, Hu et al have demonstrated that ϳ7% of B. cereus/B. thuringiensis species can have a pXO1-like plasmid and ϳ1.5% a pXO2-like plasmid (15). This was particularly underlined for some virulent B. cereus strains (i.e.…”

Chromosome-Directed PCR-Based Detection and Quantification of Bacillus cereus Group Members with Focus on B. thuringiensis Serovar israelensis Active against Nematoceran Larvae

“…For strain VD045, potential plasmid transfer through conjugation has been reported, and the identification at the species level has been questioned (45). Thus, at least VD045 may actually represent a B. cereus strain which is able to take up the plasmid carrying the genes responsible for insect toxicity.…”