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

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.