Distribution, diversity and potential mobility of extra-chromosomal elements related to the Bacillus anthracis pXO1 and pXO2 virulence plasmids.

The presence of pXO1- and/or pXO2-like plasmid(s) in clinical isolates of B. cereus (sensu stricto) and in strains of the biopesticide Bacillus thuringiensis have been reported recently, and the pXO2-like plasmid pBT9727 was found conjugative as well as another pXO2-like plasmid pAW63. 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 pXO2-related conjugative modules. The occurrence frequency of pXO1- and pXO2-like replicons in random environmental samples was ca. 6.6% and 7.7%, 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 pXO1- and/or pXO2-like replicon(s), large plasmid(s) with a size similar to that of pXO1-like and/or pXO2-like plasmids were also observed, but none of these isolates were found to carry the B. anthracis toxin or capsule virulence genes. Furthermore, 17 out of 22 pXO2-like plasmids containing the transfer modules displayed the capability of self-transfer and of mobilizing small plasmids. No pXO1-, or pXO2-like plasmid lacking the cognate transfer modules has been found to have transfer potential. Among the strains possessing the putative pXO2-like conjugative apparatus, variations on the presence of the group II introns B.th.I.1 and B.th.I.2 were observed, suggesting an important flexibility of the conjugation modules and their regulation. There was no consistent correlation between pXO2-like repA dendrogram and presence/absence of tra region, or between virB4 dendrogram and transfer capabilities. Discrepancies between pXO2-like repA and virB4 dendrograms were also observed, indicating that the evolution of pXO2 is an active process.