Sympatric soil communities of<i>Bacillus cereus sensu lato</i>: population structure and potential plasmid dynamics of pXO1- and pXO2-like elements

Eighty soil-borne Bacillus cereus group isolates were collected from two neighbouring geographical sites in Belgium. Their genetic relationships and population structure were assessed using Multilocus sequence typing analysis of five chromosomal genes, while the contribution of extrachromosomal elements to the population dynamics was gauged by the presence, diversity and transfer capacity of pXO1- and pXO2-like plasmids. Globally, the bacterial population displayed a broad diversity, including an important subpopulation of psychrotolerant isolates related to Bacillus weihenstephanensis. pXO1- and pXO2-like replicons were present in 12% and 21% of the isolates, but no Bacillus anthracis-related toxin genes were found. Furthermore, only one of the isolates containing a pXO2-related plasmid was shown to be able to mobilize small non-self-conjugative plasmids. Interestingly, several B. cereus sensu lato isolates displaying the same sequence type were observed to have different plasmid contents, suggesting the occurrence of horizontal gene exchange. Similarly, a number of pXO2-like replicons with identical sequences were found in distinct bacterial isolates, therefore strongly arguing for lateral transfers among sympatric bacteria.