Study of genetic exchanges among Bacillus spp. in the context of confined environments

(eng) Members of the Bacillus genus include potential pathogens. They are ubiquitous in the environment and contribute substantially to the bacterial communities found in confined places such as the ISS and the Antarctic Concordia station. In a context of planetary and crew health protection, it is necessary to monitor bacterial contamination and adaptation inside the confined stations designed for the exploration of new environments. The first part of this thesis consisted on the study of the Bacillus population diversity found in ISS and Concordia and to the comparison with the situation in environment, with a specific focus on members of the B. cereus group. The contribution of plasmids related to B. anthracis virulence elements to the extrachromosomal gene pool was assessed. The ability of several plasmids with a pXO2-like replicon to sustain conjugative transfer demonstrated the putative importance of horizontal gene transfer in bacterial adaptation. This was further confirmed by the isolation of a new conjugative element in a Concordia B. cereus s.l. isolate and by the detection of plasmid transfer among the Bacillus strains from the confined stations. The actual role of conjugation on bacterial adaptation was further assessed in real situation. The choice of food matrices was relevant since putative food pathogens were detected in the Bacillus population of the ISS and Concordia. Conjugative transfer of the reference conjugative plasmids pAW63 and pXO16 was successfully demonstrated in dairy products. Interestingly, transfer frequencies in foodstuffs turned out to be higher by up to 10 to 100 fold compared to those obtained in laboratory medium. Finally, the specific features of pXO16 make that it was an interesting Gram-positive model to define the mechanisms of conjugation, mobilization and retromobilization, i.e. plasmid capture. Kinetics experiments have shown that retromobilisation followed a two-step mechanism, pXO16 having first to establish in the cell harboring the mobilizable plasmid. This confirmed the previous extensive study in Gram-negative bacteria. A preliminary analysis of pXO16 sequence also suggested that it could represent the archetype of a new mobile element.