DNA exchange in the Bacillus cereus group in biofilms and under stress

The evolutionary path of bacteria is mainly influenced by horizontal gene transfers (HGT) since mobile genetic elements carry a panel of key metabolic traits to adapt to changing environments. Among these HGT, conjugation and mobilization have been described for over fifty years, in laboratory liquid media. However, it is now generally admitted that, in nature, bacteria are subjected to various stresses and are mostly found in interface-associated communities. This study focuses on members of the Bacillus cereus group which are food contaminants and characterized by a large extra-chromosomal gene pool. One of these elements, pAW63 which resembles the Bacillus anthracis virulence plasmid pXO2, is used as a model to study conjugation in Gram-positive species. This 72kb-plasmid is a self-transmissible element originating from Bacillus thurigiensis sv. kurstaki and displays an ability to mobilize other plasmids. In this work, the dynamics of pAW63 transfer in the B. cereus group was investigated under different conditions. On one hand, mating partners in liquid medium were submitted to various chemical and physical stresses including extreme osmotic environment and microgravity. On the other hand, capabilities of plasmid transfer were estimated in biofilms by performing an in situ analysis based on the detection of fluorescent proteins. Moreover, bacterial populations were analysed for their conjugation abilities on simulated food microcosms increasing the relevance of the study with regard to ecological niches of B. cereus and other food borne pathogens. The results showed that pAW63 transfer efficiency is largely affected by the chemical composition of the mating medium and was found to increase under specific stress conditions. Biofilms were also proved to be a favored environment for conjugation among members of the B. cereus group. Moreover, it appeared that the bacterial behavior inside a structured community impacts on pAW63 transfer ability resulting in “hotspots” of conjugation inside microcolonies. The food composition of the solid media used as a substrate for biofilm development was also found to dramatically affect the structure of the community and the transfer ability