Plasmid capture by the Bacillus thuringiensis conjugative plasmid pXO16.

Conjugation, mobilisation and retromobilisation are three related mechanisms of horizontal gene transfer in bacteria. They have been extensively studied in Gram-negative species, where retromobilisation, the capture of DNA from a recipient by a donor cell, was shown to result from two successive steps: the transfer of the conjugative plasmid from the donor to the recipient, followed by the retrotransfer of the mobilisable plasmid to the donor. This successive model was established for Gram-negative bacteria but was lacking experimental data from the Gram-positive counterparts. In the present work, mobilisation and retromobilisation abilities of the conjugative plasmid pXO16 from Bacillus thuringiensis sv israelensis were studied using the mobilisable plasmids pUB110 and pE194 and the "non-mobilisable" element pC194 lacking the mob and oriT features (all from Staphylococcus aureus). Experimental data suggested a successive model since different retromobilisation frequencies were observed between the small plasmids. More importantly, retromobilisation was shown to be delayed by 50 and 150 min for pUB110 and pE194, respectively, compared to pXO16 conjugation. Natural liquid foods (cow milk, soy milk and rice milk) were used to evaluate the putative ecological impact of these transfers. In cow and soy milk, conjugation, mobilisation and retromobilisation were shown to occur at frequencies of 8.0 x 10(-1), 1.0 x 10(-2) and 1.2 x 10(-4) transconjugants per recipient, respectively. These data are comparable to those obtained in LB medium and about 10-fold lower than in the case of rice milk. Taken together, these results emphasize the potential role of plasmid capture played by B. thuringiensis in natural environments.