Conjugation in the Bacillus cereus group : pAW63 transfer under stress conditions
(2012)
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- Authors
- Supervisors
- Mahillon, Jacques
- Abstract
- (en) The ability of bacteria to adapt to new environments results from the acquisition of new genetic material through Horizontal Gene Transfer. Conjugation is an efficient mechanism of such transfer, which has profound evolutionary implications. Unlike Gram-negative bacteria, the conjugation machinery is not well known in Gram-positive bacteria. pAW63, originating from Bacillus thuringiensis sv. kurstaki, was used in this study as a plasmid model to gather more insights into the conjugation mechanisms in the B. cereus s.l. group. Relaxase and origin of transfer (oriT) were previously demonstrated to be essential to plasmid transfer. The first part of this thesis consisted in the preliminary characterization of these components of the pAW63 plasmid. The putative protein encoded by pAW63 ORF76 displayed the three conserved motifs (I to III) of the DNA relaxases of mobile plasmids. In addition, sequence comparison of the pXO2-like plasmids and pHTbeta suggested the non-coding region (277 bp) between ORF2 and ORF3 as the best candidate for the pAW63 oriT region. Results of cloning and mutagenesis experiments lent further support to these hypotheses. B. cereus s.l. is an ubiquitous group of bacteria which may develop under a large set of ecological conditions. The complementary approaches of this thesis consisted in the study of the conjugative behavior of pAW63 under various stress conditions. First, several food matrices were tested and the transfer of pAW63 was found to occur with higher frequencies in full-cream milk, skimmed milk and soya milk, where the mean transfer frequencies reached 10-3 transconjugants per recipient cell. Transfer of pAW63 was also assessed in salt stress conditions and higher transfer frequencies were observed when bacteria were exposed to a severe salt stress (5% NaCl). An earlier onset of conjugation in combination with a higher transfer rate and/or a longer mating period were demonstrated by kinetics study. Finally, the conjugative behavior of pAW63 was also assessed in simulated microgravity, a more unusual stressful condition. Conjugation in Earth-based facilities revealed no significant difference in the pAW63 transfer frequencies between the three different simulated microgravitational conditions tested and the standard laboratory conditions.
- Affiliations
Citations
Beuls, E. (2012). Conjugation in the Bacillus cereus group : pAW63 transfer under stress conditions. https://hdl.handle.net/2078.5/77991