Résumé
DNA methylation in Bacteria was discovered in the 1960s, but studies linking methylation modification to phenotype changes remain rare. Deregulation of DNA-methyltransferases (DNA MTases) has been shown to be associated with significant phenotypic shift, such as virulence attenuation in Vibrio cholerae[1]. Contrary to eukaryotes, bacterial DNA methylation occurs only on specific motifs, with the majority (>94%) being methylated. Multiple studies have demonstrated the global stability of the methylome at different growth stages or under different conditions, and a few promoter-region modifications have been associated with gene regulation and phenotypic changes[2]. In marine ecosystems, where bacteria face constant environmental variations, the importance of epigenetics on pathogens cycle is still unclear. V. aestuarianus is a marine bacterial pathogen that clonally expanded over Europe in the last 20 years[3]. It causes massive mortality events for Magallana gigas oysters, leading to substantial economic losses in the European aquaculture industry. Yet the molecular mechanisms driving oyster colonization and virulence remain unknown. Environmental factors such as temperature, salinity, 02 and rainfall strongly influence the susceptibility to the bacteria. Notably, decreased salinity induces higher mortality. However, the molecular mechanisms underlying this salinity-dependent virulence remain poorly characterized. To address the effect of salinity, bacterial cultures were performed at different environmental salinities (15, 30 and 40 g/l) for methylome (SMRT-seq) and transcriptome (RNA-seq) monitoring. An initial genomic analysis revealed 10 DNA MTases in V. aestuarianus 12/016 (4 solitary, 6 in Restriction-Modification system, 8 being consistently expressed), comparable to studies showing methylome modifications associated with phenotype changes. The sequencing effort (~600X) revealed four highly methylated (>99%) m6A motifs (GATC, CAGNNNNNNTYTC, TAACNNNNRTAC, ACCNNNNNNNTTCY) and one m4C motif (>96%, GGWCC). A potential m5C motif (CGCCG) was also detected. Ongoing analyses aim to find potential methylation changes in promoter regions associated with virulence.