Plasmid Fusion and Recombination Events That Occurred during Conjugation of -Carrying Plasmids in Enterococci.

Linezolid plays a crucial role in the treatment of infections caused by multiresistant Gram-positive bacteria. The gene not only confers oxazolidinone and phenicol resistance but also decreases susceptibility to tetracycline. In this study, we investigated structural changes in mobilizable -carrying plasmids in enterococci which occurred during conjugation experiments using S1-PFGE (pulsed-field gel electrophoresis), Southern blot hybridization, and whole-genome sequencing (WGS) analysis. Two -carrying strains were identified in Enterococcus faecalis E006 and Enterococcus lactis E843, respectively. E. faecalis E006 contains the 121,520-bp conjugative plasmid pE006-121 and the 19,832-bp mobilizable -carrying plasmid pE006-19, while E843 contains the 171,930-bp conjugative plasmid pE843-171 and the 27,847-bp mobilizable -carrying plasmid pE843-27. Moreover, both -carrying plasmids were mobilized by their respective conjugative plasmid in enterococci by plasmid fusion; one was generated by homologous recombination in E. faecalis through an identical 864-bp homologous region in the plasmids of the parental strain, while another was generated by an IS-mediated plasmid integration in , involving a replicative transposition. Until now, all the genes described in enterococci, including E. faecalis, E. faecium, and E. hirae, are plasmid-borne, suggesting that plasmids play an important role in the dissemination of the gene among enterococci. This study showed that the mobilizable -carrying plasmid could transfer with the help of conjugative plasmid in enterococci via plasmid fusion, with one generated by homologous recombination in E. faecalis, and another by replicative transposition in . During both the fusion events, the -carrying plasmids changed from nonconjugative to conjugative, leading to the generation and enhanced dissemination of the larger phenicol-oxazolidinone-tetracycline resistance-encoding plasmids in enterococci.