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

Abstract

Until now, all the poxtA 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 poxtA gene among enterococci. This study showed that the mobilizable poxtA -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 E. lactis . Linezolid plays a crucial role in the treatment of infections caused by multiresistant Gram-positive bacteria. The poxtA gene not only confers oxazolidinone and phenicol resistance but also decreases susceptibility to tetracycline. In this study, we investigated structural changes in mobilizable poxtA -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 poxtA -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 poxtA -carrying plasmid pE006-19, while E. lactis E843 contains the 171,930-bp conjugative plasmid pE843-171 and the 27,847-bp mobilizable poxtA -carrying plasmid pE843-27. Moreover, both poxtA -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 1216E -mediated plasmid integration in E. lactis , involving a replicative transposition. IMPORTANCE Until now, all the poxtA 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 poxtA gene among enterococci. This study showed that the mobilizable poxtA -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 E. lactis . During both the fusion events, the poxtA -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.