The horizontal transfer of genomic islands is essential for the adaptation and evolution of Enterococcus faecalis. In this study, three porcine E. faecalis strains, each harboring a large (E)-carrying genomic island, were identified. When using the E. faecalis OG1RF as the recipient, the horizontal transfer of the (E)-carrying genomic island occurred only from E. faecalis E512, which also harbored a pheromone-responsive conjugative plasmid, but not from the other two E. faecalis strains, E533 and E509, which lacked such a plasmid. Subsequently, through plasmid curing of E. faecalis E512 and plasmid introduction into E. faecalis E533, the pheromone-responsive conjugative plasmid was identified to be indispensable for the horizontal transfer of the (E)-carrying genomic island and a subsequent homologous recombination between the chromosomal DNA of the donor and the recipient. In addition, the presence of a chromosomally-located conjugative transposon, Tn, in E. faecalis E509 could not mediate the horizontal transfer of the (E)-carrying genomic island, although Tn itself could transfer by conjugation. Thus, these data highlight the role of the pheromone-responsive conjugative plasmid in the transfer of the (E)-carrying genomic island in E. faecalis, thereby establishing the dual role of pheromone-responsive conjugative plasmids in contributing to the dissemination of both plasmid-borne resistance genes and chromosomally-located genomic islands. In this study, it was shown that a pheromone-responsive conjugative plasmid played an indispensable role in the horizontal transfer of a (E)-carrying genomic island. This finding indicates a dual role of the pheromone-responsive conjugative plasmid in disseminating both plasmid-borne resistance genes and chromosomally-located genomic islands. The role of the pheromone-responsive conjugative plasmid in disseminating chromosomal genomic islands is suggested to be essential in the genomic evolution of E. faecalis, which has become one of the leading nosocomial pathogens worldwide.
