Plasmid-Assisted Horizontal Transfer of a Large lsa(E)-Carrying Genomic Island in Enterococcus faecalis

Abstract

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 lsa(E)-carrying genomic island, were identified. When using the E. faecalis OG1RF as the recipient, the horizontal transfer of the lsa(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 lsa(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, Tn916, in E. faecalis E509 could not mediate the horizontal transfer of the lsa(E)-carrying genomic island, although Tn916 itself could transfer by conjugation. Thus, these data highlight the role of the pheromone-responsive conjugative plasmid in the transfer of the lsa(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. IMPORTANCE In this study, it was shown that a pheromone-responsive conjugative plasmid played an indispensable role in the horizontal transfer of a lsa(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.

Keywords: Enterococcus faecalis; Tn916; conjugative plasmids; genomic island; horizontal transfer.

FIG 1

The structure of the lsa(E)-carrying genomic islands in E. faecalis isolates E509, E512, and E533 as well as their comparison with E. faecalis L21. Antimicrobial resistance genes are shown in red, IS1216E in blue, and other genes in black.

FIG 2

The structure of the plasmids, pE512 from E. faecalis isolate E512 (A) and pFac74-1 from E. faecalis isolate Fac74 (B). The circles display (from the outside to inside): (i) the size scale in bp; (ii) the positions of predicted coding sequences, transcribed in the clockwise orientation; (iii) the positions of predicted coding sequences, transcribed in the counterclockwise orientation; (iv) the GC content plotted against 50%, with blue indicating >50% and purple indicating <50%; and (v) GC skew [(G-C)/(G+C)] in a 1,000 bp window. Antimicrobial resistance genes are shown in red, and other genes in black.

FIG 3

Simplified schematic diagram for the transfer of a large chromosomal segment, including a lsa(E)-carrying genomic island in two transconjugants: E512-TC1 and E512-TC2. Transferred chromosomal segments, including a 30,536 bp lsa(E)-carrying genomic island (shown in red color), were framed up, which were 116,105 bp in transconjugant E512-TC1 and 107,189 bp in transconjugant E512-TC2. The putative homologous regions that served for the integration of the chromosomal regions of E. faecalis E512 into the recipient E. faecalis OG1RF are shown in gray color. The sizes of those regions were the same on the left-hand side (8,600 bp) but varied on the right-hand side in the two transconjugants, with 172,692 bp in E512-TC1 and 64,712 bp in E512-TC2.

FIG 4

S1-PFGE analysis for the plasmid profile of E. faecalis strains used in plasmid curing and introduction assay. The plasmids’ bands were pointed by red arrows, and the explanations of them are noted following their strain names. Lane 1, E. faecalis E533 (no plasmid); Lane 2 E. faecalis Fac74-TC (pheromone-responsive conjugative plasmid pFac74-1); Lane 3, E. faecalis E533-TC (pheromone-responsive conjugative plasmid pFac74-1); Lane 4, marker; Lane 5, E. faecalis E512 (pheromone-responsive conjugative plasmid pE512); Lane 6, E512-PC (no plasmid).