pSK41-Like Plasmid Is Necessary for Inc18-LikevanAPlasmid Transfer from Enterococcus faecalis to Staphylococcus aureusIn Vitro

Abstract

ABSTRACTVancomycin-resistantStaphylococcus aureus(VRSA) is thought to result from thein vivoconjugative transfer of avanAplasmid from anEnterococcussp. toS. aureus. We studied bacterial isolates from VRSA cases that occurred in the United States to identify microbiological factors which may contribute to this plasmid transfer. First, vancomycin-susceptible, methicillin-resistantS. aureus(MRSA) isolates from five VRSA cases were tested for their ability to accept foreign DNA by conjugation in mating experiments withEnterococcus faecalisJH2-2 containing pAM378, a pheromone-response conjugative plasmid. All of the MRSA isolates accepted the plasmid DNA with similar transfer efficiencies (∼10−7/donor CFU) except for one isolate, MRSA8, for which conjugation was not successful. The MRSA isolates were also tested as recipients in mating experiments between anE. faecalisisolate with an Inc18-likevanAplasmid that was isolated from a VRSA case patient. Conjugative transfer was successful for 3/5 MRSA isolates. Successful MRSA recipients carried a pSK41-like plasmid, a staphylococcal conjugative plasmid, whereas the two unsuccessful MRSA recipients did not carry pSK41. The transfer of a pSK41-like plasmid from a successful MRSA recipient to the two unsuccessful recipients resulted in conjugal transfer of the Inc18-likevanAplasmid fromE. faecalisat a frequency of 10−7/recipient CFU. In addition, conjugal transfer could be achieved for pSK41-negative MRSA in the presence of a cell-free culture filtrate fromS. aureuscarrying a pSK41-like plasmid at a frequency of 10−8/recipient CFU. These results indicated that a pSK41-like plasmid can facilitate the transfer of an Inc18-likevanAplasmid fromE. faecalistoS. aureus, possibly via an extracellular factor produced by pSK41-carrying isolates.