Mobilization of vancomycin resistance by transposon-mediated fusion of a VanA plasmid with an Enterococcus faecium sex pheromone-response plasmid.

Abstract A striking feature of recent outbreaks of vancomycin-resistant (Vm R ) enterococci is the apparent horizontal dissemination of resistance determinants. The plasmids pHKK702 and pHKK703 from Enterococcus faecium clinical isolate R7 have been implicated in the conjugal transfer of Vm R . pHKK702 is a 41-kb plasmid that contains an element indistinguishable from the glycopeptide-resistance transposon Tn 1546 . pHKK703 is an approx. 55-kb putative sex pheromone-response plasmid that is required for conjugative mobilization of pHKK702. During experiments in which strain R7 was used as a donor, a highly conjugative Vm R transconjugant was isolated that formed constitutive cellular aggregates. Restriction analyses and DNA hybridizations revealed that the transconjugant harbored a single plasmid of approx. 92 kb and this plasmid (pHKK701) was composed of DNA from both pHKK702 and pHKK703. Results from DNA sequence analyses showed that a 39-kb composite transposon (Tn 5506 ) from pHKK702 had inserted into pHKK703. The left end of Tn 5506 contained a single insertion sequence (IS) element, IS 1216 V2, whereas the right end was composed of a tandem IS structure consisting of the novel 1065-bp IS 1252 nested within an IS 1216 V1 element. Transposition of Tn 5506 from pHKK702 to pHKK703 created an 8-bp target sequence duplication at the site of insertion and interrupted an ORF (ORFX) that was 91% identical to that of prgX , a gene proposed to negatively regulate sex pheromone response of the E. faecalis plasmid, pCF10. We propose that the interruption of ORFX by Tn 5506 led to the constitutive cellular aggregation phenotype and thereby enhanced the efficiency with which Vm R was transferred. Similar IS 1216 V-mediated transposition events may contribute to the horizontal spread of glycopeptide resistance among enterococci in nature.