Transfer of the methicillin resistance genomic island among staphylococci by conjugation

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Methicillin resistance creates a major obstacle for treatment ofStaphylococcus aureusinfections. The resistance gene,mecA, is carried on a large (20 kb to > 60 kb) genomic island, staphylococcal cassette chromosomemec(SCCmec), that excises from and inserts site-specifically into the staphylococcal chromosome. However, although SCCmechas been designated a mobile genetic element, a mechanism for its transfer has not been defined. Here we demonstrate the capture and conjugative transfer of excised SCCmec. SCCmecwas captured on pGO400, a mupirocin-resistant derivative of the pGO1/pSK41 staphylococcal conjugative plasmid lineage, and pGO400::SCCmec (pRM27) was transferred by filter-mating into both homologous and heterologousS. aureusrecipients representing a range of clonal complexes as well asS. epidermidis. The DNA sequence of pRM27 showed that SCCmechad been transferred in its entirety and that its capture had occurred by recombination between IS257/431 elements present on all SCCmectypes and pGO1/pSK41 conjugative plasmids. The captured SCCmecexcised from the plasmid and inserted site-specifically into the chromosomalattsite of both an isogenicS. aureusand aS. epidermidisrecipient. These studies describe a means by which methicillin resistance can be environmentally disseminated and a novel mechanism, IS-mediated recombination, for the capture and conjugative transfer of genomic islands.

SCCmec, the genomic island containing the gene that encodes methicillin resistance, was captured on a conjugative plasmid by recombination at IS elements and transferred by conjugation between different Staphylococcus aureus sequence types and between S. aureus and Staphylococcus epidermidis. In some of the recipients SCCmec excised from the plasmid and inserted site-specifically into the recipient chromosome. This is the first demonstration of a mechanism for the interspecies dissemination of methicillin resistance among staphylococci.

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