CHARACTERIZATION OF A CONJUGATIVE STAPHYLOCOCCAL MUPIROCIN RESISTANCEPLASMID

We studied conjugative plasmids encoding high-level mupirocin resistan ce. These plasmids were found in Staphylococcus aureus isolates from t wo geographic locations in the United States. Transfer genes on three mupirocin resistance plasmids with different restriction endonuclease profiles were indistinguishable by DNA hybridization from those on pG0 1, a conjugative aminoglycoside resistance plasmid representative of s imilar plasmids that are prevalent in the United States. One mupirocin resistance plasmid, pG0400 (34 kb), was smaller than pG01 (52 kb) bec ause of the absence from pG0400 of DNA, found on pG01, that contained genes encoding resistance to aminoglycosides, trimethoprim, and quater nary ammonium compounds flanked by directly repeated copies of the ins ertion sequence (IS)-like element IS431-IS257, The plasmids pG0400 and pG01 were otherwise indistinguishable except for the presence in pG04 00 of a 4.5-kb HindIII fragment encoding mupirocin resistance. The add ed mupirocin resistance gene was flanked by two directly repeated copi es of IS431/257. The nucleotide sequence of DNA contiguous to the outs ide of the IS elements, as well as those of the elements themselves, w as identical in both pG01 and pG0400, and there were no target site du plications flanking either copy of the element. We conclude that the m upirocin resistance gene was added to an existing conjugative plasmid in conjunction with the deletion of other resistance genes by recombin ation at IS elements. The construction of conjugative plasmids carryin g a mupirocin resistance gene may be a model for the mobility of other resistance genes newly acquired by staphylococci.