Transposition of gentamicin resistance to staphylococcal plasmids encoding resistance to cationic agents.

Plasmid pWG115 isolated from a methicillin-resistant Staphylococcus aureus encodes resistance to cationic surface-active agents and trimethoprim. It has a molecular weight of ca 14.6 megadaltons and can be transferred to other strains of staphylococci in mixed-culture transfer with propamidine isethionate as a selective agent. Gentamicin resistance in Australian methicillin-resistant Staph. aureus isolates can be either chromosomal or plasmid-borne. The most common gentamicin resistance plasmid is 18.0 megadaltons and also encodes resistance to trimethoprim and cationic surface-active agents. This suggested that pWG115 was related to gentamicin resistance plasmids and that it may provide a target for the postulated gentamicin resistance transposon. This paper demonstrates that the chromosomal gentamicin resistance determinant from WG523 can transpose into pWG115 to generate an 18.0 megadalton plasmid, phenotypically indistinguishable from the naturally occurring gentamicin resistance plasmids such as pWG53. EcoR1 restriction enzyme analysis demonstrated that gentamicin resistance can transpose into at least two sites on pWG115. One of these sites generates EcoR1 restriction fragments identical to pWG53. The 5.2 kilobase pair (3.4 megadalton) element involved confers low-level resistance to gentamicin, cross resistance to tobramycin and kanamycin, and has been designated Tn3851.