Analysis of plasmids mediating gentamicin resistance in methicillin-resistant Staphylococcus aureus

Gentamicin-resistance plasmids in methicillin-resistant Staphylococcus aureus isolated from four Australian hospitals have been studied. All the plasmids conferred resistance to gentamicin, tobramycin, kanamycin and all but one conferred resistance to quarternary ammonium compounds. Plasmids which only carried these resistance determinants were ca 15.3 megadaltons and were only found in isolates from one hospital. The most common plasmid was ca 18.0 megadaltons and in addition encoded trimethoprim resistance. Two plasmids, one of ca 19.6 megadaltons and one of ca 28.5 megadaltons were found to carry the penicillinase determinant. However only the larger of these encoded heavy metal ion resistance and was sensitive to quaternary ammonium compounds. EcoR1 analysis indicated that all but the ca 28.5 megadalton plasmid were closely related. The EcoR1 analysis of the ca 28.5 megadalton plasmid indicated that it could have resulted from recombination between a gentamicin resistance determinant and a penicillinase plasmid.