Multiple antibiotic resistance in Staphylococcus aureus and Staphylococcus epidermidis: plasmids in strains associated with nosocomial infection.

The plasmid DNA profiles were compared to phenotypically-similar, antibiotic-resistant strains of Staphylococcus aureus and Staphylococcus epidermidis associated with nosocomial infections in a Melbourne hospital. Whereas resistance to gentamicin, tobramycin and kanamycin was encoded by one of 3 plasmids [pSK1, 18 megadalton (Md); pSK4, 22 Md; pSK9, 17 Md] in S. aureus, no similar plasmids were detected in S. epidermidis. Mediated exclusively by the chromosome in S. aureus, tetracycline resistance was encoded either by the chromosome or by a 2.8 Md plasmid in strains of S. epidermidis. The inability to detect common resistance plasmids in strains of S. aureus and S. epidermidis recovered from this outbreak is in contrast to recent observations with staphylococci from other geographic areas; nevertheless, on the basis of restriction endonuclease analyses of 3 Md chloramphenicol resistance plasmids, it is suggested that a common gene pool does exist within isolates of S. aureus and S. epidermidis from Melbourne hospitals.