Antibiotic susceptibilities and plasmid profiles of nosocomial methicillin-resistant Staphylococcus aureus: A retrospective study

Abstract

Methicillin-resistant S. aureus strains isolated at a single Melbourne Hospital [Australia] between 1969 and 1981 were examined for susceptibility to a range of antimicrobial agents, and for the presence of plasmid DNA. Isolates obtained during 1969 possessed a plasmid of MW 20 .times. 106, encoding heavy metal resistance and penicillinase production, and a plasmid of MW 2.8 .times. 106, mediating tetracycline resistance. In the majority of isolates obtained after 1973, these functions were chromosomally encoded. Before 1980, both high- and low-level chromosomally-encoded gentamicin resistances were encountered; isolates from 1980 and 1981 displayed low-level gentamicin resistance only; the latter phenotype was most commonly mediated by a plasmid of MW 18 .times. 106 that also encoded resistance to tobramycin and kanamycin. Chloramphenicol resistance in strains isolated throughout the period was mediated by 1 of 3 plasmids, each of MW .apprx. 3 .times. 106.