Genetic Analysis of Gentamicin Resistance in Methicillin- and Gentamicin-resistant Strains of Staphylococcus Aureus Isolated in Dublin Hospitals

Methicillin- and gentamicin-resistant strains of Staphylococcus aureus isolated in Dublin hospitals have been classified into groups I, II, and III based on resistance to antimicrobial agents and plasmid profiles. Each group expresses a characteristic level of resistance to gentamicin, tobramycin, and sisomicin. Enzyme assays showed that resistant strains expressed 2"-aminoglycoside phosphotransferase-6'-aminoglycoside transferase activity by a determinant which is known to be chromosomally located. The gentamicin resistance (Gmr) determinants were transferred from group I, II, or III strains by transduction into a laboratory strain where each expressed the same low level of resistance. This finding suggests that high-level resistance in some clinical strains is due to a second, unlinked resistance mechanism. No evidence was obtained by hybridization experiments that clinical isolates or spontaneous mutants expressing high-level Gmr carried more than one copy of the Gmr determinant, thus eliminating the possibility that a gene dosage effect was responsible for high-level resistance. Hybridization experiments with transductants and wild strains suggested that the Gmr determinant was located at homologous sites in wild strains from different groups, although restriction site differences were observed in flanking sequences. Electron microscope analysis of a cloned Gmr determinant and genetic evidence suggested that a Dublin clinical isolate harbored a transposon very similar to Tn4001.