Genetics of drug resistance in methicillin-resistant Staphylococcus aureus from Australian hospitals

The drug-resistance determinants in methicillin-resistant Staphylococcus aureus (MRSA) from three different hospitals in Eastern Australia have been examined. With one exception, all the isolates had chromosomal determinants for penicillinase and resistance to cadmium (Cd), mercury (Hg), phenyl mercuric acetate, methicillin, tetracycline, erythromycin, lincomycin and low level streptomycin. The strain which was the exception differed in that it did not have chromosomal resistance to Cd and lincomycin. In addition, the strains often contained plasmids which belonged to one of three categories: a small cryptic plasmid of either c. 1.4 Mdal, c. 1.7 Mdal or c. 1.9 Mdal; a chloramphenicol resistance plasmid of c. 2.8 Mdal; and a gentamicin resistance plasmid within the range of c. 15.3 to c. 28.5 Mdal. The predominant gentamicin-resistant plasmid in isolates from two hospitals had a molecular weight of c. 18 Mdal, whereas the isolates from the third hospital had a plasmid of molecular weight c. 15.3 Mdal. The only other gentamicin resistance plasmids detected were associated with penicillinase determinants. In one isolate, this corresponded to a plasmid of c. 19.6 Mdal and in the other to a plasmid of c. 28.5 Mdal. These results indicate that MRSAs which are prevalent in Eastern Australian hospitals are substantially different in the location of their drug resistance determinants to earlier strains reported in the literature.