Transduction and elimination of resistance determinants in methicillin-resistant Staphylococcus aureus.

Elimination and transduction of drug resistance was examined in methicillin-resistant strains of Staphylococcus aureus. Irreversible spontaneous loss and "curing" by aging of cultures and by treatment with ethidium bromide indicated that the determinants for penicillinase production and chloramphenicol resistance, and probably also for neomycin resistance, were located extrachromosomally. On the other hand, the determinants of resistance to erythromycin, streptomycin, tetracycline, and methicillin could not be eliminated by acridines, ethidium bromide, rifampin, sodium dodecyl sulfate, ultraviolet (UV) irradiation, growth at 43.5 C, aging of cultures, or combinations of these treatments. The stimulation of transduction frequency by UV irradiation of phage in the case of the stable markers, but not in the case of the unstable ones, supported further the hypothesis of chromosomal location of the markers of methicillin, erythromycin, tetracycline, and streptomycin resistance and extrachromosomal location of the determinants for penicillinase production and chloramphenicol resistance. Neomycin resistance could not be transduced. Joint elimination and co-transduction of the determinants for penicillinase production and resistance to chloramphenicol and neomycin were not observed, indicating the location of these markers on separate, mutually compatible plasmids. Co-transduction of chromosomal resistance determinants was usually less than 1%, which makes the location of these genes in a circumscribed area of the chromosome improbable.