Drug Resistance of Staphylococci

According to genetic studies of the inducible resistance to macrolide antibiotics; erythromycin (EM), oleandomycin (OM), leucomycin (LM), spiramycin (SP), in Staphylococcus aureus, it was found that there are two types of cross-resistance; resistance to EM and to both EM and OM. In EM-resistant strains, EM is an active inducer, while both EM and OM are the inducers in those strains resistant to both EM and OM. However, the induced populations of both strains acquired high resistance (800 μg/ml or more) to all macrolide antibiotics and to lincomycin. The resistance of induced cells was lost when grown in the absence of inducers. From a strain MS537, in which EM is an inducer, a mutant MS537-1 was obtained on the plate containing OM. In strain MS537-1, OM, in addition to EM, has become an active inducer for macrolide resistance. When MS537 and MS537-1 were cultured on the plate containing LM, mutants MS537-2 and MS537-3 were obtained, respectively. They were found to be highly resistant to all macrolide antibiotics and to lincomycin, and their resistance was not inducible but constitutive. According to transductional analysis, it was found that the loci concerned with induction for macrolide resistance and those governing cross-resistance to macrolide antibiotics were co-transducible, both loci not being separated by transduction. The constitutive resistance to macrolide antibiotics and to lincomycin in strains MS537-2 and MS537-3, was also co-transducible, suggesting that the resistance to macrolide antibiotics and to lincomycin are determined by a gene or by a cluster of genes in the cocci.