Molecular analysis of constitutively expressed erm(C) genes selected in vitro by incubation in the presence of the noninducers quinupristin, telithromycin, or ABT-773.

A Staphylococcus aureus strain that harbored a plasmid-borne inducibly expressed erm(C) gene was cultivated in the presence of the noninducers quinupristin, telithromycin, and ABT-773. After overnight incubation, 78 mutants that displayed combined resistance to macrolides, lincosamides, streptogramin B antibiotics, and ketolides were analyzed for the genetic basis of this altered resistance phenotype. Because this resistance phenotype is indicative for constitutively expressed erm(C) genes, the erm(C) regulatory regions of all mutants were sequenced.

Molecular characterization of ketolide-resistant erm(A)-carrying Staphylococcus aureus isolates selected in vitro by telithromycin, ABT-773, quinupristin and clindamycin.

The aim of this study was to investigate whether a Staphylococcus aureus strain that carried an inducibly expressed erm(A) gene might exhibit resistance to the non-inducers telithromycin, ABT-773, clindamycin, quinupristin, dalfopristin or the combination quinupristin-dalfopristin after incubation in the presence of inhibitory concentrations of any of these compounds. Whenever resistant mutants were obtained, these were investigated for the molecular basis of the altered resistance phenotype. Resistant mutants were not selected with dalfopristin or quinupristin-dalfopristin, but were obtained with the other four agents.