Bactericidal activity of quinupristin-dalfopristin against strains of Staphylococcus aureus with the MLS(B) phenotype of resistance according to the erm gene type.

The bactericidal activity of quinupristin-dalfopristin was assessed by time-kill experiments against Staphylococcus aureus strains with characterized phenotypes and genotypes of MLS(B) resistance. A set of laboratory strains composed of isogenic pairs of S. aureus RN4220 derivatives containing or not the erm(A), erm(B) or erm(C) genes constitutively expressed and of 13 clinical isolates containing these genes inducibly or constitutively expressed were studied. Three of the clinical isolates with erm(B) or erm(A) genes had an unusual inducible MLS(B) cross resistance. The early bactericidal activity of quinupristin-dalfopristin was altered against strains expressing constitutive quinupristin resistance regardless of the erm(A), erm(B) or erm(C) type of gene. We conclude that the bactericidal activity of quinupristin-dalfopristin against staphylococci was dependent on the activity of quinupristin rather than on the erm genotype of the strain.