Time-kill kinetics of quinupristin/dalfopristin on Staphylococcus aureus with and without a raised MBC evaluated by two methods.

Some strains of staphylococci have raised MBCs of quinupristin/dalfopristin compared with their MICs. In this study, the time-kill kinetics of quinupristin/dalfopristin at 2 mg/L on two strains of Staphylococcus aureus were determined by viable count and intracellular ATP measurement. After 24 h exposure to quinupristin/dalfopristin, the percentage survival of the strain with a raised MBC was 5.9 and that of the strain with a normal MBC was 0.04. The time-kill kinetics of the strain with a raised MBC were analogous to those associated with phenotypic tolerance.