Abstract
Bacterial resistance studies using in vitro dynamic models are highly dependent on the starting inoculum that might or might not contain spontaneously resistant mutants (RMs). To delineate concentration-resistance relationships with linezolid-exposed Staphylococcus aureus, a mixed inoculum containing both susceptible cells and RMs was used. An RM selected after the 9th passage of the parent strain (MIC, 2 μg/ml) on antibiotic-containing media (RM9; MIC, 8 μg/ml) was chosen for the pharmacodynamic studies, because the mutant prevention concentration (MPC) of linezolid against the parent strain in the presence of RM9 at 10(2) (but not at 10(4)) CFU/ml did not differ from the MPC value determined in the absence of the RMs. Five-day treatments with twice-daily linezolid doses were simulated at concentrations either between the MIC and MPC or above the MPC. S. aureus RMs (resistant to 2× and 4×MIC but not 8× and 16×MIC) were enriched at ratios of the 24-h area under the concentration-time curve (AUC24) to the MIC that provide linezolid concentrations between the MIC and MPC for 100% (AUC24/MIC, 60 h) and 86% (AUC24/MIC, 120 h) of the dosing interval. No such enrichment occurred when linezolid concentrations were above the MIC and below the MPC for a shorter time (37% of the dosing interval; AUC24/MIC, 240 h) or when concentrations were consistently above the MPC (AUC24/MIC, 480 h). These findings obtained using linezolid-susceptible staphylococci supplemented with RMs support the mutant selection window hypothesis. This method provides an option to delineate antibiotic concentration-resistance relationships with bacteria that exhibit low mutation frequencies.