Testing the mutant selection window hypothesis with Staphylococcus aureus exposed to linezolid in an in vitro dynamic model

Abstract

Objectives: To test the mutant selection window (MSW) hypothesis applied to linezolid-exposed Staphylococcus aureus and to delineate the concentration-resistance relationship, a mixed inoculum of linezolid-susceptible S. aureus cells and linezolid-resistant mutants (RMs) was exposed to linezolid multiple dosing.

Methods: Three S. aureus strains (MIC of linezolid 2 mg/L), S. aureus 479, S. aureus 688 and S. aureus ATCC 700699, and their RMs (MIC 8 mg/L) selected by passaging on antibiotic-containing media were used in the study. RMs of S. aureus 479 and S. aureus ATCC 700699 contained a G2576T replacement (Escherichia coli numbering) in one of the copies of the 23S rRNA gene, which had been reported in clinically isolated mutants. Five-day treatments with twice-daily linezolid were simulated over a 32-fold range of the 24 h AUC (AUC24) to the MIC ratio.

Results: A pronounced enrichment of mutants resistant to 2×, 4× and 8× MIC was observed at AUC24/MIC ratios of 30 and 60 when linezolid concentrations were within the MSW for more than half of the dosing interval for each strain. The number of viable mutant cells decreased significantly at the simulated AUC24/MIC ratio of 120, while the AUC24/MIC ratio of 240 completely prevented mutant enrichment in vitro. Bell-shaped AUBCM-AUC24/MIC and AUBCM-AUC24/MPC relationships (r2 0.91 and 0.79, respectively) were strain independent.

Conclusions: The bell-shaped pattern of AUC24/MIC and AUC24/MPC relationships with S. aureus resistance to linezolid is consistent with the MSW hypothesis. 'Antimutant' AUC24/MIC ratios were predicted based on the AUC24/MIC relationship with AUBCM.