Restricting the Selection of Antibiotic‐Resistant Mutant Bacteria: Measurement and Potential Use of the Mutant Selection Window

Abstract

The selection of antibiotic-resistant mutant bacteria is proposed to occur in a drug concentration range (the mutant selection window) that extends from the minimum inhibitory concentration (MIC) of susceptible cells to the MIC of the least susceptible, single-step bacterial mutants (the mutant prevention concentration [MPC]). MPCs were estimated for tobramycin, chloramphenicol, rifampicin, penicillin, vancomycin, and several fluoroquinolones by use of Escherichia coli and Staphylococcus aureus. Comparisons among reported serum drug levels indicate that new fluoroquinolones are the least likely to enrich populations of resistant mutant bacteria during monotherapy. These data partly explain the selective enrichment of populations of resistant mutant bacteria in medical practice. The mutant selection window range (MPC:MIC) was narrowed for fluoroquinolones by structure modification, pointing to a new direction in antibiotic refinement. The mutant selection window and the MPC were determined for combinations of rifampicin and tobramycin, using S. aureus, as a guide for combination therapy with compounds that alone cannot block enrichment of mutant bacterial populations.

Mutant Selection Window Hypothesis Updated

“…Although a broad plateau in mutant recovery is observed for fluoroquinolones with mycobacteria [4,10], for erythromycin and rifampicin with Staphylococcus aureus [3,14], and for miconazole with Candida yeast [12], many other antimicrobialpathogen combinations exhibit only a small inflection in the selection curve, rather than a distinct plateau [4,9,15]. A small inflection is expected if 2 independent targets having similar drug affinity are present (e.g., gyrase and topoisomerase for fluoroquinolones): drug concentrations that trap 1 enzyme on DNA (MICs) would be close to those that trap both (MPCs).…”

“…Issues concerning dosing are addressed by the mutant selection window hypothesis [2,3]. This hypothesis maintains that drug-resistant mutant subpopulations present prior to initiation of antimicrobial treatment are enriched and amplified during therapy when antimicrobial concentrations fall within a specific range (the mutant selection window).…”

Validation of the mutant selection window hypothesis with fosfomycin against Escherichia coli and Pseudomonas aeruginosa: an in vitro and in vivo comparative study

“…32 Briefly, high-density cultures were prepared from overnight cultures grown in liquid medium followed by a 10-fold dilution and 4 h of incubation with shaking at 35°C…”

Activity of sitafloxacin against extracellular and intracellular Staphylococcus aureus in vitro and in vivo: comparison with levofloxacin and moxifloxacin

“…MICs and MBCs were measured at pH 5.4 and 7.4 by microdilution, with the bacterial suspension set at 1B5Â10 5 cfu ml À1 and by using as the endpoint the drug concentration that caused a 5-log decrease in the original inoculum. Mutant prevention concentration (MPC) 20 was defined as the lowest drug concentration that prevented bacterial colony formation from a culture containing 410 10 bacteria. The determination was similar to that for MIC, except that 410 10 cells were tested at high drug concentrations, inoculated plates were incubated for 72 h, and colonies were counted at 24 h intervals until colony numbers became constant.…”

“…In vivo extracellular and intracellular dose-response relationships Figure 5 shows the full pharmacological response of extracellular and intracellular isolates of strains to quinolone at 4 h when they were exposed to a wide range of extracellular concentrations (10,20,50, 100, 200 mg kg À1 ). Significant dose-response correlations were recorded both for the total count and when the counts were separated into the extra-and intracellular compartments.…”