Selection of Antibiotic‐Resistant Bacterial Mutants: Allelic Diversity among Fluoroquinolone‐Resistant Mutations

Abstract

To obtain a general framework for understanding selection of antibiotic-resistant mutants, allelic diversity was examined with about 600 fluoroquinolone-resistant mutants of mycobacteria. Selection at low fluoroquinolone concentration produced many low-level resistance mutants. Some of these contained mutations that conferred unselected antibiotic resistance; none contained alterations in the quinolone-resistance-determining region of the GyrA protein, the principal drug target. As selection pressure increased, a variety of GyrA variants became prevalent. High concentrations of antibiotic reduced the variety to a few types, and eventually a concentration was reached at which no mutant was recovered. That concentration defined a threshold for preventing the selection of resistance. The pattern of variants selected, which was also strongly influenced by antibiotic structure, readily explained the variants present in clinical isolates. Thus, resistance arises from selection of mutants whose identity depends on drug concentration and structure, both of which can be manipulated to restrict selection.

Parallel evolution and local differentiation in quinolone resistance in Pseudomonas aeruginosa

“…For example, selection of mutants at different quinolone concentrations -chosen to reflect known concentrations in patientswould test the hypothesis that nfxB mutants are more prevalent at lower drug concentrations. A previous study in Mycobacterium has shown that gyrA mutations are more likely to be selected at high quinolone concentrations, with non-gyrA mutations prevalent at low concentrations (Zhou et al, 2000). However, this study did not identify the nongyrA mutations, and so does not bear directly on the differences in non-gyrase mutations described here.…”

“…Drug concentrations can vary dramatically amongst tissues and, as previously noted, different resistance mutations can be selected at different antibiotic concentrations (Zhou et al, 2000). Studies of in vivo ciprofloxacin concentrations in CF patients have found systematically lower drug levels in sputum than in blood (sputum concentrations of 0.2-1.9 mg l 21 versus blood concentrations of 2.4-4 mg l 21 , depending on dose and study; Pedersen et al, 1987, and references therein).…”

“…Thus, even though a gyrA T83I variant is no more likely to arise in a population than any other resistance (point) mutation, the fitness benefit that it affords in the presence of a high concentration of antibiotic virtually guarantees its success once it does arise. At lower antibiotic concentrations, however, other mutations could play important roles (Zhou et al, 2000).…”

Mutant Selection Window Hypothesis Updated

“…When drug concentration increases, the fraction of cells that are recovered as colonies decreases, levels to a broad plateau, and then decreases a second time [4,10,11]. The initial decrease arises from the inhibition of wild-type growth and occurs at concentrations approximated by the MIC.…”

“…The plateau is the result of mutant subpopulations that are present at low frequencies (e.g., 10 Ϫ6 to 10 Ϫ8 ). Colonies that are recovered at low fluoroquinolone concentrations are predominantly caused by nontarget (nongyrase) mutants [10,12,13], whereas at moderate quinolone concentrations, several different gyrA (gyrase) mutants are recovered [10]. When drug concentration increases, only the more protective gyrA mutations are observed; when drug concentration is high enough to block the growth of the least susceptible single-step mutant, colony recovery decreases sharply a second time.…”