FIG. 1.

Induction of quinolone-resistant mutants by nalidixic acid. (A) Effect of incubation time on the accumulation of mutants. Wild-type E. coli (strain DM4100; 2.5 × 10⁷ CFU) was applied to agar containing nalidixic acid at 5 μg/ml (filled circles; 1.5 times the MIC₉₉), 6.6 μg/ml (open circles; 2 times the MIC₉₉), 8.25 μg/ml (filled squares; 2.5 times the MIC₉₉), and 9.9 μg/ml (open squares; 3 times the MIC₉₉). At the indicated times, the numbers of colonies present on the agar plates were recorded. (Inset) Relationship between the rate of mutant recovery and the nalidixic acid (Nal) concentration. The rate of colony appearance was estimated as the number of colonies at day 6 minus the number at day 1 from the data in panel A. (B) Effect of inoculum size on the accumulation of resistant mutants. Strain DM4100 was applied to agar containing 8.25 μg/ml nalidixic acid at inocula of 2.5 × 10⁷ CFU (filled circles), 1.3 × 10⁷ CFU (open circles), 6.3 × 10⁶ CFU (filled squares), and 3.1 × 10⁶ CFU (open squares); the numbers of colonies appearing by the indicated times were recorded. (C) Effect of inoculum size on the fraction of input cells recovered in the presence of various concentrations of nalidixic acid. The symbols are as described for panel A. Error bars represent the standard deviations of the means; similar results were obtained in at least three replicate experiments.