Fig 3.
Effect of drug structure on induction of quinolone resistance. (A) Comparison of fluoroquinolones. An exponentially growing culture of M. smegmatis KD1163 (4.5 × 106 CFU) was applied to agar containing the following fluoroquinolones at 2 times the MIC: ciprofloxacin at 0.4 μg/ml, empty circles; levofloxacin at 0.25 μg/ml, filled circles; gatifloxacin at 0.06 μg/ml, empty triangles; moxifloxacin at 0.06 μg/ml, filled triangles. Plates were incubated at 37°C, and at daily intervals, the cumulative number of colonies was determined and expressed as a fraction of the input number of CFU. Replicate experiments produced similar results, as did similar experiments at inocula ranging from 4.5 ×105 to 4.5 ×107. (B) Comparison of quinazoline-2,4-dione and cognate fluoroquinolone. M. smegmatis KD1163 (5 × 106 CFU) was applied as described for panel A to agar containing 2.5 times the MIC of either dione UING5-207 (5 μg/ml, filled squares) or the fluoroquinolone UING5-249 (0.08 μg/ml, open squares). Replicate experiments gave similar results. (C) Stability of fluoroquinolones and 2,4-dione. Sets of agar plates were prepared with various concentrations of each compound and incubated at 37°C, and at the indicated times, drug-containing plates were used to determine the MICs for M. smegmatis KD1163. The symbols are the same as in panels A and B. Similar results were obtained with replicate experiments. Error bars represent standard deviations.