Mutant prevention concentration for ciprofloxacin and levofloxacin with Pseudomonas aeruginosa.

The mutant prevention concentration (MPC) was determined by population analysis using six fluoroquinolone-susceptible isolates of Pseudomonas aeruginosa applied to fluoroquinolone-containing agar plates. The MPCs were 3 mg/L and 9.5 mg/L for ciprofloxacin and levofloxacin, respectively. At high concentrations of either compound, single-step gyrA resistance mutants were recovered. Using a modified method for estimating the MPC, 151 clinical isolates were surveyed. Modal MPCs were 2 mg/L and 8 mg/L, respectively, for ciprofloxacin and levofloxacin. Thus, ciprofloxacin is three to four times more active than levofloxacin against resistant mutant subpopulations. For individual isolates, the MPC correlated poorly with the minimum inhibitory concentration (r(2) = 0.41 and 0.39 for ciprofloxacin and levofloxacin, respectively).