Investigation into the selection frequency of resistant mutants and the bacterial kill rate by levofloxacin and ciprofloxacin in non-mucoid Pseudomonas aeruginosa isolates from cystic fibrosis patients.

The frequency by which resistant Pseudomonas aeruginosa strains could be selected was compared for two antibiotics, levofloxacin and ciprofloxacin. Seven distinct strains were cultured on plates containing 1x, 2x, 4x and 8x the minimum inhibitory concentration (MIC) of the antibiotic under investigation. Resistant mutants were more readily isolated by growth on culture plates that contained ciprofloxacin, and the resulting MIC of the resistant mutant was also more frequently increased. Time-kill studies on comparable strains where the MIC for both antibiotics had increased by at least fourfold showed no difference between the two agents.