Antimicrobial activity of subinhibitory concentrations of ciprofloxacin against Pseudomonas aeruginosa as determined by the killing curve method and the postantibiotic effect.

This investigation used the postantibiotic effect (PAE) and killing curves to examine the antimicrobial activity of subinhibitory (1/8x, 1/4x and 1/2x MIC) and inhibitory (1x MIC) concentrations of ciprofloxacin against mucoid (M) and nonmucoid (NM) urinary isolates of Pseudomonas aeruginosa. Subinhibitory concentrations (1/8x, 1/4x and 1/2x MIC) of ciprofloxacin produced PAEs with no difference between M and NM strains. For NM strains, those with low MICs (< or = 1.0 mg/l) to ciprofloxacin produced significantly longer PAEs than isolates with high MICs (> 1 mg/l). Killing curve studies demonstrated that subinhibitory concentrations of ciprofloxacin produce little effect (1/8x MIC) or stasis (1/4x and 1/2x MIC) of growth for several hours. Only 1x MIC was bactericidal for several strains. At 1/2x and 1x MIC, bacterial inhibition was greater against NM versus M isolates. The M phenotype of P. aeruginosa reduces killing by ciprofloxacin but not the PAE.