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 (1/8 x, 1/4 x and 1/2 x MIC) and inhibitory (1 x MIC) concentrations of ciprofloxacin against mucoid (M) and non-mucoid (NM) urinary isolates of Pseudomonas aeruginosa. Subinhibitory concentrations (1/8 x, 1/4x and 1/2 x MIC) of ciprofloxacin produced PAEs with no difference between M and NM strains. For NM strains, those with low MICs (≤1.0 mg/1) to ciprofloxacin produced significantly longer PAEs than isolates with high MICs (> 1 mg/1). Killing curve studies demonstrated that subinhibitory concentrations of ciprofloxacin produce little effect (1/8 x MIC) or stasis (1/4 x and 1/2 x MIC) of growth for several hours. Only 1 x MIC was bactericidal for several strains. At 1/2 x and 1 x MIC, bacterial inhibition was greater against NM versus M isolates. The M pheno-type of P. aemginosa reduces killling by ciprofloxacin but not the PAE.