Post-antibiotic effect of three quinolones against gram negative isolates from urine

The post-antibiotic effect (PAE) is defined as the bacterial growth suppression which persists after a limited exposure to an antimicrobial agent. The PAE and the bactericidal effect of the quinolones ciprofloxacin, norfloxacin and nalidixic acid have been studied against several urinary isolates of Gram-negative bacteria. The PAE was determined after one hour's exposure to the antimicrobial agent using an initial inoculum of 10(5) to 10(6) cfu/ml; the drug was rapidly removed by a 10(-2) dilution technique in antibiotic-free medium. When ciprofloxacin was used at four times its MIC the PAEs were 1.37 +/- 0.09; 2.45 +/- 0.63 and 2.86 +/- 0.15 h against Esch. coli, Klebs. pneumoniae and Pseudomonas aeruginosa, respectively. We found lower values for norfloxacin under the same conditions, and nalidixic acid did not induce a significative PAE. These results could support changes in dosing intervals of norfloxacin and ciprofloxacin, with possibly greater intervals between doses.