Influence of human urine on the in vitro activity and postantibiotic effect of ciprofloxacin against Escherichia coli

The purpose of this investigation was to study the effects of human urine on the minimum inhibitory concentration (MIC) and the postantibiotic effect (PAE) of ciprofloxacin against Escherichia coli. MICs and the PAE were performed in Mueller-Hinton broth (MHB; pH 7.3 and 5.5) and in human urine (pH 5.5 and 7.3). In urine, pH 5.5, MICs increased 64-fold (from 0.016 to 1.024 μg/ml) and the PAE was abolished (from 101.6 to 3.7 min), when compared to MHB, pH 7.3. An acidic pH demonstrated the greatest effect on reduced susceptibility and PAE. Using ciprofloxacin concentrations adjusted for a higher MIC obtained in pH-adjusted urine and MHB, PAE values were similar for urine and MHB (approximately 280 min at 40 x MIC). This study demonstrated that the MIC and PAE of ciprofloxacin against E. coli are influenced by human urine and in particular its pH.