Influence of pH and human urine on the antibacterial activity of ciprofloxacin, norfloxacin and ofloxacin

The influence of pH on the antibacterial activity of ciprofloxacin, norfloxacin and ofloxacin was studied in broth and pooled human urine by microdilution susceptibility tests. Selected strains of E. coli, Staphylococcus aureus and Pseudomonas aeruginosa were used as test organisms. The results show that cultivation at pH 5.7 in urine increased the MIC values for all three quinolones 8, 16 and 32-fold compared with broth at pH 7.1. Killing curves show that in urine with 10 mcg/ml ciprofloxacin, rapid killing of E. coli and Pseudomonas aeruginosa occurred, whereas ofloxacin and especially norfloxacin were less effective.