The influence of urine on the in vitro antimicrobial activity of various antibiotics against different Escherichia coli phenotypes

Background: The influence of urine on the in vitro activities of various antibiotics used in the therapy of urinary tract infections was assessed by the microbroth dilution method in this study. Methods: Thirty Escherichia coli strains were used: 10 E. coli strains susceptible to ampicillin, 10 strains resistant to ampicillin and ampicillin+sulbactam and ten extended spectrum beta-lactamase producer strains. The minimum inhibitory concentrations (MICs) of ampicillin, ampicillin + sulbactam, cephalothin, cefuroxime, ceftriaxone, amikacin, ciprofloxacin, sulfamethoxazole and trimethoprim were performed parallel in Mueller-Hinton broth and human urine by the microbroth dilution method. Results: The MIC90 of all antibiotics except cephalothin were higher in the urine. MICs performed in the urine were found significantly higher than those performed in broth. Conclusions: This study demonstrated that MICs of antibiotics are influenced by the human urine and that MICs of some antibiotics used in the treatment of urinary tract infections may be overestimated by the standard antibiotic testing methods. Copyright (C) 2004 S. Karger AG, Basel.