Comparative activity of ofloxacin and seven other antimicrobials against urea-splitting microorganisms

Urinary tract infections caused by urea-splitting bacteria are severe clinical conditions and very difficult to treat due to their association with calculi, and because such bacteria form ammonium hydroxide raising the urinary pH and thereby creating an unfavourable condition for the action of most antimicrobials. We tested ofloxacin, norfloxacin, nalidixic acid, amoxicillin, amoxicillin-clavulanic acid, gentamicin, co-trimoxazole and nitrofurantoin against 143 gram-negative and 99 gram-positive bacteria, all urea-splitting, isolated from patients with urinary tract infections. All drugs were tested using media at two different pHs (pH 7.4 and PH 8.5) and two inoculum sizes (10(4) and 10(6) cfu). Although ofloxacin and norfloxacin had a similar spectrum of activity, ofloxacin had somewhat greater intrinsic activity against gram-positive organisms. MICs of ofloxacin for 90% of Proteus mirabilis, indole-positive Proteus spp., Klebsiella pneumoniae, Staphylococcus aureus, coagulase-negative staphylococci and the Corynebacterium group D2 were greater than or equal to 4 mg/l. The activity of the other drugs varied, but there were many strains resistant to these antimicrobials. The pH and inoculum size did not significantly affect the activity of ofloxacin so that this drug should be useful for the treatment of urinary tract infections caused by the commonest urea-splitting bacteria involved in such infections.