In-vitro studies with ciprofloxacin, a new 4-quinolone compound.

Ciprofloxacin is a new antibacterial agent of the 4-quinolone group. With an agar dilution technique we compared its activity on 365 clinical isolates with those of norfloxacin, nalidixic acid, ampicillin, mezlocillin, cefadroxil, cefuroxime, ceftazidime, ceftriazone , cefotaxime, latamoxef (moxalactam), and gentamicin. Ciprofloxacin was overall the most active agent tested against aerobic Gram-negative species, with the MIC90 values for all species being below 1 mg/l (excepting Providencia stuartii with 4 mg/l), and the large majority being below 0.12 mg/l. Many of the strains were selected on the basis of resistance to beta-lactam agents or gentamicin, and ciprofloxacin was also active against these. There was little difference in the activity of ciprofloxacin at inocula of 10(4) or 10(6) cfu. Strains with higher MIC's of the related agents norfloxacin and nalidixic acid were less susceptible to ciprofloxacin . Ciprofloxacin was less active against Gram-positive species (typical MIC90 values were 0.5 or 1 mg/l) and obligate anaerobes (4 mg/l for Bacteroides fragilis). The activity of ciprofloxacin in broth dilution tests was little affected by pH over the range 6.0-8.0, or by human serum or tissue fluid; its activity was reduced by the presence of urine. Binding to human serum protein was 20-28%. Ciprofloxacin was rapidly bacterial in broth at concentrations near to its MICs. By exposure to subinhibitory concentrations of ciprofloxacin it was possible to increase its MIC for bacteria in daily subcultures. The final MIC values after ten days were often about 16-fold greater than those observed initially.