In vitro activity of ciprofloxacin, a new carboxyquinoline antimicrobial agent.

The in vitro activity of ciprofloxacin (Bay o 9867), a new carboxyquinoline antimicrobial agent, was compared with those of norfloxacin, nalidixic acid, and several other oral and parenteral antimicrobial agents. Ciprofloxacin was substantially more active than nalidixic acid or cinoxacin against all gram-negative bacteria tested. Virtually all strains of Enterobacteriaceae were inhibited by the new drug at concentrations of less than or equal to 0.125 micrograms/ml. Ciprofloxacin was more active than norfloxacin against Klebsiella sp., Enterobacter sp., and Serratia marcescens, and it was the most active agent against Pseudomonas aeruginosa (MIC90, 0.5 micrograms/ml). The new drug also demonstrated significant activity against gram-positive cocci, inhibiting all strains of staphylococci at concentrations of less than or equal to 1.0 microgram/ml. Ciprofloxacin was bactericidal at concentrations near the MIC against most isolates tested. Although stepwise increases in resistance were seen with Escherichia coli and P. aeruginosa during serial passage on plates containing incremental concentrations of the drug, significant resistance did not emerge during incubation of strains in broth containing concentrations of ciprofloxacin above the MBC.