In-vitro activity of two new quinolone antimicrobial agents, S-25930 and S-25932 compared with that of other agents.

S-25930 and S-25932, two new 4-quinolones, were compared to ciprofloxacin, enoxacin, ofloxacin, norfloxacin, cefotaxime, gentamicin, trimethoprim, and ampicillin. S-25930 and S-25932 inhibited 90% of Enterobacteriaceae at less than or equal to 1 mg/l, usually differing only two-fold. The MIC90 for Pseudomonas aeruginosa was 8 mg/l for S-25930 and 16 mg/l for S-25932, compared to MICs of 1 to 8 for the other four quinolones. Both drugs inhibited Enterobacter cloacae, Serratia marcescens and Citrobacter freundii resistant to cefotaxime and Klebsiella species resistant to gentamicin and trimethoprim. The MICs90 were 0.125 and 0.25 mg/l against staphylococci, including methicillin-resistant Staphylococcus aureus and were superior to the MICs of other quinolones; activity against haemolytic streptococci at 1-2 mg/l was also superior. S-25930 and S-25932 showed rapid bactericidal activity at the MBC concentration and both agents showed post-antibiotic suppression of growth of bacteria. The agents were active in acid medium, but activity was reduced by magnesium at 9 mM. A stepwise increase in resistance was produced by serial passage in increasing concentrations of drug.