In vitro comparison of norfloxacin with nalidixic acid, cinoxacin and oxolinic acid.

1-Ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-3-quinolinecarbo xylic acid (Norfloxacin, MK-0366), a new nalidixic acid analog was shown to be significantly more active against Enterobacteriaceae than nalidixic acid and cinoxacin and about four times as active as oxolinic acid. The compound was highly effective against Pseudomonas aeruginosa (MIC less than or equal to 1 microgram/ml). In contrast to the other compounds, norfloxacin inhibited group B and D streptococci, whereas against staphylococci, both norfloxacin and oxolinic acid were shown to be active. The new compound proved to be bactericidal at minimum inhibitory concentrations. Nalidixic acid-resistant strains of various species were less sensitive to norfloxacin than nalidixic acid-sensitive bacteria, although the MICs for these strains remained well within therapeutically obtainable levels. Variants with decreased sensitivity could easily be obtained in vitro with all compounds; however, high-level resistance was not observed with norfloxacin in contrast to the other three compounds.