Comparative in vitro activity of three new quinolone antibiotics against recent clinical isolates

New orally absorbable quinolone derivatives, ciprofloxacin, norfloxacin, and ofloxacin demonstrated excellent activity in vitro against clinical isolates of Escherichia coli, Klebsiella, Enterobacter, Proteus mirabilis, Proteus sp. indole-positive, Pseudomonas aeruginosa, Serratia marcescens, Staphylococcus aureus, Streptococcus pyogenes and enterococci. None of the 3 drugs was more than moderately effective against Bacteroides fragilis. Ciprofloxacin was 2-4 times more effective against most gram-negative strains than was either norfloxacin or ofloxacin, and was also the most effective against gram-positive strains, though the difference was less marked. Ciprofloxacin and ofloxacin were almost equally effective against S. aureus. Results with agar and with broth were comparable. The activities of all 3 drugs were essentially independent of inoculum size, as the MIC values increased less than one dilution step when the inoculum was increased from 10(3) to 10(6). The new quinolone derivatives would appear to be promising alternatives to injectable drugs such as the aminoglycosides and cephalosporins.