In-vitro selection of bacteria resistant to the 4-quinolone agents.

The activity of five 4-quinolone agents: norfloxacin, ciprofloxacin, ofloxacin, enoxacin and pefloxacin was tested against 121 distinct clinical bacterial isolates. Ciprofloxacin was the most active agent against Gram-negative bacteria and streptococci and pefloxacin was the most active against staphylococci. By serial sub-culture in sub-inhibitory concentrations of antibiotic we were able to select resistance in almost all Gram-positive and Gram-negative species, with the exception of Escherichia coli. Resistance to norfloxacin was more readily produced than resistance to the other four agents. There was almost complete cross-resistance among the five agents tested. The proportional increases in MIC were higher in Gram-negative than in Gram-positive organisms.