Studies on the emergence of resistance to lomefloxacin in vitro

The emergence of resistance to lomefloxacin, norfloxacin and ciprofloxacin was investigated in nalidixic acid sensitive and resistant urinary isolates by continuous turbidimetry. A decline in susceptibility was observed after a single exposure to each of the drugs, and further increments of resistance occurred during three sequential passages. Variants resistant to one quinolone were cross-resistant to the others. The level of resistance selected by norfloxacin in three of the five test strains was greater than that observed with lomefloxacin or ciprofloxacin. In experiments in a model of the treatment of bacterial cystitis, concentrations of lomefloxacin well within those readily achievable in urine suppressed growth of nalidixic acid sensitive and resistant strains for more than 20 h without causing any decline in susceptibility of surviving bacteria.