Frequency of Appearance of Resistant Variants to Norfloxacin and Nalidixic Acid

The clinical use of nalidixic acid has been hampered by the rapid appearance of resistant variants. This study compares the frequency of appearance of resistant variants to nalidixic acid and norfloxacin, a new quinolone-carboxylic acid with superior antimicrobial activity. We found a lower frequency of appearance of resistant variants with norfloxacin, although there was some cross-resistance between the two drugs. A gradation in this frequency was also noted between the species of Gram-negative rods tested, so that larger numbers of resistant variants appeared with the non-fermenters.