title: In vitro selection of Escherichia coli mutants with decreased susceptibility to norfloxacin.
abstract: In 10 clinical isolates of Escherichia coli the frequency of spontaneous mutation to high levels of resistance to nalidixic acid was greater than 300-fold higher than that to norfloxacin. Norfloxacin resistant mutants could not be detected (mutation frequency: less than 10(-12). However, nalidixic acid resistant mutants of E. coli developed decreased susceptibility to norfloxacin at a rate of approximately 10(-9). Mutants with decreased susceptibility to norfloxacin could also be obtained when the same clinical isolates of E. coli were exposed to norfloxacin in 2 steps. However, mutants with MIC values greater than 8 micrograms/ml for norfloxacin were never obtained even after repeated exposure. Widespread use of cinoxacin or nalidixic acid may select resistant strains and reduce the efficacy of norfloxacin and other 4-quinolones.
authors.affiliation: Department of Medical Microbiology, University of Lund, Malmö General Hospital, Sweden.
authors.forename: A
authors.initials: A
authors.lastname: Forsgren
authors.forename: A C
authors.initials: AC
authors.lastname: Striby
mesh_terms: D004352:Drug Resistance, Microbial; D004926:Escherichia coli / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology; D006801:Humans; D009154:Mutation; D009268:Nalidixic Acid / Q000494:pharmacology; D009643:Norfloxacin / Q000494:pharmacology*
chemical_list: D009268:Nalidixic Acid; D009643:Norfloxacin