title: Isolation and characterization of an Escherichia coli strain exhibiting partial tolerance to quinolones.
abstract: Quinolone antimicrobial agents rapidly kill bacteria by largely unknown mechanisms. To study this phenomenon, a strain of Escherichia coli inhibited but inefficiently killed by (i.e., partially tolerant to) norfloxacin was isolated and characterized. E. coli KL16 (norfloxacin MIC, 0.10 microgram/ml; MBC, 0.20 microgram/ml) was mutagenized with nitrosoguanidine and cyclically exposed to 3 micrograms of norfloxacin per ml. After five cycles, a bacterial strain (DS1) which was killed 1,000-fold less than KL16 during 3 h of drug exposure was isolated. The MIC and MBC of norfloxacin for DS1 were 0.20 and 1.5 micrograms/ml, respectively. Over a range of norfloxacin concentrations, DS1 was killed 2 to 4 orders of magnitude less than KL16. DS1 grew more slowly than KL16 but after normalization for growth rate was killed four times less rapidly than KL16 at drug concentrations 10-fold higher than respective MICs. DS1 and KL16 cells filamented similarly upon exposure to norfloxacin. DS1 exhibited tolerance to other DNA gyrase A subunit antagonists (ofloxacin and ciprofloxacin) and DNA gyrase B subunit antagonists (novobiocin and coumermycin) but not to the aminoglycoside gentamicin, suggesting involvement of DNA gyrase. DS1 also appeared to be minimally tolerant to the beta-lactam cefoxitin. DS1 exhibited increased susceptibility to the mutagen methyl methanesulfonate, implying a defect in DNA repair. This report describes the first use of quinolone enrichment for isolation of a bacterial strain partially tolerant to quinolones. The study of defects in such tolerant strains offers an approach to an increased understanding of the mechanisms of bacterial killing by quinolones.
authors.affiliation: Infectious Disease Unit, Medical Services, Massachusetts General Hospital, Boston 02114.
authors.forename: J S
authors.initials: JS
authors.lastname: Wolfson
authors.forename: D C
authors.initials: DC
authors.lastname: Hooper
authors.forename: D J
authors.initials: DJ
authors.lastname: Shih
authors.forename: G L
authors.initials: GL
authors.lastname: McHugh
authors.forename: M N
authors.initials: MN
authors.lastname: Swartz
mesh_terms: D000890:Anti-Infective Agents / Q000494:pharmacology*; D004352:Drug Resistance, Microbial; D004926:Escherichia coli / Q000302:isolation & purification*; D004927:Escherichia coli Infections / Q000382:microbiology*; D008741:Methyl Methanesulfonate / Q000494:pharmacology; D008826:Microbial Sensitivity Tests; D009643:Norfloxacin / Q000494:pharmacology
chemical_list: D000890:Anti-Infective Agents; D008741:Methyl Methanesulfonate; D009643:Norfloxacin