Ineffectiveness of Topoisomerase Mutations in Mediating Clinically Significant Fluoroquinolone Resistance in Escherichia coli in the Absence of the AcrAB Efflux Pump

Fluoroquinolone-resistant mutants, selected from a wild-type Escherichia coli K-12 strain and its Mar mutant by exposure to increasing levels of ofloxacin on solid medium, were analyzed by Northern (RNA) blot analysis, sequencing, and radiolabelled ciprofloxacin accumulation studies. Mutations in the target gene gyrA (DNA gyrase), the regulatory gene marR , and additional, as yet unidentified genes (genes that probably affect efflux mediated by...