High‐Level Fluoroquinolone Resistance in Pseudomonas aeruginosa Due to Interplay of the MexAB‐OprM Efflux Pump and the DNA Gyrase Mutation

Fluoroquinolone resistance in Pseudomonas aeruginosa is mainly attributable to the constitutive expression of the xenobiotic efflux pump and mutation in DNA gyrase or topoisomerase IV. We constructed cells with a double‐mutation in gyrA and mexR encoding DNA gyrase and repressor for the mexAB‐oprM operon, respectively. The mutant showed 1,024 times higher fluoroquinolone resistance than cells lacking the MexAB‐OprM. Cells with a single mutation in gyrA and producing a wild‐type level of the MexAB‐OprM efflux pump showed 128 times higher fluoroquinolone resistance than cells lacking the MexAB‐OprM. In contrast, a single mutation in gyrA or mexR caused only 4 and 64 times higher resistance, respectively. These findings manifested the interplay between the MexAB‐OprM efflux pump and the target mutation in fluoroquinolone resistance.