High-Level Fluoroquinolone-Resistant Clinical Isolates of Escherichia coli Overproduce Multidrug Efflux Protein AcrA

Immunoblotting with antibody against AcrA, an obligatory component of the AcrAB multidrug efflux system, showed that this protein was overexpressed by ≥170% in 9 of 10 clinical isolates of Esherichia coli with high-level ciprofloxacin resistance (MICs, ≥32 μg/ml) but not in any of the 15 isolates for which the MIC was ≤1 μg/ml.