Inhibition of the Mycobacterium tuberculosis reserpine-sensitive efflux pump augments intracellular concentrations of ciprofloxacin and enhances susceptibility of some clinical isolates

Background/PurposeActive efflux is known to play a major role in the resistance of many bacteria to antibiotics. To evaluate the possibility of overcoming resistance by suppressing the efflux, we determined the effect of reserpine, an efflux pump inhibitor.MethodsIntracellular accumulations and the minimal inhibitory concentrations (MICs) of ciprofloxacin in M. tuberculosis H37Rv and 16 clinical isolates were determined, compared, and analyzed. Nine of the clinical isolates were resistant to isoniazid and rifampin (multiple-drug resistant MDR). Five of these were resistant to ciprofloxacin.ResultsA reserpine-inhibited efflux system was identified in the H37Rv control and 10:1 (90.9%) of ciprofloxacin-susceptible and 4:1 (80%) of ciprofloxacin-resistant clinical isolates. The MIC of ciprofloxacin decreased in the presence of reserpine in 3/10 (30%) of the ciprofloxacin-susceptible and 2/4 (50%) of the MDR ciprofloxacin-resistant strains that expressed efflux pumps. Two of the efflux-positive, ciprofloxacin-resistant strains in which the MIC of ciprofloxacin was not decreased by reserpine were found to carry a D94A gyrA mutation. In contrast, two strains with the D94G gyrA mutation were susceptible to ciprofloxacin in the presence of reserpine. An efflux-negative strain, highly resistant to multiple antibiotics, was found to have a novel G247S mutation that differs from known mutations in the QRDR region of the gyrA gene.ConclusionThese findings indicate t hat reserpine can increase intracellular concentrations of ciprofloxacin, but is unable to overcome other mechanisms of resistance in clinical isolates.