A novel role for colistin as an efflux pump inhibitor in multidrug-resistant Klebsiella pneumoniae.

Antimicrobial resistance (AMR) is a global challenge that demands new strategies to maintain the effectiveness of current treatments. AMR arises through various mechanisms, including overexpression of drug efflux pumps. The AcrAB-TolC efflux pump is used by Enterobacterales such as Klebsiella pneumoniae to expel antibiotics and overcome antimicrobial toxicity. Here we explored a novel secondary activity of colistin as an efflux pump inhibitor. Experiments were conducted to determine antimicrobial susceptibility, selection of a resistant mutant, assess the function of efflux machinery under various treatment conditions, and measure the inhibition of extrusion by colistin. Colistin augmented the efficacy of various antibiotics against resistant K. pneumoniae strains and reversed clinically relevant antibiotic resistance caused by acrAB overexpression. This effect was demonstrated via increased uptake of efflux pump substrates such as N-phenyl-1-napthylamine, ethidium bromide, and Hoechst dye in K. pneumoniae overexpressing the AcrAB efflux pump. Molecular docking models indicated that colistin likely binds to the transmembrane region of K. pneumoniae AcrB, further validating colistin’s function as an efflux pump inhibitor at low concentrations. Scanning electron microscopy showed that sub-nephrotoxic concentrations of colistin had no effect on bacterial membrane integrity. These novel findings highlight the therapeutic potential of sub-nephrotoxic concentrations of colistin as an adjuvant to overcome efflux-mediated resistance in clinically problematic Enterobacterales K. pneumoniae.