A novel role for colistin as an efflux pump inhibitor in multidrug-resistant <i>Klebsiella pneumoniae</i>

<p dir="ltr">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 <i>Klebsiella pneumoniae</i> 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 <i>K. pneumoniae</i> strains and reversed clinically relevant antibiotic resistance caused by <i>acrAB</i> 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 <i>K. pneumoniae</i> 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 <i>K. pneumoniae</i>.</p>