Types and Mechanisms of Efflux Pump Systems and the Potential of Efflux Pump Inhibitors in the Restoration of Antimicrobial Susceptibility, with a Special Reference to <i>Acinetobacter baumannii</i>

Bacteria express a plethora of efflux pumps that can transport structurally varied molecules, including antimicrobial agents and antibiotics, out of cells. Thus, efflux pump systems participate in lowering intracellular concentrations of antibiotics, which allows phenotypic multidrug-resistant (MDR) bacteria to survive effectively amid higher concentrations of antibiotics. <i>Acinetobacter baumannii</i> is one of the classic examples of pathogens that can carry multiple efflux pump systems, which allows these bacteria to be MDR-to-pan-drug resistant and is now considered a public health threat. Therefore, efflux pumps in <i>A. baumannii</i> have gained major attention worldwide, and there has been increased interest in studying their mechanism of action, substrates, and potential efflux pump inhibitors (EPIs). Efflux pump inhibitors are molecules that can inhibit efflux pumps, rendering pathogens susceptible to antimicrobial agents, and are thus considered potential therapeutic agents for use in conjunction with antibiotics. This review focuses on the types of various efflux pumps detected in <i>A. baumannii</i>, their molecular mechanisms of action, the substrates they transport, and the challenges in developing EPIs that can be clinically useful in reference to <i>A. baumannii</i>.