Broad-Spectrum Naphthyl-Substituted Diaminoquinolines Inhibiting the AdeG Efflux Pump of Acinetobacter baumannii

AdeFGH and AdeIJK, the two homologous multidrug efflux pumps of the resistance-nodulation-division superfamily of transporters, play distinct roles in Acinetobacter baumannii physiology and antibiotic resistance. Unlike ubiquitous AdeIJK, AdeFGH is strain-specific, typically expressed at low levels, and if overproduced, it enables resistance to a narrow spectrum of antibiotics, e.g., fluoroquinolones or chloramphenicol. In this study, we report that representatives of naphthyl-substituted diaminoquinolines targeting AdeIJK are also active against AdeFGH. We isolated AdeFGH-overproducing strains from the clinical AYE and Ab5075 isolates lacking AdeIJK and AdeABC pumps and demonstrated that these inhibitors are active in A. baumannii strains with different genetic backgrounds. The inhibitors potentiate the antibacterial activities of various antibiotics and enhance the bactericidal properties of the fluoroquinolones. We further analyzed how amino acid substitutions in the substrate translocation tunnels of AdeG affect the efflux properties of this pump and its sensitivity to inhibitors and compared them to the analogous substitutions in AdeJ. Our results suggest that the inhibitors engage similar contacts within the deep binding pockets of the two pumps but differ in their interactions in the entrance and the proximal binding sites. We conclude that the broad-spectrum activities of the diaminoquinolines as well as other inhibitors likely arise from the interactions within the deep-binding pockets, but their specificity is determined in the proximal-binding sites of the pumps.