Synergistic Antibacterial Activity of Amorolfine Combined with Colistin Against Acinetobacter baumannii

Emerging resistance to colistin in <i>Acinetobacter baumannii</i> is concerning because of the limited therapeutic options for this important clinical pathogen. Given the shortage of new antibiotics, one strategy that has been proven to be therapeutically effective is to overcome antibiotic-resistant pathogens by combining existing antibiotics with another antibiotic or non-antibiotic. This study was designed to investigate the potential synergistic antibacterial activity of amorolfine, a morpholine antifungal drug, in combination with colistin against <i>A. baumannii</i>. In this work, antibiotic susceptibility testing, checkerboard assays, and time-kill curves were used to investigate the synergistic efficacy of colistin combined with amorolfine. The molecular mechanisms of combination therapy were analyzed using fluorometric assays, UV-vis spectroscopy, and molecular docking. Finally, we evaluated the in vivo efficacy of combination therapy against <i>A. baumannii</i>. In brief, the combination therapy showed significant synergistic activity against <i>A. baumannii</i> (FICI = 0.094). In addition, the combination of amorolfine improved the membrane disruption of colistin, and amorolfine exhibited the capacity of binding to DNA. Moreover, in a mouse sepsis model, this combination therapy increased survival compared to colistin monotherapy. Our findings demonstrated that amorolfine serves as a potential colistin adjuvant against <i>Acinetobacter baumannii</i>.