Combinations of colistin and bacteriocins prevent the selection of colistin resistance in Acinetobacter baumannii.

Acinetobacter baumannii is a multidrug-resistant nosocomial pathogen responsible for infections that are often difficult to treat. Here, we show that exposure of A. baumannii to the last-resort antibiotic colistin, which disrupts the outer membrane of Gram-negative bacteria, results in inner membrane permeabilization and depolarization, ultimately inhibiting ATP synthesis. Nevertheless, under these conditions, colistin-resistant mutants are rapidly and frequently selected. In addition, A. baumannii is able to tolerate colistin, most likely due membrane depolarization and ATP depletion, which are hallmarks of antibiotic-tolerant subpopulations. In this context, we investigated whether bacteriocins can potentiate colistin activity. We identified and characterized two bacteriocins that inhibit the growth of multidrug-resistant clinical isolates, albeit at high concentrations. In vitro analyses showed that these small α-helical bacteriocins permeabilize phospholipid vesicles, highlighting their potential to potentiate antibiotics that compromise cell envelope integrity. Importantly, low concentrations of these bacteriocins combined with colistin leads to a substantial reduction in survival. Moreover, bacteriocin-colistin combinations limit the emergence of colistin-resistant mutants and partially restore susceptibility in colistin-resistant strains. These findings highlight the potential of combining bacteriocins and antibiotics to disrupt cell envelope homeostasis and support further evaluation of this strategy in vivo.