Bakuchiol, a Natural Antioxidant, Synergizes with Colistin Against Multidrug-Resistant Gram-Negative Bacteria by Disrupting Iron Homeostasis and Membrane Integrity

The rapid emergence and global dissemination of colistin resistance pose a critical threat to public health by limiting therapeutic options against Gram-negative infections. In this study, we report that bakuchiol (BAK), a natural antioxidant meroterpenoid, significantly restores colistin (COL) efficacy against a range of clinically relevant Gram-negative pathogens, including colistin-resistant strains. The combination of BAK and COL reduced the minimum inhibitory concentrations (MICs) of colistin by 8- to 32-fold, indicating strong synergistic antibacterial activity. Mechanistic studies revealed that BAK disrupts bacterial iron homeostasis via chelation-mediated intracellular iron depletion and concurrently compromises membrane integrity through interaction with phospholipids. This dual action induces collapse of the proton motive force and severe metabolic dysfunction. Importantly, the BAK–COL combination exhibited no detectable toxicity and demonstrated potent in vivo efficacy in aGalleria mellonellainfection model. Furthermore, formulation with polyvinylpyrrolidone (PVP) markedly improved the solubility and sustained the synergistic effects of BAK over a broad concentration range. Our findings highlight the potential of bakuchiol as a novel antioxidant adjuvant and provide a promising combinatory strategy for overcoming multidrug-resistant bacterial infections.