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 a <i>Galleria mellonella</i> infection 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.