Direct antibacterial activity of allicin and restoration of carbapenem susceptibility by inhibiting New Delhi metallo-β-lactamase

BACKGROUND: Carbapenem-resistant Enterobacterales (CRE), particularly strains producing New Delhi metallo-β-lactamase (NDM), represent a critical global health threat and underscore an urgent need for effective NDM inhibitors. Allicin, a major bioactive compound in freshly crushed garlic, shows broad-spectrum antibacterial activity, but its detailed mechanisms and its capacity to reverse NDM-mediated carbapenem resistance remain to be fully elucidated. PURPOSE: This study aimed to evaluate the antibacterial effects of allicin alone and in combination with meropenem both in vitro and in vivo to uncover its mechanisms of action, thereby providing new mechanistic insights and a potential therapeutic strategy against NDM-mediated resistance. METHODS: The antibacterial activity of allicin and its mechanisms were assessed using microbroth dilution, membrane-permeability assays, ROS and H₂S detection, and comet assays. The in vivo efficacy was evaluated using a Galleria mellonella infection model and a rat Pasteurella pneumonia model. Synergy was assessed in vitro via checkerboard assays and in vivo using a rat wound infection model. The interaction with NDM-5 was studied through molecular docking, site-directed mutagenesis, enzyme kinetics, and microscale thermophoresis. RESULTS: Allicin exhibited potent activity against clinical isolates of Pasteurella and NDM‑positive Escherichia coli (MICs 8-64 μg/ml, MBCs 16-128 μg/ml) both in vitro and in vivo. It disrupted bacterial redox homeostasis by inducing ROS accumulation and suppressing endogenous H₂S production. Moreover, allicin restored meropenem activity both in vitro and in vivo and acted as a non‑competitive inhibitor of NDM‑5. Its disulfide bond covalently bound to key zinc‑coordinating residues in the active site, leading to Zn²⁺ release and the loss of NDM enzymatic activity. CONCLUSION: Allicin possesses a dual pharmacological role: exerting direct antibacterial activity through oxidative stress induction and reversing carbapenem resistance via the non‑competitive inhibition of NDM. These findings provide a promising lead compound for developing natural product-derived antibiotic adjuvants to address the global challenge of antimicrobial resistance (AMR).