Metal--Antibiotic Combinations as a One Health Strategy Against Multidrug-resistant Pathogens: In Vitro and Milk Matrix Evaluation

Introduction: The rise of antimicrobial resistance (AMR) poses a major threat to food safety, public health, and sustainable agriculture. This study investigates the potential of metal-based antimicrobials as innovative agents to combat multidrug-resistant (MDR) foodborne pathogens within the One Health framework.

Material and methods: We evaluated five metal salts—silver nitrate (AgNO₃), zinc sulfate (ZnSO₄), gold chloride (AuCl₃), copper sulfate (CuSO₄), and aluminum sulfate (Al₂(SO₄)₃) for their antibacterial activity against ESBL-producing Escherichia coli, methicillin-resistant Staphylococcus aureus (MRSA), and vancomycin-resistant Enterococcus (VRE). Using a suite of microbiological and physicochemical assays, Zone of Inhibition (ZOI), MIC, MBC, time-kill kinetics, and propidium iodide (PI) membrane integrity analysis, and analysing the toxicity of metals, we observed strong concentration-dependent antibacterial effects.

Results: AgNO₃ showed the highest efficacy, with MIC values as low as 0.318 mM and >3-log reduction in bacterial counts within 4–8 hours. PI uptake assays indicated substantial membrane damage, supporting disruption-based mechanisms of action. Checkerboard assays revealed synergistic interactions (FICI ≤ 0.5) between metals and antibiotics. Importantly, all metals retained antibacterial activity in raw milk without causing precipitation or discoloration, demonstrating compatibility with complex food matrices. While metal toxicity is still under evaluation, the heatmaps and radar plots were used to visualize strain-specific responses and rank antimicrobial performance, with AgNO₃ > AuCl₃ > ZnSO₄.

Conclusion: These findings highlight the promise of metal-based antimicrobials as functional, broad-spectrum agents for use in dairy systems and food biotechnology. Their potential to enhance food safety, reduce AMR, and support sustainable food systems makes them valuable candidates for integration into next-generation food engineering and "foods as medicine" strategies.