Nanomaterials enhanced the antimicrobial effect of microwave treatment: understanding the role of nanomaterial properties

Unique outcomes ensuing nanomaterial (NM) interactions with discrete wavelengths of electromagnetic radiation have been utilized in various biological applications. We investigated the antibacterial effect and dissolution of five NMs (gold nanospheres (AuNSs), two gold nanorods (AuNRs636 and AuNRs772), silver nanoparticles (AgNPs), and titanium dioxide nanoparticles (TiO₂ NPs)) in saline and milk during microwave (MW) treatment. AuNSs, AgNPs, AuNRs636, and AuNRs772 improved the antibacterial effect of MW not only by increasing the temperature of the suspending media but also due to oxidative stress. Notably, the damage to bacterial membrane, measured as a reduction in the membrane potential, and reactive oxygen species generation in Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria was higher during MW treatment in the presence of AuNRs in comparison to AuNSs. AuNRs636 (4 µg/mL) combined with MW (40 s) achieved ~ 5 Log₁₀(CFU/mL) reduction of E. coli and S. aureus in milk. MW enhanced the dissolution of AuNRs636 in milk, while AgNPs and TiO₂ NPs showed aggregation after MW. Apart from elucidating the increased temperature and oxidative stress on bacterial elimination, this work highlighted the differential effects of MW on NMs of different chemical composition and shape.