Differential Roles of Ag and ZnO in the Bactericidal Activity of Ag/ZnO-Hydroxyapatite Nanocomposites against Multidrug-Associated Methicillin-Resistant Staphylococcus aureus.

Methicillin-resistant Staphylococcus aureus (MRSA) is often resistant to multiple antibiotic classes and extremely difficult to treat, highlighting the urgent need for new drugs or alternative strategies. Hydroxyapatite (HAp) combined with silver (Ag) and zinc oxide (ZnO) to form an Ag/ZnO-HAp nanocomposite has been widely studied for its antibacterial properties. This nanocomposite is often assumed to produce additive or synergistic antibacterial effects. Four Ag/ZnO-HAp nanocomposites with different Ag-to-ZnO mole ratios: nanocomposite-A (0.005:0.005), nanocomposite-B (0.01:0.005), nanocomposite-C (0.005:0.01), and nanocomposite-D (0.01:0.01) were used to understand the roles of Ag and ZnO in antibacterial activity against MRSA ATCC700699. Agar diffusion tests of nanocomposites A, B, C, and D produced inhibition zones of 8, 9, 6, and 8 mm, respectively, whereas unmodified HAp showed no inhibition zone. Bactericidal assays revealed log reductions of 5.27, 7.16, 5.00, and 5.81 for nanocomposites A, B, C, and D at 0.0625 mg/mL over 18 h, respectively. Specific death rates (min-1) for nanocomposites A, B, C, and D over 240 min were 0.062, 0.075, 0.058, and 0.064; D-values (minutes) were 16.1, 13.3, 17.2, and 15.6, respectively. Minimum bactericidal concentrations for 9 MRSA and 1 methicillin-sensitive S. aureus clinical isolates ranged from ≤ 0.06 to > 2 mg/mL, regardless of antibiotic susceptibility profiles. Overall, the nanocomposites exhibit bactericidal activity in the order B > D> A > C. Ag content primarily drives bactericidal activity, whereas ZnO reduces Ag's bactericidal effectiveness. These findings suggest that optimizing the Ag-to-ZnO ratio is crucial for maximizing bactericidal activity. Additionally, customizing the nanocomposites to clinical isolates may be essential to maximize biomedical applications.