Antibacterial effects and resistance induction of silver and gold nanoparticles against Staphylococcus aureus‐induced mastitis and the potential toxicity in rats

Staphylococcus aureus (S. aureus) is one of the prevalent mastitis‐inducing pathogens worldwide. The resistance of S. aureus to antibiotics is a common issue for dairy farms. Recently, nanoparticles (NPs) have been used for treating antibiotic‐resistant bacteria. We therefore aimed to investigate the antimicrobial effect of silver and gold NPs (AgNPs and AuNPs, respectively) and the resistance developed by S. aureus as well as the toxic effects of both NPs in rats. We used 198 S. aureus strains to determine the antibacterial effects of AgNPs and AuNPs. The microdilution method was used to establish the minimum inhibitory concentrations (MICs) of both NPs. To induce resistance, 20 S. aureus strains were passaged 10 times in broth medium with sublethal doses of NPs and an additional 10 times without NPs to examine the stability of resistance. Histopathology was performed after oral administration to the rats with the study doses of 0.25, 0.5, 1, and 2 mg/kg of NPs for 30 days. The MICs of 10‐nm AgNPs, 20‐nm AgNPs, 10‐nm AuNPs, and 20‐nm AuNPs against S. aureus were 14.70 ± 1.19 μg/ml, 9.15 ± 0.13 μg/ml, 24.06 ± 2.36 μg/ml, and 18.52 ± 1.26 μg/ml, respectively. Most strains developed strong resistance when treated with 20‐nm or 10‐nm AgNPs, whereas only two strains were resistant to 10‐nm AuNPs and three strains to 20‐nm AuNPs. No cross‐resistance between NPs and various antibiotics was identified in any of the adapted S. aureus strains. Organ histopathology revealed that 0.25, 0.5, and 1 mg/kg doses of AgNPs and AuNPs were not toxic to rat tissue. In contrast, a higher dose (2 mg/kg) of NPs impaired all organs tested. This study demonstrates the antibacterial effects of NPs. S. aureus strains develop resistance less frequently against AuNPs than AgNPs, and neither AuNPs nor AgNPs was toxic to rats at low doses.