Introduction: mastitis is one of the most costly disease in dairy cows, which greatly decreases milk production. Use of antibiotics in cattle leads to antibiotic-resistance of mastitis-causing bacteria. The present study aimed to investigate synergistic effect of Zinc oxide nanoparticles (ZnONPs) with
Amikacin or Dihydrostreptomycin antibiotic on mastitis-causing Staphylococcus aureus
Methods: In this study, 75 samples of milk were taken from the cows with clinical and subclinical mastitis during the august-October 2018 sampling period. In addition to biochemical tests, nuc gene amplification by PCR was used to identify strains of Staphylococcus aureus. Disk diffusion test and microdilution were performed to determine minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). Fractional Inhibitory Concentration (FIC) index was calculated to determine the interaction between a combination of ZnONPs and each one of the antibiotics.
Results: Twenty strains of Staphylococcus aureus were isolated from 75 milk samples and were confirmed by PCR. Based on disk diffusion test, 35%, 15% and 50% of the strains were respectively susceptible, moderately susceptible and resistant to Amikacin In addition, 30%, 20% and 50% of the strains were respectively susceptible, moderately susceptible and resistant to Dihydrostreptomycin According to FIC index, Amikacin antibiotic and ZnONPs had synergistic effects in 50% of the strains. Furthermore, Dihydrostreptomycin antibiotic and ZnONPs had synergistic effects in 45% of the strains.
Conclusion: It can be concluded that a combination of ZnONPs with either Amikacin or Dihydrostreptomycin showed synergistic antibacterial properties in Staphylococcus aureus isolates from mastitis. In addition, some hypotheses were proposed to explain antimicrobial mechanism of the combination.