Antimicrobial resistance and toxin gene profiles of Staphylococcus aureus strains from Holstein milk.

UNLABELLED: Isolation of Staphylococcus aureus (Staph. aureus) from Holstein milk samples with mastitis and nonmastitis was conducted to estimate its prevalence, antimicrobial resistance and toxin genes. A total of 353 milk samples were collected from three Chinese Holstein herds. Fifty-three Staph. aureus isolates collected from 29 Staph. aureus-positive samples were characterized via antimicrobial susceptibility, toxin genes and Pulsed-field Gel Electrophoresis (PFGE) profiles. The prevalence of Staph. aureus was 4·0-9·5% in mastitic and 7·3-11·5% in nonmastitic samples in the analysed herds. Approximately 61·0% of Staph. aureus strains isolated from mastitis cows were resistant to ≥10 antimicrobials compared with 0% of isolates with nonmastitis. The most frequently observed super antigenic toxin gene was pvl (41·5%) followed by seh + pvl (13·2%). We did not find mecA-positive methicillin-resistant Staph. aureus (MRSA) strains, while mecA-negative MRSA strains were identified in the three herds. PFGE results suggested potential transmission of Staph. aureus strains in different farms. These results open new insights into Staph. aureus transmission and antimicrobial resistance of Holstein dairy cows and into developing strategies for udder health improvement of dairy cattle. SIGNIFICANCE AND IMPACT OF THE STUDY: Significantly higher levels of antibiotic resistance of Staphylococcus aureus strains were detected in the lactating Holstein milk with clinical mastitis compared with that without clinical mastitis. PFGE results suggest potential transmission of Staph. aureus strains in different dairy farms. The results imply that the dairy farms need to select effective antibiotics in the treatment of Staph. aureus-infected cows and prevent the transmission of Staph. aureus strains from one herd to another.