Objective To investigate the prevalence distribution of Staphylococcus aureus isolated from mastitis cases in large-scale dairy farms in Changji Prefecture, Xinjiang, and conduct strain isolation and identification, antimicrobial susceptibility testing, PCR detection of virulence genes and mouse pathogenicity tests. This study aims to clarify the epidemiological characteristics, antimicrobial resistance phenotypes, virulence gene profiles and pathogenic capacity of bovine-derived S. aureus in the local area, and provide theoretical references for clinical prevention and control as well as rational antimicrobial use against bovine mastitis.
Methods A total of 184 milk samples from mastitic dairy cows were collected. Bacterial isolation and purification, Gram staining and 16S rRNA sequencing were performed for species identification. The Kirby-Bauer (K-B) disk diffusion method was used to detect the antimicrobial resistance spectrum of isolates. PCR amplification was applied to screen six key virulence genes, and intraperitoneal challenge tests in mice were carried out to verify bacterial pathogenicity.
Results Twenty-six strains of Staphylococcus aureus were isolated from 184 milk samples, with an isolation rate of 14.13%. The isolates exhibited susceptibility rates above 90% to cefotaxime, tetracycline, chloramphenicol, polymyxin B, gentamicin, florfenicol, cotrimoxazole and ciprofloxacin. Six virulence genes including nuc, PVL, hla, hlb, FnBPA and FnBPB were detected, with positive rates of 100%, 76.9%, 100%, 84.6%, 100% and 80.8%, respectively. Five distinct virulence gene profiles were identified, among which the nuc-PVL-hla-hlb-FnBPA-FnBPB genotype accounted for 53.8%. Strains carrying multiple virulence genes showed strong pathogenicity.
Conclusion There is an epidemic risk of mastitis caused by Staphylococcus aureus on this dairy farm. The isolated strains show low overall antimicrobial resistance, but widely carry diverse virulence genes with abundant genotypes, and possess strong pathogenicity.
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