Objective: Streptococcus dysgalactiae  is a major pathogen in bovine mastitis. The purpose of this study was to survey the prevalence, antimicrobial resistance, as well as the spread of resistance and virulence-associated gene of S. dysgalactiae .
Methods: A total of 60 S. dysgalactiae  strains were obtained from 830 milk samples from Holstein cows with clinical mastitis. Antimicrobial resistance was examined by the disk diffusion method. Antimicrobial resistance and virulence genes were investigated by PCR, agarose gel electrophoresis and 16S rRNA gene sequencing.
Results: All isolates were resistant to tetracycline and showed a high level of resistance to aminoglycoside antibiotics, where 81.67% of the strains were multi-resistant to these ten sorts of antibiotics. In addition, the most prevalent resistance gene in S. dysgalactiae  was aphA-1  (98.33%), followed by blaTEM  (96.67%), ermB  (83.3%), aadA1/aadA2  (78.33%) and tetL  (73.33%). Totally, seven virulence genes with 25 combination patterns were detected in these isolates, and each isolates harbored at least one virulence gene. 21.67% of the isolates carried three or more virulence genes, while one strain with seven virulence-related genes and belonged to cfb +lmb +eno +napr +bca +scpB +cyl .
Conclusion: These findings indicate that S. dysgalactiae  isolated from clinical bovine mastitis cases in Northwest China show a variety of molecular ecology and are highly resistant to antibiotics commonly used in dairy farms. This research will help investigators better understand the pathophysiology S. dysgalactiae  in bovine mastitis and choose the appropriate antibiotics to treat mastitis.
Keywords: Streptococcus dysgalactiae , bovine mastitis, antimicrobial resistance, virulence gene