Phenotypic and genotypic characterization of antimicrobial resistance profiles in Streptococcus dysgalactiae isolated from bovine clinical mastitis in 5 provinces of China

Bovine mastitis is among the most prevalent and costly diseases of dairy animals and is caused by a variety of bacterial pathogens including Streptococcus dysgalactiae. However, comprehensive studies reporting the prevalence and antimicrobial resistance profiles of S. dysgalactiae isolated from bovine mastitis are scarce. Therefore, this study was to investigate the occurrence of S. dysgalactiae associated with bovine clinical mastitis, to assess their antimicrobial resistance profiles, and to analyze the phenotypic and genotypic profiling of resistant isolates. In total, 1,180 milk samples were collected from dairy cows with clinical mastitis belonging to 74 commercial dairy herds located in 14 provinces of China from January 2014 to May 2016. Overall S. dysgalactiae isolates were recovered from 88 (7.5%) of the mastitic milk samples. The antimicrobial susceptibility of these isolates was tested against 8 antimicrobial agents by using minimum inhibitory concentrations. Results showed that 82 (93.2%) isolates expressed resistance to more than one antimicrobial agent. Antimicrobial resistance was highest against kanamycin (89.8%), sulfonamide (83.0%), and streptomycin (58.0%), which can be attributed to the intrinsic resistance for most of Streptococcus spp. against those antimicrobial substances. Strikingly, 30 (34.1%) and 12 (13.6%) isolates were found resistant to cephalexin and ceftriaxone, respectively. BlaTEM, ermB, and tetM were the most prevalent resistance genes. All isolates carried at least one of all tested resistance genes. Also, 1.1, 12.5, 18.2, 36.4, and 31.8% of isolates were positive for at least one tested resistance gene in 1, 2, 3, 4, or 5 classes of antimicrobials. Survival analysis showed a significant association between ermB and survival of the S. dysgalactiae isolates at increasing erythromycin concentrations. No other statistically significant associations were observed between the phenotypic and genotypic resistance profiles. This study concludes a considerable prevalence of S. dysgalactiae associated with bovine mastitis in dairy herds of China and these isolates exhibited high resistance rates to tested antimicrobials, coupled with high occurrence of resistance genes. Both the prevalence of S. dysgalactiae and their antimicrobial resistance profiles strongly varied among dairy herds, demonstrating the need for antimicrobial susceptibility surveillance at the herd level to ensure optimal therapeutic results