Antimicrobial resistance of Streptococcus dysgalactiae, Streptococcus agalactiae, and Streptococcus canis in quarter milk samples from Bavaria, Southern Germany, between 2012 and 2022.

The objective of this study was to analyze the in vitro antimicrobial resistance (AMR) of Streptococcusdysgalactiae, Strep. agalactiae, and Strep. canis over a 10-yr period from 2012 to 2022 against the most commonly used antimicrobial agents. For this purpose, all quarter milk samples (QMS) submitted to the milk laboratory of the Bavarian Animal Health Service were analyzed. Each QMS was tested using the California Mastitis Test and categorized as negative, subclinical, or clinical mastitis if the milk character was abnormal. Samples with Strep. dysgalactiae, Strep. agalactiae, or Strep. canis were included and a subset of isolates were further tested for in vitro AMR by breakpoint analysis with broth microdilution. Streptococcus dysgalactiae (61%, n = 65,750) was the most abundant pathogen among those 3 species, followed by Strep. agalactiae (28%, n = 30,486), and Strep. canis (11%, n = 11,336). All 3 species showed the highest resistance to the same 4 antimicrobial agents: erythromycin, marbofloxacin, pirlimycin, and cefalexin/kanamycin with varying degrees of resistance. Throughout the study period, Strep. dysgalactiae, Strep. agalactiae, and Strep. canis were largely susceptible to the remaining antimicrobial agents tested (penicillin, amoxicillin-clavulanate, oxacillin, cefazolin, cefoperazone, cefquinome). Only less than 14% of isolates of Strep. dysgalactiae and Strep. canis were resistant against any of the antimicrobials tested. Streptococcus agalactiae was the species with the highest percentage of resistant isolates. Although the percentage of resistant isolates from Strep. canis and Strep. dysgalactiae decreased, the percentage of resistant Strep. agalactiae isolates increased after 2017. In summary, most isolates were not resistant to the most commonly used antimicrobial agents for mastitis therapy, including β-lactam antibiotics and penicillin should remain the first-choice therapy against streptococcal mastitis.