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

Abstract

The objective of this study was to analyze the in vitro antimicrobial resistance (AMR) of Streptococcus (Sc.) dysgalactiae, Sc. agalactiae, and Sc. canis over a 10-year 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 (TGD) were analyzed. Each QMS was tested using the California Mastitis Test (CMT) and categorized as negative (N), subclinical (S), or clinical (C) mastitis if the milk character was abnormal. Samples with Sc. dysgalactiae, Sc. agalactiae, or Sc. canis were included and a subset of isolates were further tested for in vitro antimicrobial resistance by breakpoint analysis with broth microdilution. Sc. dysgalactiae (61%, n = 65,750) was the most abundant pathogen among those 3 species, followed by Sc. agalactiae (28%, n = 30,486), and Sc. 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, Sc. dysgalactiae, Sc. agalactiae, and Sc. canis were largely susceptible to the remaining antimicrobial agents tested (penicillin, amoxicillin-clavulanate, oxacillin, cefazolin, cefoperazone, cefquinome). Only less than 14% of isolates of Sc. dysgalactiae and Sc. canis were resistant against any of the antimicrobials tested. Sc. agalactiae was the species with the highest percentage of resistant isolates. While the percentage of resistant isolates from Sc. canis and Sc. dysgalactiae decreased, the percentage of resistant Sc. agalactiae isolates increased since 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.