In Vitro Susceptibility of Mastitis Pathogens Isolated from Clinical Mastitis Cases on Northern German Dairy Farms

The present research study investigated the susceptibility of common mastitis pathogens&#8212;obtained from clinical mastitis cases on 58 Northern German dairy farms&#8212;to routinely used antimicrobials. The broth microdilution method was used for detecting the Minimal Inhibitory Concentration (MIC) of <i>Streptococcus agalactiae</i> (n = 51), <i>Streptococcus dysgalactiae</i> (n = 54), <i>Streptococcus uberis</i> (n = 50), <i>Staphylococcus aureus</i> (n = 85), non-<i>aureus</i> staphylococci (n = 88), <i>Escherichia coli</i> (n = 54) and <i>Klebsiella</i> species (n = 52). Streptococci and staphylococci were tested against cefquinome, cefoperazone, cephapirin, penicillin, oxacillin, cloxacillin, amoxicillin/clavulanic acid and cefalexin/kanamycin. Besides cefquinome and amoxicillin/clavulanic acid, Gram-negative pathogens were examined for their susceptibility to marbofloxacin and sulfamethoxazole/trimethoprim. The examined <i>S. dysgalactiae</i> isolates exhibited the comparatively lowest MICs. <i>S. uberis</i> and <i>S. agalactiae</i> were inhibited at higher amoxicillin/clavulanic acid and cephapirin concentration levels, whereas <i>S. uberis</i> isolates additionally exhibited elevated cefquinome MICs. Most Gram-positive mastitis pathogens were inhibited at higher cloxacillin than oxacillin concentrations. The MICs of Gram-negative pathogens were higher than previously reported, whereby 7.4%, 5.6% and 11.1% of <i>E. coli</i> isolates had MICs above the highest concentrations tested for cefquinome, marbofloxacin and sulfamethoxazole/trimethoprim, respectively. Individual isolates showed MICs at comparatively higher concentrations, leading to the hypothesis that a certain amount of mastitis pathogens on German dairy farms might be resistant to frequently used antimicrobials.