Comparison of phenotypic and genotypic antimicrobial resistance patterns associated with Staphylococcus aureus mastitis in German and Danish dairy cows

摘要

Staphylococcus aureus is one of the most commonpathogens associated with bovine mastitis in Germanyand Denmark. Successful therapy is strongly linked tothe susceptibility of the pathogen to the administeredantimicrobial. An increase in resistant pathogens in humanand veterinary medicine has become a concernworldwide and hampers therapy due to reduced susceptibility.In the present study, susceptibility testingwas performed for 85 and 93 S. aureus isolates originatingfrom mastitis cases on 12 German and 8 Danishdairy farms, respectively. Phenotypic examination wasperformed by detection of minimal inhibitory concentration(MIC) values using the broth microdilutionmethod, followed by genotypic investigations of the blaZand mecA resistance genes via PCR. The tested antimicrobialswere the most frequently used β-lactams inGerman and Danish dairy farms, including cefquinome,cefoperazone, cephapirin, penicillin, oxacillin, cloxacillin,amoxicillin-clavulanic acid, and cephalexin-kanamycin.Special attention was paid to varying therapyconcepts because, in Germany, third- and fourth-generationcephalosporins have been predominantly usedin mastitis therapy, whereas in Denmark, restrictiveuse of penicillin is followed by a general avoidance ofcephalosporins. Differences in MIC values were mainlybased on determined MIC_(90) values (MIC at which 90%of isolates are inhibited). In general, Danish S. aureusisolates were inhibited at comparatively lower MIC_(90)values than S. aureus isolated from German dairyfarms for most β-lactams. No differences were observedregarding cefquinome, because both German and Danishisolates exhibited MIC_(50) and MIC_(90) values of 0.5and 1 μg/mL, respectively. In contrast, the MIC_(90) forpenicillin against German and Danish S. aureus were0.5 and ≤0.06 μg/mL, respectively. Resistance genes(blaZ, mecA) were only detected in German S. aureusisolates on 3 dairy farms in Germany. A total of 5 isolatestested positive for both blaZ and mecA, whereas1 isolate carried the blaZ resistance gene only. A directcorrelation between frequently used antimicrobials andreduced susceptibility could not be determined basedon results of the present study. In addition to furtherresearch to determine factors associated with resistancedevelopment, we emphasize the urgent need for internationallystandardized clinical breakpoints to assessresistance situations more accurately.