[Resistance patterns of bovine mastitis-associated Escherichia coli isolates to selected antimicrobial agents in Austria].

Escherichia (E.) coli is one of the most important environmentally associated mastitis pathogens worldwide. Gram-negative bacteria can cause numerous clinical symptoms. Lipopolysaccharide (cell wall component of gram-negative bacteria) triggers an immune reaction in the mammary gland. Individual cow-associated factors such as lactation stage and age also play a major role. Due to the different forms of mastitis, there are also different therapeutic approaches. Severe cases of mastitis are treated with systemic antimicrobial therapy. Antimicrobial susceptibility testing in compliance with international standards is essential for this approach. One major focus is a more targeted use of antimicrobials and a reduction of critically important antimicrobials in practice to avoid development of multidrug resistant bacteria.To determine the antimicrobial susceptibility of mastitis-associated E. coli, isolates originating from quarter milk samples of cows with mastitis were tested using different phenotypic methods. The antimicrobial resistance was tested using agar disk diffusion test (ADD, ampicillin, cefoperazone, amoxicillin/clavulanic acid, kanamycin/cefalexin, ciprofloxacin, cefepime, gentamicin, kanamycin, tetracycline, and trimethoprim/sulfamethoxazole) and a commercially available broth microdilution method (MICRONAUT-S Mastitis 3, Bruker Corporation, Billerica, Massachusetts, USA) for determination of the minimum inhibitory concentration (MIC, ampicillin, cefazolin, cefoperazone, cefquinom, amoxicillin/clavulanic acid, kanamycin/cefalexin, and marbofloxacin). To investigate extended-spectrum β-lactamase (ESBL) production and AmpC-producing the Mast D68C test (D68C MASTDISCS, Mast Group Ltd, UK) was used.In both methods isolates showed a high in vitro susceptibility to quinolones and fluoroquinolones, 3rd and 4th generation cephalosporins and amoxicillin/clavulanic acid. In contrast, high resistance rates were found against trimethoprim/sulfamethoxazole (24.0%), tetracycline (17.5%) and ampicillin (ADD 19.5%). Four isolates were phenotypically ESBL positive; none of the isolates showed phenotypically AmpC β-lactamase activity.Based on the results, amoxicillin/clavulanic acid can be considered an alternative option for antimicrobial treatment of E. coli mastitis in bovines. However, further studies are necessary to confirm in vivo efficacy. The targeted use of antimicrobial substances in human and veterinary medicine is essential to restrain the development of ESBL -producing bacteria.