Characterization of Escherichia coli isolates associated with bovine mastitis in Austria.

Despite advances in dairy husbandry, mastitis-associated Escherichia (E.) coli (MAEC) remains a primary causative agent of bovine mastitis, contributing significantly to economic losses in the cattle industry. This study characterized 50 E. coli isolates from severe, moderate, mild, and subclinical mastitis by antimicrobial susceptibility testing and genotypic analysis, including PCR, DNA microarray, E. coli phylotyping and whole-genome sequencing of selected strains. Antimicrobial resistance rates were highest for ampicillin (30%), tetracycline (20%), amoxicillin-clavulanate (20%), and trimethoprim-sulfamethoxazole (14%); The majority of isolates (66%) was susceptible to antibiotics. One isolate displayed an ESBL phenotype, carrying bla CTX-M-3 and bla TEM-1 genes. Phylogenetic analysis indicated that members of phylogroups B1 (50%) and A (16%) are predominant among the isolates, followed by group C (12%), with the remaining isolates belonging to groups D, E, and F, and two unassigned. The most prevalent virulence-associated genes detected were fimH (100%), and genes of the ferric dicitrate uptake locus (fecIRABCDE). Overall, MAEC isolates exhibited diverse and heterogeneous genetic profiles. The proliferation of ESBL-producing E. coli poses a threat within a One Health framework, given the risk of multidrug-resistant strains at the human-animal-environment interface, compromising critical antimicrobials in both veterinary and human medicine.