Short communication: Bovine mastitis caused by a multidrug-resistant, mcr-1-positive (colistin-resistant), extended-spectrum β-lactamase-producing Escherichia coli clone on a Greek dairy farm.

We performed a survey aimed at analyzing milk samples collected from cows with mastitis for the presence of extended-spectrum β-lactamase (ESBL)-producing Escherichia coli. Single-quarter mastitic milk samples obtained from 400 cows in 23 Greek dairy herds with a history of E. coli mastitis were processed for the selective isolation of ESBL-producing E. coli. The antimicrobial susceptibility of the ESBL-producing isolates was analyzed using agar disk diffusion, and minimum inhibitory concentrations of colistin were determined by broth microdilution. We used PCR followed by DNA sequencing to characterize the β-lactamases and mcr-1 (colistin resistance) genes, and for phylotyping and multilocus sequence typing. We found a total of 89/400 (22.25%) E. coli isolates from 12/23 (52%) farms. Six isolates originating from 6 cows on a single farm were ESBL producers and were resistant to cefquinome, amoxicillin-clavulanic acid, aztreonam, ampicillin, and colistin. Five of these isolates were resistant to trimethoprim-sulfamethoxazole, and 5 to streptomycin. The 6 ESBL producers were mcr-1-positive and carried bla

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