Mastitis is a global disease occurring in dairy cows, causing notable economic losses. Extensive use of antibiotics could allow the emergence of mobile antimicrobial resistance genes in mastitis-causing pathogens. This study aimed to investigate the prevalence and characterization of colistin resistance genes in E. coli recovered from bovine mastitic milk. A total of 74 E. coli isolates were investigated for antimicrobial resistance. The presence of mcr-1, mcr-2, mcr-3, mcr-4, and mcr-5 plasmid-mediated resistance genes, as the most crucial contributors to resistance to colistin, was examined by Multiplex PCR. Antimicrobial susceptibility patterns of all isolates to the seven most common antibiotics applied in dairy herds, including colistin, ceftriaxone, ampicillin, tetracycline, gentamicin, enrofloxacin, and trimethoprim-sulfamethoxazole were determined by the DD test. Among all samples, 70 isolates (94.6%) were resistant to colistin. In the MIC test, all isolates were also resistant to colistin, which was in agreement with the DD test. None of the E. coli isolates carried plasmid-mediated colistin resistance mcr-1 to 5 genes in Multiplex PCR. Despite the important role of food-producing animals in the transfer of antibiotic resistance, mastitis-causing E. coli isolates were not the source of mcr 1 to 5 genes in this study. The present research showed a high level of phenotypic resistance to colistin, while there was no agreement with their genotypic resistance. Consumption of polymyxins in dairy calves and the probable existence of other more effective resistance genes could be the reason for this high rate of phenotypic resistance.

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