Evaluation of extended-spectrum β-lactamase production and plasmid-mediated quinolone resistance in Escherichia coli isolated from raw milk

This study aimed to determine the antibiotic resistance and extended-spectrum β-lactamase production profiles of Escherichia coli isolates obtained from raw milk samples. To this end, the study used a chromogenic-based culture method, MALDI-TOF-MS, 16S rRNA gene sequencing, disc diffusion, and combined disc techniques. PCR was also performed to detect ESBL ( bla SHV , bla TEM , and bla CTX-M ), quinolone ( qnrA , qnrB , and qnrS ) genes, and the presence of class I integrons. A total of 47 (78.33%) E . coli isolates were identified by morphological, biochemical, and molecular tests. All isolates exhibited susceptibility to gentamicin and tobramycin. Resistance rates to different antibiotics were observed as follows: ampicillin, 51.06%; nalidixic acid, 23.40%; trimethoprim/sulfamethoxazole, 17.02%; meropenem, 14.89%; ceftazidime, 8.51%; cefotaxime, 8.51%; piperacillin-tazobactam, 4.25%; aztreonam, 4.25%; imipenem, 4.25%; and levofloxacin, 2.12%. ESBL production was positive in 3 (6.38%) of the isolates. Bla genes were detected in 11 isolates ( bla SHV = 9 [19.14%], bla TEM = 4 [8.5%], and bla CTX = 3 [6.38%]). The qnrB , qnrS , and qnrA genes were found in 5 (10.63%), 4 (8.51%), and 1 (2.12%) isolates, respectively. The intI1 gene was detected in 14 (29.78%) isolates. • Escherichia coli can be commonly found in unhygienic milk and dairy products. • Intensive use of antibiotics in animals causes the formation of resistant isolates. • Food is a potential vector for the transmission of antibiotic-resistance genes.