We studied 50 antimicrobial resistance mechanisms associated with Escherichia coli strains isolated from five species of commercial fish to evaluate the carriage and type of extended-spectrum β-lactamases (ESBLs) in these isolates. One hundred fifty samples of fresh raw fish were collected from large supermarkets and fish markets in northern Portugal during 2017. Thirty samples from each species were screened for the presence of E. coli and ESBL-producing E. coli. The samples were spread onto Levine plates with or without cefotaxime (2 μg/mL). The sensitivity of the isolates was determined for 16 antibiotics. The resistance and virulence genes were investigated by PCR. From the 150 samples, 45 E. coli isolates were recovered, and 3 of these isolates produced ESBL. All of these ESBL-producing isolates were resistant to β-lactams and harbored the blaCTX-M-1 and blaTEM genes, and all belonged to phylogenetic group A. One ESBL-producing E. coli isolate also was resistant to tetracycline and harbored the tet(A) gene, and another isolate was resistant to streptomycin and harbored the aadA gene. The remaining E. coli isolates were resistant to individual or combinations of the following antibiotics: tetracycline (16 isolates), sulfamethoxazole-trimethoprim (10 isolates), streptomycin (9 isolates), ampicillin (8 isolates), and chloramphenicol (2 isolates). The blaTEM, tet(A), aadA, and cmlA genes and combinations of sul genes were detected among most isolates that were resistant to ampicillin, tetracycline, streptomycin, chloramphenicol, and sulfamethoxazole-trimethoprim, respectively. The unmonitored and excessive use of antibiotics in aquatic environments promotes the development and propagation of antimicrobial resistance, and fish can be reservoirs of antibiotic resistance genes, which can easily be transmitted to humans through the consumption of raw fish, creating a public health problem.

  • Forty-five E. coli isolates were recovered from fresh fish.

  • Three of the E. coli isolates produced ESBL.

  • ESBL-producing E. coli isolates harbored the blaCTX-M-1 and blaTEM genes.

  • The presence of ESBL-producing E. coli may represent a serious public health problem.

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