Isolation and characterization of Shiga toxin-producing Escherichia coli (STEC) in retail edible beef by-products

The prevalence of Shiga toxin-producing Escherichia coli (STEC) was investigated in 350 edible beef intestinal samples, including omasum (n=110), abomasum (n=120), and large intestines (n=120), collected from traditional beef markets in Seoul, Korea. A total of 23 STEC strains were isolated from 15 samples (four strains from three omasa, 10 from five abomasa, and nine from seven large intestines). The O serotypes and toxin gene types of all STEC isolates were identified, and antimicrobial resistance was assessed using the disk diffusion method. The isolation rates of STEC from edible beef intestines were 2.8% in omasum, 4.2% in abomasums, and 5.9% in large intestines. All STEC isolates harbored either stx1, or both stx1 and stx2 genes simultaneously. Among the 23 isolates, 13 strains were identified as 11 different O serogroups, and 10 strains were untypable. However, enterohemorrhagic Esherichia coli O157, O26, and O111 strains were not isolated. The highest resistance rate observed was against tetracycline (39%), followed by streptomycin (35%) and ampicillin (22%). Of the 23 isolates, 12 isolates (52%) were resistant to at least one antibiotic, nine (39%) isolates were resistant to two or more antibiotics, and one isolate from an abmasum carried resistance against nine antibiotics, including beta-lactam/beta-lactamase inhibitor in combination and cephalosporins. This study shows that edible beef by-products, which are often consumed as raw food in many countries, including Korea, can be potential vehicles for transmission of antimicrobial-resistant pathogenic E. coli to humans.