Isolation and Characterization of Non-O157 Shiga Toxin-Producing Escherichia coli in Foods Sold at Retail Markets in China

Non-O157 Shiga toxin-producing Escherichia coli (STEC) strains are significant foodborne pathogens that can cause acute diarrhea in humans. This study was conducted to investigate the contamination by non-O157 STEC in different types of food sold at retail markets in the People's Republic of China and to characterize non-O157 STEC strains. From May 2012 to April 2014, 1,200 retail food samples were collected from markets in 24 cities in China. Forty-four non-O157 isolates were recovered from 43 STEC-positive samples. Of the isolates, 22 and 19 carried the stx(1) and stx(2) genes, respectively, and 3 harbored both stx(1) and stx(2). stx(1a) and stx(2a) were the most prevalent stx subtypes. Other virulence genes, ent, hlyA, astA, eae, espB, iha, subAB, and tia, were commonly detected. Diverse O serogroups were identified among these isolates. Multilocus sequence typing indicated the high genetic diversity of the isolates. Thirty-two sequence types (STs) were identified among the 44 isolates, with ST383 (9.09%), ST134 (6.82%), and ST91 (6.82%) the most prevalent. Nine new STs were found. The isolates had a high prevalence of resistance to cephalothin, ampicillin, tetracycline, trimethoprim- sulfamethoxazole, nalidixic acid, streptomycin, and chloramphenicol. Twenty isolates (45.45%) were resistant to at least three antibiotics. This study provides updated surveillance data for non-O157 STEC isolates from foods sold at retail markets. Virulent and multidrug-resistant non-O57 STEC strains were isolated from all types of food. Our findings highlight the need for increased monitoring of non-O157 STEC in retail foods.