High prevalence of multidrug-resistant Escherichia coli and first detection of IncHI2/IncX4-plasmid carrying mcr-1 E. coli in retail ready-to-eat foods in China

Antibiotic-resistant bacteria in food pose an important threat to public health. Multidrug-resistant strains in ready-to-eat (RTE) foods can be transferred to humans through diet, which increases their health risk. This study systematically investigated antibiotic resistance and antibiotic resistance genes in E. coli isolated from retail RTE foods and characterized plasmid-mediated colistin-resistant E. coli strains. A total of 1118 RTE food samples were collected from markets in 39 cities in China, and 126 E. coli strains, >95% of which were multidrug-resistant, were isolated. The isolates showed a high prevalence of resistance to tetracycline (95.24%), ampicillin (82.54%), trimethoprim-sulfamethoxazole (77.78%), nalidixic acid (74.60%), cephalothin (72.22%), chloramphenicol (66.67%), and streptomycin (53.97%). Twenty-two extended-spectrum beta-lactamase (ESBL)-producing E. coli and four colistin-resistant E. coli were identified. The resistance genes TEM, CTX-M, tetA, sul2, strA/strB, aadA, and qnrS were the most frequently detected. CTX-M-55 and CTX-M-14 were the predominant CTX-M types. All the four colistin-resistant E. coli isolates were positive for mcr-1. The mcr-1 gene can be transferred to E. coli C600 through conjugation and transformation. Whole-genome sequencing revealed that the mcr-1 genes were found in IncX4 and IncHI2 plasmids. To the best of our knowledge, this is the first report of IncHI2/IncX4 plasmid-bearing mcr-1-positive E. coli strains in RTE foods sold in markets, and the first report of the isolation of the international epidemic E. coli clone ST101 and mcr-1-carrying ESBL-producing E. coli from RTE foods. These results provide valuable information for assessing antibiotic-resistant E. coli infections and controlling antibiotic-resistant E. coli.