Abstract: To investigate the level of Escherichia coli contamination in frozen foods in Shaanxi province, China, and analyze the potential food safety issues caused by it, a total of 360 frozen food samples (120 frozen dumpling samples and 240 ice cream samples) were collected from four cities (Baoji, Xianyang, Xi’an and Weinan) in Shaanxi province. E. coli strains were isolated and identified by selective culture and polymerase chain reaction (PCR). The 21 virulence genes, 23 antibiotic resistance genes, resistance to 15 common antibiotics and phylogenetic groups of the isolates were investigated. Total 13.6% (49/360) of the 360 samples were contaminated with E. coli, of which five samples (10.2%, 5/49) had coliform counts > 100 CFU/g. Nine virulence genes were detected, among which the detection rate of the extraintestinal pathogenic E. coli-related genes FyuA and iss was the highest (14.3%, 7/49 each). The results of drug susceptibility test showed that the isolates were most commonly resistant to β-lactams and folate metabolism inhibitors (98.0%, 48/49 each), followed by tetracyclines (20.4%, 10/49). The major coding gene of β-lactams was blaCTX and the major coding gene of tetracyclines was tetA. This study also identified one isolate carrying the polymyxin resistance gene mcr-9. Moreover, 20.4% of the strains were multi-drug resistant, and one of them was resistant to up to 13 antibiotics. Four phylogenetic groups (A, B1, C, and F) were detected in all isolates, and group A was the dominant one. The results indicate that there is a risk of the contamination of pathogenic E. coli and multidrug-resistant strains in frozen foods and their transmission to humans through the food chain in Shaanxi, China.

Key words: frozen food; Escherichia coli; virulence gene; phylogenetic groups; antibiotic resistance