Characterization of Extended-Spectrum β-Lactamase-Producing Escherichia coli Strains Isolated from Retail Foods in Shaanxi Province, China

Extended-spectrum beta-lactamase (ESBL) producing Escherichia coli strains have been reported worldwide; however, the incidence and characterization of foodborne ESBL-producing E. coli strains have been rarely reported in the People's Republic of China. Among a collection of 659 E. coli isolates recovered from retail foods in Shaanxi Province, People's Republic of China, 223 cefoxitin-resistant and/or cefoperazone-resistant isolates were screened for ESBL production with the double disk diffusion test. The ESBL-producing isolates were characterized for antimicrobial resistance and the presence of bla(TEM), bla(SHV), and bla(CTX-M) genes. Isolates with bla(CTX-M) were further classified by PCR as having bla(CTX-M-1), bla(CTX-M-2), bla(CTX-M-8), bla(CTX-M-9), or bla(CTX-M-25). One hundred forty-seven isolates were identified as ESBL positive. PCR detection revealed that 146 isolates (99.3%) contained the bla(CTX-M) gene. Among these isolates, 42 (28.8%) were positive for the enzyme CTX-M-1, 5 (3.4%) for CTX-M-2, and 99 (67.8%) for CTX-M-9. No CTX-M-8 and CTX-M-25 were found in this study. One hundred fifteen isolates (78.2%) were positive for the bla(TEM) gene, but bla(SHV) was not detected. Among the 147 ESBL-producing a coli isolates, 75 (51.0%), 35 (23.8%), and 4 (2.7%) isolates were positive for bla(TEM) and bla(CTX-M-9), bla(TEM) and bla(CTX-M-1), and bla(TEM) and bla(CTX-M-2), respectively. All of the 147 ESBL-producing isolates were resistant to three or more non-beta-lactam antibiotics. This study provides evidence that foodborne E. coli can harbor ESBL-encoding genes. Thus, food could be a vehicle for the dissemination of ESBL-producing E. coli strains, a situation that requires surveillance and appropriate management strategies.