Molecular Characterization of Escherichia coli Strains Isolated from Retail Meat That Harbor bla_CTX-M and fosA3 Genes

A total of 55 cefotaxime-resistant Escherichia coli isolates were obtained from retail meat products purchased in Shenzhen, China, during the period November 2012 to May 2013. Thirty-seven of these 55 isolates were found to harbor a bla_CTX-M gene, with the bla_CTX-M-1 group being the most common type. bla_CMY-2 was detected in 16 isolates, alone or in combination with other extended-spectrum β-lactamase (ESBL) determinants. Importantly, the fosA3 gene, which encodes fosfomycin resistance, was detected in 12 isolates, with several being found to reside in the conjugative plasmid that harbored the bla_CTX-M gene. The insertion sequence IS26 was observed upstream of some of the bla_CTX-M-55 and fosA3 genes. Conjugation experiments showed that bla_CTX-M genes from 15 isolates were transferrable, with Inc I1 and Inc FII being the most prevalent replicons. High clonal diversity was observed among the bla_CTX-M producers, suggesting that horizontal transfer of the bla_CTX-M genes among E. coli strains in retail meats is a common event and that such strains may constitute an important reservoir of bla_CTX-M genes, which may be readily disseminated to other potential human pathogens.