Prevalence and characterisation of CTX-M β-lactamases amongst Escherichia coli isolates from healthy food animals in China

The impact of extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae of food animal origins on human health has caught considerable attention worldwide. Intestinal Escherichia coli obtained from healthy food animals (pigs, cattle and poultry) in China were tested for the presence of ESBL genes. CTX-M-producing isolates were further characterised by pulsed-field gel electrophoresis (PFGE), phylogenetic grouping, genetic environment analysis, conjugation and plasmid replicon typing. A total of 127 of the 896 E. coli isolates showed reduced susceptibility to cefotaxime (minimal inhibitory concentration ≥ 2 μg/mL). blaCTX-M genes were detected in 111 of the 127 isolates. The most common CTX-M types were CTX-M-14 (n = 40), CTX-M-55 (n = 29) and CTX-M-65 (n = 22), followed by CTX-M-27, -15, -98, -24, -3, -102 and -104. CMY-2 was detected in two isolates. High clonal diversity was found amongst CTX-M-producing isolates. Insertion sequence ISEcp1 was observed 42 bp upstream of the start codon of all CTX-M-9 group genes, whereas the spacer region between the right inverted repeats and CTX-M-1 group genes varied from 45 bp to 127 bp. Most blaCTX-M genes were transferable by conjugation. IncFII, IncI1, IncFIB, IncN and IncA/C replicons were detected in 28, 21, 7, 5 and 1 of the 70 transconjugants carrying blaCTX-M, respectively. This study demonstrates that commensal E. coli from healthy food animals can be important reservoirs of blaCTX-M genes and may contribute to the dissemination and transfer of these β-lactamase genes throughout China.