Genomic Characteristics of mcr-1and blaCTX-M-typein a Single Multidrug-Resistant Escherichia coliST93 from Chicken in China

This study was undertaken to discern the transmission characteristics of mcr-1and blaCTX-M-typein one multidrug-resistant Escherichia coliLWY24 from chicken in China. The genetic profiles of LWY24 isolate were determined by conjugation, S1-pulsed-field gel electrophoresis, southern blot hybridization and whole genome sequencing analysis. Meanwhile, co-transfer of plasmids in LWY24 isolate was screened by dual conjugation assays. The LWY24 isolate was identified as ST93, and harbored three conjugative plasmids, pLWY24J-3 (blaCTX-M-55-bearing IncFⅡ), pLWY24J-mcr-1(mcr-1-carrying IncI2) and pLWY24J-4 (non-resistance-conferring IncI1), and one non-conjugative plasmid pLWY24 (blaCTX-M-14-containing IncHI2/IncHI2A). Numerous resistance genes, insertion sequences (especially IS26), and transposons were found in the four plasmids, suggesting that horizontal transmission have occurred by plasmid mating, homologous recombination and transpositions. Under the selection pressure of cefotaxime and colistin or cefotaxime alone, the mcr-1-bearing plasmid and the blaCTX-M-55-harboring plasmid could be co-transferred at a similar frequency, with 8.00×10-4or 9.00×10-4transconjugants per donor cell, respectively. The specific shufflon region in mcr-1-encoding plasmid could generate up to six diverse PilV structures, which may further accelerate the horizontal transfer of plasmid. In conclusion, the transmission characteristics of mcr-1and blaCTX-M-typein LWY24 isolate could due to clonal spread of ST93, selective pressure of cefotaxime, IS26-mediate homologous recombination and transposition, and the specific shufflon region.