Background: CMY-2 is the most prevalent pAmpC β-lactamase, but the chromosomal bla _CMY-2 gene transfer via horizontal transmission has been seldom reported. This study aimed to describe an IS Ecp1 -mediated transposition of a chromosomal bla _CMY-2 gene from Escherichia coli  into a small endogenous ColE1-like plasmid, resulting in elevated resistance to extended-spectrum cephalosporins. 
Methods: Three ESCs-resistant ST641 E. coli  strains EC6413, EC4103 and EC5106 harbored the bla _CMY-2 gene. S1- PFGE, I-ceu  I-PFGE, Southern blotting and electroporation experiments were performed to investigate the location and transferability of bla _CMY-2. The genetic context and gene expression of bla _CMY-2 in the original isolates and the corresponding electroporants were explored by PCR mapping, primer walking strategy and RT-qPCR. 
Results: The bla _CMY-2-containing region (ISEcp1 -bla _CMY-2-∆blc -∆yggR -∆tnp1 -orf7 -orf8 -orf9 -∆tnp2 -∆hsdR ) was transposed into endogenous ColE1-like plasmid pSC137 in the process of electroporation at very low frequencies (10^–8–10^–9). The transpositions resulted in novel larger bla _CMY-2-harboring ColE1-like plasmids with size of 14,845 bp, enabling increase in MICs of 2 to 8-fold for cefotaxime, ceftiofur, and ceftazidime in recipient strains over their respective original counterparts. Transcriptional level analysis revealed that the increased bla _CMY-2 expression was correlated with elevated MIC values of cephalosporins. The bla _CMY-2 transposition unit was identical to that in a clinical isolate E. coli  TN44889 from France isolated in 2004. 
Conclusions: Our results firstly demonstrated that ISEcp1  mediated a transposition of chromosome-borne bla _CMY-2 into an endogenous ColE1-like plasmid by electroporation. Amplification of the bla _CMY-2 gene facilitates the strain adaptation to a changed environment with an elevated antibiotic pressure.
Keywords: bla _CMY-2, chromosome-borne, ColE1-like plasmid, ISEcp1 -mediated transposition, extended-spectrum cephalosporin