Liang-Xing Fang,1,2,* Xing-Ping Li,1,2,* Liang Li,1,2 Mu-Ya Chen,1,2 Cai-Yan Wu,3 Lu-Lu Li,4 Xiao-Ping Liao,1,2 Ya-Hong Liu,1,2 Jian Sun1,2 1National Risk Assessment Laboratory for Antimicrobial Resistance of Animal Original Bacteria, South China Agricultural University, Guangzhou, People’s Republic of China; 2Guangdong Provincial Key Laboratory of Veterinary Pharmaceutics Development and Safety Evaluation, South China Agricultural University, Guangzhou, People’s Republic of China; 3Institute of Animal Health, Guangdong Academy of Agricultural Sciences, Guangzhou, People’s Republic of China; 4Institute of Animal Science and Veterinary Medicine, Shandong Academy of Agricultural Sciences, Jinan, People’s Republic of China *These authors contributed equally to this work Background: CMY-2 is the most prevalent pAmpC β-lactamase, but the chromosomal blaCMY-2 gene transfer via horizontal transmission has been seldom reported. This study aimed to describe an ISEcp1-mediated transposition of a chromosomal blaCMY-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 blaCMY-2 gene. S1- PFGE, I-ceu I-PFGE, Southern blotting and electroporation experiments were performed to investigate the location and transferability of blaCMY-2. The genetic context and gene expression of blaCMY-2 in the original isolates and the corresponding electroporants were explored by PCR mapping, primer walking strategy and RT-qPCR. Results: The blaCMY-2-containing region (ISEcp1-blaCMY-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 blaCMY-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 blaCMY-2 expression was correlated with elevated MIC values of cephalosporins. The blaCMY-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 blaCMY-2 into an endogenous ColE1-like plasmid by electroporation. Amplification of the blaCMY-2 gene facilitates the strain adaptation to a changed environment with an elevated antibiotic pressure. Keywords: blaCMY-2, chromosome-borne, ColE1-like plasmid, ISEcp1-mediated transposition, extended-spectrum cephalosporin
