ISEcp1-mediated transposition of chromosome-borne bla(CMY-2) into an endogenous ColE1-like plasmid in Escherichia coli

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 ISEcp1-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.