IS-mediated transposition of chromosome-borne into an endogenous ColE1-like plasmid in .

Background: CMY-2 is the most prevalent pAmpC β-lactamase, but the chromosomal gene transfer via horizontal transmission has been seldom reported. This study aimed to describe an IS-mediated transposition of a chromosomal gene from into a small endogenous ColE1-like plasmid, resulting in elevated resistance to extended-spectrum cephalosporins.

Methods: Three ESCs-resistant ST641 strains EC6413, EC4103 and EC5106 harbored the gene. S1-PFGE, I- I-PFGE, Southern blotting and electroporation experiments were performed to investigate the location and transferability of . The genetic context and gene expression of in the original isolates and the corresponding electroporants were explored by PCR mapping, primer walking strategy and RT-qPCR.

Results: The -containing region (IS--∆-∆-∆-∆-∆) was transposed into endogenous ColE1-like plasmid pSC137 in the process of electroporation at very low frequencies (10-10). The transpositions resulted in novel larger -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 expression was correlated with elevated MIC values of cephalosporins. The transposition unit was identical to that in a clinical isolate TN44889 from France isolated in 2004.

Conclusions: Our results firstly demonstrated that IS mediated a transposition of chromosome-borne into an endogenous ColE1-like plasmid by electroporation. Amplification of the gene facilitates the strain adaptation to a changed environment with an elevated antibiotic pressure.