Different IncI1 plasmids from Escherichia coli carry ISEcp1-blaCTX-M-15 associated with different Tn2-derived elements

- We sequenced 3 different IncI1 plasmids encoding the important CTX-M-15 β-lactamase.
- All three plasmids have a typical IncI1 organisation, but one is a quite novel type.
- Detailed comparative analysis reveals more subtle differences in IncI1 backbones.
- blaCTX-M-15 is inserted in different Tn2 derivatives in the same backbone region.
- Tn2- and IS26-mediated events may contribute to the spread of blaCTX-M-15.

The blaCTX-M-15 gene, encoding the globally dominant CTX-M-15 extended-spectrum β-lactamase, has generally been found in a 2.971-kb ISEcp1-blaCTX-M-15-orf477Δ transposition unit, with ISEcp1 providing a promoter. In available IncF plasmid sequences from Escherichia coli, this transposition unit interrupts a truncated copy of transposon Tn2 that lies within larger multiresistance regions. In E. coli, blaCTX-M-15 is also commonly associated with IncI1 plasmids and here three such plasmids from E. coli clinical isolates from western Sydney 2006-2007 have been sequenced. The plasmid backbones are organised similarly to those of other IncI1 plasmids, but have insertions and/or deletions and sequence differences. Each plasmid also has a different insertion carrying blaCTX-M-15. pJIE113 (IncI1 sequence type ST31) is almost identical to plasmids isolated from the 2011 E. coli O104:H4 outbreak in Europe, where the typical blaCTX-M-15 transposition unit interrupts a complete Tn2 inserted directly in the plasmid backbone. In the novel plasmid pJIE139 (ST88), ISEcp1-blaCTX-M-15-orf477Δ lies within a Tn2/3 hybrid transposon. Homologous recombination could explain movement of ISEcp1-blaCTX-M-15-orf477Δ between copies of Tn2 on IncF and IncI1 plasmids and generation of the Tn2/3 hybrid. pJIE174 (ST37) is almost identical to pESBL-12 from the Netherlands and in these plasmids blaCTX-M-15 is flanked by two copies of IS26 that truncate the transposition unit within a larger region bounded by the ends of Tn2. blaCTX-M-15 and the associated ISEcp1-derived promoter may be able to move from this structure by the actions of IS26, independently of both ISEcp1 and Tn2.