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

Abstract

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.