Recombination in IS26 and Tn2 in the evolution of multiresistance regions carrying blaCTX-M-15 on conjugative IncF plasmids from Escherichia coli.

CTX-M-15 now appears to be the dominant extended-spectrum β-lactamase worldwide, and a number of different factors may contribute to this success. These include associations between bla(CTX-M-15) and particular plasmids (IncF) and/or strains, such as Escherichia coli ST131, as well as the genetic contexts in which this gene is found. We previously identified bla(CTX-M-15) as the dominant ESBL gene in the western Sydney area, Australia, and found that it was carried mainly on IncF or IncI1 plasmids. Here, we have mapped the multiresistance regions of the 11 conjugative plasmids with one or more IncF replicons obtained from that survey and conducted a limited comparison of plasmid backbones. Two plasmids with only an IncFII replicon appear to be very similar to the published plasmids pC15-1a and pEK516. The remaining nine plasmids, with multiple IncF replicons, have multiresistance regions related to those of pC15-1a and pEK516, but eight contain additional modules previously found in resistance plasmids from different geographic locations that carry a variety of different resistance genes. Differences between the multiresistance regions are largely due to IS26-mediated deletions, insertions, and/or rearrangements, which can explain the observed variable associations between bla(CTX-M-15) and certain other resistance genes. We found no evidence of independent movement of bla(CTX-M-15) or of a large multiresistance region between different plasmid backbones. Instead, homologous recombination between common components, such as IS26 and Tn2, appeared to be more important in creating new multiresistance regions, and this may be coupled with recombination in plasmid backbones to reassort multiple IncF replicons as well as components of multiresistance regions.