The CTX-M-15-producing Escherichia coli diffusing clone belongs to a highly virulent B2 phylogenetic subgroup

Objectives: A clone of CTX-M-15-producing Escherichia coli has recently been reported to be spreading through Europe and Africa. The aim of this work was to thoroughly characterize this clone. Materials and methods: Representative isolates of this clone were subjected to multilocus sequence typing, O typing, virulence gene detection, adhesion assay on human cells, biofilm production assay and mouse lethality assay. Results: The clone: (i) belongs to a unique B2 phylogenetic subgroup encompassing the pyelonephritogenic diffusely adhering EC7372 strain; (ii) exhibits a specific O25b molecular subtype; (iii) is identical to the E. coli clone O25:H4-ST131 producing CTX-M-15; (iv) produces biofilm; and (v) is highly virulent in mice despite lacking classical extraintestinal pathogenicity islands (except for high pathogenicity island) and the afa/dra gene. Conclusions: The CTX-M-15-producing E. coli diffusing clone is associated with a high level of antibiotic resistance and with high virulence, showing that, under certain selective pressures, the previously observed trade-off between resistance and virulence may not apply.