['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.', '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.', '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.'] Does the CTX-M-15-producing Escherichia coli diffusing clone belong to a highly virulent B2 phylogenetic subgroup?