Cladograms of major evolutionary steps in the divergence of EPEC and EHEC clones. The two cladograms are based on the presence of the LEE at the selC (a) or pheU (b) loci. The diagrams are models of a branching order for the ancestry of the chromosomal backgrounds or clonal frames inferred from multilocus analysis. Branch lengths are arbitrary and not set to an evolutionary scale. Points of acquisition of principal virulence factors that define EPEC and EHEC are marked on the branches. Gains and losses of genes or phenotypes are marked below branches. The circles designate ancestral nodes referred to in the text. The EPEC (EAF) plasmid has two arrows to denote the possibility that it may have been acquired multiple times, a hypothesis to account for the α and β bundlin (bfpA) alleles occurring in both EPEC groups. HPI, high pathogenicity island.