Abstract
Escherichia coli O157:H7 is a human pathogen that has emerged from its less pathogenic progenitor, E. coli O55:H7, to form the EHEC 1 clade. In its emergence, E. coli O157:H7 formed three distinct clusters, each of which exists today. Sequencing and SNP analysis of Cluster 1 of this clade demonstrated constrained radiation from the cluster founder. Here we investigated the diversity of Cluster 2 strains by sequencing signature SNPs in six strains collected throughout Washington State. Our results suggest that successful Cluster 2 strains have radiated on only two branches from their founder; one of these two branches leads to Cluster 3. Constrained radiation appears to be a common theme among this pathogenic clade.