Figure 4. Phylogenetic relationships of 17 locus of enterocyte effacement (LEE)–negative and 13 LEE-positive Shiga toxin–producing Escherichia coli (STEC) strains (highlighted in gray) compared with a cohort of reference E. strains. Phylogeny was demonstrated by a neighbor-joining algorithm from 7 housekeeping gene sequences. Each isolate has been assigned a sequence type (ST) (in boldface), and assigned clonal groups (CGs) are displayed. The scale bar demonstrates the branch length that corresponds to 2 nucleotide substitutions per 1,000 nucleotide sites. Significant nodes were identified by bootstrapping (Monte Carlo randomization); nodes were present in >70% of the 1,000 bootstrap trees highlighted and identified as significant.