FIG. 2.

Phylogenetic trees of Shigella and related enterobacteria. (A) Classical taxonomy of Shigella spp. based on phenotypic and numerical classification. (B) Evolutionary tree of Shigella and nonpathogenic E. coli strains (EcoR strains and K-12) based on comparative genomics. Shigella spp. fall into three main clusters, wherein the traditionally classified species are intermingled. The distribution of several E. coli EcoR strains between the Shigella strains indicates that the Shigella strains evolved from multiple ancestral origins. (Adapted from references 57a and 146 with permission from the Society for General Microbiology [United Kingdom] and Elsevier, respectively.)