Figure 1

Phylogenetic tree representing E. coli and Shigella strains. The tree was built based on an alignment of the concatenated sequences of 100 genes that were selected at random from a list of 1,214 E. coli K12 genes that are present and can be fully aligned in all the pathogenic E. coli strains, the Shigella strains and in S. typhimurium. Bootstrap analysis was carried out with 100 replicas. The number of replicas that agree with each branch assignment are indicated. Branches of the tree that lead towards a pathogenic E. coli strain are colored blue. Branches of the tree that lead towards a Shigella strain are colored red. If a node in the tree is a direct ancestor only of pathogenic E. coli strains, it was considered to be a pathogenic E. coli strain itself. Similarly, if a node is a direct ancestor only of Shigella strains it is considered to be a Shigella. Under these assumptions, it is not clear whether the direct ancestor of the EHEC strains and of S. dysenteriae should be classified as a pathogenic E. coli or rather as a Shigella. The branch connecting this group to its ancestor is colored yellow.