Parallel evolution of virulence in pathogenic Escherichia coli

The mechanisms underlying the evolution and emergence of new bacterial path ogens are not well understood. To elucidate the evolution of pathogenic Esc herichia coli strains, here we sequenced seven housekeeping genes to build a phylogenetic tree and trace the history of the acquisition of virulence g enes. Compatibility analysis indicates that more than 70% of the informativ e sites agree with a single phylogeny, suggesting that recombination has no t completely obscured the remnants of ancestral chromosomes(1-3). On the ba sis of the rate of synonymous substitution for E. coli and Salmonella enter ica (4.7 x 10(-9) per site per year(3)), the radiation of clones began abou t 9 million years ago and the highly virulent pathogen responsible for epid emics of food poisoning, E. coli O157:H7, separated from a common ancestor of E. coli K-12 as long as 4.5 million years ago. Phylogenetic analysis rev eals that old lineages of E. coli have acquired the same virulence factors in parallel, including a pathogenicity island involved in intestinal adhesi on, a plasmid-borne haemolysin, and phage-encoded Shiga toxins. Such parall el evolution indicates that natural selection has favoured an ordered acqui sition of genes and the progressive build-up of molecular mechanisms that i ncrease virulence.