Order and Disorder during Escherichia coli Divergence

Abstract

Escherichia coli is a single species with numerous recognized roles, from lab workhorse to beneficial intestinal commensal or deadly pathogen. The extant strains have disparate lifestyles as a result of differential niche expansion since their divergence 25–40 million years ago, ten times longer than the estimated divergence between chimpanzees and humans [1,2]. Not only do these roles vary by strain (variant) of the species, but the recognition of a strain’s role in one context does not exclude radically different behaviour in another, due to differential gene expression [3]. These are organisms adapting on evolutionary and lifetime scales to myriad environments and pressures. How do these strains differ from one another and what sustains their identification as a single species? To address these questions, Touchon et al. have completely sequenced and annotated six strains of E. coli while reannotating previously sequenced strains, as discussed in this issue of PLoS Genetics [4]. Comparative genomics analyses of 20 E. coli strains and one out-group provided insights into the contributions of horizontal gene transfer (HGT) and mutation on evolution in this species. In addition, the strains were tested in a mouse model to compare their virulence.