Shigella loses genes at a very high rate

Shigella strains are ecotypes of E.coli and were only given a separate name because all Shigella strains cause a distinct disease (dysentery). Ruth Hershberg led a study (just published in Genome Biology) that demonstrated that Shigella strains lose genes at much higher rates than other E. coli strains and that this is largely due to a genome-wide reduction in the strength of purifying selection. This reduction in the strength of selection might be a result of the different lifestyle of Shigella strains compared to other E. coli strains.