Not only did it gain the ability to invade cells, but a whole suite of genes was simultaneously activated. Preeti Koli and her colleagues found that the mutation of a single gene was enough to reactivate these virulence genes, transforming K-12 into a very different kind of bacterium. We generally expect unused genes to disappear or acquire many mutations over time, but neither of those seems to be the case here. Although K-12 isn’t normally virulent, there are virulence genes in its genome, a fact which has puzzled scientists. As a result of being at the center of molecular biology for nearly 40 years, it may well be the best characterized life form on the planet. In 1997, it was one of the first organisms to have its genome sequenced. It became widely used throughout molecular genetics research and is still used in many labs today (along with derivative strains). These characteristics are the reason that the pioneers of the genetic age selected K-12 for their work. We know that under a wide range of conditions and in various animals, K-12 doesn’t invade its host’s cells or behave as a pathogen in fact, the lab strain has even lost the ability to thrive in the human gut. In the 1970s, it became the workhorse for molecular biology because of its noninvasive, extracellular, benign commensal nature - in other words, because it’s a harmless bug that lives in our gut but doesn’t get into our cells. coli K-12 is a strain isolated from the human gut in 1922.
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