Via: Ars Technica:
The bacteria behind the Black Death has a very unusual history. Its ancestor is an unassuming soil bacterium and the current strains of Yersinia pestis still infects thousands of people annually, but no longer cause the suite of horrifying symptoms associated with the medieval plagues. The radical differences between the two versions, in fact, led some to suggest that we have been blaming the wrong bacteria. Now, researchers have obtained DNA from some of London’s plague victims and confirmed that Y. pestis appears to be to blame. But the sequences also suggest that the strains of bacteria we see today may be different from the ones that rampaged through Europe.
What transformed soil bacteria into a human pathogen? One key event seems to have been the fact that it picked up a plasmid, a short, circular piece of DNA that can be copied separately from the rest of the organism’s DNA. In the case of Y. pestis, that plasmid contained three key genes: two that helped it kill off competing bacteria, and a third that helped it manipulate the human blood clotting system. So, when presented with the opportunity to obtain the DNA of plague victims, this is the DNA the authors decided to target.
The DNA came from 53 bones and 46 teeth from the East Smithfield, a mass burial site in London that dates to the first appearance of the Black Death in Europe, from 1347-1351. This is a key resource, since the different waves of plague that swept through Europe had somewhat different behavior, suggesting that Y. pestis was already adapting to its human hosts. To serve as controls, the authors obtained bones from a set of 10 human remains that predate the appearance of the plague.
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