Scientists sequence black plague DNA, admit previously they may have been looking at the wrong bacteria

image

Studying the bacteria that caused the Black Plague isn’t as easy as one might think. Scientists can study the bones of plague victims, but the bacteria that caused the plague is a normal naturally-occurring bacteria that has lived in dirt for millennia and still lives in dirt. So trying to trace how the plague evolved is a bit tricky, but scientists now think they’ve pinned down the real culprit.

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.

To figure out the degree of contamination, each of the sets of bones were used to search for the DNA of human mitochondria, which should provide a degree of information about the levels of damage and contamination. Some of the samples contained no DNA, but a significant number did. Contamination levels were manageable, and the sequence had the sorts of changes that are typical for older, damaged DNA. With the samples looking good, the authors turned to sequencing the Y. pestis plasmid DNA.

As an added layer of precaution, they sent their samples to two different sequencing facilities, neither of which had previously handled Y. pestis DNA. It was relatively easy to pick up plasmid DNA from the tooth samples, which the authors explain as being derived from the pulp, which is rich in the blood vessels that the bacteria prefer to inhabit. The DNA they obtained also showed indications of the same changes seen in the human sequence, indicating it was also likely to be old and have suffered the same damage over time. With sufficient sequences (they got as many as 37,000 from a single sample), they were able to reconstruct the sequence of the plasmid.

For the most part, the sequence is similar to that found in modern strains of the bacteria. In contrast, some of the DNA they obtained from the bacteria’s chromosome showed some distinct differences, none of which are present in modern strains. Nevertheless, the sequence was clearly still from Y. pestis.

The authors conclude that this provides a clear indication that a single type of bacteria has been responsible for the Black Death and several other plague outbreaks, and is still causing modern diseases.

Read more here

comments powered by Disqus

Notes

  1. iheartchaos posted this
ihc featured sites
most popular tags