The Black Death infamously wiped out about a third of Europe's population in the 14th century, but until now there was no firm evidence that bubonic plague was the cause.
Some researchers have suggested that the epidemic was caused by a virus such as Ebola, but an analysis of DNA from a London plague pit seems to settle the argument in favour of the "plague" bacterium Yersinia pestis.
Hendrik Poinar at McMaster University in Hamilton, Ontario, Canada, and colleagues developed a technique to look for Yersinia DNA in the bones of Black Death victims. The task was made tricky because of the possibility of contamination, Poinar says. "When we extract DNA from the skeletons, we also get DNA from their environment."
To pick out the signature of Y. pestis, Poinar's team took DNA from a modern strain and made a molecular "probe" that would bind to DNA from this type of bacterium. The team attached a magnetic chip to the probe and tested it on around 100 samples of teeth and bone excavated from a London plague pit.
Then they used a magnet to fish out the chips, which carried bacterial DNA belonging to a strain of Y. pestis unlike any known today. The DNA was not present in teeth from skeletons buried elsewhere in London before the Black Death.
The findings are the first confirmation that these Black Death victims were infected with Y. pestis, Poinar says. The evidence was strong enough to convince two team members who had previously argued that the bug was not the cause.
Tom Gilbert at the University of Copenhagen, Denmark, who was not involved with the work, is convinced too. "I've been sceptical of previous research on this topic," he says. "What makes this work stand out is its very clever approach."
Both Gilbert and Poinar reckon the technique could uncover the full genetic sequence of the bacterial strain behind the Black Death, which could help explain both why it was so virulent and how it evolved – as well as whether similarly devastating strains might appear in future.
"This technology will open up a brave new world of ancient pathogen identification," Poinar says.
Hamzelou, Jessica. 2011. "Black Death bug identified from medieval bones ". New Scientist. Posted: August 30, 2011. Available online: http://www.newscientist.com/article/dn20844-black-death-bug-identified-from-medieval-bones.html