The Black Death: DNA extracts of the plague bug Yersinia pestis

These are skeletons of victims of the Black Death from East Smithfield, London.

Scientists have extracted fragments of bubonic plague DNA from their teeth.

Fragments of 700-year-old DNA from the bug responsible for the Black Death have been pulled from the teeth of four plague victims buried in east London.

Scientists used the degraded strands to reconstruct the entire genetic code of the deadly bacterium. It is the first time experts have succeeded in drafting the genome of an ancient pathogen, or disease-causing agent.

The researchers found that a specific strain of the plague bug Yersinia pestis caused the pandemic that killed 100 million Europeans - between 30% and 50% of the total population - in just five years between 1347 and 1351.

Picture: Museum of London Archaeology /PA

"This is the first time a human pathogen more than a century old has ever been fully sequenced," says Johannes Krause at the University of Tübingen, Germany. 

The teams used DNA from modern Yersinia in an array that bound to similar DNA in victims' teeth. That DNA carried telltale chemical changes showing it was indeed ancient plague. Differences reported earlier between it and modern Yersinia were not confirmed.

The sequence also shows the ancient bacteria started infecting humans at the right time, between 1240 and 1340, just before the disease exploded (Nature, DOI: 10.1038/nature10549).

Yersinia has for some time been the prime suspect because some of its symptoms are similar to the Black Death. But questions were raised because modern Yersinia is a slow-spreading, rat-borne disease that is very different from the Black Death. 

Its DNA doesn't explain why. "There are almost no genetic differences between the ancient and modern Yersinia," says Krause.

He speculates that the Black Death behaved differently from modern Yersinia infection due to Europeans' total lack of previous exposure. 

Another possibility is co-infection with other pathogens, a so-called syndemic. The team hopes to learn more about the evolution of human disease by probing plague pits and other ancient samples for different pathogens.