The DNA of the Black Death bacterium has been sequenced, a development that could allow researchers to track how it evolved to become such a killer.

The Black Death plague killed an estimated 50 million Europeans between 1347 and 1351 — about half of the population in five years. It's thought that the bacterium Yersinia pestis was the cause.

In Wednesday's online issue of the journal Nature, researchers in Hamilton, Ont., and the University of Tubingen in Germany describe its entire genome.

"We sort of built, methodologically, a time machine to go back and be able to access diseases of past human population, which is something we never could do before," said geneticist Prof. Hendrik Poinar, director of the McMaster Ancient DNA Centre. "It's sort of been a pipedream for the last many many years."

The data showed that the ancient bacterial strain is the ancestor of all modern plagues.

Comparing ancient strain

In the study, researchers analyzed over 100 skeletal remains from victims buried in the East Smithfield "plague pits" in London, located under what is now the Royal Mint.

First the scientists checked for the presence of Y. pestis in the dental pulp of five bodies and then searched for more in bones.

Next they extracted and purified 99 per cent of the microbe's DNA and compared it with modern strains.

"Our data reveal that the Black Death in medieval Europe was caused by a variant of Y. pestis that may no longer exist," the study's authors concluded. "Given the growing body of evidence implicating this bacterium as responsible for the pandemic, we believe scientific debates should now shift to addressing the genetic basis of the epidemic's unique characteristics."

'Incredibly exciting'

The researchers speculated that the Black Death wiped out so many because it was new — no one had a chance to develop an immune response against it.

Unlike today when antibiotics and sanitation exist, medieval conditions differed sharply, with crowded homes and cities and more malnourished residents.

During the experiments, scientists also analyzed DNA from control teeth taken from another site in London. No Black Death DNA was found in those samples, which the researchers said circumvents concerns over the possibility of microbial contamination from other soil-dwelling bacteria.

They also said they found one important gene that seemed to enhance the bubonic form of the disease by facilitating the spread of the bacteria and encouraging their growth in the respiratory system during cases of pneumonia.

The modern strain of the Black Death microbe is thought to be responsible for about 2,000 cases worldwide, the researchers noted.

The researchers found relatively few changes in the genome. Those changes might account for why the ancient bug was so deadly. Their next step is to see whether that's the case.

Future outbreak clues

"Getting an effectively complete genome sequence of a bacterium that lived nearly 700 years ago is incredibly exciting," said Julian Parkhill, a disease genome expert at the Wellcome Trust Sanger Institute in Britain. Parkhill was not involved in the research but has studied the bacteria.

The work could also offer important clues for future disease outbreaks, said Dr. Allison McGeer, an infectious disease specialist at Mount Sinai Hospital in Toronto.

"Once you know how bacteria make you sick then you can start building blocks of 'can we treat this better, can we prevent it more effectively?' "

The study was funded by the Canadian Institutes for Health Research, the Social Sciences and Humanities Research Council, Canada Research Chairs, the Ontario government, the Michael G. DeGroote Institute of Infectious Disease Research, the Wenner Gren Foundation, and the medical faculty at University of Tubingen.