Black Death-causing bacterium′s DNA sequenced | Science| In-depth reporting on science and technology | DW | 13.10.2011
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Science

Black Death-causing bacterium's DNA sequenced

Scientists say the Black Death hasn't changed much since the 14th century, but humans have. They examined human remains more than 700 years old to extract the needed genetic material.

14th-century Black Death skeletons

The scientists pulled teeth from 14th century skeletons

Most students of European history know that the "Black Death," swept across Europe in the 14th century, killing between one-third and half of the European population by the middle of that century.

Nearly 700 years later, researchers have conclusively shown that a known bacterium was the definitive origin of that plague, and that its genetic descendant that exists today has only changed very slightly.

In a scientific paper published on Wednesday in the journal Nature, a team of researchers from Germany, Canada, the United States and Colombia sequenced the genome of Yersinia pestis, the bacterium that caused the plague nearly 700 years ago.

DNA

Findings showed only very slight genetic differences between the 14th century sample and modern Y. pestis

Fortunately though, modern antibiotics and improved hygiene standards have made it harder for most humans to succumb to the fatal disease.

"In order to understand why [the Black Death was so virulent], one can compare this ancient genome with that of the modern plague," said Hernan Burbano, a researcher at the Max Planck Institute for Evolutionary Anthropology in Leipzig, and one of the co-authors of the paper, in an interview with Deutsche Welle.

"This is Y. pestis, but this is a disease that is not as severe as the Black Death. You can try to infer what happened in the past. But the best way is not to do inferences, but to sequence it."

Genetically speaking, the ancient sample is only different in 97 DNA letters out of the entire 4.6 million for the entire bacterium’s genome - a tiny fraction.

Dental extraction

For years, scientists have debated whether or not Y. pestis was in fact the Black Death’s original infection source.

Solving that problem was one of the goals of this research, Jonathan Fletcher, a senior lecturer in microbiology at the University of Bradford in the United Kingdom, who was not part of the study.

The Spanish flu

The Spanish flu was a deadly infection in the early 20th century

If Y. pestis was the infection source of those who died in the Black Death, then it was unlike Y. pestis today, Fletcher said.

"The analogy with that is the Spanish influenza outbreak in 1918 where 20 million people across Europe died. But influenza as we know it today doesn’t kill two million people in the space of two years. So, the other reason is to look at the genetic strain, is there anything about that? Can we say that this is different from Y. pestis today?"

This new study differed from earlier ones as it examined previously exhumed skeletons from the East Smithfield cemetery in east London, a well-known site of Black Plague victims. The scientists took samples from intact teeth in these skeletons to get at dried blood at their bottom, or the pulp of the tooth, in hopes to find some genetic material of the bacterium that caused the 14th century plague.

The teeth were then returned to their respective skeletons following the extraction.

An encouraging find

Other scientists in the field who have also studied Y. pestis and the Black Death have been encouraged by the new paper.

"Understanding the evolution of pathogens over time allows us to see how the pathogen has changed and adapted to humans," wrote Julian Parkhill, a pathogen genomics researcher at the Wellcome Trust Sanger Institute in Hinxton, in the United Kingdom, in an e-mail sent to Deutsche Welle. He was not part of the study.

"This should tell us about which genes may be involved in interactions with the human host," Parkhill added.

Scientists also say there is little risk that the Black Death could return with such deadly force as once happened many centuries ago, given that modern science has come up with sophisticated techniques to detect, treat and analyze such infections.

"Pathogens can evolve, and become more or less pathogenic," he said. "I don’t think that we’re going to have a new black death, but there are pathogens evolving all the time. It’s difficult to forsee."

Author: Cyrus Farivar
Editor: Stuart Tiffen

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