Scientists have, for the first time, achieved a complete reconstruction of the genome responsible for the bubonic plague, which struck the Eastern Roman Empire 1,500 years ago, and is said to have been responsible for between 25-50 million deaths worldwide. The new study published in the journal Lancet Infectious Diseases, revealed that the pathogen is linked with the ‘Black Death’ pandemic that killed 50 million people in medieval Europe 800 years later, and warned that it could strike again.

Scientists have previously analysed DNA samples taken from plague victims to determine that the bubonic plague, otherwise known as the ‘Plague of Justinian’, was likely caused by the bacterium Yersinia pestis. In the new study, researchers built on this previous work and sampled DNA from the teeth of two 1,500-year-old plague victims in Bavaria. With those fragments, they reconstructed the genome of the oldest bacteria known. They confirmed that the Justinian plague was indeed caused by a strain of Yersinia pestis, and that this is the same pathogen responsible for the Black Death that struck medieval Europe.

"This is the first complete genome from one of the most significant disease events in human history," said Professor Edward Holmes, from the University of Sydney.

The Justinian Plague, which occurred in the sixth century AD, was one of the first recorded plague pandemics in the world. Named after the Byzantine emperor Justinian I, the outbreak was nearly worldwide in its scope, striking central and south Asia; north Africa and Arabia, and Europe all the way to Denmark and Ireland. It is believed that the pandemic originated from Asia and then spread to Europe along trade routes such as the Silk Road.

Holmes said one of their objectives of the study was to determine why the Justinian plague was so severe. "Was there something about the genome of these ancient pathogens that made them especially virulent, or was it the way that people lived in the past, conditions were not so good, general health wasn't as good, that made them die in higher numbers?" He added that there were some hints of gene mutations affecting virulence in the Justinian plague but said more research was required to confirm any specific mechanisms.

However, the results of the study did show that the strains of Yersinia pestis from the plague victims were distinct from those involved in the Black Death, the later pandemic which killed an estimated 60% of the European population. According to Associate professor Jeremy Austin, from the Australian Centre for Ancient DNA, this suggests that “catastrophic diseases aren't things that evolve once, and then lurk around waiting for an opportunity to reappear – they actually evolve multiple times from different ancestors,"

"What this shows is that the plague jumped into humans on several different occasions and has gone on a rampage," said Tom Gilbert, a professor at the Natural History Museum of Denmark who wrote an accompanying commentary. "That shows the jump is not that difficult to make and wasn't a wild fluke."

Researchers have warned that this means another outbreak could occur. “If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggest it could happen again,” said Dave Wagner, professor in the Centre for Microbial Genetics and Genomics at Northern Arizona University.

However, Hendrik Poinar, director of the Ancient DNA Centre at McMaster University in Canada, who led the new research, believes that it is unlikely a modern plague epidemic would be so devastating. “Plague is something that will continue to happen but modern-day antibiotics should be able to stop it," he said. The real danger would be if the plague manages to transform into an airborne version. According to Poinar, this would be much harder to snuff out.

By April Holloway