Bubonic plague: seriously bad news for the Romans (Image: NIAID-NIH/Phanie/REX)

What caused the fall of the Roman Empire? A devastating plague that struck during the reign of Emperor Justinian in 541 AD, killing a quarter of the population, seems to have landed the final blow, but the identity of the infection was a mystery.

Now sequencing of DNA taken from two skeletons buried in Bavaria, Germany, in the 6th century has uncovered the complete genome of Yersinia pestis, the bacteria also blamed for the Black Death that struck Europe in 1348. The find suggests that Y. pestis may have emerged to ravage humanity several times.

Hendrik Poinar at McMaster University in Hamilton, Canada, who led the team that sequenced the German bacteria, also helped sequence Y. pestis bacteria from Londoners killed by the Black Death. He says the new finds don’t prove Y. pestis was the sole cause of both plagues, but “make it more likely Yersinia was part of the larger story”.


Ancient cemetery

The team analysed 12 skeletons from a large Iron Age cemetery at Aschheim in Bavaria. While there were low levels of Y. pestis DNA in 10 of them, says Poinar, the teeth of two skeletons had enough to allow the team to reconstruct the entire DNA sequence of the bacteria.

Video: Ancient teeth reveal origin of the Justinian plague

The beads buried with women in the cemetery were used to date the burials – previous work had shown that fashions changed quickly in Iron Age Bavaria, fast enough that the styles can date graves to the nearest 25 years with them. Such beads date one of the two skeletons to 525-550 AD, very close to the first wave of Justinian’s plague. The position of the other grave, says Poinar, suggests a similar date.

Bavaria was outside the Roman Empire, but records of the time report that the highly contagious disease spread beyond its borders, into Persia and across the Roman frontier at the Rhine.

With the complete sequence the team can position the bacteria on the family tree of all Y. pestis taken from human infections. They were surprised to find that the German bacteria merit their own branch, with no known descendants. In contrast, the Y. pestis DNA from a mass grave dug during the 1348 plague in Spitalfields, London, suggest those bacteria are ancestors of all modern human infections worldwide.

This suggests the plagues emerged separately, and repeatedly, from the bacteria’s usual hosts, ground-dwelling rodents such as marmots. That also means, the team warns, that Y. pestis could emerge again.

Different character

But the gene sequences cannot yet explain why both plagues behaved so differently from disease caused by modern Y. pestis.

Both the plague of Justinian and the Black Death raced across Europe, as if spreading from human to human. When Y. pestis travelled worldwide in the so-called “third pandemic” at the turn of the 20th century, however, it spread slowly, and rarely from person to person. This remains the case today in places such as Madagascar where the bacteria frequently cause human disease.

It could be, says Poinar, that Y. pestis was not the sole cause of the two devastating plagues, but merely the “final straw” that killed people weakened by another, fast-spreading infection – the way bacterial pneumonia often strikes after flu. “We are looking at other, co-infecting pathogens as well,” says Poinar. “That is the million dollar question.”

The team has developed a method to screen for 1000 human pathogens at once, he says, which will allow them to search for causes of death in ancient bones comprehensively.

Journal reference: The Lancet Infectious Diseases, DOI: 10.1016/S1473-3099(13)70323-2