Researchers have for the first time mapped the genome of Yersinia pestis, the bacterium that caused the Black Death - the plague between 1347 and 1351 that killed up to 30 million people - 50% of the population of Europe at the time.
This bacterium is still around today and continues to cause cases of bubonic plague, the other name for the Black Death, though not with the same virulence or scale that it did in mediaeval times. Why? No one knows for sure.
One possibility is that the more ancient forms of the bacteria carried mutations or genetic changes that could account for their more aggressive behaviour.
To find out whether this was the case, McMaster University scientist Kirsten Bos, together with colleagues in Germany, the US and Canada, looked at bacterial DNA extracted from teeth collected from four victims buried in 1348 in a “plague pit” in what is now East Smithfield in London.
The DNA they recovered from the teeth was in quite poor condition having been broken up by the passage of time into many short fragments. But by making millions of copies of the individual pieces and then using a computer to work out which bits overlapped with each other, they were nonetheless able to reassemble - for the first time - the complete genome of the chromosome of the 14th century strain of the bacterium as well as the sequences of two smaller rings of DNA, known as plasmids, that are also present in the bug’s cells.
The big surprise for the scientists, when they compared the sequence of the 14th century Black Death strain with its milder, currently-circulating counterparts, was how few differences there were between them.
The results from the London victims also show that, by the time of the Black Death, the plague bacterium had only recently emerged in the human population – perhaps a century or so before the Black Death occurred; in fact, owing to the close relationship with strains from across the world, the scientists think the spread of the Black Death plague was the first main dissemination of the bacterium globally.
So if the Black Death strain of the bacterium is so similar to the Yersinia pestis isolated from plague sufferers today, why was the outbreak in the middle ages so devastating?
At the moment scientists don’t know. It’s clear that the bacterium is essentially the same, so something else must account for the increased past virulence. It may be that those who died historically had a genetic vulnerability to the bug, social or economic factors may have played a role. Maybe a second, different infection was co-circulating alongside the plague which boosted its transmission and virulence? Or perhaps climate or even the behaviour of the vector – the fleas on rats – that spread the disease had a role to play. Those are the questions that need to be addressed next.