News in Science

Gene tweak turned gastro bug into Black Death killer

Tracing the plague The bacterium Yersinia pestis has inflicted almost unimaginable misery upon humankind over the centuries, killing an estimated 200 million or more people and triggering horrific plagues in the 6th and 14th centuries.

Sometime in the last 10,000 years it evolved from minor genetic changes to a more benign pathogen that caused a mild gastrointestinal infection.

The transition occurred through mutations in a single gene, report scientists in the journal Nature Communications.

The bacterium causes different types of plague including bubonic, septicemic and the most deadly and infectious form, pneumonic plague.

The researchers conducted mouse experiments that retraced the fateful genetic change in the bacterium.

They took an ancestral form of the bacterium that still circulates in the wild -- isolated in a rodent called a vole from Asia -- and inserted into it a gene called Pla, which is involved in breaking down blood clots. This addition empowered the bacterium to produce a fatal lung infection.

The results indicate that addition of the gene long ago transformed the gastro pathogen Yersinia pseudotuberculosis into Yersinia pestis, which causes pneumonic plague.

They also found that a single mutation of the same gene -- a mutation present in modern strains of the bacterium -- enabled it to spread in the body and invade the lymph nodes as occurs in bubonic plague.

Yersinia pestis caused two of the deadliest pandemics in human history: the 6th century Justinian Plague, named for the Byzantine emperor who was sickened but survived, and the 14th century Black Death. Rats with fleas carrying the germ spread the plague to people.

The Justinian Plague is estimated to have killed 25 million to 50 million people and the Black Death at least 150 million people, says microbiologist Wyndham Lathem of Northwestern University's Feinberg School of Medicine in Chicago, who led the study.

"It's just remarkable what Yersinia pestis has done to the course of human civilisation," says Lathem.

It is hard to know with certainty when the bacterium, which has gained and lost various genes over time, added the Pla gene, he says, but "it's certainly likely to have occurred at least more than 1500 years ago."

That would mean it could have occurred in the century before the Justinian Plague.

"That's something to keep in mind when we're studying other bacterial pathogens," Lathem adds. "A small change is all that's needed and suddenly we may be faced with a new pandemic of some sort."

Plague is still endemic in many tropical and sub-tropical countries of the world, in particular Madagascar, the Democratic Republic of Congo and Peru.

Dr Karl: Black death

Related: Black death burial site uncovered