Yersinia pestis is a bacteria most famous for its role in the bubonic plague that devastated Europe, Asia and North Africa in the 1300s, an event known as the Black Death that killed as much as half of Europe's population. Researchers have now found the earliest known strain of this plague-causing bacteria, through genetic analysis of teeth and bone from a 5,000-year-old hunter gatherer.
This oldest known strain of Y.pestis was found in a 20 to 30-year-old man whose skeleton was excavated from present-day Latvia in the late 1800s, known as "RV 2039." Scientists sequenced the man's genome through the teeth and bone samples and tested it for bacterial and viral pathogens, discovering evidence of Y.pestis.
Through a subsequent reconstruction of the bacteria's genome and then a comparison to other ancient strains, the scientists concluded that this was indeed the oldest strain of Y.pestis ever discovered. The team's investigations revealed that this early strain was only missing a few genes from the same bacteria behind the bubonic plague, but one of those was key to its success all those years later.
"What's so surprising is that we see already in this early strain more or less the complete genetic set of Y. pestis, and only a few genes are lacking," says senior author Ben Krause-Kyora. "But even a small shift in genetic settings can have a dramatic influence on virulence."
The scientists found that the ancient strain was missing the gene that enabled fleas to act as vectors and spread the plague to human hosts. They believe it took more than 1,000 years for the bacteria to take on all the mutations required for this flea-based transmission, and therefore conclude that the illness experienced by RV 2039 was likely far milder than the medieval version.
Y.pestis was present in his bloodstream, indicating he probably died from a bacterial infection though the course of the disease was likely a slow one. People buried alongside were not infected with the plague, indicating it was unlikely to be highly contagious, with the scientists suspecting he contracted the illness through the bite of an infected rodent.
"Isolated cases of transmission from animals to people could explain the different social environments where these ancient diseased humans are discovered," says Krause-Kyora. "We see it in societies that are herders in the steppe, hunter-gatherers who are fishing, and in farmer communities – totally different social settings but always spontaneous occurrence of Y. pestis cases."
The discovery could reshape our understanding of the way infectious diseases took hold in large cities in Europe and Asia, with the low transmissibility of this early strain suggesting it mightn't have played such a big role in historic population declines. Moreover, understanding its history can also inform our understanding of the history of the human genome.
"Different pathogens and the human genome have always evolved together," says Krause-Kyora. "We know Y. pestis most likely killed half of the European population in a short time frame, so it should have a big impact on the human genome. But even before that, we see major turnover in our immune genes at the end of the Neolithic Age, and it could be that we were seeing a significant change in the pathogen landscape at that time as well."
The research was published in the journal Cell Reports.
Source: Cell Press via EurekAlert