On the trail of the worst pathogen of all time: Researchers have discovered the oldest known strain of the plague bacterium in the remains of a hunter-gatherer from Latvia. The genetic studies show that this version of Yersinia pestis was comparatively less contagious and aggressive. The man was probably infected by a rodent bite around 5,000 years ago, the researchers say. The characteristics of the early plague strain also indicate that the evolutionary history of Yersinia pestis goes back around 7,000 years.
The Covid-19 pandemic hit mankind hard – but one thing is clear: other infectious diseases have raged much worse in history – above all the plague. In the 14th century in particular, the Black Death was merciless and probably killed almost half of the European population. It is now certain that the bacterium Yersinia pestis caused the various epidemics. Similar to the current case of the corona viruses, researchers are also asking themselves the question of how the responsible pathogen could develop into a plague of mankind when it comes to the plague. Findings in human remains from the Bronze Age have already shown that the history of Yersinia pestis’ development goes back a long way. But the researchers around Krause-Kyora from the University of Kiel have now come even closer to the origin of the infamous bacterium.
Surprise find in old DNA
They discovered the early strain of the pathogen in the remains of a 20 to 30-year-old man who lived in what is now Latvia around 5000 years ago. He was buried next to three other people who probably belonged to his group of hunters and gatherers. Actually, Krause-Kyora and his colleagues were initially primarily interested in the genome of these people. So they elicited remains of DNA from the teeth and bones of all four dead so that the genomes could be sequenced. In addition, they also looked for genetic traces of potential pathogens in the samples. So, to their surprise, they finally came across genetic material from Yersinia pestis in the individual named RV 2039.
The researchers then succeeded in reconstructing the genome of the bacterium, which enabled analyzes and comparisons with other known strains of the pathogen. In this way, they were able to show that the bacterium from the remains of RV 2039 is the most primitive of all Yersinia pestis strains ever discovered. According to the characteristics, it belonged to a line that originated about 7000 years ago – probably only a few hundred years after the pathogen split off from its predecessor Yersinia pseudotuberculosis. “Thanks to this finding, we can now trace the occurrence of Yersinia pestis much further back than before,” says Krause-Kyora. “It seems that we are getting very close to the origin of the bacterium.”
No killer potential yet
As can be seen from the genetic analyzes, the early strain already possessed the genetic set that identifies it as a representative of Yersinia pestis – but it still lacked crucial genetic traits associated with the virulence of later versions. Most importantly, it did not have the genetic traits that enabled the bacterium to survive in fleas, the scientists report. These bloodsuckers were the notorious little helpers of the Black Death: In the case of the medieval bubonic plague, they ensured the transmission of the disease from rats to humans. It would probably be more than a thousand years from RV 2039 until Yersinia pestis acquired all of the mutations required for flea transmission, the researchers say.
But how did the man get infected 5000 years ago and how has it fared? As Krause-Kyora and his colleagues report, the findings show that RV 2039 was very heavily infested with the bacteria and therefore most likely also died from the infection. As they explain, however, the strong infestation indicates a slow course of the disease and a comparatively low level of aggressiveness of the pathogen. In earlier studies, a high bacterial load from Yersinia pestis was associated with less intense courses. The researchers assume that it was also not a highly contagious form of the plague that spread via droplet infection. In addition to genetic clues, the fact that the three other dead from the place where they were found were apparently not sick suggests this.
The researchers conclude that the trace of a prehistoric plague pandemic is not reflected in the results. According to them, the man may have been infected directly by a rodent bite and did not subsequently spread the disease. As they finally emphasize, the study is therefore also of importance for assessing previously unclear population developments in human history. The less contagious and deadly nature of the early Y. pestis strain therefore contradicts, for example, an assumption that the pathogen could have led to population declines in Western Europe, which became apparent at the end of the Neolithic.
Source: Cell Press, technical article: Cell Reports, doi: 10.1016 / j.celrep.2021.109278