Recent genomic evidence has shown that the contemporary strain of the Y. pestis bacterium, commonly associated with human infection, is a direct descendant of the bacterium that caused inconceivable devastation in the Middle Ages, when people were convinced the human race was at its end. Genomic structures indicate no significant discrepancies between the two strains — the ancient and the modern — forcing researchers to wonder what made Y. Pestis unthinkably lethal back in the Middle Ages.
The Black Death, one of the most devastating epidemics in our history, killed a vast population — from thirty to sixty percent — of Europeans from 1348 to 1350. Also called the Great Plague, the disease wreaked chaos, profoundly influencing Europe's religious, social and economic history to an unimaginable degree. It was commonly associated with buboes, or inflammation of lymph nodes, in the armpit, neck and groin, accompanied with purulence or pus.
With the stagnation of the expansion of medical knowledge during their time, those in the Middle Ages attributed blame of the drastic spread of the sickness to the alignment of three planets in 1345, which they believed caused a "great pestilence in the air." It was discovered in 1894 that the pathogen involved was Yersinia Pestis, a type of bacteria. Today, Y. Pestis is classified as a category A pathogen, or a bioagent that can potentially cause mass chaos and sickness in the world.
To the contrary of common misconception, Y. Pestis still infects humans today. In Sept. 2009, a professor at the University of Chicago was infected while working with a weakened strain of the bacterium in his lab. The World Health Organization (WHO) reports 1,000 to 3,000 cases of the plague in the world annually. However, the infections of modern Y. Pestis are much less severe, sparing everyone from another massive catastrophe.
Thus, the question still lingers in the scientific community: what contributed to the lethality of Y. Pestis during the Middle Ages? What made it so virulent to have felled a third of Europe's population?
Many epidemiological differences have been observed between modern Y. Pestis and the agent that caused the Black Death — simply put, the modern strain causes symptoms on a much milder scale. This has raised controversies to whether Y. Pestis was, in fact, the bacterial agent that wreaked havoc in the late 1340s.
Thankfully for the DNA sequencing technologies we utilize vastly in many fields today, the evolutionary adaptations of microbes can be analyzed and elucidated with great accuracies. 46 teeths and 53 bones were collected from the East Smithfield, which has been discovered to be a burial ground, specifically for those who were infected by the plague in the late 1340s. Through laborious work of isolating the DNA of Y. Pestis while removing human and other bacterial DNA collected over hundreds of years, researchers have found genetic architectures confirming that the plague was caused by a bacterium very closely related to Y. Pestis. A resulting phylogenic tree also indicated that the Y. Pestis strains we see today are direct descendants of the Black Death's pathogen.
A genetic comparison with the modern strain of Y. Pestis indicates no significant changes in genetic identity. This suggests that the Black Death was further worsened by the environment, including climate or social changes, such as transportation and host susceptibility. Thus, the gene of the Black Death bacterium did not have a dominant role in increasing the virulence of Y. Pestis involved in the Middle Ages.
A confluence of factors may have contributed to the Black Death in 1348, including climate, transportation and the immunity of humans. Over 660 years, our species has been exposed to countless of pathogens, changing our susceptibilities towards certain diseases. Y. Pestis is also known for residing in flees, which in turn transports the bacterium through rats. Thankfully again, the increased sanitation of our modern society has blocked many means of transport for bacteria as Y. Pestis.
These recent findings suggest examining not only the genetic contributions to lethality, but also external influences that could potentially make bacteria a destructive agent.
