How the Black Death Evolved to be So Deadly: The Bacterial Family Tree

The Black Death was one of the most devastating sicknesses to spread across Europe. Now for the first time ever, scientists have studied the Black Death bacterium's entire family tree to understand how some family members evolved to be so deadly.

The Yersinia family of bacteria has many sub species; while some of these can be deadly, others aren't. Two of the most feared members of this bacterial family are Yersinia pestis, which caused the bubonic plague or the Black Death, and Yersinia enterocolitica, which is a major cause of gastroenteritis.

In this case, the researchers sequenced 224 strains of the different bacterial family members from across the world. This allowed them to better understand how the individual strains could evolve to be harmful.

"In order to understand how an organism becomes dangerous or pathogenic, we need to understand their non-pathogenic family members to see what makes them different to the pathogenic forms," said Sandra Reuter, one of the researchers, in a news release. "Our dataset has allowed us to redefine the family structure of this unique set of bacteria and give us a full view of how an individual bacterial species can become harmful."

So what did they find? It turns out that Yersinia enterocolitica and Yersinia pestis both acquired a segment of DNA known as plasmids independently of each other. They also gained the gene ail that allowed them to be pathogenic.

"Before this study, there was uncertainty about what path these species took to become pathogenic: had they split from a shared common pathogenic ancestor? Or had they evolved independently?" said Nicholas Thomson, one of the researchers, in a news release. "What we found were signatures in their genomes which plot the evolutionary path they took."

The findings reveal how this bacterial family grew and evolved over time. While some strains became dangerous, others remained harmless. This, in particular, may help inform future studies on bacteria.

The findings are published in the journal Proceedings of the National Academy of Sciences.