October 2010


Unique Strains of Yersinia pestis Caused the Black Death

Anyone who has studied the history of medieval Europe surely knows of the Black Death, the extremely deadly disease that killed so many people in the mid-fourteenth century (and beyond). This plague is commonly attributed to the bacteria Yersinia pestis, but there's plenty of reason to question this assumption.

For example, Mark Welford and Brian Bossak (Georgia Southern University) recently reported that the epidemiology of the Black Death doesn't match that of modern Yersinia pestis outbreaks. Their conclusion was that the causative agent of the Black Death was likely either genetically distinct from modern variants of the bacteria, or was something else entirely.

In the blog post previously cited, I lamented that a modern biology lab could not be sent back in time to answer this question. As it turns out, scientists have recently done something very similar.

Barbara Bramanti (Johannes Gutenberg University, Germany) and coworkers have analyzed the skeletons of past victims of the Black Death to identify the causative lethal agent. Based on DNA and protein analysis, they conclude that the Black Death was caused by at least two different strains of Yersinia pestis which may now be extinct, a finding that explains the anomalous epidemiology results of Welford and Bossak.

Biochemical analysis.

The scientists extracted DNA and proteins from the teeth or bones of 76 skeletons buried in ancient plague pits, dating from the fourteenth to seventeenth centuries, in five European nations. The controls in this case were from skeletons of people who died either before or after the Black Death, due to something else.

They went to great effort to avoid contamination of their samples. This was confirmed by a number of control experiments and observations, including but not limited to their inability to detect Yersinia pestis DNA in skeletons buried in the same cemeteries as past Black Death victims, but during a time of no such bacterial outbreak based on available archaeological evidence.

The cause of the Black Death.

The scientists found genes unique to Yersinia pestis in the skeletal remains of Black Death victims, and proteins unique to the bacteria when genetic analysis was unsuccessful, confirming bacterial identification. Similarly to the genetic analysis, these unique proteins were not found in skeletal remains of people who did not die of the Black Death.

Perhaps most importantly, unique elements of the primary gene of interest (pla, specific to Yersinia pestis) were absent in the DNA extracted from the skeletal remains. Furthermore, at least two subsets of the bacteria can be detected in the remains, distinct from modern strains.

What these results clearly indicate is that Yersinia pestis -- of multiple, distinct lineages that may now be extinct -- was the cause of the Black Death. However, to my knowledge, such experiments on long-expired remains is still unlikely to uncover the biochemistry underlying its distinct epidemiology, which if known may provide biochemical insight into its extreme virulence.

NOTE: The scientists' research was funded by the Deutsche Forschungsgemeinschaft, the University of Mainz, and the Science Foundation of Ireland. Additional support was provided by the Compagnia di San Paolo.

Haensch, S., Bianucci, R., Signoli, M., Rajerison, M., Schultz, M., Kacki, S., Vermunt, M., Weston, D. A., Hurst, D., Achtman, M., Carniel, E., & Bramanti, B. (2010). Distinct Clones of Yersinia pestis Caused the Black Death PLoS Pathogens, 6 (10) DOI: 10.1371/journal.ppat.1001134