The origins of the plague go back to the Neolithic Age, the oldest finds of the causative pathogen Yersinia pestis come from human bones around 5,000 years old. In the history of the plague, the late antique Justinian plague from the sixth century and the so-called black death of the late Middle Ages are particularly prominent. They have been shown to be caused by Y. pestis and are estimated to have wiped out up to half the population in parts of Europe. While smaller, regionally limited outbreaks recurred over the centuries on different continents, a third plague pandemic occurred from the mid-19th to the early 20th century. It initially affected Asia in particular, with a focus on India, and subsequently spread globally. With around 15 million confirmed fatalities, it is one of the deadliest pandemics in human history. Even today, the plague continues to occur regionally and is almost always fatal if not treated promptly with antibiotics.
Over thousands of years, the bacterium Y. pestis has evolved into numerous related strains through both the acquisition and loss of genes. Researchers around the world are studying the evolution of Y. pestis to learn more about the causes of historical pandemics and the continuing threats posed by the plague. To this end, they are investigating in particular the genetic properties of the pathogen, which are responsible, for example, for transmission, geographical distribution and the severity of the disease. In a new research project, a research team from the Christian-Albrechts-Universität zu Kiel (CAU) and the Max Planck Institute for Evolutionary Biology in Plön (MPI-EB) analyzed ancient and modern Y. pestis genomes from a period from the Neolithic to the investigated in the modern pandemic. The researchers around Dr. Daniel Unterweger, research group leader at the MPI-EB and at the CAU, and Professors Almut Nebel and Professor Ben Krause-Kyora from the Institute for Clinical Molecular Biology (IKMB) at the CAU found that between the Middle Ages and the modern pandemic, Y. pestis developed a new must have absorbed a genetic element, the so-called YpfΦ prophage, which is related to the virulence of the pathogen, i.e. its pathogenic effect. This prophage produces a protein that is very similar to certain cytotoxins from other pathogens, such as the cholera pathogen. The researchers published their results in the Kiel Evolution Center (KEC) work at the CAU, recently together with colleagues from the University of Southern Denmark in Odense (SDU) in der Fachzeitschrift Proceedings of the Royal Society B: Biological Sciences.
New genetic elements increased virulence of the pathogen
The Kiel research team obtained the genetic samples thanks to a collaboration with the SDU’s Department of Forensic Medicine, which manages skeletal material from various Danish museums. In this specific case, the scientists examined the remains of 42 deceased who were buried in two Danish municipal cemeteries between the 11th and 16th centuries. The genetic information contained in the samples was sequenced and the Y. pestis genes contained therein compared with other known genomes from the Neolithic, the Middle Ages and modern times.
“Previous research has shown that the pathogen, in its early stages of development, lacked the genetic makeup required for effective flea transmission typical of modern-day bubonic plague. During its evolution, Y. pestis attained remarkable virulence that contributed to later outbreaks of some of the deadliest pandemics in human history,” says Dr. Joanna Bonczarowska, first author of the work, who carried out this research as part of her doctorate at the IKMB with the support of the International Max-Planck-Research School for Evolutionary Biology (IMPRS) performed.
“In our study, we show that all known Y. pestis strains before the 19th century lacked a specific genetic element, the YpfΦ prophage,” says Bonczarowska, who now works as a postdoc at the IKMB, where she is also from Cluster of Excellence »Precision Medicine in Chronic Inflammation« (PMI) is supported. The prophage was probably taken up from the environment by lateral gene transfer. This genetic information influences the virulence of the pathogen, i.e. the severity of the disease resulting from an infection. Y. pestis strains that carry the prophage have been shown to have a significantly lower lethal dose than those without YpfΦ. Thus, this incorporation of new genetic elements could represent an evolutionary advantage for Y. pestis during the modern plague pandemic.
How has the increased virulence come about since the Middle Ages?
How the prophage contributes to the increased virulence of the modern plague pathogen has not yet been researched in detail. Previous studies suggest that such new genetic information can help the pathogen infect body tissues far from the original site of infection. In search of such a mechanism, the Kiel researchers examined all proteins produced by the new DNA in question. They discovered that one of these proteins is very similar to a toxin known from other pathogens.
“The structure of this protein is similar to that of the so-called zonula occludens toxin (ZOT), which facilitates the exchange of harmful substances between infected cells and has a damaging effect on the mucous membranes, particularly in the intestinal area. This connection was first discovered in the cholera pathogen, where it is responsible for the typical gastroenteritis symptoms,” explains Bonczarowska. The Kiel researchers therefore want to investigate this ZOT-like protein in Y. pestis in more detail in the future, as it offers a plausible explanation for the increased virulence of the plague pathogen in the present and in the recent past.
Continue researching the evolution of the plague pathogen and other pathogens
Such rapid evolution of Y. pestis also contributes to the continuing threat of a pandemic. “The acquisition of new genetic elements can lead to new symptoms of the infection. These misleading signs of illness can make it more difficult to diagnose the plague in good time and thus delay the rapid treatment that is essential for survival,” emphasizes Unterweger. “In addition, some strains of the plague pathogen are already resistant to various antibiotics, which further increases the risk potential of this disease,” Unterweger continues.
An important aspect of the work are the newly discovered parallels to other bacterial species, because genetic elements that are very similar to YpfΦ have also been found in other bacteria. These findings provide clues to their future evolution towards increased virulence.
Overall, the research results underscore that investigating the historical evolution of diseases using aDNA, which goes back hundreds or thousands of years, is a major gain in knowledge for modern science and medical applications. “If we understand how the pathogen was able to increase its harmfulness in the past – sometimes through evolutionary leaps – this helps us to identify new forms of the disease and prevent future pandemics,” summarizes Krause-Kyora from the IKMB.