A University of Wisconsin-Madison researcher helps to unlock the genetic blueprint of ancient bacteria that commonly afflicted pregnant women.
DNA helps show cause of death
As archaeologists excavating a cemetery near Troy, the fabled city from Homer’s Iliad, carefully removed the skeleton of a woman buried eight centuries ago, they noticed two chalk-like strawberry-sized nodules near her ribs.
The unusual discovery led to a University of Wisconsin-Madison researcher helping to unlock the genetic blueprint of ancient bacteria that commonly afflicted pregnant women, and most likely killed the woman whose skeleton was found in the late Byzantineera graveyard.
In an article published Tuesday in the journal eLife, Caitlin Pepperell and other researchers reveal the molecular portrait of a fatal infection. The woman, estimated to be about 30 years old, likely died of a bacterial infection of the placenta, amniotic fluid and membranes surrounding her male fetus.
With practically no archaeological evidence of human maternal health and death, finding a case of maternal sepsis is an incredible discovery.
“I see patients in the hospital as an infectious disease specialist, but there was a medical detective aspect to this story,” said Pepperell, an assistant professor of medicine and medical microbiology as well as an expert on the evolution of pathogens. “I was thinking of her as a physician” as well as a researcher.
Though it’s rare to tease out more than a tiny bit of DNA from ancient material, scientists were shocked to get a very large sample of wellpreserved genetics from the nodules that would have formed when the woman was still alive. Because she was pregnant and a lot of calcium was flowing to her fetus, the bacterial DNA was preserved through calcification that formed the nodules. In a way, Pepperell said, tiny suitcases were created to hold the DNA.
At first, because of the nodules’ location, the German archaeologist curating the skeleton, Henrike Kiesewetter, thought she might have suffered tuberculosis. Kiesewetter sent the nodules to William Aylward, a UW classics professor and expert on the Trojan War. Aylward contacted Pepperell because of her expertise with tuberculosis. She, in turn, sought the help of Hendrik Poinar, an ancient DNA expert at McMaster University in Canada whose lab extracted the DNA.
They determined it wasn’t tuberculosis or kidney stones. Instead, they found ghost cells within the two nodules with DNA of common bacterial pathogens Staphylococcus saprophyticus and Gardnerella vaginalis. While the Gardnerella vaginalis sample is the same as modern day bacteria, the Staphylococcus saprophyticus that afflicted the woman who died 800 years ago is more closely aligned with strains found in livestock — perhaps not surprising since peasants of that era typically lived closely with their animals.
“What’s fascinating is that we learned about Staphylococcus saprophyticus by comparing it to modern data and it seems to be a very fluid organism. It can cause urinary tract infections but also get into the environment,” Pepperell said. “It seems really adept at moving between environments and pretty diverse environments.”
Why go to all this trouble? As bacteria and viruses evolve and sicken humans, one way to determine how that happens is by studying ancient bacteria samples and using DNA sequencing to see how their modern counterparts have changed over time to resist antibiotics created to kill them.
“Bacteria become pathogens and cause disease because they figure out how to deal with our immune systems,” Pepperell said.
A contemporary example is the Zika virus, which has been linked to women giving birth to babies with microcephalitis.
Though it was tempting to call her Helen, researchers usually referred to her as the woman of Troy, Pepperell said.
The woman’s life was difficult — she had lost her teeth, suffered from chronic infection of her bones and like many females of childbearing age, died from complications during pregnancy. More than half of the skeletons found in the cemetery show signs of spine and joint degeneration, probably from hard physical labor.
“I thought about what a short difficult life it must have been. On a personal scale I would say I feel a lot of gratitude to her for allowing us to learn so much,” Pepperell said.