In a medieval cemetery in southwestern Norway, human skeletons buried centuries ago are still telling stories—not through their bones alone, but through the invisible communities of microbes that have shaped their decay. Researchers at the University of Stavanger have discovered something remarkable: well-preserved medieval bones and heavily degraded ones host entirely different microbial populations, offering the first concrete glimpse into how microbes sculpt the fate of archaeological remains over hundreds of years.
For over 150 years, scientists have understood that microbes contribute to bone degradation. Yet the specifics remained a mystery—which microbes? How do they work? A new study published in PLOS One, led by Damla Kaptan and colleagues, finally begins answering these questions by combining microscopic analysis of bone damage with genetic sequencing of the microbial communities living within the bones themselves.
The researchers examined bone samples from medieval cemeteries dating back between the 11th and 19th centuries, some originally buried outdoors and others deposited indoors beneath church naves. The pattern was clear: bones from outdoor locations showed more extensive degradation, as did older samples. But the real breakthrough came when they identified the microbial inhabitants. Well-preserved bones tended to host bacteria from the genus Streptomyces, while more degraded samples contained Lysobacter and other genera. This distinction matters because some Streptomyces and Streptosporangium bacteria produce enzymes known to break down collagen—one of bone's crucial structural components.
The team also found that microbial diversity was consistently higher in newer bones, indoor-buried remains, and better-preserved samples. Counterintuitively, this suggests that bones may maintain their integrity even while hosting diverse microbial communities—perhaps because well-preserved bones retain more nutrients and physical structures that support microbial life. In other words, a bone's survival may depend less on avoiding microbes entirely and more on hosting the right microbial balance.
Hege Ingjerd Hollund, who has spent more than fifteen years documenting the tunnels and traces that bacteria leave in buried bone under the microscope, describes the significance with genuine excitement: the team has finally been able to match those microscopic signatures with actual bacterial species. "By combining microscopy with analyses of old DNA found in the same bone sample, I've been able to give a name to potential perpetrators for the first time," Hollund explains. Finding Streptomyces in nearly all their samples suggests they may have identified key players in bone preservation processes.
The research does carry limitations. DNA degrades over time, so the team couldn't always distinguish specific bacterial species from degraded samples. Contamination from excavation, handling, and storage also poses challenges. Yet Kaptan emphasizes the larger truth: "Ancient bones are not biologically silent remains. They contain rich microbial signatures that can reveal how bones change over centuries after burial." The same mystery that has puzzled archaeologists for generations—why some bodies disappear rapidly while others remain preserved—may finally yield answers through understanding the microbial processes at work below the surface. These medieval skeletons aren't just artifacts. They're records of invisible conversations between bone and microbe, written in the very chemistry of preservation itself.