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The mechanisms by which bone degrades in the burial environment are poorly understood, with large amounts of variation seen even within the same cemetery containing the same environmental and soil conditions. While it has been hypothesised since the 19th century that microbes, either bacterial or fungal, play an important role, the exact way in which this happens has rarely been a focal point of research. To redress this, a team from the University of Stavanger and the University of Copenhagen has carried out a preliminary study, recently published in PLOS One (https://doi.org/10.1371/journal.pone.0340244), looking at whether any perceptible patterns in microbiome variation could be associated with bone preservation or lack thereof.
In total, the team took samples from 83 individuals from six different cemeteries, all located along the south-west coast of Norway and dating to between the 11th and 19th centuries. Thin transverse sections were taken from long bones and embedded in epoxy resin before being examined using light microscopy to assess the level of bioerosion seen. To do this they used a scale called the Oxford Histological Index, which is used to quantify erosion patterns. The results were then analysed temporally (by time period), spatially (by site and by location within the cemetery), and by excavation year. In addition to the morphological analysis, the team took DNA samples from the part of the bone immediately adjacent to where they took the histological samples. These were then analysed for microbial DNA in order to determine what might be driving bone bioerosion.
The results showed that 70% of the samples displayed some level of bioerosion, with 43% showing heavy damage, while 30% showed no erosion at all. There were also a few notable patterns in the data: individuals buried in indoor environments had less bioerosion than those buried outside, while individuals from earlier time periods showed more damage. In terms of the microbial results, the team identified 30 bacterial species and one fungal one. The most prevalent genera were Streptomyces, which was found in 86% of individuals, followed by Streptosporangium and Lysobacter. In particular, Streptosporangium was most abundant in individuals who also displayed high levels of bioerosion, while Lysobacter was more prominent in those with moderate bioerosion, and Streptomyces was most common in the samples with the least amount of damage. It was observed, too, that individuals with the most microbial diversity were among the best-preserved. As the team note in their paper: ‘these results show that well-preserved bones tend to support richer and more evenly distributed microbial communities’.
Importantly, they also observed a trend towards higher microbiome diversity in museum-stored samples compared with more recently excavated bones. Because of the limited sample size, however, this pattern was not statistically significant and requires further investigation.
Even though this is a fairly small-scale study, these correlations in the data were strong enough to warrant further research. The team stress, however, that the techniques they used were unable to differentiate between inactive or dead bacteria, and so the results could be showing a cumulative microbial record instead of active communities causing bone degeneration. To extend this work further and determine if these bacteria actually play a causative role, the team suggest extending the research to analyse RNA as well, which would potentially be able to identify active bacteria. More work is also required to determine the role of fungi as, despite there being microscopic remains of fungi, the genetic analysis could confirm its presence in only one sample, which was from the genus Serpula. This study, however, has provided a crucial first step in this area of research.
Text: Kathryn Krakowka / Image: Museum of Archaeology, University of Stavanger, CC-BY 4.0
