Plaque that forms on teeth, also called calculus, can be preserved for a very long time, and traps all types of biomolecules and bacteria that are found in our mouths. An article published by Nature Genetics explores a study conducted to observe the oral microbiome on calculi of ancient human teeth and its differences from the oral microbiome that exists today. In this study, the ancient calculi showed the historical importance of gene transfer in the microbiome, which has been, and still is a rich consortium of bacteria that readily and rapidly exchange gene.

The researchers studied the teeth of four adult human skeletons from the medieval city of Dalheim, Germany, who showed signs of mild to severe periodontal disease, as well as the current teeth of nine people. Interestingly, the same bacteria, proteins, and pathogens were identified in both the ancient and modern calculus, despite the differences in oral hygiene and diet between each time period. Researchers also found evidence of antibiotic resistance genes, like efflux pumps, in many members of the ancient plaque. This means that horizontal gene transfer was clearly occurring between members of the oral microbiome, and the genes for some forms of antibiotic resistance have ancient roots. However, other genetic adaptations for antibiotic resistance against modern drugs were not identified, even those that are ubiquitous in the oral microbiome today. This should be no surprise, as modern antibiotics were not in use in the Middle Ages.

Medieval dental calculi, and fecal samples for that matter, give us a peak at what the ancient microbiome looked like. By studying it, we can learn how the human oral microbiome has adapted over time in response to changes in human behavior, diets, hygiene, and antibiotic use.