Clinical takeaways
- Calculus, although a nuisance, is also a time capsule offering a rich picture of humans’ oral health history.
- Methanobrevibacter oralis, the archaeon associated with periodontal disease, appears in ancient calculus samples, which raises the question of how modern diets and hygiene practices have altered our microbiome.
- Regional differences tied to diet, cultural practices, and regional variation offer clues about patients’ oral microbiomes.
Clinicians see the negative effects of calculus: bad breath, inflammation, and tooth loss. But there may be an upside to tartar: It offers significant clues about human history. A study published in Scientific Reports by researchers at Toho University, the University of Tokyo, and Kyushu University used DNA extracted from ancient dental calculus to reconstruct oral microbiomes spanning centuries of Japanese history, finding meaningful differences between past and present populations.
The researchers analyzed calculus samples from human skeletal remains excavated from sites in Tokyo, Saitama, Yamanashi, Fukuoka, and Okinawa, primarily from the Edo period (1603-1868) and then compared the samples with modern dental calculus. Using existing oral microbiome databases to exclude soil contamination, the researchers assessed the microbial composition, functional gene profiles, and phylogenetic relationships (the study of the evolutionary relatedness among groups of organisms) across time periods, including Jomon-era samples.
Key findings for clinicians
The oral microbiome of Edo-period individuals differed substantially from that of modern individuals. Notably, Methanobrevibacter oralis (M. oralis), an archaeon associated with periodontal disease, was frequently found in ancient samples. Regional variation also emerged, with microbiome differences observed between samples from Honshu-Kyushu and those from Okinawa, suggesting that local diet and environment shape oral microbial communities.
Phylogenetic comparison of Jomon-period and Edo-period samples revealed further differences among several oral bacterial species, which the researchers said is consistent with changes in human mobility, agricultural practices, and diet over time.
The ohaguro connection
Perhaps the most intriguing finding from the study involves ohaguro, the traditional Edo-period practice of tooth blackening common among married women, which involved applying iron-containing and plant-derived substances to the teeth.
Phylogenetic analysis of M. oralis identified two major lineages -- clade A and clade B -- and all females with documented ohaguro traces belonged to the same clade. The researchers also found lineage-specific variants in metabolic genes related to iron utilization, raising the possibility that the altered oral chemical environment created by ohaguro may have favored particular microbial strains.
The authors cautioned that confirming this link will require systematic documentation of ohaguro alongside chemical analysis of calculus samples.
