Our Inherited Oral Microbiome
Our mouths today are home to a diverse community of bacteria known as the oral microbiome. Some of these microscopic guests are beneficial, while others can contribute to cavities and gum disease if allowed to overgrow. But how did humans acquire these particular microbes, and how have they evolved over our species’ history?
Unlocking Ancient Dental Plaque Secrets
A fascinating new study published in Molecular Biology and Evolution1 provides some answers by analyzing bacteria preserved in 4,000-year-old human teeth from an ancient cave in Ireland. And the insights reveal just how much our oral microbiome has been shaped by major dietary shifts over the millennia.
The Ancestral Origins of Tooth Decay
The researchers made two groundbreaking discoveries by retrieving and analyzing ancient bacterial genomes from the teeth. First, they uncovered the oldest genome yet identified of Streptococcus mutans – the primary culprit behind tooth decay and cavities in modern mouths.
Remarkably, this ancient S. mutans genome came from a tooth that showed no visible signs of decay. This suggests the bacteria existed at high levels long before the modern caries epidemic, which only started after our diets grew richer in sugar and carbohydrates able to nourish S. mutans’ tooth-decaying lifestyle.
Evolving to Exploit Dietary Changes
Further analysis hints that S. mutans strains only acquired key tooth-decaying capabilities like acid resistance and enamel adhesion factors in more recent times, coinciding with the popularization of sugar. Some deep ancestral lineages of S. mutans devoid of these factors still persist today across diverse populations.
Divergent Strains of Gum Disease Bacteria
The second key finding was that the two teeth sampled, which belonged to the same ancient individual, harbored distinct strains of another oral bacterium called Tannerella forsythia. T. forsythia is one of the major microbial players in gum disease and periodontitis.
Strikingly, these two ~4,000-year-old T. forsythia strains were more divergent from each other than any pair of modern strains. This points to much greater diversity of T. forsythia in prehistoric human populations – diversity that has been largely lost over time.
Tracing Microbial Evolutionary Impacts
By comparing the ancient genomes to modern bacteria, the researchers were able to reconstruct how S. mutans and T. forsythia evolution was impacted by major dietary revolutions. Both species underwent expansions and turnovers in their gene repertoires, including key shifts in virulence factors, in the last 750 years as sugars and refined carbohydrates became more widespread.
For T. forsythia, the turnover was particularly dramatic, with most prehistoric diversity going extinct and functionally divergent modern strains arising. In contrast, the greater genome flexibility of S. mutans allowed some deeply ancestral lineages to persist alongside newly emergent ones.
Our Teeth Record Dietary Legacies
The findings emphasize how our oral microbiome today is a product of our recent dietary history as a species. Our prehistoric ancestors, with lifestyles shaped more by hunting, gathering and early agriculture, harbored a richer community of microbial diversity in their mouths.
It was our species’ more modern penchant for sugar and refining starches and carbohydrates that transformed oral ecologies, selecting for hardier microbes like S. mutans and new strains of T. forsythia capable of exploiting these new food sources – setting the stage for tooth decay and periodontal disease as collateral damage.
So next time you’re enjoying a sweet treat, remember: you’re not just feeding yourself! The microbes in your mouth are the descendants of bacteria that long ago adapted to capitalize on our ancestral cravings for sugar and carbohydrates.
- Molecular Biology and Evolution, Volume 41, Issue 3, March 2024, msae017, https://doi.org/10.1093/molbev/msae017 ↩︎
