Bronze Age teeth from an Irish cave hide secrets about today's tooth decay

An oral microbiome has been uncovered from 4,000 year-old teeth from remains concealed in an Irish limestone cave. The teeth still harboured bacteria that can cause gum disease.

Scientists discovered from the remains a high quality ancient genome has been extracted for the oral bacterium S. mutans - still rampant today as one of the biggest causes of tooth decay. Based on the skeletal remains excavated from the cave in Ireland’s County Limerick, at Killuragh, a Bronze Age man’s tooth root harboured much of the bacteria. These findings are the result of a paper published recently in Molecular Biology and Evolution by Oxford University Press.

A senior author of the study, Lara Cassidy who is Assistant Professor at Trinity College Dublin, said: “We were very surprised to see such a large abundance of C. mutans in this 4,000 year old tooth. It is a remarkably rare find and suggests this man was at high risk of developing cavities right before his death.”

The skeletal finds help us understand diets through history

The secrets hidden in the teeth of the Bronze Age man provide information to scientists about how past diets affected the oral microbiome over thousands of years. The researchers' results support the idea that microbiomes of our ancestors were more diverse than we see today. This is despite our understanding of them having a more “limited” diet of food available, compared to the diversity of food products we see on supermarket shelves today.

“The two sampled teeth contained quite divergent strains of T. forsythia.” explained Iseult Jackson, a PhD candidate and first author of the study. “These strains from a single ancient mouth were more genetically different from one another than any pair of modern strains in our dataset, despite these modern samples deriving from Europe, Japan, and the USA. This is interesting because a loss of biodiversity can have negative impacts on the oral environment and human health.”

Microbiome of the mouth shaped by diet heavily

The bacteria S. mutans is common to find in this modern day, contained in oral microbiomes, and the main perpetrator of dental cavities that cause us to have to go to the dentist and get a filling. Compared to today, this bacterium is extremely rare to find in history, particularly the ancient genomic record.

It is possible to look at the ancient teeth to see how their oral health different from our own in the modern day. With the advent of industrialisation hundreds of years ago, the introduction of sugar caused major changes to the way our oral microbiome is presented today. Agricultural practices also show their mark in the history of our microbiome.

Sugar and cereal changed the oral microbiome

Sugar is seen everywhere in our shops. In England, almost 90% of tooth extractions seen in youth are from decay. To get an idea of how impactful it is, we can look to the 2018 levy that encouraged manufacturers to cut their products’ sugar levels.

At the end of 2023 researchers at the University of Cambridge stated that the UK soft drinks industry levy in 2018, could have saved over 5,500 tooth extraction admissions that year, based on their study published in BMJ Nutrition, Prevention & Health.

There was a slight rise of dental cavities present with the rise of cereal crop farming practices in history, and then another marked rise in around 1500, the Early Modern period, but it is little compared to today.

Since the 18th and 19th centuries, sugar became an “illness of the middle classes” for this newly introduced ingredient was no longer restricted to those of the upper class, as a luxury, prestigious good. This led to the concept of “Royal Cavities”.

As sugar beet became massively produced in Europe, and then with with industrial refinement of sugar post-1850, new, affordable products brought sugar into being a staple of the lower class, which brought health consequences.

Very few genomic sequences have been uncovered in full from before the Medieval era. So not much has been known about the bacterial diversity of the oral microbiome or how their diets’ impacts compared to ours.

Genomic value for the future

The researchers could see that broadly, dramatic changes were seen across 750 years in the oral “microenvironment” of the microbiome, when oral bacteria altered greatly since the Bronze Age. This may relate to us having less diverse microbiome, and poorer oral health, which worsened with the rise of sugar in our diets.

The study also fills in gaps of knowledge about the genomics of harmful oral bacteria. Very few genomic sequences have been uncovered in full from before the Medieval era. So not much has been known about the bacterial diversity of the oral microbiome or how their diets’ impacts compared to ours.

Even so, the bacteria in our mouth is not always harmful, in the case of “good bacteria”. Healthline advises that harmful bacteria are attracted to sugar and saliva attempts to combat this with the build up of acid. However, this continual fight takes its toll on tooth enamel, leading to dental cavities.

Once again, looking to the past reminds up of the hidden world of bacteria inside our mouths and the importance of diet and external cultural influences on our inner landscape.