Dental calculus is the calcified plaque, or tartar, that is removed with a dental scalar during regular dentist visits. It cannot be removed by brushing. It is analogous to concrete, and is composed of millions of bacterial cells calcified together with calcium phosphate which comes from the saliva.
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Oral bacteria are inherited from your primary caregiver in early childhood, and transferred between family members later in life. Importantly, dental plaque and calculus are increasingly linked to a range of systemic diseases including cardiovascular disease, premature birth, arthritis, and diabetes. For example, bacteria from dental plaque leak through the inflamed gums into the bloodstream, where they stimulate a permanent inflammation response, and are found in clumps on coronary artery walls.
However, it is important to note that oral bacteria are also an important part of health, and are equally involved in repairing damage to teeth and preventing pathogens from colonizing. Prof. Alan Cooper, direct of the University of Adelaide’s Centre for Ancient DNA, comments, “The problem is that the natural human bacterial ecosystem has been massively altered by our adoption of the farming diet, which is enormous amounts of carbohydrates, and recently, processed sugar.”
The idea for the project — Ancient tooth decay DNA reveals effects of changing diets — came 17 years ago when Prof. Cooper first met archaeologist Prof. Keith Dobney, now at the University of Aberdeen.
While previous studies by Prof. Dobney and others had used microscopes to identify the range and number of bacteria present in dental calculus, and also the large amount of food preserved amongst the bacteria, it had proved impossible to accurately characterize the species present.
"Over the past decade we tried several times to extract bacterial DNA from ancient calculus, but contaminating bacterial DNA in the enzymes and components impacted the experiments, so we had to wait for improvements in manufacturing” said Prof. Cooper. “We also had to develop ultra-clean laboratories at ACAD to provide a bacterial-free workspace.”
This study is the first to show that DNA survives within dental calculus for long periods of time (>8 kyr), and that the DNA sequences recovered from ancient skeletons accurately relate to the oral bacteria present in the mouth of the individual during life.
To test whether the microbial signal originated from the calculus alone, and not from other bacterial in the soil or water surrounding the specimen, the team analysed the microbial diversity inside the same teeth they collected calculus deposits from. If bacteria in the soil or water was penetrating the specimen and contaminating the results, the DNA sequences inside the tooth should resemble those recovered from the calculus. Instead, the DNA results from inside the tooth were completely different from the calculus, and were closer to environmental controls.
While ancient microbial DNA has also been recovered from coprolites (preserved faeces), it is thought that much of the bacterial diversity relates to changes taking place after deposition, as both internal and external microbial communities start to break down the coprolite. In contrast, the microbes in dental calculus are already calcified in place at the time of death, and don’t undergo subsequent modification. This makes dental calculus a far more powerful record of ancient human microbiomes.
Previous studies of early skeletons have shown a range of pathologies associated with the introduction of farming during the Neolithic (farming age), including a marked reduction in average population height, nutritional stress during youth, and increases in diseases such as arthritis and tuberculosis. These are thought to relate to the change from the diverse hunter-gatherer diet to one based mainly on carbohydrates from crops such as wheat, barley, and rye.
The studies of dental calculus on the last hunter-gatherers, and early farming skeletons reveal a major change in oral microbes, with an overall reduction of diversity and an increase in bacteria associated with gum disease (periodontitis) and dental caries (holes in the teeth).
However a far bigger, and more detrimental change occurred only 150 years ago, when the Industrial Revolution made processed sugar and flour readily available. At this point, the study shows that the oral microbial diversity in European skeletons plummeted, and became dominated by disease causing bacterial such as Streptococcus mutans, the main caries-causing microbe.
Prof. Cooper described the modern oral environment a permanent disease state, where the reduced diversity of the ecosystem and large amounts of carbohydrates (especially sugar) allows pathogens to colonise and dominate.
Prof. Corey Bradshaw, an ecologist, says that the same principles apply to both bacterial and standard ecosystems, and that reduced diversity is strongly associated with a reduced resilience to environmental change, such as invasive species.
“Oral bacteria have been associated with a range of diseases, including diabetes, heart disease, and cancers” said Dr Adler. “One common cause of gum disease, Porphyromonas gingivalis, had been suggested to lie behind recent rises in heart disease. However, we were able to show it had not increased in prevalence over the past 7,000 years, suggesting it was not likely to be causative. However, it may well contribute to the disease by stimulating a permanent state of inflammation.”