Should we blame our genes for the bacteria behind tooth decay?
By Catharine Paddock PhD
The composition of our oral bacteria when we are very young is predominantly influenced by our genetic background. But as we age, this heritable factor wanes and non-heritable ones such as diet and oral hygiene play a stronger role in shaping the oral microbiome.
This idea is supported by a new study - led by the J. Craig Venter Institute (JCVI) in La Jolla, CA - which has been published in the journal Cell Host & Microbe. It investigated the oral microbiomes of identical and non-identical twins in childhood.
Senior study author Karen E. Nelson, Ph.D., president of the JCVI, and her colleagues chose to investigate twins because, since they are likely to have had a very similar upbringing, they offer ideal subjects for studying the "nature versus nurture" question with respect to oral health.
Dr. Nelson and team note that they "investigated a large cohort of twin children to shed light on the contributions of host genotype and the early shared environment in shaping the oral microbiome in the context of oral health."
Tooth decay, or dental caries, is a major global health issue. Up to 90 percent of school-age children and nearly 100 percent of adults worldwide have dental cavities. Up to 20 percent of middle-aged adults have severe gum, or periodontal, disease, which can lead to tooth loss and other health problems.
In the United States, statistics on tooth decay show that 37 percent of children aged 2 to 8 have it in their primary teeth, 58 percent of teenagers currently have or have had it, and more than 90 percent of adults have it.
Oral microbiome and disease
In their study paper, the authors explain that tooth decay commonly results when certain types of bacteria metabolize "frequent sugar intake." This leads to an acid environment in the mouth that attacks tooth enamel and causes cavities.
They also note that in adults with gum disease, specific groups of bacteria trigger inflammation that leads to destruction of gum tissue, the formation of "pockets," and tooth loss.
There is also mounting evidence of links between the oral microbiome - that is, the collection of oral microbes and their genetic material - and other illnesses, including oral cancer and cardiovascular disease.
However, while there have been numerous studies on the links between the gut microbiome and health and the extent to which this might be influenced by host genetic background, there have been hardly any on the oral microbiome.
For their study, Dr. Nelson and colleagues used mouth swabs to profile the oral microbiomes of 485 pairs of twins aged between 5 and 11 years. Of these, 205 were identical twins and 280 were non-identical, or fraternal, and there was also one set of triplets.
Link to high added sugar intake
As expected, they found that the oral microbiomes of identical twins were more similar to each other than those of non-identical twins. This, the researchers say, suggests that the host genetic background influences the types of bacteria present in the mouth.
But they also found that the types of bacteria most closely linked to host genetic background - the so-called heritable bacteria - were not those that play a role in tooth decay.
Also, when they compared the results from children aged 5 with those of age 11, they found that "the most heritable bacteria decrease in abundance with age."
Finally, they also found that twins whose diet included a lot of added sugar had fewer of the types of bacteria that are linked to lower rates of tooth decay and more of the types that are linked to higher rates of tooth decay.
The researchers plan to continue following the twins and study changing patterns in their oral microbiomes. They also want to compare the health of identical and non-identical twins with functional differences in their oral microbiomes.
"Limiting sugar consumption and acid buildup in the mouth have been part of the dogma of the dental community for some time. This work introduces specific taxa of bacteria that can be acquired through the environment and that have the ability to induce cavities."
Karen E. Nelson, Ph.D.
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