Environment a bigger factor than genes in oral bacteria

The makeup of bacteria in the oral cavity is driven more by environmental factors than genetics, according to a new long-term study of human twins by University of Colorado Boulder researchers (Genome Research, October 12, 2012).

The study compared saliva samples from identical and fraternal twins to see how much "bacterial communities" in saliva vary from mouth to mouth at different points in time, according to lead author Kenneth Krauter, PhD, a professor in the molecular, cellular, and developmental biology department at the university.

The twin studies show that the environment, rather than a person's genetic background, is more important in determining the types of microbes that live in the mouth.

For the study, doctoral student Simone Stahringer sequenced the microbial DNA present in the saliva samples of twins. She and the research team then determined the microbes' identities through comparison with a microbe sequence database. The saliva samples came from the university's Longitudinal Twin Study and Colorado Adoption Project, which has involved hundreds of identical and fraternal twin pairs.

After determining the oral microbiomes of identical twins, who share the same environment and genes, and the microbiomes of fraternal twins who share only half their genes, the researchers found the salivary microbes of the identical twins were not significantly more similar to each other than to those of fraternal twins.

"We concluded the human genome does not significantly affect which bacteria are living in a person's mouth," Krauter said. "It appears to be more of an environmental effect."

While the twin data from the oral microbiome study indicate that genetics plays a more minor role, it's possible the genes still affect the oral microbiome in more subtle ways, he added.

The researchers also found that the salivary microbiome changed the most during early adolescence, between the ages of 12 and 17. This discovery suggests that hormones or lifestyle changes at this age might be important, according to the team.

A core community of oral bacteria appear to be present in nearly all humans studied, Krauter noted.

"Though there are definitely differences among different people, there is a relatively high degree of sharing similar microbial species in all human mouths," he said.

This study has established a framework for future studies of the factors that influence oral microbial communities, he added.

"With broad knowledge of the organisms we expect to find in mouths, we can now better understand how oral hygiene and environmental exposure to substances like alcohol, methamphetamines, and even foods we eat affect the balance of microbes," Krauter said.

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