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Because the human body harbors 10 times more bacterial cells than human cells, scientists since ages have been trying to fully understand the bacteria we carry.

The human mouth is a major gateway for bacteria into the body and it contains a diverse array of microbial species. Yet scientists know little about this diversity and how it relates to diet, environment, health and disease.

Now, in an attempt to find out more about this microbiome researchers analysed a unique sample set–DNA and saliva from an extended, Ashkenazi Jewish family living in various households spread across four cities on three continents. That allowed the team ask how much of the variation seen in salivary microbiomes is due to host genetics and how much is due to environment.

“It’s generally becoming known that there’s a link between our microbiomes and our health and that’s reason enough to find out what’s in there, how they arrived there, and what they are doing,” says Adam P. Roberts, senior lecturer in antimicrobial chemotherapy and resistance at the Liverpool School of Tropical Medicine. Roberts co-led the study, which was conducted during his previous post at the UCL Eastman Dental Institute. UCL Genetics Institute graduate student Liam Shaw adds, “The oral cavity is naturally colonized by hundreds of bacterial species, which stop external pathogens from establishing a foothold, but they also can themselves cause oral disease.”

Because the family members are ultra-orthodox Ashkenazi Jews, they share cultural diets and lifestyles that control for many confounding factors.

Shaw and the team sequenced the bacterial DNA signatures present in saliva samples from 157 family members and 27 unrelated Ashkenazi Jewish controls. Across all samples, they found the core salivary microbiome made up of bacteria from the genera Streptococcus, Rothia, Neisseria, and Prevotella.

They found conclusively that spouses and parents and children younger than 10 living in a household together had the most similar saliva microbiomes. The team also looked carefully at whether genetic relatedness drove the makeup of the saliva microbiome. When they used a measure of relatedness based on family tree relationships alone, they saw a small, but statistically significant effect.

However, when they used the genetic sequence information, a more accurate measure of relatedness, the effect disappeared. In other words, a person’s genetics played virtually no role in shaping their saliva microbes.

This study shows that environments shared during upbringing play a major role in determining what community of bacteria gets established. And knowing that the shared environment drives the microbiome, Roberts says, may gives us the ability to one day modulate it.

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