For years, scientists suspected that genetics shaped individual salivary microbiomes, but new research published in the journal mBio counters that theory. Instead, it appears that the mix of microorganisms inhabiting each human’s mouth is largely determined by the person’s household environment.

Consequently, roommates of any kind — be they relatives or former strangers — can likely affect what’s living in each other’s mouths.

Adam Roberts and Andrew Smith, along with their research team, analyzed a unique sample set: DNA and saliva from an extended family of Ashkenazi Jews living in households spread across four cities on three continents, plus additional DNA and saliva from unrelated individuals.

Smith, an immunologist at University College London, knew about the collected Ashkenazi data that had been used for other studies. Because the individuals are all ultra-orthodox Ashkenazi Jews, they share cultural diets and lifestyles that control for many confounding factors. As a result, the dataset offers an opportunity to investigate the effects of environment and genetics separately.

The researchers sequenced the bacterial DNA signatures present in the saliva samples from 157 family members as well as 27 unrelated Ashkenazi Jews. They then compared factors — such as shared household, city of residence, age and genetic relatedness — to find out which had the greatest influence on each person’s saliva microbes. The greatest determinant by far was household environment.

Residences turn out to be surprisingly full of spit residue, even if people in the homes are frequent hand washers.

“Normal activities lead to hand–mouth contact quite frequently,” Roberts, a senior lecturer in antimicrobial chemotherapy and resistance at the Liverpool School of Tropical Medicine, told Seeker.

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Roberts, formerly of University College London’s Eastman Dental Institute and now at the Liverpool School of Tropical Medicine, explained that just eating a snack or drinking a beverage — and touching the cup’s rim — can transfer saliva to hands and then to objects that others might also touch.

Smith said tooth brushing will transfer saliva from the brush to the hands and covering the mouth with hands during coughs, sneezes, and certain other moments can also lead to transfers of saliva.

He and his colleagues found that spouses, parents, and children younger than 10 living in a household had the most similar salivary microbiomes. It is easy to see how prior scientific teams thought that genetics could heavily shape these collections of microorganisms within an individual’s mouth. Family members, however, tend to have more direct contact with each other, as well as with common shared objects, such as doorknobs, computer keyboards, and TV remotes.

Unrelated household members touch these kinds of shared objects too, permitting many opportunities for transfer of salivary microorganisms.

Children younger than 10 in the study were found to have more similar saliva bacteria to their parents than to older children, possibly reflecting that older children are becoming “more independent individuals,” Roberts said. This is likely, at least in part, due to their growing exposure to environments outside of the household.

Lead author Liam Shaw of University College London said the establishment of the salivary microbiome earlier in life could affect its long-term composition.

“Despite repeated disruption, the oral microbiome seems able to persist," Shaw said. “This is true even after a course of antibiotics where it returns quickly to its previous diversity.”

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The salivary microbiome stabilizes somewhat in response to early household environmental factors, even though it can undergo future changes in response to new settings with probable different species of microorganisms.

Smith said from birth and throughout your early childhood, a person develops an adaptive immune response to a host of microbes, foods, and environmental antigens. “This tolerance also develops with the organisms that make up our microbiomes and results in the symbiosis between host and microbial communities,” he said.

“As we become adults,” he continued, “we maintain both immunological memory to things such as chicken pox, and tolerance to our microbiome.”

Bacteria that can potentially cause disease clearly are not always tolerated, though. Some bacterial strains are more dangerous to human health than others.

Numerous studies indicate that balanced microbiomes, as opposed to the presence or absence of any particular microbe, are important to good health. This appears to be true for the colonies of organisms that exist not only in the mouth, but also on the skin and in the gut. Mouth and gut microbiomes are surprisingly very different from each other, according to the researchers.

Such colonies of microorganisms appear to be communicable.

“It may be that the microbiome itself is contagious — or transferable — which is why we see more similarity at the household level,” Roberts said. “If you could promote a microbiome associated with health as opposed to disease by behavior, for example, then this could affect all household members.”

Smith added that maintaining good oral hygiene benefits an entire household, not just the individual brusher.

“Setting up a healthy oral microbiome in early life could provide the most benefit,” he said.

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Although environmental factors seem to shape saliva more than DNA does, prior research determined that genetics can affect the microbial communities living in another ubiquitous substance: mucus.

The difference probably has to do with the fact that “saliva is an open system, which means that environmental contact leads to more similar compositions,” Shaw said.

Many questions remain about the link between saliva and health.

Smith is currently investigating the potential benefits of probiotics in the treatment of an oral inflammatory disease. For a separate project, Roberts is studying the role of the oral microbiome as a source of antimicrobial-resistant genes, which may be acquired by pathogens as they transmit through the oral cavity.

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