In a study of newlywed couples, researchers found that depression and anxiety can be passed from one spouse to the other via the oral microbiome, which is transferred during close contact, such as kissing.
We are gaining a greater understanding of how, like the gut microbiome, our oral microbiome, the diverse community of bacteria in our mouths, can affect our physical and mental health. Studies have linked an out-of-whack oral microbiome with autism spectrum disorder, dementia, Parkinson’s disease, schizophrenia, anxiety, and depression.
In an interesting new study, researchers have investigated whether the oral microbiome can be transferred between people through kissing, and whether such transfer can affect mental health.
“Given that bacterial transmission can occur between spouses, this study aimed to investigate whether the transmission of oral microbiota between newlywed couples mediates symptoms of depression and anxiety,” the researchers said.
Researchers from Iran, India, Italy and the UK were involved in the study, which first screened 1,740 couples for the presence or absence of insomnia. Of those, 268 couples who’d been married in the past six months and were living together were recruited into the study. One spouse was a healthy control, while the other had insomnia with anxiety and depression. On day one and day 180 of the study, all participants had their oral microbiome composition and salivary cortisol (stress hormone) levels taken. Participants were asked not to change their baseline dietary habits, oral hygiene practices, or exercise routines during the study.
The researchers found that after six months of marriage to a partner with insomnia with features of depression and anxiety, healthy spouses showed increased depression and anxiety scores, and poorer sleep quality. The changes were more pronounced in female spouses. While salivary cortisol levels were significantly higher in insomniac spouses, they found that healthy spouses also experienced elevated cortisol levels after six months, indicating stress-related physiological changes. Interestingly, the researchers observed that the composition of the bacteria found in the mouths of healthy spouses changed considerably, becoming increasingly similar to their affected partners.
“Oral microbiota transfer between individuals in close contact, such as couples in the present study, may mediate depression and anxiety,” said the researchers. “We found that changes in oral microbiota composition are associated with changes in the severity of insomnia, salivary cortisol levels, and depression and anxiety scores. Our findings align with previous studies on salivary cortisol levels as well as depression and anxiety scores.”
The researchers said that oral microbiome transfer between couples makes sense, given that other forms of physiological synchrony have been documented, including synchronized heartbeats and sleep concordance. They said their findings have implications for holistic, personalized and preventive medicine, especially in the context of mental health treatment.
Some limitations of the study include the use of self-reporting to measure insomnia, depression, and anxiety; the measurement of only morning salivary cortisol levels; and the researchers' failure to control for shared lifestyle factors such as diet. Additionally, due to financial restraints, bacterial samples were only collected from the tonsils and pharynx, rather than the full oral microbiome.
Nonetheless, the study raises some interesting questions for future research.
“Since this study is associational, further research is needed to establish whether this association is causal,” the researchers said. “If it is determined that this association is indeed causal, it could have significant implications for contemporary research.”
The study was published in the journal Exploratory Research and Hypothesis in Medicine.