A new study suggests that close contact between newlyweds, such as kissing, could transfer bacteria in the mouth that may contribute to anxiety and depression.
Researchers found that the oral microbiome—a complex mix of bacteria in the mouth—might play a role in how mental health issues like depression and anxiety can spread between partners. Much like the gut microbiome, the oral microbiome is now understood to influence both physical and mental health. Previous studies have linked disruptions in oral bacteria to conditions including autism, dementia, Parkinson’s disease, schizophrenia, and mood disorders.
This latest research, led by an international team from Iran, India, Italy, and the UK, focused on how oral microbiota are exchanged between partners and whether this affects their mental well-being.
The study began with a screening of 1,740 couples, identifying 268 newlywed pairs who had been married less than six months and were living together. In each couple, one partner suffered from insomnia along with anxiety and depression, while the other was considered mentally healthy. Researchers collected samples of their oral bacteria and salivary cortisol—an indicator of stress—on the first day and again six months later. Participants were asked not to change their diet, oral hygiene, or exercise habits during the study.
After six months, the researchers found noticeable changes in the healthy spouses. Their levels of anxiety and depression increased, and their sleep quality declined—especially among women. Their cortisol levels also rose, reflecting elevated stress. At the same time, their oral microbiomes began to resemble those of their affected partners, suggesting that bacterial transfer had occurred.
“Oral microbiota transfer between individuals in close contact, such as couples in the present study, may mediate depression and anxiety,” the researchers wrote. They noted that these changes were associated with worsening symptoms, higher cortisol levels, and disrupted sleep.
The findings also align with previous research on physiological synchrony between couples, including shared sleep patterns and synchronized heartbeats. According to the researchers, this highlights the potential for using oral microbiome data in personalized and preventive approaches to mental health care.
However, the study had several limitations. It relied on self-reported data for insomnia and mood symptoms, only tested cortisol in the morning, and did not fully control for shared lifestyle factors like diet. Also, due to funding limits, samples were only taken from the tonsils and throat, not the entire oral cavity.
Despite these constraints, the study opens the door for further investigation into how close relationships and shared environments influence mental health.
“Since this study is associational, further research is needed to establish whether this link is causal,” the researchers said. “If proven, it could change the way we think about mental health transmission and treatment.”
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