An international team of scientists from Italy, India, Iran, and Great Britain conducted a study involving 268 newly married couples, focusing on the role of the oral microbiome in the transmission of mental health conditions. In each couple, one partner suffered from insomnia and exhibited signs of anxiety-depressive disorder, while the other was initially in good mental health.
According to Bild, the researchers determined the composition of the oral microbiome and measured the stress hormone cortisol in the saliva of all participants on the first day of the study and after six months. By analyzing these samples, they linked changes in stress levels to differences in the oral flora.
At the end of the observation period, the researchers detected changes, including an increase in symptoms of depression and anxiety in the originally healthy spouses. The study provided evidence that the oral microbiome seems to play a role in the transmission of symptoms of depression and anxiety, with healthy partners experiencing increased symptoms when living with spouses who had mental health issues.
The study supports the idea of physiological synchrony in close relationships, suggesting an unexpected form of "emotional contagion" mediated by bacteria. This indicates that psychological symptoms in relationships are transmitted not only emotionally or socially but probably also microbiologically.
Groups of bacteria such as Clostridia, Veillonella, and Bacillus were found to be more common among spouses with a matching symptom complex. Most importantly, the composition of the oral microbiota of the healthy individuals began to resemble that of their affected partners.
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The greatest changes were observed in women. The level of the stress hormone cortisol increased, particularly in women, from 11.3 nanograms per milliliter to 20.8 nanograms per milliliter.
The oral microbiome is an invisible ecosystem of microorganisms that resides in the mouth and can be transferred during intimate contact, such as kissing. This may partially explain the development of symptoms of depression, anxiety, and sleep disorders in partners.
The researchers stated that understanding the relationship between microbiota and psychological disorders could transform current therapeutic models. In the future, microbiome-based therapeutic approaches may be tested as complementary treatments for mental illnesses.
The study acknowledges its limitations, including reliance on self-reports to assess symptoms of insomnia, anxiety, and depression. Additionally, cortisol levels were measured only once in the morning, which could introduce biases in the results. The researchers also recognized that bacterial samples were collected exclusively from the tonsils and pharynx, without covering the entirety of the oral microbiome.
Despite these limitations, the researchers emphasize that the study opens new questions. These include whether the oral microbiome can really influence mental health causally, what role it plays in emotional connection between people, and whether future therapies could include interventions in the microbiome as part of the treatment of psychological disorders.
The transfer of oral microbiota is not an outlandish idea; it has already been documented how other physiological functions can synchronize between couples, such as heart rhythms and sleep patterns. The researchers are confident that bacterial exchange through kissing and shared meals could play a role in this transmission. Oral flora may be a previously unnoticed factor affecting mental health in relationships.
Previous studies have shown that oral microbiota—bacteria in the mouth—are linked to neuropsychiatric disorders such as depression, anxiety, and schizophrenia. The study opens perspectives in the area of the so-called microbiota-brain axis, which is the interaction between microorganisms and neurological functions.
The researchers did not investigate or explain in detail how exactly the harmonization of the oral microbiome occurred. However, it would be logical that kissing or other intimate closeness, such as sharing food, could be factors. Shared diet, environmental factors, and the type and frequency of intimate behaviors may also have affected the microbiome.
The study is associative, meaning that a causal relationship between the transfer of microorganisms and psychological stress cannot yet be proven. The researchers acknowledge that more studies are needed to understand the real connection between microbiota composition and mental health.