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In a nutshell
- Depression and anxiety can spread between married couples through shared mouth bacteria — healthy spouses showed increased mental health symptoms after just six months of marriage to a depressed partner.
- Women appear more susceptible to this bacterial transmission — female spouses experienced more pronounced changes in both their oral microbiome and mental health scores.
- This research could change how we treat mental health in relationships — healthcare providers may need to consider treating both partners as a unit rather than focusing on individuals alone.
TEHRAN — Marriage means sharing your life with someone special. But new research out of Iran reveals couples might be sharing more than they bargained for: the bacteria in their mouths — and potentially their mental health struggles along with it.
A study of newlywed couples in Iran found that when one spouse suffers from depression and anxiety, their partner’s oral bacteria actually changes to mirror theirs. As those microbes shift, so do the healthy partner’s mood and sleep patterns.
After just six months of marriage, previously healthy spouses showed significantly higher rates of depression, anxiety, and sleep problems if their partner had what researchers called the “depression-anxiety phenotype” — essentially, someone dealing with both conditions simultaneously along with insomnia.
“Oral microbiota transmission between individuals in close contact partially mediates symptoms of depression and anxiety,” the researchers concluded in their paper published in Exploratory Research and Hypothesis in Medicine. The journal notes that exploratory research published by it “does not necessarily need to be comprehensive and conclusive, but the study design must be solid, the methodologies must be reliable, the results must be true, and the hypothesis must be rational and justifiable with evidence.”
How Bacteria Jump Between Partners
Every time you kiss your spouse, share a meal, or breathe the same air in close quarters, you’re potentially swapping millions of bacteria. Most bacterial exchange is harmless or even beneficial. But this study points to a pathway for mental health conditions to spread between intimate partners.
The research team, led by independent researcher Reza Rastmanesh, tracked couples married for an average of six months. They recruited 1,740 couples from two private sleep clinics in Tehran between February and October 2024, comparing 268 healthy spouses with 268 partners who had depression, anxiety, and sleep problems. Both groups took standardized mental health tests, provided saliva samples to measure stress hormones, and had their mouth bacteria analyzed using advanced DNA sequencing techniques.
At the study’s start, healthy spouses scored normally on tests for depression, anxiety, and sleep quality. After six months of living with an affected partner, their scores climbed significantly higher — not quite matching their troubled spouse, but moving in that direction.
Can depression or anxiety really be transmitted through a kiss? New research suggests the swapping of oral bacteria could make it possible (Photo by PeopleImages.com – Yuri A on Shutterstock)
More revealing was what happened to their mouth bacteria. The oral microbiome—the community of microorganisms living in our mouths—of healthy spouses began resembling that of their depressed and anxious partners. Using a technique called linear discriminant analysis, researchers found that specific bacterial families like Clostridia, Veillonella, Bacillus, and Lachnospiraceae became more abundant in both partners.
These bacteria aren’t random. Previous research has linked these same microorganisms to brain disorders, including depression, anxiety, and sleep disturbances. Scientists believe these bacteria may affect the brain by compromising the blood-brain barrier or through what’s called the “oral microbiota-brain axis”—essentially, a pathway of communication between mouth bacteria and the brain.
Women Show Stronger Effects
Female spouses appeared more susceptible to these changes than men, showing more pronounced increases in depression, anxiety, and sleep problems after six months, along with greater shifts in their oral bacteria composition.
The study also measured cortisol, often called the “stress hormone,” in participants’ saliva. Healthy spouses married to depressed and anxious partners showed significant increases in their cortisol levels over six months, indicating their stress response systems were being activated.
This bacterial transmission theory builds on existing research showing how couples synchronize biologically — their heart rates can align during conversations, sleep patterns often mirror each other, and they share similar daily cortisol rhythms. What’s new here is the suggestion that mental health conditions might spread through bacterial exchange.
What This Means for Treatment
These findings could reshape how we think about treating depression and anxiety in relationships. Instead of focusing solely on the individual patient, healthcare providers might need to consider the couple as a unit, potentially treating both partners even when only one shows obvious symptoms.
The research builds on compelling evidence from animal studies where scientists successfully transferred depression-like behaviors to healthy mice by transplanting gut bacteria from depressed animals. Other studies show that probiotics—beneficial bacteria—can improve mood and reduce anxiety in both animals and humans.
Of course, this study has limitations. Researchers only followed couples for six months and didn’t account for other factors like shared stress from major life events, similar diets, or lifestyle changes that come with marriage. Participants were also aware they were being studied, which could have influenced their responses to mental health questionnaires.
The research was conducted in Iran with Persian-speaking couples, so findings might not apply universally. While the study found strong correlations between bacterial changes and mental health symptoms, it can’t definitively prove the bacteria caused the mood changes—just that they occurred together.
For couples, the findings offer both concern and hope. While no one wants to think they might “catch” their partner’s depression, understanding this potential mechanism could lead to better treatments. If bacterial imbalances contribute to mental health problems, then targeted probiotics or other microbiome interventions might offer new therapeutic options.
The research also reinforces what marriage counselors and therapists have long observed—that mental health conditions affect entire families, not just individuals. This study provides biological evidence for that clinical experience.
In close relationships, couples may share more than they realize—including microscopic influences on their mental health.
Paper Summary
Methodology
Researchers conducted a cross-sectional study of 536 Iranian couples (268 couples total) who had been married for an average of six months. They recruited participants from two private sleep clinics in Tehran between February and October 2024. One spouse in each couple had what researchers called the “depression-anxiety phenotype”—meaning they scored high on standardized tests for depression (Beck Depression Inventory-II score of 14 or higher), anxiety (Beck Anxiety Inventory score of 16-25), and sleep problems (Pittsburgh Sleep Quality Index indicating poor sleep). The other spouse was healthy at the study’s start. Participants completed mental health questionnaires, provided saliva samples for cortisol measurement, and had oral bacteria collected from their tonsils and throat. The bacterial samples were analyzed using advanced DNA sequencing techniques to identify and quantify different bacterial species.
Results
After six months, healthy spouses showed significant increases in depression, anxiety, and sleep problem scores compared to their baseline measurements, though they remained lower than their affected partners’ scores. Their oral bacteria composition changed to become more similar to their depressed and anxious spouses, with specific bacterial families (Clostridia, Veillonella, Bacillus, and Lachnospiraceae) becoming more abundant. Salivary cortisol levels also increased significantly in the healthy spouses. Female spouses showed more pronounced changes than male spouses across all measures. The researchers found that oral bacteria patterns could predict depression-anxiety status and that bacterial changes correlated with mood and stress hormone alterations.
Limitations
The study followed couples for only six months, which may not capture long-term effects. Participants knew they were being studied, potentially influencing their questionnaire responses. The research didn’t control for confounding factors like shared diet, stress exposure, or frequency of intimacy between couples. The study used self-report questionnaires rather than clinical psychiatric diagnoses. Only morning saliva samples were collected for cortisol measurement, and bacterial sampling was limited to tonsils and throat due to budget constraints. The sample consisted entirely of Persian-speaking Iranian couples, limiting generalizability to other populations.
Funding and Disclosures
The study was funded by a private donation from Dr. Javid Azizi. The authors declared no conflicts of interest. The research was approved by the ethics committee of the Iran National Science Foundation and followed Declaration of Helsinki guidelines.
Publication Information
The study was published in Exploratory Research and Hypothesis in Medicine, Volume 10, Issue 2, pages 77-86, in April 2025. The paper was authored by Reza Rastmanesh and colleagues from institutions in Iran, India, Italy, and the UK. The digital object identifier is 10.14218/ERHM.2025.00013.