Newlywed Couples May Share Depression and Anxiety Through Bacterial Exchange, Study Finds

A recent study has found that healthy spouses who live with partners experiencing depression and anxiety gradually develop similar symptoms themselves, potentially due to the transmission of oral bacteria between couples.

According to research published in Exploratory Research and Hypothesis in Medicine, this bacterial exchange may partially explain how mood disorders are transmitted between intimate partners.

Researchers led by Reza Rastmanesh followed 268 newlywed couples over six months. They discovered that healthy spouses married to partners with combined depression-anxiety symptoms showed significant increases in their own depression, anxiety, and sleep problems. Simultaneously, their oral bacteria composition underwent complex changes that showed similarities to their affected partners.

The study addresses an important gap in understanding how psychological conditions might be transmitted between people in close contact.

While previous research has documented various forms of physiological synchronization between couples, including heart rhythms and stress hormone patterns, this is the first study to examine whether bacterial transmission could mediate the spread of mood disorders.

What Triggers Depression and Anxiety in This Context

According to the study, the primary trigger appears to be close physical contact (such as kissing) that enables bacterial transmission between partners.

The researchers focused on couples with what they termed a “depression-anxiety phenotype”—individuals who simultaneously experienced insomnia, moderate depression, and moderate anxiety.

The study suggests that living in the same household and sharing intimate contact creates conditions for oral bacteria to transfer from one partner to another, potentially affecting brain chemistry through what researchers call the “oral microbiota-brain axis.”

Research Methodology and Limitations

The researchers conducted a longitudinal study of 1,740 couples at two private sleep clinics in Tehran, Iran, between February and October 2024. From this larger group, they identified 296 couples where one partner had the depression-anxiety phenotype and the other was initially healthy. After excluding 28 couples due to factors like antibiotic use, pregnancy, divorce, or missing data, 268 couples remained in the final analysis. All couples had been married and living together for an average of six months.

The team collected oral bacteria samples specifically from the palatine tonsils and pharynx at the beginning of the study and again six months later. They also measured salivary cortisol levels—a stress hormone indicator—and administered validated questionnaires measuring depression, anxiety, and sleep quality.

However, the study has important limitations that the researchers acknowledge. They couldn’t control for shared factors like diet, stress exposure, or frequency of intimate contact between couples.

Additionally, participants were aware of the study’s purpose, which may have influenced their responses. Critically, the methodology only examined correlation, not causation, meaning the findings suggest association rather than proof that bacterial transmission directly causes mood changes.

Key Findings: Complex Bacterial and Mood Changes

Transmission of Mental Health Symptoms

Healthy spouses married to partners with anxiety and depression symptoms started developing similar symptoms within six months. Specifically, their sleep quality worsened, and they scored significantly higher on depression and anxiety assessments compared to their initial baseline scores.

Changes in Oral Microbiota

Significant changes were detected in the oral microbiota of healthy spouses. After six months, their microbiota composition closely resembled that of their affected partners. The study specifically identified increased abundance of certain bacteria such as Clostridia, Veillonella, Bacillus, and Lachnospiraceae associated with the DA phenotype.

Cortisol Levels and Anxiety-Depression Link

Alongside microbiota changes, cortisol levels (an indicator of stress) significantly increased in healthy spouses, correlating with higher anxiety and depression scores. This suggests that oral bacteria might contribute indirectly by affecting stress-related hormones.

Gender Differences

Interestingly, women showed more pronounced increases in anxiety, depression, and cortisol levels than men, suggesting potential gender differences in susceptibility or reporting of mental health symptoms.

Implications and Study Limitations

These findings suggest that mood disorders might be partially transmitted through bacterial exchange, challenging traditional views of mental health as purely individual conditions. The research has potential implications for family medicine, couple therapy, and personalized treatment approaches that consider both partners in a relationship.

The study’s limitations are significant and extensively acknowledged by the researchers. Most importantly, this was an observational study that cannot establish whether bacterial transmission actually causes mood changes or whether both phenomena result from other shared factors. The researchers emphasize that their findings show association, not causation. The couples weren’t randomly selected, limiting generalizability, and the researchers only measured morning cortisol levels rather than tracking patterns over multiple days.

The authors also note they used self-reported questionnaires rather than clinical psychiatric diagnoses based on the Diagnostic and Statistical Manual of Mental Disorders. They couldn’t control for important variables like shared diet, environmental stressors, or the frequency of intimate contact between partners. The researchers acknowledge that “deriving a conclusive understanding from our preliminary findings is challenging without controlling for confounders.”

Broader Significance and Future Research

While these findings require replication and more rigorous testing, they align with previous research showing that fecal bacteria transplants from depressed patients can induce depression-like behaviors in laboratory animals. This suggests the relationship between bacteria and mood may indeed be causal, though the researchers caution that “translating findings from animal models to human studies” requires careful consideration.

The research opens new questions about how infectious disease principles might apply to mental health conditions traditionally considered non-communicable. The authors suggest that future studies should employ randomized controlled designs and animal models to determine whether bacterial transmission actually causes mood changes or simply accompanies them. They specifically recommend that “future research may involve the recruitment of clinical samples” and note that “animal models will be instrumental in determining whether such relationships are causal.”

Study Citation:Rastmanesh, R., Vellingiri, B., Isacco, C. G., Sadeghinejad, A., & Daghnall, N. (2025). Oral Microbiota Transmission Partially Mediates Depression and Anxiety in Newlywed Couples. Exploratory Research and Hypothesis in Medicine, 10(2), 77–86. DOI: 10.14218/ERHM.2025.00013.