Relationships are often described as the merging of two lives – emotionally, physically, and practically. We accept the idea that couples influence each other’s habits, moods, and routines. But what if those changes extend far beyond conscious behaviors?
What if even depression could shift between partners – not just through shared struggles, but through the very bacteria in your body beginning to mirror those of your spouse?
A new study led by Iranian researcher Reza Rastmanesh presents a surprising theory: depression and anxiety might be contagious among couples – not just through shared emotional experiences, but through oral bacteria.
After six months of marriage, healthy spouses living with a partner suffering from both depression and insomnia began to show measurable changes in their mood, sleep, and microbiota.
“Oral microbiota transmission between individuals in close contact partially mediates symptoms of depression and anxiety,” noted the authors of the study.
This research, while preliminary, opens a new frontier in how we understand emotional well-being within intimate relationships.
The researchers focused on couples married for under a year, recruited from two private sleep clinics in Tehran. They selected 268 couples in which one spouse exhibited the “depression-anxiety phenotype” – defined by moderate depression, moderate anxiety, and significant sleep issues.
Each of these individuals lived with a partner who was initially healthy, free from emotional and sleep disturbances.
The study measured mental health using validated Persian versions of standard questionnaires: the Beck Depression Inventory-II, Beck Anxiety Inventory, and Pittsburgh Sleep Quality Index. The researchers collected saliva to assess cortisol – a stress hormone – and mouth swabs to analyze bacterial composition.
Six months later, the healthy spouses had changed. Their depression, anxiety, and sleep scores increased, and their oral bacteria began to resemble those of their affected partner. Even more revealing, women appeared to be more vulnerable to these shifts than men.
Mouth-to-mouth bacterial transfer sounds unusual, but it is a daily part of close contact. Whether through kissing, sharing food, or simply breathing the same air, couples swap millions of microbes.
In most cases, this poses no risk. But the Iranian study suggests that these exchanges may influence mental health.
The microbiota of a healthy spouse started to show elevated levels of bacterial groups previously linked to mental illness.
These included Clostridia, Veillonella, Bacillus, and Lachnospiraceae. The researchers used advanced DNA sequencing and linear discriminant analysis to track these shifts.
The findings suggested a microbial convergence. The oral ecosystems of the healthy partners grew more similar to those of their depressed and anxious spouses. Over time, emotional and biological states seemed to synchronize as well.
The team measured cortisol, a hormone known to spike under stress. Cortisol can indicate how the body reacts to emotional burdens.
At the start of the study, healthy partners had low levels. But after living with a troubled spouse, their cortisol levels rose dramatically.
Female spouses saw particularly steep increases. In one part of the analysis, women’s cortisol nearly doubled. These hormonal changes paralleled shifts in depression and anxiety scores, reinforcing the theory that stress responses are shared on a physiological level in close relationships.
This adds weight to the idea that psychological synchrony in couples may not just be behavioral – it might also have microbial and hormonal dimensions.
The oral microbiome is more than just an ecosystem of bacteria. It communicates with the brain through what scientists call the oral microbiota-brain axis.
Changes in this microbial network may influence neurological pathways, potentially altering mood and cognitive states.
Insomnia severity in this study correlated with the presence of certain bacterial groups. Higher levels of Fusobacteria and Spirochaetota, for example, tracked closely with disrupted sleep. Healthy spouses began to show increased levels of these microbes after six months.
One especially interesting find was that a bacterium called Dialister – belonging to the Firmicutes family – was more abundant in women than men.
This suggests that women may absorb and reflect microbial changes more intensely than their male counterparts.
Scientists have long known that couples synchronize in fascinating ways. They may mirror each other’s heart rates, align their sleep cycles, or even exhibit similar cortisol rhythms.
This new research suggests that oral bacteria may form another point of connection, influencing depression among couples.
The study discusses “social closeness” as a factor in microbial exchange. This refers not just to physical proximity but emotional bonds that enable deeper contact.
The closer the relationship, the more likely it becomes for microbial and emotional states to influence one another.
This may explain why couples often seem to echo each other’s mental states over time. Depression and anxiety could ripple through households, not just via conversations and behaviors, but via silent bacterial messengers.
This research was conducted with care. The team excluded any participants taking antibiotics, pregnant women, or couples that had separated during the study period.
Oral samples were collected from the palatine tonsils and throat. The DNA was extracted and sequenced using industry-standard protocols.
Participants provided samples at the start and again six months later. Cortisol was measured using liquid chromatography-tandem mass spectrometry.
Statistical analyses included logistic regression, correlation testing, and microbial diversity evaluation. All procedures followed ethical guidelines set by the Iran National Science Foundation.
Despite these strengths, the researchers acknowledge limitations. They only collected morning saliva, didn’t account for shared dietary changes, and relied on self-reported mental health scores. Still, the trends they uncovered were consistent and compelling.
One implication is clear: emotional health within a couple may be more interconnected than we realized. Traditionally, doctors treat individuals.
But this study raises the possibility that treating both partners may be more effective when mental health symptoms appear in one.
The study references animal research that supports this view. Transferring gut bacteria from depressed mice into healthy ones produced depressive behaviors. Similarly, probiotics have improved sleep and mood in mice exposed to stress.
The findings suggest that microbial environments might trigger emotional shifts, at least under certain conditions.
“Fecal microbiota-induced insomnia and depression-like behaviors in mice can be alleviated by microbiota-modulating probiotics,” wrote the researchers.
If this effect translates to humans, we may need to rethink mental health therapy – not just for individuals, but for families.
This study hints at larger networks. The oral microbiome is linked to both the gut and ocular microbiomes.
People suffering from dry eye syndrome, for example, often have depression or anxiety. These conditions may share more than emotional roots – they may share microbial ones.
Such overlaps suggest that personalized medicine could one day include microbiome mapping. Couples might receive probiotic therapies together or undergo screenings that look at bacterial harmony in addition to emotional wellness.
This idea aligns with the goals of predictive and personalized care. If bacterial shifts can predict emotional changes, then we might have a new tool for early diagnosis and intervention.
Relationships, as it turns out, are not just a merging of hearts. They’re also a merging of ecosystems. When one partner carries the emotional burden of depression or anxiety, the other may feel it not just psychologically, but biologically.
This research does not suggest people should avoid relationships with depressed partners. Instead, it encourages a more compassionate and scientifically informed view of mental health.
If bacterial imbalances contribute to emotional distress, then treating those imbalances could offer hope.
Future studies will need to confirm whether these effects are causal. But if they are, they could shift how we define and support emotional health – not only in individuals, but in the shared space between them.
In the end, couples may influence each other in ways we’re only beginning to understand – and some of those influences might be microscopic.
The study is published in the journal in Exploratory Research and Hypothesis in Medicine.
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