A recent study published in Translational Psychiatry has found a link between the bacteria in our mouths and mental health. The researchers analyzed the salivary microbiome of people experiencing mental health symptoms, comparing it to the microbiome of mentally healthy people. Their analysis revealed remarkable differences in the composition of oral bacteria, suggesting that the oral microbiome may play an important role in mental well-being. These findings add to the growing body of evidence supporting the concept of the ‘mouth-brain axis’.
Mental health disorders are a major cause of the global health burden, affecting hundreds of millions of people worldwide. While advances in treatment have improved outcomes for many, barriers such as treatment resistance, partial responses, and high relapse rates underscore the need for innovative and complementary treatment strategies.
Recent research has turned attention to the human microbiome, the vast ecosystem of microorganisms that live in and on our bodies, as a potential factor influencing mental health. The gut microbiome, in particular, has been extensively studied for its role in the “gut-brain axis,” a two-way communication network that connects the gut to the brain. Changes in the composition and function of gut bacteria have been linked to various mental health conditions, suggesting that targeting the gut microbiome may offer new therapeutic opportunities.
However, the oral microbiome—another critical and diverse microbial community—has received comparatively less attention, despite its potential to influence systemic and brain health. The oral cavity harbors hundreds of bacterial species that interact with the host and other microbial communities.
“My research initially focused on how the gut microbiome affects mental health,” said the study’s author Stefanie Malan-Müllerpostdoctoral fellow at the Complutense University of Madrid. “Over the years, many studies have shown how the gut and brain communicate through the ‘gut-brain axis’, influencing mental well-being. But our bodies harbor microbes in many places, not just the gut. One of those key areas is the mouth, which is directly connected to the gut.”
“We also know that oral health affects the whole body – research shows, for example, that gum disease can increase the risk of heart disease. This got me wondering: could certain oral bacteria also play a role in mental health? That curiosity led to this study.”
The study analyzed saliva samples from 470 participants recruited from two population-based microbiome studies in Spain. Inclusion criteria ensured that all participants were residents of Spain aged 18 years or older, fluent in Spanish, and not recently exposed to antibiotics, which could alter their microbiome.
The sample included both mentally healthy individuals and individuals with symptoms of anxiety, depression, or post-traumatic stress disorder. Participants provided detailed self-reports of mental health and periodontal health and submitted saliva samples for analysis using 16S ribosomal RNA sequencing, a technique that identifies and categorizes bacteria based on their genetic material.
The researchers also collected blood samples from a subset of participants to measure levels of tryptophan and serotonin—compounds linked to mental health—and analyzed associations between these levels and oral bacterial composition.
The researchers found significant differences in the composition of the salivary microbiome between people with mental health symptoms and healthy controls. Participants with symptoms of post-traumatic stress disorder and depression showed higher levels of certain bacteria, such as Prevotella histicola. This bacteria, associated with oral inflammation, was also associated with lower interpersonal quality of life scores. Conversely, participants with anxiety disorders had lower levels Neisseria elongataa bacterium associated with oral and cardiovascular health.
“What happens in the mouth doesn’t stay in the mouth,” Malan-Müller told PsyPost. “Our results suggest a role for the mouth-brain axis—an idea that suggests the bacteria in our mouths could also play a critical role in mental well-being.”
Another key finding was the association between oral microbiota and early life trauma. For example, higher levels Streptococcus mutansa bacterium often associated with tooth decay, was found in people who reported experiencing emotional neglect in childhood.
“We also confirmed that gum health affects the oral microbiome,” explained Malan-Müller. “People with conditions such as gingivitis or periodontitis had distinct patterns of bacteria, including higher levels of Upright and Shuttleworthiaand lower levels Capnocytophaga.”
In addition, participants with anxiety disorders showed higher levels Oribacterium asaccharolyticumwhich has been implicated in both mouth and gut inflammation. Interestingly, periodontal health also influenced mental health treatment outcomes. Two types of oral bacteria, Upright and Haemophilus parainfluenzaewere associated with both self-reported gingivitis (periodontitis or gingivitis) and perceived effectiveness of psychotherapy.
Importantly, the researchers found evidence of a common biological pathway linking periodontal health and mental health, involving tryptophan metabolism. Participants with symptoms of depression, anxiety or post-traumatic stress disorder showed lower levels of tryptophan breakdown, which is critical for serotonin production. Reduced tryptophan metabolism was also observed in participants with severe gingivitis, linking periodontal health and mental health through a common metabolic pathway.
“A surprising finding was that we did not identify a single bacterium that was directly linked to both mental health and gum health.” Malan-Müller said. “However, we discovered a possible common biological process performed by microbes that links the two: the breakdown of tryptophan, an important amino acid.”
“Using the PICRUSt assay, we identified microbial functions associated with mental health symptoms and gingivitis symptoms, revealing reduced tryptophan breakdown in individuals with PTSD, childhood trauma, poor social relationships, and in individuals with possible gum disease. Reduced breakdown of tryptophan can lead to lower levels of serotonin – a key chemical involved in mood regulation. “Interestingly, we also found lower levels of serotonin in the blood of participants with mental health symptoms, suggesting a possible metabolic link between oral bacteria and mental health.”
However, it is important to note that the study’s cross-sectional design also limits the ability to determine causality—whether changes in the oral microbiome contribute to or result from mental health symptoms. It is also possible that a third factor influences both.
“An important caveat is that our study only shows a relationship between the types of bacteria in the mouth and mental health symptoms at a specific point in time,” Malan-Müller said. “To determine whether bacteria actually contribute to these symptoms, future research will need to track changes over time and explore how these relationships evolve.”
Future studies could also explore the mechanisms linking oral bacteria to mental health, such as their role in systemic inflammation or neurotransmitter regulation. Investigating whether improving oral health through better hygiene or targeted interventions could enhance mental well-being is another promising direction.
“Based on our findings, an exciting area for future research is to explore the potential of dental care and oral hygiene as therapeutic tools to improve mental health.” explained Malan-Müller. “I am currently involved in a randomized controlled trial investigating this approach. Furthermore, it is important to understand the biological pathways linking the oral microbiome to mental health. We need to investigate how specific types of bacteria in the mouth can affect brain function and emotional regulation.”
“As we unravel the mysteries of the mouth-brain axis, we invite you to consider the importance of maintaining good oral health—not only for a bright smile but also for mental well-being,” he added.
The study, “Exploring the mouth-brain connection: Oral microbiome patterns in a large community with anxiety, depression and trauma symptoms and periodontal outcomes”, written by Stefanie Malan-Müller, Rebeca Vidal, Esther O’Shea, Eduardo Montero, Elena Figuero, Iñaki Zorrilla, Javier de Diego-Adeliño, Marta Cano, Maria Paz García-Portilla, Ana González-Pinto and Juan. Word.
