A recent study published in Translational Psychiatry has found a connection between the bacteria in our mouths and mental health. Researchers analyzed the salivary microbiome of individuals experiencing mental health symptoms comparing it to the microbiome of mentally healthy individuals. Their analysis revealed notable differences in the composition of oral bacteria, suggesting that the oral microbiome may play a significant role in mental well-being. These findings contribute to the growing body of evidence supporting the concept of the “oral-brain axis.”
Mental health disorders are a leading cause of global health burden, affecting hundreds of millions of people worldwide. While advancements in treatment have improved outcomes for many, obstacles such as treatment resistance, partial responses, and high relapse rates highlight the need for innovative and complementary therapeutic strategies.
Recent research has turned attention to the human microbiome, the vast ecosystem of microorganisms living 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 bidirectional communication network connecting the gut and 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 might 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 is home to hundreds of bacterial species that interact with the host and other microbial communities.
“My research originally focused on how the gut microbiome influences mental health,” said study author Stefanie Malan-Müller, a postdoctoral 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,’ affecting mental well-being. But our bodies host microbes in many places, not just the gut. One of these key areas is the mouth, which is directly connected to the gut.”
“We also know that oral health impacts the whole body—research shows, for example, that gum disease can increase the risk of heart disease. This made me wonder: 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 Spanish population-based microbiome studies. The recruitment criteria ensured that all participants were Spanish residents aged 18 or older, fluent in Spanish, and not recently exposed to antibiotics, which could alter their microbiome.
The sample included both mentally healthy individuals and those experiencing symptoms of anxiety, depression, or posttraumatic stress disorder. The participants provided detailed self-reports on 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 individuals with mental health symptoms and healthy controls. Participants with symptoms of posttraumatic stress disorder and depression exhibited higher levels of certain bacteria, such as Prevotella histicola. This bacterium, associated with oral inflammation, was also linked to lower interpersonal quality of life scores. Conversely, participants with anxiety disorders had lower levels of Neisseria elongata, a 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 oral-brain axis—a concept proposing that 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 instance, higher levels of Streptococcus mutans, a bacterium often linked to dental decay, were found in individuals who reported experiencing childhood emotional neglect.
“We also confirmed that gum health impacts the oral microbiome,” Malan-Müller explained. “People with conditions like gingivitis or periodontitis had distinct patterns of bacteria, including higher levels of Eggerthia and Shuttleworthia, and lower levels of Capnocytophaga.”
Additionally, participants with anxiety disorders showed higher levels of Oribacterium asaccharolyticum, which has been implicated in both oral and gut inflammation. Interestingly, periodontal health also influenced mental health treatment outcomes. Two types of oral bacteria, Eggerthia and Haemophilus parainfluenzae, were linked to both self-reported gum disease (periodontitis or gingivitis) and the perceived effectiveness of psychotherapy.
Importantly, the researchers found evidence of a shared biological pathway linking periodontal health and mental health, involving tryptophan metabolism. Participants with symptoms of depression, anxiety, or posttraumatic stress disorder exhibited lower levels of tryptophan degradation, which is critical for serotonin production. Reduced tryptophan metabolism was also observed in participants with severe gum disease, linking periodontal health and mental health through a common metabolic pathway.
“One surprising finding was that we didn’t identify a single bacteria directly linked to both mental health and gum health,” Malan-Müller said. “However, we discovered a potential common biological process performed by microbes that connects the two: the breakdown of tryptophan, an important amino acid.”
“Using PICRUSt analysis, we identified microbial functions associated with mental health symptoms and symptoms of gum disease, revealing reduced tryptophan breakdown in people with PTSD, childhood trauma, poor social relationships, and those with possible gum disease. Reduced tryptophan breakdown can lead to lower serotonin levels—a key chemical involved in regulating mood. Interestingly, we also found lower serotonin levels 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 cross-sectional design of the study also limits the ability to determine causation—whether changes in the oral microbiome contribute to mental health symptoms or result from them. It is also possible that a third factor influences both.
“One important caveat is that our study only shows a link between the types of bacteria in the mouth and mental health symptoms at a single point in time,” Malan-Müller said. “To determine whether the 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 investigate the mechanisms linking oral bacteria to mental health, such as their role in systemic inflammation or neurotransmitter regulation. Exploring whether improving oral health through better hygiene or targeted interventions could enhance mental well-being is another promising direction.
“Based on our findings, one exciting area for future research is exploring the potential of dental care and oral hygiene as therapeutic tools for improving mental health,” Malan-Müller explained. “I am currently involved in a randomized controlled trial investigating this approach. Additionally, it’s important to understand the biological pathways linking the oral microbiome to mental health. We need to explore how specific types of bacteria in the mouth may influence brain function and emotional regulation.”
“As we unravel the mysteries of the oral-brain axis, we invite you to consider the importance of maintaining good oral health—not just for a bright smile but also for mental well-being,” she added.
The study, “Probing the oral-brain connection: oral microbiome patterns in a large community cohort with anxiety, depression, and trauma symptoms, and periodontal outcomes,” was authored 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 C. Leza.
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