New study links bacteria in the mouth to mental health symptoms

A recent study published in Translational psychiatry discovered a link between the bacteria in our mouth and mental health. Researchers analyzed the salivary microbiome of individuals with mental health symptoms by 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 an important 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 the global health burden, affecting hundreds of millions of people worldwide. Although therapeutic advances have improved outcomes for many people, barriers such as treatment resistance, partial responses, and high relapse rates highlight the need for innovative and complementary therapeutic strategies.

Recent research has focused attention on 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 widely studied for its role in the “gut-brain axis,” a bidirectional communication network connecting the gut and brain. Changes in the composition and function of gut bacteria have been associated with a variety of mental health conditions, suggesting that targeting the gut microbiome may offer new therapeutic opportunities.

However, the oral microbiome – another essential 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 initially focused on how the gut microbiome influences mental health,” said study author Stefanie Malan-Müller, a postdoctoral researcher at the Complutense University of Madrid. “Over the years, many studies have shown how the gut and brain communicate via the ‘gut-brain axis’, thereby affecting mental well-being. But our bodies harbor microbes in many places, not just the gut. One of these key areas is the mouth, which is directly connected to the intestine.

“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 got me thinking: could certain oral bacteria also play a role in mental health? This curiosity led to this study.

The study analyzed saliva samples from 470 participants recruited in two microbiome studies of the Spanish population. Recruitment criteria ensured that all participants were Spanish residents aged 18 years or older, fluent in Spanish, and not recently exposed to antibiotics that could alter their microbiome.

The sample included both mentally healthy people and people with symptoms of anxiety, depression or post-traumatic stress disorder. Participants provided detailed self-assessments 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.

Researchers found significant differences in the composition of the salivary microbiome between individuals with mental health symptoms and healthy controls. Participants with symptoms of post-traumatic stress disorder and depression had 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 of Neisseria elongateda bacteria 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 bacteria in our mouths may also play a critical role in mental well-being.”

Another key finding was the association between the oral microbiota and early life trauma. For example, higher levels of Streptococcus mutansa bacteria often linked to tooth decay, have been found in people who reported suffering from emotional neglect as children.

“We also confirmed that gum health impacts the oral microbiome,” Malan-Müller explained. “People with conditions such as gingivitis or periodontitis had distinct bacteria profiles, including higher levels of Eggerthie And Shuttleworthiaand lower levels of Capnocytophaga.”

Additionally, participants with anxiety disorders showed higher levels of Oribacterium asaccharolyticumwhich has been implicated in oral and intestinal inflammation. Interestingly, periodontal health also influenced mental health treatment outcomes. Two types of oral bacteria, Eggerthie And Haemophilus parainfluenzaewere related to both self-reported gum disease (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 had lower levels of breakdown of tryptophan, essential 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.

“A surprising finding is that we did not identify any bacteria directly linked to mental health and gum health.” » says Malan-Müller. “However, we discovered a potential common biological process carried out by microbes that connects the two: the degradation of tryptophan, an important amino acid.

“Through PICRUSt analysis, we identified microbial functions associated with mental health symptoms and gum disease symptoms, revealing reduced tryptophan breakdown in people with PTSD, childhood trauma, poor relationships social and in those suffering from 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 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 causality, that is, 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 link between the types of bacteria in the mouth and mental health symptoms at one 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 investigate 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 improve 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.” Malan-Müller explained. “I am currently participating in a randomized controlled trial studying this approach. Additionally, it is 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 can 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 only for a dazzling smile, but also for mental well-being,” a she added.

The study, “Probing the oral-brain connection: Oral microbiome patterns in a large community cohort with symptoms of anxiety, depression, and trauma, as well as 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.