Grace Bowland and Laura Weyrich propose an oral microbiome-brain axis in Frontiers in Psychiatry, a framework for how mouth microbes may affect the brain
This post looks at a 2022 review in Frontiers in Psychiatry on how microbes in the mouth may affect the brain and mental health. Its authors are Penn State anthropologists Grace Bowland and Laura Weyrich.
Studies of the gut-brain axis show that gut microbes can affect the brain. Bowland and Weyrich ask whether the mouth does too. Their review lays out a framework they call an oral microbiome-brain axis. Weyrich led a 2017 study in Nature that read Neanderthal diet and disease from DNA in dental plaque.
In October 2024, a team at Complutense University of Madrid compared the saliva microbiome of 306 people reporting mental health symptoms with 164 controls. Certain bacteria tracked with PTSD and depression scores and with anxiety diagnoses. On March 4, 2025, a study in the Journal of Affective Disorders of 6,212 people in a US national health survey linked higher oral microbiome diversity to lower odds of depression. Both found associations, not causes.
The authors propose four links between mouth microbes and the brain.
Figure 1 of the review links each proposed pathway to a mechanism and to outcomes in neuropsychiatric disorders. Image: Bowland and Weyrich, Frontiers in Psychiatry, CC BY 4.0
The authors are clear about the limits. They write that "the mechanisms and pathways that illuminate how oral microbiota and brain communicate in NPDs remain unknown." The paper reports no new data.
They also argue that the field studies too narrow a slice of humanity. Most microbiome studies draw on industrialized people in the United States or China. People from Western, educated, industrialized, rich and democratic (WEIRD) societies make up less than 20% of the world. Yet they have historically made up 60 to 90% of subjects in psychology studies. Diet, culture and environment shape the oral microbiome, so the authors call for anthropologists to join this research.
The review names probiotics, oral microbial transplants and environmental exposure as directions for research. The authors suggest diet as one area for future research. Of oral microbial transplants, they write that "this has not been tested in humans." These are ideas for future studies. None of them is a proven treatment yet.
Since this post, a June 2025 study of more than 15,000 US adults, reported by NYU, linked lower oral microbiome diversity to depressive symptoms. Its authors say it is not clear which way the effect runs.
The review reads as a research agenda. Its call to study people beyond the United States and China follows from its own evidence that diet and place shape the microbes in our mouths. I later applied the same ecological idea, dysbiosis, to science communication in a Comment in Nature Microbiology.
Grace B. Bowland and Laura S. Weyrich, "The Oral-Microbiome-Brain Axis and Neuropsychiatric Disorders: An Anthropological Perspective," Frontiers in Psychiatry 13:810008, published March 30, 2022. doi:10.3389/fpsyt.2022.810008.
In a Comment for Nature Microbiology's June 2026 focus issue on science communication, Dr. Raven Baxter argues that the information ecosystem around science is out of balance and can recover.
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