Human oral microbiome and its influence on mental health and brain disorders.

The human oral microbiome can affect brain functions directly through the trigeminal nerve and olfactory system and indirectly via the oral-gut-brain axis. However, the potential link between the oral microbiome and mental health remains an area that requires further investigation. Taking into consideration that gut microbiota dysbiosis plays a role in the onset and progression of several mental disorders, as well as the potential influence of the oral microbiome on mental health via direct pathways, the present narrative review explores the role of the human oral microbiome in health and disease, along with the factors that affect its composition, with a particular focus on its potential impact on mental health, including its involvement in a range of mental disorders and brain-related conditions, such as Alzheimer's disease, Parkinson's disease, autism spectrum disorder, anxiety, depression, stress, bipolar disorder, Down's syndrome, cerebral palsy, epilepsy, and schizophrenia. Chronic oral diseases can impair the oral mucosal barrier, allowing microorganisms and endotoxins to enter the bloodstream, triggering systemic inflammation, and affecting the blood-brain barrier. This pathway can lead to neuroinflammation and cognitive dysfunction and contribute to adverse mental health effects. Additionally, translocation of oral bacteria to the gut can drive persistent inflammation and thereby affect brain health. Multiple studies suggest a potential relationship between the oral microbiome and several mental disorders, but further research is needed to strengthen the evidence surrounding these associations and to fully clarify the underlying mechanisms linking the oral microbiome to these conditions. Given the promising implications, future research should focus on elucidating the biological mechanisms through which alterations in the oral microbiome influence the development and progression of determinate neurodegenerative and neuropsychiatric disorders. Additionally, identifying reliable biomarkers linked to the oral microbiome could enhance early detection, diagnosis, and monitoring of these conditions.