The oral cavity harbours a plethora of bacteria and dysbiosis can result in disease. Periodontitis is a chronic multi-bacterial infection that affects nearly 50% of the global population and elicits low-grade systemic inflammation via pro-inflammatory cytokine release and the invasion of periodontal pathogens. Periodontitis thus causes and exacerbates other chronic systemic inflammatory diseases. A common feature of most neuropsychiatric disorders (NPDs) is neuroinflammation and researchers reported associations between periodontitis and psychiatric disorders, thereby proposing a role for the oral-brain axis. A large-scale cohort study (n > 60 000 with a 10-year follow-up) supports the causal link between periodontitis and major depression: a higher incidence of subsequent depression was observed in a periodontitis group compared to a non-periodontitis group. In addition, the stress hormone cortisol not only influences immune responses but also alters the oral microbial community. We hypothesize that the bidirectional oral-microbiota-brain axis may be involved in the mechanisms that drive the inflammatory profile observed in NPDs.

The importance of the microbiota (all the microorganisms such as bacteria, eukaryotes, archaea, and viruses within/on the human body) in health and disease has been in the spotlight in recent years, with the gut-microbiota-brain axis being of particular interest in mental health research. Investigations of the gut microbiota is currently leading in terms of the number of publications. However, the significance of the oral microbiota and the mechanisms whereby it can influence immune signaling, neuro-inflammation and behavior, and how stress, in turn, influences the oral microbiota within the oral-microbiota-brain axis, is now coming to the fore. This Research Topic aims to clarify the role of the bidirectional oral-microbiota-brain axis in mental health disorders and to determine whether the oral microbiota could be targeted for adjunctive therapies to improve psychiatric symptoms.

We welcome articles that focus on the oral microbiota and mechanisms whereby it can influence the central nervous system, behaviour and mental health; how stress alters the oral microbial landscape; or ways the oral microbiota could be targeted as a therapeutic intervention to improve psychiatric symptoms.
Types of articles include:

· Original research articles (pre-clinical and clinical)

· Clinical trials (where effects of the drug on the oral microbiota is included in the outcomes)

· Review articles related to this specific topic

· Methods papers related to this specific topic

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Frontiers in Psychiatry

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