This systematic review aims to investigate the role of the oral microbiome in the pathophysiology of mental health disorders and to appraise the methodological quality of research of the oral–brain axis which is a growing interest area. The PRISMA guideline was adopted, to carry out an electronic search through the MEDLINE database, to identify studies that have explored the role of the oral microbiome in the pathophysiology of mental health disorders published from 2000 up to June 2020. The search resulted in 140 records; after exclusions, a total of 22 papers were included in the present review. In accordance with the role of the oral microbiome in the pathophysiology of mental disorders, four mental disorders were identified: Alzheimer’s disease, dementia, and cognitive disorders; autism spectrum disorder; Down’s syndrome and mental retardation; and Bipolar disorders. Studies argue for correlations between oral microbiota and Alzheimer’s disease, autism spectrum disorders, Down’s syndrome, and bipolar disorders. This field is still under-studied, and studies are needed to clarify the biological links and interconnections between the oral microbiota and the pathophysiology of all mental health disorders. Researchers should focus their efforts to develop research on the oral–brain axis in the future.

This review aims to clarify the nature of the link between Alzheimer’s disease and the oral microbiome on an epidemiological and pathophysiological level, as well as to highlight new therapeutic perspectives that contribute to the management of this disease. We performed a systematic review, following the Preferred Reporting Items for Systematic Reviews checklist, from January 2000 to July 2021. The terms “plaque,” “saliva,” and “mouth” were associated with the search term “oral diseases” and used in combination with the Boolean operator “AND”/”OR”. We included experimental or clinical studies and excluded conferences, abstracts, reviews, and editorials. A total of 27 articles were selected. Evidence for the impact of the oral microbiome on the pathophysiological and immunoinflammatory mechanisms of Alzheimer’s disease is accumulating. The impact of the oral microbiome on the development of AD opens the door to complementary therapies such as phototherapy and/or the use of prebiotic compounds and probiotic strains for global or targeted modulation of the oral microbiome in order to have a favourable influence on the evolution of this pathology in the future.

This systematic review aims to identify probiotics and prebiotics for modulating oral bacterial species associated with mental disorders. Using the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guideline, we search the electronic MEDLINE database published till January 2021 to identify the studies on probiotics and/or prebiotics for preventing and treating major oral dysbiosis that provokes mental disorders. The outcome of the search produces 374 records. After excluding non-relevant studies, 38 papers were included in the present review. While many studies suggest the potential effects of the oral microbiota on the biochemical signalling events between the oral microbiota and central nervous system, our review highlights the limited development concerning the use of prebiotics and/or probiotics in modulating oral dysbiosis potentially involved in the development of mental disorders. However, the collected studies confirm prebiotics and/or probiotics interest for a global or targeted modulation of the oral microbiome in preventing or treating mental disorders. These outcomes also offer exciting prospects for improving the oral health of people with mental disorders in the future.

Psychobiological research is a systems approach that aims to integrate the biological, psychological and social systems that may influence health or pathology, particularly in chronic diseases and physical and/or psychiatric disorders. In this approach, we can expect to be able to deduce a ‘biological signature’ associated with particular symptom clusters. Similarly, psychosocial factors such as life events, health attitudes and behaviours, social support, psychological well-being, spirituality and personality are to be considered in terms of their influence on individual vulnerability to disease. At the psychophysiological level, it is important to understand, for example, the pathways that link the effects of chronic stress, social support and health, through the neuroendocrine and autonomic mechanisms that determine stress responses. At the macroscopic level, the role of individual socio-demographic variables such as personality, treatment modalities and health promotion through psycho-educational interventions needs to be explored.

The Schizophrenia Oral Health Profile questionnaire was developed to assess the oral health-related quality of life among individuals with schizophrenia based on their perceptions rather than from caregivers. A 5-point Likert scale was used to self-report on 42 items. In the present study, different analyses were conducted to determine the dimensional structure of the final scale: (1) inter-item correlation analysis and Cronbach’s α coefficient, (2) Rasch model analysis, (3) exploratory factor analysis and (4) confirmatory factor analysis. The final version of the Schizophrenia Oral Health Profile questionnaire consisted of 20 items and an internal structure composed of three dimensions: (1) emotions related to oral health, (2) oral pain and discomfort and (3) self-image, others’ views and the need for care. We showed that the difficulty and discrimination indices of each of the 20 selected items were acceptable according to the Rasch model, as well as their inter-item and inter-score correlations (α = 0.875). The psychometric study of the Schizophrenia Oral Health Profile questionnaire is still in progress to investigate reproducibility, sensitivity to change and external structure.