Oral Microbiome Disturbance, Salivary Dysfunction, and Cognitive Health in Older Adults: An Academic Review of the Oral-Brain Axis

Multimorbidity, polypharmacy, decreased salivary flow, tooth loss, periodontal disease, denture use, changes in diet, diminished self-care, and immunosenescence represent the oral physiology changes related to ageing. All of these factors cause reconfiguration of the oral microbial ecosystem. Biological state relevant isn't just reduced amount of ‘normal flora’ but depleted ecological stability, reduced in numbers of good flora, increased presence of ‘pathobionts', depleted in numbers of nitrate-reducing flora and chronic periodontal inflammation. Thus, this review aims at exploring the potential linkage which may exist between age-associated, oral dysbiosis and cognitive impairment, dementia and Alzheimer disease. Mechanisms for their spread have been proposed to involve systemic exposure to inflammatory mediators and microbial products, microperfusion, blood-brain barrier disruption, activation of microglia, amyloidogenic responses, tau-related injury, involvement of vascular pathways, nitric-oxide pathways, nutritional and/or quality of life decline, and reduced mastication after tooth loss. There is general support for a relationship between periodontitis and loss of teeth, oral microbial signatures and cognitive decline in observational studies and systematic reviews, although causality has not been shown with the majority of the decline in cognition thought to cause oral decline. Although till now, evidence does not support microbiome-directed interventions, they should be part of oral health assessment and periodontal care that forms part of healthy ageing. Future longitudinal studies need to include standardized dental phenotyping, saliva measurements, shotgun metagenomics, inflammatory measures and trajectories, cognitive trajectories, medication data and trials of intervention.