Oral Microbiota in Patients with Alzheimer’s Disease: A Systematic Review

Oral microorganisms have been found in the cerebral milieu, suggesting the involvement of oral microbiota in the onset and course of Alzheimer’s Disease (AD) through mechanisms such as amyloid-beta accumulation, tau phosphorylation and neuroinflammation. It is still uncertain whether and which oral bacteria are associated with AD. Therefore, the aim of this systematic review was to assess the current evidence for associations between oral microbiota and AD. A database search in Pubmed and Embase resulted in 623 hits. After removing duplicates, 437 papers remained. Of these, 13 papers passed the inclusion criteria and were included for quality/risk of bias assessment and data extraction. Analysis of these 13 studies revealed high heterogeneity in terms of sample size, age, study design and microbiological methods. Quality assessment using the MINORS criteria indicated reasonable to good quality across studies. As a result of the omission of two of the criteria, the quality results may have been biased. There is no conclusive evidence as to if and which oral microbiota are associated with AD since many conflicting results were reported. Although the overall quality of the studies was acceptable, the studies differed in study design and protocol. Further research is needed to clarify this association.