A mechanistic framework linking the oral microbiome to Alzheimer's disease through neuroinflammation

Alzheimer's disease (AD) is a growing problem in our society and the most common form of dementia. This neurodegenerative disease is characterized by neuroinflammation and the accumulation of amyloid-β (Aβ) and tau. Previous studies have found associations between the oral microbiome and AD. This review aims to elucidate the role of the oral microbiome in AD, through neuroinflammation, and reviews the relationship between AD and bacteria and fungi. Studies have found bacteria (e.g., Porphyromonas gingivalis) and fungi (e.g., Candida albicans) in postmortem AD brains. Moreover, mice models have shown that oral microbes are able to cross the blood-brain barrier (BBB), and were correlated with activated microglia, neuroinflammation, and Aβ load. This review introduces a mechanistic framework that describes how oral microbes cause an inflammatory response resulting in AD pathology. Specifically, oral dysbiosis causes oral pathogens to disseminate into the bloodstream, this triggers an inflammatory response, subsequently activating microglia, ultimately resulting in AD pathology. This process can follow two pathways: First, there is a direct response of the immune system in the brain to oral pathogens that migrate through the bloodstream and cross the BBB, which causes neuroinflammation and activates microglia, leading to AD pathology. Second, an early-life systemic inflammation causes microglia to get into a "hyperactive" state, in which they respond in an exaggerated way to normal stimuli triggering immune responses throughout a person's life that result in AD pathology. This mechanistic framework provides new line of thought for future research on the question of causality of AD.