Is there a link between oral bacteria and the development of Alzheimer's disease?

Poor gum health and tooth loss are associated with an approximately 20 percent higher risk of cognitive decline and dementia. This is the finding of a meta-analysis of 47 longitudinal studies. The overall quality of evidence was low, however: the studies differed considerably from one another, and the risk of reverse causality is real. After all, Alzheimer's patients forget their brushing habits, so poor oral health may be a consequence of the disease rather than a cause.

One specific oral bacterium, P. gingivalis, recurs across multiple studies as a suspect. Post-mortem brain tissue from Alzheimer's patients contained this bacterium, and the quantity of toxic enzymes (gingipains) it produces was associated with the severity of the characteristic protein accumulations in the brain. In mouse experiments, infection via the mouth caused brain colonisation and increased amyloid production. A systematic review of 23 studies confirms this pattern. Importantly, causality in humans has not yet been proven, and one of the early publications on gingipains came from researchers affiliated with a pharmaceutical company that had a commercial interest in the findings.

An observational cohort study of 60 Alzheimer's patients found that those who had gum inflammation at baseline declined six times faster on a cognitive test over six months. This association was accompanied by higher inflammatory markers in the blood. It is a single small study of only six months, so the finding needs to be replicated in larger groups.

Multiple reviews describe how bacteria, or the substances they produce, can reach the brain via the bloodstream or along nerve pathways and trigger inflammation there. That mechanism is biologically plausible, but there are still no large clinical trials in humans demonstrating that gum disease treatment slows or prevents Alzheimer's disease. Researchers explicitly call for such trials.