Gum bacteria imbalance linked to Alzheimer's disease biomarker, study finds

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Older adults with more harmful than healthy bacteria in their gums are more likely to have evidence for amyloid beta, a key biomarker for Alzheimer's disease, in their cerebrospinal fluid (CSF), according to new research by New York University published in the journal Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring. However, this imbalance in oral bacteria was not associated with another Alzheimer's biomarker, tau.

Alzheimer's disease is characterized by two hallmark proteins in the brain: amyloid beta, which clumps together to form plaques and is believed to be the first protein deposited in the brain as Alzheimer's develops, and tau, which builds up in nerve cells and forms tangles, the researchers said.

For the study, the researchers studied 48 healthy, cognitively normal adults ages 65 and older. Participants underwent oral examinations to collect bacterial samples from under the gumline, and lumbar puncture was used to obtain CSF to determine the levels of amyloid beta and tau. To estimate the brain's expression of Alzheimer's proteins, the researchers looked for lower levels of amyloid beta, which translate to higher brain amyloid levels) and higher levels of tau (which reflect higher brain tangle accumulations, in the CSF.

Analyzing the bacterial DNA of the samples taken from beneath the gumline, the researchers quantified bacteria known to be harmful to oral health (e.g. Prevotella, Porphyromonas, Fretibacterium) and pro-oral health bacteria (e.g. Corynebacterium, Actinomyces, Capnocytophaga).

The results showed that individuals with an imbalance in bacteria, with a ratio favoring harmful to healthy bacteria, were more likely to have the Alzheimer's disease signature of reduced CSF amyloid levels. The researchers hypothesize that because high levels of healthy bacteria help maintain bacterial balance and decrease inflammation, they may be protective against Alzheimer's disease.

The study adds to the growing evidence of a connection between periodontal disease or gum disease and Alzheimer's disease. Periodontal disease, which affects 70 percent of adults 65 and older, according to estimates from the U.S. Centers for Disease Control and Prevention, is characterized by chronic and systemic inflammation, with pockets between the teeth and gums enlarging and harboring bacteria.

The researchers did not find an association between gum bacteria and tau levels in this study, so it remains unknown whether tau lesions will develop later or if the subjects will develop the symptoms of Alzheimer's disease. The researchers plan to conduct a longitudinal study and a clinical trial to test if improving gum health, through deep cleanings to remove deposits of plaque and tartar from under the gumline, can modify brain amyloid and prevent Alzheimer's disease.

"Our results show the importance of the overall oral microbiome--not only of the role of 'bad' bacteria, but also 'good' bacteria--in modulating amyloid levels," said Angela Kamer, DDS, PhD, lead authorof the study and associate professor of periodontology and implant dentistry at NYU College of Dentistry, in a statement. "These findings suggest that multiple oral bacteria are involved in the expression of amyloid lesions."