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 from the US. However, this imbalance in oral bacteria was not associated with another Alzheimer’s biomarker called tau.
The study by a team from NYU College of Dentistry and Weill Cornell Medicine—and published in the journal Alzheimer’s & Dementia: Diagnosis, Assessment & Disease Monitoring—adds to the growing evidence of a connection between periodontal disease and Alzheimer’s.
“To our knowledge, this is the first study showing an association between the imbalanced bacterial community found under the gumline and a CSF biomarker of Alzheimer’s disease in cognitively normal older adults,” the study’s lead author Angela Kamer said.
“The mouth is home to both harmful bacteria that promote inflammation and healthy, protective bacteria. We found that having evidence for brain amyloid was associated with increased harmful and decreased beneficial bacteria.”
Alzheimer’s disease is characterised 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 mechanisms by which levels of brain amyloid accumulate and are associated with Alzheimer’s pathology are complex and only partially understood. The present study adds support to the understanding that proinflammatory diseases disrupt the clearance of amyloid from the brain, as retention of amyloid in the brain can be estimated from CSF levels,” the study’s senior author Professor Mony de Leon said.
“Amyloid changes are often observed decades before tau pathology or the symptoms of Alzheimer’s disease are detected.”
The researchers studied 48 healthy, cognitively normal adults aged 65 and older. Participants underwent oral examinations to collect bacterial samples from under the gumline, and lumbar puncture was used to obtain CSF in order 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.
Analysing the bacterial DNA of the samples taken from beneath the gumline, the researchers quantified bacteria known to be harmful to oral health and pro-oral health bacteria.
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 signature of reduced CSF amyloid levels. The researchers hypothesise that because high levels of healthy bacteria help maintain bacterial balance and decrease inflammation, they may be protective against Alzheimer’s