Presence of Mouth Bacteria Linked to Risk of Alzheimer’s, Study Shows

A connection discovered between Alzheimer’s and the bacteria that cause gum disease suggests that improved oral hygiene may decrease the risk of developing the disorder, a study reports.

The study, “Porphyromonas gingivalis in Alzheimer’s disease brains: Evidence for disease causation and treatment with small-molecule inhibitors,” was published in the journal Science Advances.

Figuring out exactly what causes Alzheimer’s has long been a goal of researchers, and some have suggested that the disease may be at least partially attributed to infections. This association is based largely on the fact that Alzheimer’s brains tend to have inflammation comparable to what might be expected in an infected brain.

The new study implicates the bacterium Porphyromonas gingivalis in Alzheimer’s. This bacterium is one of the main causes of periodontitis — a disease characterized by inflamed gums. The research suggests that it can migrate from the mouth to the brain and, once in the brain, secrete toxins that cause damage to brain cells.

Previous research had actually demonstrated this migratory capacity in mice — this new study shows that P. gingivalis is also detectable in the brains of humans with Alzheimer’s.

P. gingivalis produces major virulence factors known as gingipains, which consist of lysine-gingipain (Kgp), arginine-gingipain A (RgpA), and arginine-gingipain B (RgpB). Gingipains are secreted by the bacteria and released to the outside cellular environment inside small vesicles.

Researchers tested 53 brain samples for the presence of bacterial proteins. Depending on which of two proteins they looked for, Kgp or RgpB, 91% or 96% of Alzheimer’s brains were positive, respectively. In contrast, among control non-demented brains, only 52% or 39% were positive for either Kgp or RgpB, respectively.

Additionally, on average, Alzheimer’s brains had higher levels of bacterial protein, although this was just a trend; some Alzheimer’s brains had relatively low levels, and vice versa.

In addition, the researchers identified P. gingivalis DNA in the cerebrospinal fluid (CSF) — the liquid surrounding the brain and spinal cord — of living patients diagnosed with probable Alzheimer’s, “suggesting that CSF P. gingivalis DNA may serve as a differential diagnostic marker,” they wrote.

They also used molecular tests to demonstrate that bacterial DNA could be detected in samples of Alzheimer’s brains, as well as in most of the control brains tested. This solidifies the notion that these bacteria can and do infect human brains.

“We discovered DNA-based proof that the bacteria causing gingivitis can move from the mouth to the brain,” said Piotr Mydel, MD, PhD, professor at the University of Bergen in Norway and co-author of the paper, in a press release.

Of course, these data do not suggest that P. gingivalis is the lone cause of Alzheimer’s — but they do suggest that infections with this species of bacteria could increase a person’s risk of developing it.

This, in turn, suggests that improved oral hygiene could reduce Alzheimer’s risk: “Brush your teeth and use floss,” Mydel said.

Additionally, the detailed understanding of how these bacteria cause damage may eventually lead to strategies to actually treat Alzheimer’s, or at least mitigate some of the damage.

“We have managed to develop a drug that blocks the harmful enzymes from the bacteria, postponing the development of Alzheimer’s. We are planning to test this drug later this year,” Mydel said.