No, we don’t know that gum disease causes Alzheimer’s

Do you floss regularly? A study published January 23 in Science Advances — and the news stories that it inspired — might have scared you into better oral hygiene by claiming to find a link between gum bacteria and Alzheimer’s disease.

Those experiments hinted that the gum disease–causing bacteria Porphyromonas gingivalis was present in the brains of a small number of people who died with the degenerative brain disease. Some headlines trumpeted that the cause of Alzheimer’s had finally been found.

Enzymes made by P. gingivalis, called gingipains, interact with key Alzheimer’s proteins called amyloid-beta and tau in test tube experiments and in the brains of mice, the researchers found. Gingipains prod A-beta to accumulate and tau to behave abnormally, both signs of Alzheimer’s disease in people, the experiments suggest. And compounds that block gingipains seemed to reduce the amount of A-beta in the infected mice. The findings “offer evidence that P. gingivalis and gingipains in the brain play a central role” in the development of Alzheimer’s disease, the researchers write in their study. The research was paid for and conducted in part by employees of Cortexyme, Inc., a San Francisco–based biotech company that’s developing these compounds.

The results fit with an idea that’s gaining traction among Alzheimer’s researchers — that bacteria, viruses and even fungi could spark the disease (SN: 7/21/18, p. 10). But the Science Advances study is far from conclusive, cautions Rudolph Tanzi, an Alzheimer’s researcher at Massachusetts General Hospital in Boston. Science News asked him what the study can, and can’t, answer about Alzheimer’s disease. His responses are edited for length and clarity.

Do we now know what causes Alzheimer’s disease?

No. “It would be a complete fantasy to say that now we’ve solved Alzheimer’s based on this,” Tanzi says. “People need to know that this was a small study.... It’s way too early to say that this result is valid.  We need to see many more samples. We need much more replication.”

Headlines that claim gum bacteria causes Alzheimer’s disease stretch the science way too far, he says. “It got out of hand. People should not be freaking out just because they didn’t floss enough. It doesn’t mean you’re going to get Alzheimer’s.”

But did it make sense to look at whether gum bacteria play a role in Alzheimer’s?

Yes. Tanzi and his colleagues suspect that Alzheimer’s is kicked off by brain inflammation, perhaps prodded along by bacteria, viruses or fungi. His team has been looking at large swaths of genetic material found in brains to figure out exactly which infectious entities might be in the brains of people with Alzheimer’s.

“We went in expecting to see periodontal bacteria in the brain. That was a leading hypothesis. One of the biggest pools of bacteria in your body lives in your gums if your gums are not clean. We expected to find them, but we didn’t.” (Those negative results, from dozens of brains, are unpublished.)

“The bottom line is that the jury is still out on this,” Tanzi says.

Should we still floss our teeth?

“You really should, just for the sake of your breath,” Tanzi says. “It’s just courtesy. But to say that you’re going to get Alzheimer’s because you don’t floss your teeth, people should not be thinking that.”