Updated July 27, 2021

The evidence for a microbial origin of Alzheimer’s disease is stacking up, particularly since the Alzheimer’s biomarker beta-amyloid was recognized as an antimicrobial peptide1 and a common oral pathogen was described as a “central driver of Alzheimer’s pathology.” I updated this blog to include the recent science on Alzheimer’s and the oral microbiome. You’ll also find treatments in this blog to promote a healthier oral microbiome and optimize brain health.

We all have been touched by Alzheimer’s disease. It’s estimated to affect almost 6 million people in the United States. In Alzheimer’s disease, certain proteins accumulate and interfere with healthy brain cell activity. Those brain cells become damaged and eventually die. That is why Alzheimer’s patients don’t recognize the people they love. They may also lose the ability to walk, eat, or swallow. They literally lose the brain cells to do those things.  Alzheimer’s can be deadly for those same reasons.

Did you ever suspect that the 20 billion bacteria that live in your mouth could be the explanation for brain inflammation and brain damage like that seen in Alzheimer’s?  If you have red, bleeding gums, or chronic gum disease, then you may have a significantly higher risk of getting Alzheimer’s disease.2

Indeed, the oral microbiome is not only a player when it comes to heart disease, diabetes, rheumatoid arthritis, cavities, periodontal disease, preterm birth, and cancers, but also Alzheimer’s disease.

Alzheimer’s disease is a neuroinflammatory and neurodegenerative brain disease. It is characterized by accumulation and deposition of beta-amyloid, a protein fragment found in the brain, and the presence of neurofibrillary tangles, which are made of clumps of damaged tau proteins.1 Amyloid-beta 42 and tau species in spinal fluid are diagnostic markers of Alzheimer’s disease.

Periodontitis, or gum disease, is oral dysbiosis that leads to inflammation and progressive tissue destruction. It is associated with various systemic diseases such as cardiovascular disease, diabetes, preterm birth, and more.1

A clear link between Alzheimer’s disease and periodontitis has emerged over the last 15 years, but scientists didn’t understand how or why. After a landmark 2019 study found the common oral pathogen Porphyromonas gingivalis in the spinal fluid and brains of Alzheimer’s patients, one researcher said, “P. gingivalis may link these two inflammatory and degenerative conditions.”1

Bad Oral Bacteria Are Linked to Alzheimer’s, but Good Bacteria Prevent It

In a 2019 landmark study, a bacteria known to cause gum disease, Porphyromonas gingivalis, was found in the brain, cerebrospinal fluid, and saliva of people with Alzheimer’s. P. gingivalis is a keystone pathogen in chronic periodontitis, meaning that it influences the oral microbiome to promote the disease and inflammation.1 The researchers concluded that P. gingivalis is a central driver of Alzheimer’s pathology.3

Animal studies confirm the connection between oral disease and brain disease. When animals have periodontal infections, their brains suffer. They show brain inflammation, amyloid build-up, tau pathology, and neurodegeneration.3,4

Image by Sabrina B. from Pixabay

Subsequent studies of brain cells show that P. gingivalis can invade and persist in them. The oral pathogen triggers Alzheimer’s-like disease in these cells: it changes the cell organelles, damages the cell walls, increases tau phosphorylation, and cuts off communication between brain cells (known as synapse loss).5 After infection with this bacterium, 25% of cells die.5

That’s not all. A 2021 study showed that people who had amyloid deposits were more likely to have “bad” periodontal bacteria, or dysbiosis under their gums. In fact, in people with unfriendly bacteria under their gums, the chance of being amyloid-positive was 4 times higher! Unlike the earlier study, these researchers said that multiple unfriendly oral bacteria were to blame, not just P. gingivalis. However, they didn’t see the same relationship for another Alzheimer’s biomarker, tau protein. Therefore, they concluded that oral dysbiosis and increased amyloid may be an early event in the course of Alzheimer’s.4 Perhaps tau protein abnormalities come later in the disease.

Interestingly, bacteria have many sneaky ways of getting into the brain. They can infect immune cells which later gather in the brain, they can damage and infect the endothelial cells that protect the blood-brain barrier, or they can infect and spread to the brain through nerves (like the ones in your nose or jaw). Bad bacteria in the mouth can reach the brain through the blood and directly cause amyloid production. They can turn on brain inflammation, which encourages unhealthy amyloid plaques.4 So, the bugs in your mouth have access to your brain (unfortunately).3

Can Good Oral Bacteria Help Your Brain?

Image by Pete Linforth from Pixabay

Funny you should ask. The 2021 study showed that friendly bacteria below the gumlines reduced amyloid deposits. In other words, the more good bacteria in your mouth, the lower your risk of Alzheimer’s pathology.

Good bacteria may protect your brain in these ways:4

  • Crowd out bad species such as P. gingivalis
  • Decrease inflammation in the mouth and throughout the body
  • Improve nitric oxide production, which relaxes the blood vessels and lowers blood pressure
  • Inhibit harmful toxins (virulence factors) made by bad bugs
  • Strengthen the oral lining so that bacteria can’t escape into the bloodstream
  • Calm inflammation in the brain

How the Bacteria in Your Mouth are Linked to Brain Health and Systemic Health

Three common factors seem to crop up when we look closely at the oral microbiome and the long list of associated diseases.

  • Bacterial dysbiosis– an imbalance of bacteria causing illness.
  • Inflammation-an immune system response intended to fight infection, which can inadvertently harm the host.
  • Leaky mouth-weakening and damage of the mouth lining which can allow bacteria into the bloodstream, where they don’t belong.

In Alzheimer’s, we have a few other ingredients added to the mix. One is that as we age, the immune system weakens. That allows bacteria to overgrow in the mouth, more so than in youth. More bacterial dysbiosis means more inflammation and higher risk for gum disease. And just in terms of geography, that could open the brain up to more bacteria.6

Another issue in Alzheimer’s is that inflammation and dysbiosis can damage the blood-brain-barrier. The “BBB” is a vital barrier that separates the bloodstream from the brain. It is essential for keeping the brain safe from all of the junk that tends to circulate in the blood. If the BBB is broken down, then the brain is defenseless against bacterial invasion and inflammatory chemicals.

When beta-amyloid proteins clump together and interfere with brain cells, it’s called an amyloid plaque. It’s a hallmark feature of Alzheimer’s. Researchers have shown that the beta-amyloid plaques that accumulate in the brains of Alzheimer’s patients are also antimicrobial. Could it be that the characteristic substance involved in Alzheimer’s is actually an attempt to protect the brain from pathogenic bacteria?7,8


Keep Your Oral Health on Point and Boost Your Brain Power

What all this means is your oral health is more important than ever. The list of diseases linked to your oral health grows longer and longer. By encouraging a healthy oral microbiome throughout your life, it could improve your brain function later. However, even small steps in oral health could improve brain function in real time. In a small study of older people with dementia, eight months of regular oral hygiene treatments improved markers of cognitive function, as well as improved their oral health.

Image by klimkin from Pixabay

One of the most powerful ways to cultivate a healthy oral microbiome is to eat a healthy diet. Low in sugar and in refined carbohydrates, high in fruits and vegetables; this is the kind of diet that feeds healthy bacteria, which protect you from disease. When your good bacteria grow at high levels, they can crowd out potentially harmful bacteria, like P. gingivalis.

Oral probiotics, probiotic toothpaste, and regular dental hygiene will help you keep not only your mouth happy, but also your brain, heart, joints, and more. Nasal breathing- not mouth breathing- is key for nourishing a healthy oral bacteria community. Tongue scraping, oil pulling, and avoiding toxic dental products can further balance and promote the good bugs in your mouth. Learn more about the magnificent microbiome- the bacteria that live in your mouth in, “Heal Your Oral Microbiome,” available on Amazon and on my website.

There is more to learn about Alzheimer’s and the oral microbiome and we look forward to the new developments. Stay tuned!


  1. Ryder MI. Porphyromonas gingivalis and Alzheimer disease: Recent findings and potential therapies. Journal of periodontology. 2020;91 Suppl 1:S45-S49.
  2. Choi S, Kim K, Chang J, et al. Association of Chronic Periodontitis on Alzheimer’s Disease or Vascular Dementia. J Am Geriatr Soc. 2019;67(6):1234-1239.
  3. Dominy SS, Lynch C, Ermini F, et al. Porphyromonas gingivalis in Alzheimer’s disease brains: Evidence for disease causation and treatment with small-molecule inhibitors. Sci Adv. 2019;5(1):eaau3333.
  4. Kamer AR, Pushalkar S, Gulivindala D, et al. Periodontal dysbiosis associates with reduced CSF Abeta42 in cognitively normal elderly. Alzheimers Dement (Amst). 2021;13(1):e12172.
  5. Haditsch U, Roth T, Rodriguez L, et al. Alzheimer’s Disease-Like Neurodegeneration in Porphyromonas gingivalis Infected Neurons with Persistent Expression of Active Gingipains. J Alzheimers Dis. 2020;75(4):1361-1376.
  6. Shoemark DK, Allen SJ. The microbiome and disease: reviewing the links between the oral microbiome, aging, and Alzheimer’s disease. J Alzheimers Dis. 2015;43(3):725-738.
  7. Pritchard AB, Crean S, Olsen I, Singhrao SK. Periodontitis, Microbiomes and their Role in Alzheimer’s Disease. Front Aging Neurosci. 2017;9:336.
  8. Aguayo S, Schuh C, Vicente B, Aguayo LG. Association between Alzheimer’s Disease and Oral and Gut Microbiota: Are Pore Forming Proteins the Missing Link? J Alzheimers Dis. 2018;65(1):29-46.


Cass Nelson-Dooley, M.S.

Cass Nelson-Dooley, M.S.

Cass Nelson-Dooley, MS, is a researcher, author, educator, and laboratory consultant. She studied medicinal plants in the rain forests of Panama as a Fulbright Scholar and then launched a career in science and natural medicine. Early on, she studied ethnobotany, ethnopharmacology, and drug discovery at the University of Georgia and AptoTec, Inc. She joined innovators at Metametrix Clinical Laboratory as a medical education consultant helping clinicians use integrative and functional laboratory results in clinical practice. She owns Health First Consulting, LLC, a medical communications company with the mission to improve human health using the written word. Ms. Nelson-Dooley is an oral microbiome expert and author of Heal Your Oral Microbiome. She was a contributing author in Laboratory Evaluations for Integrative and Functional Medicine and Case Studies in Integrative and Functional Medicine. She has published case studies, book chapters, and journal articles about the oral microbiome, natural medicine, nutrition, laboratory testing, obesity, and osteoporosis.