Let’s be honest, most of us probably aren’t flossing nearly as often as our dentist tells us to. As long as our teeth look fairly white and our breath smells fine, we assume that our oral care routine is good enough. However, this becomes particularly problematic as we get older. Elderly people often neglect their oral hygiene, leading to a variety of oral infections including gum disease. Not only can gum disease make it painful to chew food and increase the risk of tooth loss, but research suggests it may also contribute to the development of Alzheimer’s disease.
Microbes and Dementia
The idea that microbial infections may act as a trigger or cofactor for Alzheimer’s disease, known formally as the pathogen hypothesis, has been around since the 1980s, though only recently has it moved from the fringes to the forefront of mainstream neuroscience. The hypothesis suggests that microbes can enter the central nervous system by passing through a selective membrane called the blood-brain barrier, which tends to become more leaky as we age, particularly in the hippocampus (the brain area most associated with memory).
Once inside, the microbe triggers an immune response that results in the deposition of amyloid beta plaques, the toxic protein clumps that are the hallmark of Alzheimer’s disease. As more microbes slip past the barrier, the brain is unable to clear away the plaques faster than they’re being created, causing them to accumulate and eventually leading to neurodegeneration.
Support for the pathogen hypothesis comes from studies that show positive associations between infections and the development of Alzheimer’s disease. In particular, meta-analyses have shown that the herpes virus, as well as the bacteria that cause pneumonia, syphilis, and Lyme disease, are found at significantly increased rates in Alzheimer’s patients compared to healthy controls of the same age and genetic risk profile. Other studies show that infecting lab-cultured neurons with these viruses and bacteria causes an Alzheimer’s-like pathology to develop.
Researchers now believe certain bacteria that can cause oral infections may be the next microbes to join the list of Alzheimer’s disease risk factors.
Your Oral Microbiome and You
The microbes living in our mouths and throats, known collectively as the oral microbiome, are different for every person and can include nearly 900 unique species of bacteria. Changes in the microbiome’s composition as a result of poor oral hygiene can lead to periodontitis, a severe form of gum disease that gradually destroys the bone sockets holding your teeth in place, resulting in tooth loss. The CDC reports that nearly half of all adults in America suffer from mild, moderate, or severe periodontitis. For adults over 65, that number increases to more than 70%.
The bacteria implicated in periodontitis can cause further complications when they enter the bloodstream and travel to other organs throughout the body. Studies suggest that periodontal bacteria can initiate harmful inflammatory reactions in multiple organ systems, increasing the risk or severity of various conditions including diabetes, cardiovascular disease, and rheumatoid arthritis.
Oral Bacteria Inside the Brain
Due to their ability to travel through the bloodstream, periodontal pathogens can also enter the brain via a leaky blood-brain barrier. A 2002 study observed that DNA from Treponema, a family of bacteria implicated in periodontitis, was more likely to be present in Alzheimer’s brains compared to healthy brains. A more recent study found virulence factors for the periodontal bacteria P. gingivalis in postmortem brain tissue of Alzheimer’s patients.
Multiple studies have repeatedly correlated periodontitis with reduced cognitive function, as well as higher levels of amyloid beta in the brain. Another study of elderly Chinese adults reported the loss of more than 16 teeth being associated with severe cognitive impairment. In addition, a 2016 study presented evidence that periodontitis might worsen dementia symptoms after observing that Alzheimer’s patients diagnosed with periodontitis had a six-fold increase in the rate of cognitive decline during a six month period.
A recent study, published in January 2019, added more evidence to support a role for oral bacteria in Alzheimer’s. The researchers showed that when mice were infected with P. gingivalis, a common type of periodontal bacteria, they developed many key hallmarks of Alzheimer’s. The group then designed a new drug that inhibited a specific protein from P. gingivalis and administered it to the mice. This treatment halted the progression of Alzheimer’s-like pathology in the mice, suggesting that these kinds of drugs could be useful for treating some cases of Alzheimer’s in humans.
In conclusion, more research is still needed to conclusively support a causal relationship between Alzheimer’s disease and oral infections. However, based on the strong correlational evidence with comorbidity of these two conditions, as well as the more definite links between periodontitis and other health problems, it’s clear that we need to place greater emphasis on the importance of oral hygiene for our overall health. This is particularly true for individuals over the age of 65, who have the greatest risk for both dementia and gum disease. If protecting your brain could be as simple as twice daily brushing, daily flossing, and regular trips to the dentist’s office, there’s no reason we shouldn’t all be taking this initiative.