For decades, Alzheimer\u2019s disease has been studied largely as a disorder of the brain. Increasingly, however, researchers have been looking somewhere less obvious for part of the story: the mouth.<\/h3>\n
At the centre of that research is Porphyromonas gingivalis<\/em>, a bacterium strongly associated with chronic periodontitis, the inflammatory disease that damages the tissues and bone supporting the teeth. Scientists have detected the bacterium, and the toxic enzymes it produces, in the brains of people with Alzheimer\u2019s disease. In animal experiments, infection with P. gingivalis has also produced changes associated with Alzheimer\u2019s pathology.<\/p>\n The findings have opened an intriguing line of inquiry into whether chronic infection and inflammation might contribute to the development or progression of Alzheimer\u2019s in some people. They have not established that gum disease causes Alzheimer\u2019s, and attempts to translate the theory into treatment have so far produced mixed results. But seven years after one of the most influential studies brought the connection to wider attention, researchers are still pursuing it.<\/p>\n In 2019, a team led by Stephen Dominy published a study in Science Advances examining post-mortem brain tissue from people with Alzheimer\u2019s disease. The researchers identified P. gingivalis and gingipains, toxic protein-cutting enzymes produced by the bacterium, in Alzheimer\u2019s brains. Higher levels of the gingipains were associated with Alzheimer\u2019s diagnosis and with proteins involved in the disease, including tau.<\/p>\n The researchers then infected mice orally with P. gingivalis. The bacterium reached the brain and was accompanied by increased production of amyloid-beta 1-42, one of the proteins that accumulates in the characteristic plaques seen in Alzheimer\u2019s disease. Gingipains were also found to damage tau and neurons in laboratory and animal experiments.<\/p>\n It was a provocative result because it suggested a possible sequence connecting a common chronic infection with processes already known to occur in Alzheimer\u2019s.<\/p>\n Periodontitis itself is not caused by a single organism. It develops within a complex microbial community, influenced by the immune system and other factors. But P. gingivalis is regarded as an important pathogen because it can disrupt that microbial environment and produce gingipains capable of damaging tissue and interfering with immune responses.<\/p>\n The biological argument is plausible. Inflamed gums can allow bacteria and bacterial products to enter the bloodstream. Researchers have proposed several routes by which P. gingivalis, its toxins or the inflammation it provokes could ultimately affect the brain, including changes to the blood-brain barrier and prolonged activation of immune cells within the nervous system. A 2026 review described P. gingivalis-driven systemic and neurological inflammation as a credible mechanism, while emphasising that much of the evidence remains preclinical or epidemiological rather than proof of causation in humans.<\/p>\n Human population studies have added another layer. A US National Institute on Aging analysis followed more than 6,000 people using data spanning as long as 26 years and found associations between periodontal disease, antibodies to oral bacteria including P. gingivalis, and subsequent Alzheimer\u2019s disease and other dementias. The researchers were careful about what that meant. Observational studies can identify an association, but they cannot prove that one condition caused the other.<\/p>\n
\u00a0<\/p>\n That distinction is especially important in Alzheimer\u2019s research. People developing dementia may become less able to maintain their oral health, making severe gum disease a potential consequence as well as a possible contributor. Age, smoking, diabetes, cardiovascular health, socioeconomic factors and access to dental care can further complicate the relationship.<\/p>\n A systematic review of clinical evidence published in 2022 found moderate evidence for an association between oral bacteria and Alzheimer\u2019s disease, with a particularly strong association when bacteria were detected in brain tissue. But the authors concluded that further research was needed to establish how, or whether, those organisms participate in the development of the disease.<\/p>\n The most revealing test of the theory came when researchers attempted to treat Alzheimer\u2019s by targeting the bacterium itself.<\/p>\n Cortexyme developed COR388, later known as atuzaginstat, to block the gingipain enzymes produced by P. gingivalis. In the original animal experiments, gingipain inhibitors reduced bacterial levels in the brain, lowered amyloid-beta production and reduced neuroinflammation.<\/p>\n That early promise led to the GAIN trial, a Phase 2\/3 study involving 643 people with mild to moderate Alzheimer\u2019s disease. The result was considerably less straightforward.<\/p>\n Atuzaginstat failed to produce a statistically significant benefit on the trial\u2019s two primary measures of cognition and daily functioning across the overall study population. Cortexyme reported encouraging results in a pre-specified subgroup of participants who had detectable P. gingivalis DNA in their saliva, but the improvement was not reproduced across both primary endpoints. The treatment was also associated with dose-related elevations in liver enzymes. In January 2022, the US Food and Drug Administration placed the Alzheimer\u2019s programme on full clinical hold.<\/p>\n Rather than proving the infection theory, the trial left scientists with a more complicated question: could a microbial mechanism be relevant only to a particular subset of Alzheimer\u2019s patients?<\/p>\n That possibility is now being investigated more directly.<\/p>\n![\"Could](\"https:\/\/greekcitytimes.com\/wp-content\/uploads\/2026\/09\/Could-Gum-Disease-Bacteria-Play-a-Role-in-Alzheimers-1024x683.webp)
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