Bacteria in your mouth linked to memory and dementia risk

Cognitive decline is a growing public health concern, particularly in aging populations. While some decline is a normal part of aging, a subset of individuals experience mild cognitive impairment (MCI), a condition that affects about 15% of older adults.

Those with MCI are at a heightened risk for developing dementia or Alzheimer’s disease, with approximately 10% progressing to dementia annually. Identifying risk factors and preventative strategies for cognitive decline remains a critical research focus.

Recent studies highlight a potential connection between oral health and cognitive function. Periodontitis, a severe gum disease, has been associated with worsened cognitive performance.

Missing teeth have also been linked to lower Mini Mental State Examination test scores. One hypothesis suggests that pathogenic oral bacteria, such as Porphyromonas gingivalis, Treponema denticola, and Prevotella intermedia, contribute to cognitive decline by triggering inflammation and damaging the brain.

Higher levels of P. gingivalis have been detected in individuals with Alzheimer’s disease compared to those without. Researchers propose two primary routes through which oral bacteria may influence brain health.

The direct route involves bacteria entering the bloodstream through oral trauma, crossing the increasingly permeable blood-brain barrier, and colonizing the brain.

The indirect route involves bacterial metabolites and inflammatory responses that compromise the oral mucosal barrier, allowing harmful substances to enter circulation and contribute to neuroinflammation.

A significant mechanism linking oral health to cognition involves nitric oxide (NO), a molecule crucial for various physiological functions, including vasodilation, neurotransmission, and immune defense.

NO is primarily produced through two pathways: the endogenous L-arginine pathway, which relies on NO synthase enzymes, and the nitrate–nitrite–NO pathway, in which commensal oral bacteria reduce dietary nitrate to nitrite, further converting it into NO in the body.

Research suggests that NO plays a vital role in brain function, particularly in synaptic plasticity and memory retention. It binds to guanylyl cyclase and acts as a signaling molecule within the hippocampus, the brain region responsible for learning and memory. Studies in preclinical models show that inhibition of NO production impairs memory, underscoring its importance in cognitive function.

As people age, endogenous NO production declines due to reduced NO synthase activity and increased degradation of arginine. This decline is associated with cardiovascular diseases such as hypertension and vascular dementia. Reduced NO biomarkers, including nitrate and nitrite, have been observed in individuals with Alzheimer’s disease, further linking NO availability to cognitive decline.

A recent study led by the University of Exeter explored the relationship between oral bacteria and cognitive function in older adults.

Researchers analyzed mouth rinse samples from 110 participants over the age of 50, categorizing them into two groups: those with normal brain function and those with MCI. The results revealed notable differences in bacterial composition between these groups.

Higher levels of Neisseria and Haemophilus were associated with better memory, attention, and executive function. These bacteria also correlated with elevated nitrite levels in the mouth, suggesting a beneficial role in NO production.

Conversely, individuals with higher levels of Porphyromonas exhibited more pronounced memory problems, while those with increased Prevotella had lower nitrite levels. The presence of Prevotella was particularly common in individuals carrying the APOE4 allele, a known genetic risk factor for Alzheimer’s disease.

APOE4 carriers experience a weakened blood-brain barrier and are more susceptible to systemic conditions such as hypertension and atherosclerosis.

These conditions are linked to NO deficiency, indicating a potential interaction between NO bioavailability, oral bacteria, and cognitive decline. However, it remains unclear whether shifts in oral microbiome composition precede cognitive impairment or merely accompany it.

The findings suggest that maintaining a healthy oral microbiome could play a role in cognitive health.

Lead author Dr. Joanna L’Heureux of the University of Exeter Medical School emphasized the potential of routine dental checkups to assess bacterial levels and detect early signs of cognitive decline. Such screenings could provide an accessible and non-invasive method for identifying at-risk individuals before symptoms of dementia manifest.

Professor Anne Corbett, a co-author of the study, highlighted the broader implications of these results. If specific bacteria contribute to cognitive decline while others support brain function, then modifying the balance of oral bacteria through dietary changes, probiotics, improved oral hygiene, or targeted treatments could offer a novel approach to dementia prevention.

The research, published in PNAS Nexus, provides a foundation for further studies examining how interventions in oral health might mitigate cognitive decline.

While more research is needed to establish causality, the growing evidence suggests that a healthy mouth may be key to a healthy mind.

Note: Materials provided above by The Brighter Side of News. Content may be edited for style and length.

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