Researchers Revealed Biological Mechanisms That Link Oral Microbiome And Cognitive Decline

Today, various studies conclude the correlation between poor oral health and Alzheimer's disease. Still, only a few of them explain the exact mechanisms of this phenomenon in detail. Fortunately, recent research from the University of Exeter shed light on the biological processes of how specific oral bacteria are linked to cognitive decline.

Researchers focused on mild cognitive impairment (MCI) – a disorder that can progress into Alzheimer's disease and dementia in older adults.

How do bacteria affect the brain?

Besides the most obvious way – a vast amount of pathogens may enter the bloodstream, causing inflammatory processes and damaging brain tissues directly – there is another crucial factor: some bacteria can disrupt nitric oxide (NO) levels – an essential compound for cognitive function.

This happens when specific bacteria species affect NO's key precursors – nitrate and nitrite molecules – and the process of their transformation. In simple terms, bacteria convert nitrate into nitrite, and then nitrite takes part in the process of NO production.

To determine which bacteria species in the mouth cavity are related to this process, scientists involved 115 participants (mostly female) with an average age of about 68 years (+/- 8 years). They were divided into two groups depending on the presence or absence of MCI. Scientists analyzed oral microbiome elements of both groups to find how exactly they affected participants' cognitive abilities.

Key findings

The low levels of NO are strongly associated with declined cognitive functions. As it turns out, one of the main causes that may lead to it – is the bacteria called Prevotella intermedia, which transforms nitrate into ammonia and thus disrupts the NO production process. Additionally, scientists identified a few other bacteria that related to MCI:

Porphyromonas gingivalis –– was more prevalent in the group with MCI and associated with cognitive decline. Additionally, they may cause neuroinflammation when they enter the bloodstream.
Fusobacterium nucleatum – co-occurs with P. gingivalis, helping it to spread across the human mouth and potentially worsening neuroinflammation.

Besides this group, researchers also identified bacterial species that linked with a positive impact on cognitive capabilities:

Neisseria – high levels of this microorganism in people with MCI are associated with better executive functions and visual attention. This bacteria correlated with working memory in healthy groups (they have a higher abundance of Neisseria on average).
Haemophilus co-occurs with previous bacteria and is linked to better executive functions in people with MCI, potentially helping to inhibit cognitive decline.
Capnocytophaga is associated with higher nitrate concentration and usually persists in healthy participants. In the MCI group, in turn, scientists observed low levels of the bacteria, which may lead to lower NO production due to nitrate absence.

What's next?

In further research, scientists plans to find out, how changes in oral nitrite may affect cognitive decline to discover potential treatment that can lower risk of Alzheimer Disease and dementia.

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