Mild cognitive impairment (MCI) is the greatest risk factor for the development of dementia or Alzheimer's disease, with an estimated 10% of people affected progressing to dementia each year. As cognitive decline and dementia represent a major public health issue, there is an urgent need to understand the risk factors to enable risk reduction.
A recent study published inPNAS Nexus (Volume 4, Issue 1, January 2025) examined bacteria living in the saliva of 115 older adults: 60 were in the healthy group and 55 in a group with mild cognitive impairment (MCI), which affects about 15% of older adults.
One of the authors of the study, Joanna L'Heureux, Postdoctoral Researcher, Public Health and Sport Sciences, University of Exeter, summarised the findings in an accessible way in the Conversation (published 5 February 2025).
The bacteria in the saliva samples taken from the healthy group and the mild cognitive impairment group were examined. It was found that people who had large numbers of Neisseria and Haemophilus bacteria present in their saliva performed better in brain health tests. In particular, people with these bacteria had better memory and better ability to pay attention and perform complex tasks.
These people also had higher levels of nitrite in their mouths; nitrite is made by bacteria when they break down nitrate, which is a natural part of a vegetable-rich diet. Bacteria can also break down nitrite to produce nitric oxide, which improves circulation, including blood flow to the brain. This suggests that eating lots of nitrate-rich vegetables, such as leafy green spinach and rocket, could boost levels of healthy bacteria and help improve brain health, which might be especially important as people age.
As a result of this, the authors are investigating whether nitrate-rich beetroot juice can improve brain function in older adults by boosting healthy bacteria in the mouth.
Conversely, the study found two groups of bacteria, Porphyromonas and Prevotella, were potentially linked to worse brain health. Porphyromonas, which is often associated with gum disease, was more common in people with memory problems than people who were healthy. Prevotella was linked to low nitrite, which in turn could mean poorer brain health. Prevotella was also more common in people who carry the gene APOE4, which is associated with an increased risk of Alzheimer’s disease.
These findings suggest that some bacteria might play a detrimental role in changes in brain health as people age. It also raises the question of whether routine tests to measure levels of these bacteria could be used to detect very early signs of declining brain health as part of dental check-ups in the future.
Encouraging the growth of nitrite-producing bacteria like Neisseria, while reducing Prevotella and Porphyromonas, could help maintain brain function as we age. This could be achieved through dietary changes, probiotics, oral hygiene routines or even targeted treatments that reshape the microbiome.
These findings provide a strong rationale for further research. If future studies confirm that the oral microbiome plays a role in maintaining a healthy brain, then by paying closer attention to the bacteria in our mouths we may unlock new possibilities for detecting and potentially delaying dementia.
In the meantime, the best advice is to keep your teeth clean, see your dentist regularly and eat food with lots of nitrate, like leafy green vegetables, to keep feeding the good bacteria in your mouth!
