Longevity Briefs: Could Bacteria In The Nose Influence Cognitive Health?

Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

The problem:

As we age, our sense of smell (olfaction) declines, with a significant decline being known as hyposmia. This isn’t just an inconvenience; research increasingly suggests that there is a link between declining smell and cognitive decline, and hyposmia is an early and close to universal sign of Alzheimer’s disease. What could explain this relationship? One promising area of investigation is the nasal microbiome – the community of bacteria and other microorganisms living in our noses. The nasal microbiome plays a crucial role in maintaining a healthy nasal environment, and disruptions to this delicate ecosystem could potentially contribute to both olfactory and cognitive problems, with implications for how we look after our cognitive health. In this study, researchers attempt to further our understanding of this emerging relationship.

The discovery:

Researchers studied 510 Chinese participants with an average age of 77.9 years from a long-term study of ageing (the Rugao Longitudinal Ageing Study). These participants took the brief Chinese Smell Identification Test (B-CSIT), designed to assess olfactory function, and were classified as hyposmic or normosmic (normal sense of smell). They also underwent several cognitive tests to detect cognitive impairment or mild cognitive impairment (MCI). Nasal swabs were collected to analyse the composition of the nasal microbiome.

Researchers found that, after controlling for confounding factors like age and smoking, there was a statistically significant correlation between bacterial diversity and hyposmia: people with more diverse nasal microbiomes had, on average, a poorer sense of smell. In fact, of the variables measured by the researchers, only age and lower education status showed stronger correlations with hyposmia than bacterial diversity. Some specific bacteria like Acidovorax and Morganella seemed to be particularly important, being more strongly correlated with hyposmia. Researchers also developed a scoring method – the Nasal Olfactory Index or NOI – that they were able to use to accurately predict whether someone would be normosmic or hyposmic.

Researchers also found that certain microbial biotypes – clusters of similar microbial communities – were associated with a lower incidence of MCI, suggesting that their might be a relationship between the nasal microbiome and cognitive decline. For example, a biotype dominated by Corynebacterium was associated with reduced incidence of MCI when compared to one dominated by Moraxella.

The implications:

This research suggests that the nasal microbiome might play an underappreciated role in both olfactory function and cognitive health. However, because this study was observational in nature, it doesn’t tell us how these three factors are related. Changes in the nasal microbiome could lead to pathogenic bacteria penetrating the brain via the nasal cavity, thereby causing inflammation and other changes in the brain that could impact cognitive function. However, we know that in the case of the better studied (though still not fully understood) gut microbiome, the brain can affect bacteria in the gut by altering their environment, and the same may be true of the nasal microbiome.

The immune system is another factor to consider as it influences both cognitive health and nasal bacterial population. It is even possible that the nasal microbiome has no real impact on the brain, and that the ageing immune system is responsible for the decline of both cogntive function and the nasal microbiome. The nasal microbiome is much smaller than the gut microbiome, but then again, it is a lot closer to the brain.

More research is needed to understand these relationships fully, but assuming the nasal microbiome does influence cognitive function, how might we use this to our advantage? Avoiding the introduction of foreign objects into the nose (including fingers!) may be beneficial for limiting the introduction of pathogenic bacteria. Additionally, breathing chemicals like cigarette smoke can damage nasal passages and disrupt the microbiome.