Billions of microorganisms, including bacteria, fungi, and archaea, are present in the human nose, which collectively form the nasal microbiome.
Several studies have indicated that the nasal microbiome is an important factor in personal and overall health. In fact, considerable evidence has been reported regarding the links between the gut microbiome and Parkinson’s disease (PD). Some studies have also indicated that signals from the nasal microbiota reach the brain through the olfactory system and affect the functioning of the nervous system.
Study: The Microbiome-Nose-Brain Axis in Health and Disease. Image Credit: goa novi / Shutterstock.com
A recent Trends in Neuroscience The study provides insight into how the nasal bacterial population influences olfactory processing and neurodegeneration, particularly in Parkinson’s disease (PD).
Composition of the nasal microbiome
Among the various microorganisms present in the nose, bacteria are considered to be the main component. Although 104 different strains of bacteria are present in the nose, only two to ten species make up 90% of an individual’s nasal microbiome. Most bacterial strains are symbiotic; however, some opportunistic pathobionts are also present and can cause various diseases.
Colonization of symbiotic microbes has been detected since birth and continues to develop during the first year of life. Firmicutes and Proteobacteria are abundantly present in children, while Actinobacteria are overrepresented in adults.
Bacterial pathogens, such as Haemophilus influenzae, Moraxella catarrhalisand Streptococcus pneumoniae, are also sometimes detected in the nasal microbiota. These bacteria have the potential to cause upper respiratory tract infections.
The bacterial composition associated with an individual’s nasal microbiome is influenced by geographic factors, proximity to animals, climate, composition of drinking water, air quality, diet, and prevalence of infectious diseases . Moreover, age and host immunity are also major determinants that influence the composition of the nasal microbiota.
The role of the nasal microbiome in olfaction
Several studies have indicated that the nasal microbiota limits pathogen colonization and modulates host immune responses associated with respiratory tract infections and disease severity. However, a limited number of studies related to the role of the nasal microbiome in olfactory performance have been conducted.
invivo experiments using a mouse model revealed that the nasal microbiome plays a vital role in the function of the nasal epithelium as odor responses. A stronger and faster odor response was recorded by electroolfactogram in germ-free mice.
The amplitude and speed of odorant-evoked responses were different depending on the nature of the odorant in mice without nasal microbiota. Therefore, alterations in the nasal microbiome could significantly affect an individual’s health due to the influence of olfaction on nutrition and general health.
Odors are detected by olfactory sensory neurons (OSNs) in mammals. OSNs are present in the olfactory epithelium of the posterior nasal cavity.
To determine whether the nasal microbiota impacts human olfaction, individuals aged 18 to 46 in a previous study were classified into three groups: good, normal, and poor sense of smell. This study found that although a similar nasal microbial population was present in all groups, the presence of butyric acid-producing bacteria in the nasal microbiome caused alterations in olfactory functions.
Nasal microbiota and neurodegenerative diseases
OSNs are extremely vulnerable to external factors, including toxins like herbicides and pesticides, as well as other microorganisms like coronaviruses and staphylococci. These factors can lead to mitochondrial damage, inflammation-induced axonal damage, and oxidative stress. These changes can alter OSNs and therefore cause olfactory bulb dysfunction.
Olfactory deficits can occur due to differential nasal bacterial colonization. Olfactory deficits are closely associated with neuroinflammation, inflammation, and neurodegenerative diseases such as Zika, coronavirus disease 2019 (COVID-19), PD, and Alzheimer’s disease.
PD is a neurodegenerative disease characterized by the loss of dopaminergic neurons in the substantia nigra.
The current study discusses how the nasal microbiome effects the incidence and prognosis of PD. Olfactory dysfunction has been observed in the early phase of PG progression. Thus, it was hypothesized that the nasal microbiome may influence the neuronal accumulation of misfolded OSNs.
In most Parkinson’s patients, misfolded alpha-synuclein (aSyn) is a vital component of Lewy bodies that is considered a marker for PD. Chronic nasal dysbiosis induced by local inflammation can significantly increase neurodegenerative damage to OSNs, leading to accumulation of misfolded aSyn.
This pathological condition could progress to the olfactory bulb and have a significant impact on cognitive, motor and psychiatric expressions in patients with PD. Misfolded aSyn commonly occurs in olfactory bulb tissues and neuroepithelium samples from patients with PD.
In a recent US-based study, nostril swabs taken during endoscopy showed that nasal dysbiosis is strongly associated with PD. Here, a distinct nasal microbiome was found in Parkinson’s patients compared to healthy individuals.
The nasal microbiome of Parkinson’s patients is characterized by an increased concentration of bacteria belonging to the phylum Proteobacteria. Nevertheless, many studies contradict the findings of this study and report that no significant difference in the nasal microbiome was observed between PD patients and controls.
Nasal dysbiosis leads to aSyn aggregation, which has been associated with neurodegenerative processes. In the future, more research is needed to better elucidate the role of the microbiome-nose-brain axis in the development and progression of PD.
- Lazarini, F., Roze, E., Lannuzel, A. & Lledo, P. (2022) The microbiome-nose-brain axis in health and disease. Trends in Neuroscience. doi:10.1016/j.tins.2022.08.003