Nasal Microbiota, Olfactory Health, Neurological Disorders and Aging—A Review

Abstract

The nasal region is one of the distinct environments for the survival of various microbiota. The human microbial niche begins to inhabit the human body right from birth, and the microbiota survive as commensals or opportunistic pathogens throughout the life of humans in their bodies in various habitats. These microbial communities help to maintain a healthy microenvironment by preventing the attack of pathogens and being involved in immune regulation. Any dysbiosis of microbiota residing in the mucosal surfaces, such as the nasal passages, guts, and genital regions, causes immune modulation and severe infections. The coexistence of microorganisms in the mucosal layers of respiratory passage, resulting in infections due to their co-abundance and interactions, and the background molecular mechanisms responsible for such interactions, need to be considered for investigation. Additional clinical evaluations can explain the interactions among the nasal microbiota, nasal dysbiosis and neurodegenerative diseases (NDs). The respiratory airways usually act as a substratum place for the microbes and can act as the base for respiratory tract infections. The microbial metabolites and the microbes can cross the blood–brain barrier and may cause NDs, such as Parkinson’s disease (PD), Alzheimer’s disease (AD), and multiple sclerosis (MS). The scientific investigations on the potential role of the nasal microbiota in olfactory functions and the relationship between their dysfunction and neurological diseases are limited. Recently, the consequences of the severe acute respiratory syndrome coronavirus (SARS-CoV-2) in patients with neurological diseases are under exploration. The crosstalk between the gut and the nasal microbiota is highly influential, because their mucosal regions are the prominent microbial niche and are connected to the olfaction, immune regulation, and homeostasis of the central nervous system. Diet is one of the major factors, which strongly influences the mucosal membranes of the airways, gut, and lung. Unhealthy diet practices cause dysbiosis in gut microbiota and the mucosal barrier. The current review summarizes the interrelationship between the nasal microbiota dysbiosis, resulting olfactory dysfunctions, and the progression of NDs during aging and the involvement of coronavirus disease 2019 in provoking the NDs.

Extra-axial inflammatory signal and its relation to peripheral and central immunity in depression

“…The investigation was prompted by the proximity of facial bones to the nasal passages and sinuses, housing the nasal microbiota crucial for immune regulation and general CNS homeostasis. 52,53 The TSPO signal in facial bones may be sensitive to changes in these spaces; however, further research is needed to elucidate the connection between facial bones and nasal passages and its implications for the brain-periphery connection.…”

Deleterious Effect of Air Pollution on Human Microbial Community and Bacterial Flora: A Short Review

“…A few studies suggested that the nasal microbiota has been significantly associated with the central nervous system and immune system. Dysbiosis in the nasal microbiota may lead to inflammatory diseases and pathological invasion of the nasal cavity, which may damage the olfactory system [ 34 ].…”

Nasal Microbiota and Neuroinflammation: Relationship between Nasal Flora and Multiple Sclerosis Onset/Progression

“…The microbes, like a Trojan-horse, cross the BBB either transcellularly or paracellularly or with the help of infected phagocytes. Then, the presence of these elements alters the BBB function causing increased permeability, encephalopathy, or pleocytosis [4].…”

“…The role of nasal microbiota has been investigated in neurodegenerative diseases such as Parkinson's Disease (PD) [1]; however, neither Pereira [2] nor Heintz-Buschart [3] identified any differences in the nasal microbiota composition comparing PD patients and healthy controls. Thangaleela in 2022 speculated that metabolites from nasal microbiota could pass through the Blood Brain Barrier (BBB) [4]. If this hypothesis could be confirmed, the effect of nasal microbiota in the development of neuro-inflammation and neurodegeneration could be like that of gut microbiota [1,5].…”

“…Other host factors can also influence the nasal flora, such as host hygiene, immunity, and genetics, as well as physical factors, such as oxygen, pH, moisture, and other microbial interactions. The colonization of opportunistic pathogens results in the onset of respiratory infections and changes in the innate immune mediators [4].…”