The Nasal Microbiome Can Alter Our Smell Perception


“If people experienced what I have, if only for a month, they’d know how much difference it makes to your life,” says June Blythe who lived 37 years without any sense of smell due to chronic sinusitis. “Without a sense of smell or taste life really does lose its colour. Cooking was drained of all pleasure. Gardening became purely visual; all the glorious scents of roses, honeysuckle and lavender were lost on me.”* Image by jiao tang from Pixabay

“For normosmic people, it is hard to imagine what life would be like without olfaction. Many physicians seem rather helpless in what to tell patients with olfactory disorders,” writes Prof. Dr. Ilona Croy et al. “From the extensive body of literature, it can be concluded that loss of the sense of smell leads to disturbances in important areas, mainly in food enjoyment, detecting harmful food and smoke, and to some extent in social situations and working life.”

“Medical students get very little training on this subject — probably a week or two out of five years at medical school,” asserts Andrew McCombe, an ear, nose, and throat (ENT) consultant surgeon from Mediclinic City Hospital, Dubai. ‘This means they may trivialize anosmia — or think nothing can be done. Little do they realize that patients can suffer severe depression and emotional issues because of the detrimental effect it has on their lives.”

“For many causes of anosmia, no specific treatment is available, and the loss of smell may be permanent,” writes the U.S.Pharmacist.

Hyposmia (reduced smelling ability) and anosmia (complete loss of smell) affect ~20% of the US population. The rest are said to be normosmic, with normal olfactory function. A loss of sense of smell is often a result of aging or inflammation by, for example, viral infection, asthma or chronic rhinosinusitis. Other reasons could be traumatic brain injuries, aging, or neurodegenerative diseases (e.g., Alzheimer’s or Parkinson’s).

Nasal Microbiome

We inhale about 10,000,000 bacterial cells per cubic meter of air daily. These bacteria plus the chemicals, humidity, and toxins in the atmosphere shape our microbiome in the nasal cavity, sinuses, nasopharynx, and oropharynx. Collectively they form the upper respiratory tract or nasal microbiome. Its usual symbiotic composition is made of Bifidobacterium, Corynebacterium, Staphylococcus, Streptococcus, Dolosigranulum, and Moraxella species.

Environmental toxins like cigarette smoke — active or passive — is the biggest influencer of the nasal microbiome. Smoking deprives the nasal microbiome of oxygen needed by many friendly microbes. Cigarette toxins also impair the nasal immune system — e.g., lysozyme, antimicrobial peptides, and immunoglobulins. The bad or pathogenic bacteria, thus, thrive as they do not have to compete with the ‘friendly’ bacteria and our immune system. Examples of bad bacteria include H. influenzae, M. catarrhalis, Campylobacter species, S. pneumoniae, and S. pyogenes.

Respiratory disorders and nasal microbiome dysbiosis frequently co-occur. In chronic rhinosinusitis, for example, there is a depletion of Anaerococcus, Corynebacterium, Finegoldia, Peptoniphilus, Propionibacterium, and Staphylococcus populations — all of which promotes nasal health.

* Image by Anastasia Lavrinovich from Pixabay

Nasal Microbiome Manipulation

The first line of thought of killing bacteria is with antibiotics. But this is rather risky as common pathogenic bacteria like H. influenza, Chlamydia pneumoniae, and S. pneumoniae are resistant to many conventional antibiotics. Whereas other friendly bacteria — such as Dolosigranulum and Corynebacterium — can be killed. Antibiotics may exacerbate the situation and, as usual, have a bad reputation with microbiomes.

What about probiotics, the opposite of antibiotics? To date, only one 2017 randomized control trial tried using nasal probiotic (Lactobacillus + Bifidobacterium) spray to treat chronic rhinosinusitis in adults and the elderly; unfortunately, it failed. In children with respiratory disorders, however, oral probiotics help. Enterococcus faecalis, for example, has treated acute sinusitis many times. Several Lactobacillus and Bifidobacterium species have also been reported effective for improving asthma in children. Why oral and not nasal probiotics work still puzzles scientists.

Today, nasal rinse or irrigation — with its roots in ancient Ayurveda practice — is a popular method to prevent and even treat chronic rhinosinusitis, upper respiratory tract infections, and allergic rhinitis in both children and adults. Nasal rinse is also safe for people with a healthy respiratory system; it won’t damage their sense of smell for people.

The principle is that nasal rinse clears pro-inflammatory chemicals such as leukotrienes, prostaglandins, antigens, and other pollutants that damage the olfactory mucosa. This may, in turn, provides the opportunity for the nasal immune system and microbiome to bounce back to symbiosis. It is like a ‘reset’. Even the Centers for Disease Control (CDC) has provided guidelines for the proper preparations and procedures of nasal irrigation.

“The high frequency of positive results of nasal irrigation in several studies indicates that nasal rinsing is an effective, inexpensive, and simple method to treat sinonasal disorders alone,” writes a group of Austrian researchers in BMC Biology in November 2019, with Kaisa Koshiken (stay tuned) as a co-author. Given their respiratory disorders are ameliorated, so must be their sense of smell even if the studies did not ask about it.

But what about those without respiratory ailments who suffer an impaired sense of smell?

* Image by

Nasal Microbiome — Olfaction Link

Published in 2018 in Scientific Reports from the Nature publishing group, Kaisa Koskinen, Ph.D. from the Medical University of Graz, Austria and her colleagues performed an innovative study. They recruited 67 adults that (i) did not consume antibiotics/probiotics within the last month, (ii) do not have respiratory disorders like hay fever, pollen allergies, common cold or nasal polyps, and (iii) do not have any psychiatric or neurologic diseases. Among the 67 volunteers, 29 were normosmic (normal olfaction), 10 were hyposmic (reduced olfaction) and 28 were anosmic (loss of olfaction). They measure olfaction by TDI score — smell threshold, detection, and identification.

Koshiken et al. showed that hyposmic individuals have a higher nasal microbiome diversity, enriched with several butyrate-producing bacteria found in the gut and mouth. Examples include Faecalibacterium, Enterobacteriaceae, and Lachnospiraceae belonging to the gut, and Porphyromonas species of the mouth. Notably, these effects could not be explained by age or BMI.

Koskinen suspects that the butyrate production — which normally benefits the gut and mouth — became out of place in the nasal microbiome. Indeed, not only does the olfactory receptors recognize odorant molecules, but it also responds to bacterial metabolites including butyrate.

“It seems possible that by producing strong-smelling compounds such as butyric acid, microorganisms within the nasal microbiome could impact on olfactory perception and thus also on appetite,” says Koskinen. “Our study provides first insights into a potential correlation between olfactory function and olfactory epithelium microbiome composition.”

"Although therapies are currently lacking, there is hope for future breakthroughs. Ongoing scientific work is investigating how stem cells in the nose replace dying olfactory nerve cells... Or we may be able to electrically stimulate a sensation of smell using an artificial implant," writes Eric Holbrook, MD in Harvard Health Publishing in December 2018. And targeted nasal microbiome manipulation may also be one potential way to restore the sense of smell in the future.

This article was originally published in Microbial Instincts with minor modifications.

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