Hay fever: has a nasal dysbiosis been under our nose the whole time?

The nasal microbiota’s involvement in allergic rhinitis had long been suspected. But what is an allergy? An allergy is a chronic disease caused by an exaggerated reaction of immune system cells to normally harmless foreign substances in our body, such as animal hair, food, or pollen.

Studies have already shown reduced nasal microbiota diversity in those who suffer from allergies, which is linked to antibody production typical of the condition. But which bacterial genera or species are responsible for this nasal dysbiosis? And what role do they play in allergy-related respiratory diseases? Researchers decided to compare in detail the nasal microbiota of 55 people suffering from allergic rhinitis with that of 105 healthy individuals. They were on to the right scent, with enlightening results.

Streptococcus salivarius makes itself at home in the nostrils of allergy sufferers

The researchers were able to confirm reduced microbial diversity in the subjects with allergic rhinitis compared to the healthy controls. The Streptococcus genus made all the difference, particularly the Streptococcus salivarius species, which was highly abundant in the allergic patients. In contrast, Staphylococcus epidermidis, a species considered beneficial to the nasal microbiota, dominated in the healthy subjects. However, S. salivarius is regularly found in the mouth and throat. It is even considered probiotic and therefore good for our health, since it produces antimicrobial substances known as bacteriocins. So, are they present in the noses of allergy sufferers to fight harmful germs? No, because when the researchers put the allergy patients’ S. salivarius into contact with bacteria known to colonize the nose, they found that it secreted only small quantities of bacteriocins.

Sticky and inflammatory... can allergies be relieved by dislodging this bacterium from the nose?

To better understand the role of S. salivarius, the researchers transferred these bacteria from the allergic patients to mice, together with Alternaria alternata, an allergen which causes allergic rhinitis, over three days. The sensitized mice reacted by secreting several inflammatory proteins. What’s more, when S. salivarius from allergy patients and S. epidermidis from healthy subjects were brought into contact with mouse nasal mucosa cells, only S. salivarius stimulated inflammation and a biochemical cascade associated with allergic reactions. The gene for mucin-5AC, a “sticky” substance that protects mucous membranes, was also overexpressed, a sign of respiratory hyperreactivity. Lastly, unlike S. epidermidis, S. salivarius adhered more strongly to mucous cells when exposed to the allergen, unless the mice were genetically modified not to produce this mucin. This adhesion increases contact between the bacterium’s pro-inflammatory substances and inflammation receptors in the nasal mucosa.