sense of smell

We've got microbiota in our nostrils...


Could the microbial community in our nose influence our food choices?

Men or women, young or old, we are all colonized by one to two kilos of micro-organisms. These bacteria, viruses and fungi, present inside and outside our bodies, are brought together under the increasingly familiar term microbiota. Most of them are found in our intestine, but they are also found on all surfaces of the body, including the alveoli of our lungs and our mucous membranes, including the nose.

Ahe many scientific studies devoted to the microbiota, a field of research that is in full swing, have made us aware of its importance for staying healthy. We now realize that it may play a role in our health. in our behaviour towards our fellow menthat it influences our body odour and even our relationship with food. If you're passionate about discovering the intestinal microbiotaYou will be surprised to learn that we are only just beginning to explore the influence of the microbiota on the sense of smell.

The influence of the microbiota on the intestinal epithelium (the layer of cells lining the inner surface of this organ) has led to a considerable development of research, from its genetic characterization in 2006. It is now widely accepted that the intestinal flora is involved in most of the major functions of the body, from regulating an individual's blood pressure to the amount or type of food consumed.

The influence of the microbiota on sociability

Works published in 2016 go even further. They attribute to the microbiota a major role in social behaviour. This American study concerns the development of autism in connection with maternal obesity. The authors study mice from obese mothers fed a fatty and sweet diet. These mice, although fed a standard diet after birth, have poor social relationships.

You should know that mice are social animals. In the presence of a new individual, they usually spend time exploring each other. However, mice born to obese mothers have very little interest in other mice. Thus, they are a valuable animal model for studying autism, which is known to occur more frequently in children born to obese mothers.

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First of all, the researchers found that the microbiota of these mice is different from that of their congeners from mothers on a balanced diet. Secondly, they show that by adding a major bacterium missing from their microbiota via their diet, their social relationships return to normal. They conclude that their study opens up new perspectives in the treatment of behavioural disorders in humans through the use of probiotics. These "good" bacteria are already commonly used as a supplement to food in cases of fungal infections or diarrhoea. The conclusion of these researchers raises questions, as we will see a little further on.

Surprisingly, while the intestinal microbiota and its physiological effects are much studied, little work is focused on the other organs. The author of the "microbiota" page on Wikipedia even complains that, out of more than 4,200 studies devoted to the human microbiota, only seven focus on the microbial community... of the penis. Since none of the studies have focused on the nose, my field at the Institut national de la recherche agronomique (INRA), we decided with a group of colleagues to take an interest in it.

The microbiota modifies odour detection

The olfactory epithelium, the tissue lining the inside of our nose, carries out the initial stage of odour detection, before the information is processed by the brain. We have initially chosen to study the variations in the olfactory system. in animals without any microbiota, called "axenic".... According to our observations, the absence of microbiota does alter the structure of the olfactory epithelium - less drastically, however, than the epithelium of the intestine.

First of all, its renewal is slowed down. This is probably due to the disappearance of the micro-organisms usually present in the nasal cavity. Less attacked by pathogenic micro-organisms from the environment, the epithelial cells are not pushed to renew themselves as frequently.

Secondly, in axenic animals, the ciliary layer of olfactory neurons where the detection of odorant molecules takes place is thinner. That's right, there are neurons in our noses, not just in our brains!. Despite this thinner lash layer, the neural electrical signals generated by the arrival of scents are, paradoxically, more intense in axenic animals. Without providing an explanation for this paradox, these initial studies show that the microbiota influences the structure of the nervous tissues located in our nose.

Preferences for different smells

We then conducted a new series of experiments to investigate whether the nature of the microbiota could influence the way mice perceive odours. To this end, we used mice with the same genetic profile but separated into three groups, each colonized by a different microbiota. We found that the three groups of rodents did not show the same preferences for a panel of odours selected specifically to attract their interest.

To find out more, we recorded the electrical activity of neurons in the olfactory epithelium of mice in response to the odours tested. Again, we found variations between the three groups. However, some groups of mice were able to respond differently to two different odours while the neurons in their noses responded in a similar way.

The neurons of the nose only carry out the first stage of processing the information provided by odours. The more or less pronounced interest of a mouse in an odour therefore results from the integration of information from the olfactory epithelium by numerous structures in its brain. The discrepancy observed between the behaviour of mice and the response of the neurons in their nose therefore suggests that the nature of the microbiota colonising the mouse organism influences the way its brain interprets odours.

We are familiar with our body odor

At least two hypotheses allow us to understand that the preferences of mice are influenced by their microbiota. The odours emitted by all animals are closely related to the microorganisms that colonize them. This is the case in humans, where the majority of body odours result from the metabolism of our bacteria on the surface of our skin, in our intestine and in our genitals. It is the same in rodents. Their body odour, which is familiar to them, may therefore explain the differences in attraction to the odours tested.

Furthermore, the olfactory preferences of adult rodents are highly dependent on odours emanating from the environment during their brain development, starting as early as the uterine stage, as many recent studies have shown. The smells they have been in contact with early in their lives are also familiar to them and may therefore influence their interest in the smells they encounter as adults. This very early learning of odours during foetal development seems very general in the animal world, and is also well described in humans.

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The smells we encounter very early in life influence us as adults.

At this stage of our reasoning, however, it is useful to remember that mice differ from humans in the use of their five senses. The mouse favours the sense of smell - fundamental in its social interactions - whereas humans use their senses of vision and hearing. Thus, mice rendered unable to detect odours by genetic modification (so-called anosmic mice) lose their reproductive and defensive behaviours. They also have altered parental behaviour.

If the microbiota affects the functioning of the olfactory system of mice, then we should take a fresh look at the work of teams using rodents as models to study the impact of the microbiota on behaviour. This is the case of the work of the American team on autism and the microbiota, cited at the beginning of this article. The question must be asked whether the disturbances in social behaviour observed in the mice in their experiment might not, in fact, come from a disturbance in their olfactory cues.

Mice disturbed in their olfactory cues

Indeed, the researchers applied a different diet to the mice after weaning than that of their mothers. This change in food resulted in a change in their body odour. They probably lose a good part of their initial olfactory cues! We can therefore hypothesize that the disruption of social behaviour in these rodents primarily involves their sense of smell.

Unlike mice, we humans do not give priority to olfactory information to establish our social relationships. This remark leads to relativizing the scope of the study on these animals.

On the other hand, if, as in rodents, our microbiota changes the way we perceive odours around us, it is quite possible that it influences our choice of food. Because it is well established that our sense of smell guides us in our likes and dislikes of this or that food.... A completely different line of research could be to assess the extent to which we can change our microbiota so that our cravings will naturally lead us to healthier food.

Nicolas MeunierNeurobiologist specializing in olfaction, University of Paris Saclay, INRA

The original text of this article was published on The Conversation.

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