Wisdom Wednesday: Olfaction and the Microbiome-Gut-Brain Axis

This review covers the field of olfaction and chemosensation of odorants and puts this information into the context of interactions between microbes and behavior; the microbiome – gut – brain axis (MGBA). Recent emphasis has also been placed on the concept of the holobiome which states that no single aspect of an organism should be viewed separately and thus must include examination of their associated microbial populations and their influence. While it is known that the microbiome may be involved in the modulation of animal behavior, there has been little systematized effort to incorporate into such studies the rapidly developing knowledge of the wide range of olfactory systems.

The classical olfactory system is evolutionarily conserved in multiple taxa from insects through to fish, reptiles and mammals, and is represented by the largest gene families in vertebrates. Mice have over 1000 different olfactory receptors and humans about 400. They are distributed throughout the body and even found in spermatozoa where they function in chemotaxis. Some ectopic olfactory receptors have been shown to have functional effects in the gut and kidney, highlighting the complexity of the systems engaged by odorants. However, there are, in addition to classical olfactory receptors, at least two other families of receptors involved in olfaction that are also widely found expressed on tissues in many different organs in addition to the nervous system and brain: the trace-amine associated and formyl peptide receptors.

Bacteria can make many if not most odorants and are responsible for recognition of species and relative relatedness as well as predator presence, among many other examples. Activation of different combinations of olfactory receptors by bacterial products such as B-phenylethylamine have been shown even to control expression of emotions such as fear and aggression.

Olfaction is one of the five senses, however, it is perhaps not widely appreciated that it represents a form of chemical communication which is widely distributed and has been extensively conserved evolutionarily. Linda Buck and Richard Axel jointly received the Nobel Prize in 2004 for their discoveries of ‘odorant receptors and the organization of the olfactory system’. Their description of olfactory genes occupying roughly 3% of our total gene pool was first published in 1991.

Animals use olfaction to distinguish each other, recognize individuality and kin and even use this ability for the purposes of mate selection and preference. This is not limited to vertebrates. Indeed, bacterial synthesis of phenol was first shown to account for male sexual attraction in grass grub beetles.



While it is clear that chemosensation/olfaction underlies a great deal of animal behavior both from reproductive and social standpoints, it is assumed that humans have much less capacity to detect and even respond to the odorant cues that are so evident in the animal kingdom. Port may have been the first to establish in humans that recognition of kin could occur through olfaction and characteristic body odors in 1985. Similar conclusions have been drawn for the recognition by mothers of babies, and by infants of their mothers in 2015. Although controversial, there are recent suggestions that olfaction in human communication may be important in interpersonal signaling and that this may not rely solely on auditory or visual signals. Whether these systems in humans really do reflect the ability to smell fear and even happiness remains to be confirmed.

My Take:
I apologize for the length of this blog. It’s hard to condense 13 pages of text into a single page.

If you just think of the sense of smell as one of the five senses, then these concepts seem very believable. When you expand the sense to include chemical signaling between every cell, tissue, organ and organ system in the body it makes perfect sense. However, now throw in the bacterial soup that we live in as the driving force behind olfaction and it sounds more like a Sci-Fi movie.

The Bottom Line:
We’d better get used to this concept of symbiosis because it’s real. You can start by taking better care of your microbes. The ones in the gut love fiber, so eat lots of fruits and veggies. The ones in the skin don’t survive all the artificial chemicals we use – deodorants, chemically scented lotions, etc. All react adversely to medications, including oral contraception, and HRT. We need to clean up our personal environment for without the microbes we will not survive.

Source: Biological Reviews Cambridge Philosophical Society 2018