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.
