By Anna Huang
Like everyone’s genetic makeup, body odour emitted by each individual human is unique to them and is sometimes referred to as ‘odour signature’. Although the word ‘odour’ is mostly associated with a repulsive scent, body odour per se does not smell unpleasant. In fact, body odour and olfactory perception of other human individuals plays a key role in kin detection and also mate selection.
Odour individuality is important for new-born babies to olfactorily recognise their parents and vice versa. The importance of individual odour recognition persists throughout adulthood and also occurs in sibling recognition, aids in mate choice and their recognition once the relationship is established (Havlicek & Lenochova, 2006).
Human body odour is partly genetically inherited and can also vary in line with changing psychophysiological and ecological influences. By reflecting an individual’s genetic equipment, olfactory cues are also involved in major histocompatibility complex (MHC), allele-dependent, and mate selection, where females display preference for the body odour of men with different MHC alleles (Prokop-Prigge et al., 2016; Rikowski & Grammer, 1999).
Thus, it seems that disassortative mating (where individuals prefer mates who are genotypically or phenotypically dissimilar to themselves) is induced by human female olfactory preferences to maintain MHC diversity in offspring (Rikowski & Grammer, 1999).
MHC molecules are involved in antigen presentation upon pathogen invasion and influence the susceptibility to infection as their structure determines the probability with which a given pathogen will be recognized by the individual’s immune system (Milinski et al., 2013).
A variety of other factors, other than genetics, can also influence an individual’s particular scent. Body odour can also change throughout a woman’s menstrual cycle, can be affected by diet, and can also be indicative of certain diseases (Havlicek & Lenochova, 2006).
So, what causes malodour formation (especially in the axillary region) that results in body odour taking on an unpleasant smell? The culprit behind the production of malodour originating from the skin surface, particularly the underarm (axillary) region, are bacteria. The biotransformation of so-called malodour precursors (non-odorous molecules), which are secreted by human apocrine glands (one of the two major types of sweat glands found in humans), by microbiota gives rise to axillary malodour (Bawdon et al., 2015). The main causes for axillary malodour are the bacterial transformation of N-acylglutamines into malodorous volatile fatty acids (VFAs) and the transformation of hydroxyalkylcysteinylglycines into thioalcohols (Bawdon et al., 2015; James et al., 2013). Of these two molecule classes (VFAs and thioalcohols), thioalcohols are thought to have a lower olfactory threshold and are the most pungent volatiles, as a relatively small amount is needed for the odour receptors in the human nose to be triggered (Bawdon et al., 2015; Rudden et al., 2020).
The axillary microbiota predominantly consists of Staphylococcus, Cutibacterium (formerly Propionibacterium) and Corynebacterium genera, which are involved in malodour formation. Several Staphylococcus species and strains (such as the coagulase negative Staphylococci) in particular have been identified to be the most efficient bacterial metabolisers of L-cysteinylglycine dipeptide-conjugated alcohols (odourless physiological malodour precursors) into 3-methyl-3-sulfanylhexan-1-ol (3M3SH), the most abundant thioalcohol (of a total of four) found in axillary secretions (Rudden et al., 2020). Staphylococcal species metabolise the malodour precursor (Cys-Gly-3M3SH) using a dipeptidase to release glycine and then a C-S β-lyase to liberate the volatile 3M3SH (the most pungent component of axillary malodour) (Rudden et al., 2020). Despite some remaining knowledge gaps, the last decade has seen a step change in the understanding of the microbial and biochemical origins of axillary malodour. This knowledge will ultimately be useful for the development of body odour deterrent products (i.e., deodorants) that target specific bacteria, metabolic pathways, and enzymes, instead of relying on heavy fragrances and broad-spectrum antimicrobial agents (James et al., 2013).
Furthermore, gender, age and ethnicity also seem to be factors that influence the axillary microbiome and cause variation in underarm microbiome profiles.
One of the other challenges that remain is understanding the genetic influence of genetics on malodour intensity. Only in recent years has a single nucleotide polymorphism (SNP) in the ATP-binding cassette, sub-family C, member 11 gene (ABCC11) been identified which affects the volatile organic compound (VOC) profiles between populations of different ethnicities (of Asian, African American, and Caucasian descent) (Prokop-Prigge et al., 2016). Findings of a 2016 study suggest that an individual’s ethnicity has a significant effect on axillary malodour production. Quantitative differences in the production of axillary odorous compounds have been found in individuals belonging to Asian, African American, and Caucasian descent (Parma et al., 2019; Prokop-Prigge et al., 2016). However, the exact role of the ABCC11 gene in the production of volatile compounds still remains unclear and thus more research is required to understand the influence of different ABCC11 genotypes on the production of axillary malodour.
These findings can also provide further insight into the future development of targeted odour deterrent products. Still, further exploration and research is needed to better understand the relationship between microbiome, as well as other factors (e.g., genetic) and malodour formation (Li et al., 2019).
Bawdon, D., Cox, D. S., Ashford, D., James, A. G. & Thomas, G. H. (2015) Identification of axillary Staphylococcus sp. involved in the production of the malodorous thioalcohol 3-methyl-3-sufanylhexan-1-ol. FEMS Microbiology Letters. 362 (16). Available from: doi: 10.1093/femsle/fnv111.
Havlicek, J. & Lenochova, P. (2006) The effect of meat consumption on body odor attractiveness. Chemical Senses. 31 (8), 747-752. Available from: doi: 10.1093/chemse/bjl017.
James, A. G., Austin, C. J., Cox, D. S., Taylor, D. & Calvert, R. (2013) Microbiological and biochemical origins of human axillary odour. FEMS Microbiology Ecology. 83 (3), 527-540. Available from: doi: 10.1111/1574-6941.12054.
Li, M., Budding, A. E., van der Lugt-Degen, M., du-Thumm, L., Vandeven, M. & Fan, A. (2019) The influence of age, gender and race/ethnicity on the composition of the human axillary microbiome. International Journal of Cosmetic Science. 41 (4), 371-377. Available from: doi: 10.1111/ics.12549.
Milinski, M., Croy, I., Hummel, T. & Boehm, T. (2013) Major histocompatibility complex peptide ligands as olfactory cues in human body odour assessment. Proceedings of the Royal Society of London, Series B: Biological Sciences. 280 (1757), 20130381. Available from: doi: 10.1098/rspb.2013.0381.
Parma, V., Redolfi, N., Alho, L., Rocha, M., Ferreira, J., Silva, C. F. & Soares, S. C. (2019) Ethnic influences on the perceptual properties of human chemosignals. Physiology & Behavior. 210 112544. Available from: doi: 10.1016/j.physbeh.2019.05.005.
Prokop-Prigge, K., Prokop-Prigge, K., Greene, K., Greene, K., Varallo, L., Varallo, L., Wysocki, C. J., Wysocki, C. J., Preti, G. & Preti, G. (2016) The Effect of Ethnicity on Human Axillary Odorant Production. Journal of Chemical Ecology. 42 (1), 33-39. Available from: doi: 10.1007/s10886-015-0657-8.
Rikowski, A. & Grammer, K. (1999) Human body odour, symmetry and attractiveness. Proceedings of the Royal Society of London, Series B: Biological Sciences. 266 (1422), 869-874. Available from: doi: 10.1098/rspb.1999.0717.
Rudden, M., Herman, R., Rose, M., Bawdon, D., Cox, D. S., Dodson, E., Holden, M. T. G., Wilkinson, A. J., James, A. G. & Thomas, G. H. (2020) The molecular basis of thioalcohol production in human body odour. Scientific Reports. 10 (1), 12500. Available from: doi: 10.1038/s41598-020-68860-z.