Bacteria basics: Smell like… body odor!

This year the world remembered the 25th anniversary of the death of Kurt Cobain, the former lead singer of the famous Grunge Rock band Nirvana, who tragically committed suicide on 5th of April 1994. Their epic song “Smells like teen spirit” represents one of the most influential rock songs of all times and served as a blueprint for the definition of Grunge music. As many other people, I still remember my first contact with this song. It happened in a small record store in Kassel, Germany, where the song was played in the background. As if struck by lightning, I had to buy the album immediately. Outside the Grunge Rock community it is largely unknown, that the title of this song was inspired by a Graffiti sprayed onto Kurt´s bedroom wall saying that “Kurt smells like Teen Spirit”, with “Teen Spirit” being the favorite deodorant brand of Kurt´s girlfriend at the time.

 

“On average, 100 to 200 glands per cm2 are distributed virtually all over the body surface, excreting hundreds of milliliters of sweat per day.” 

 

Apart from serving as inspiration for one of my favorite rock songs, deodorants determined my working life for quite some time. From 2006 until 2010 I worked in the Microbiology department of the Henkel headquarter in Düsseldorf, Germany. There, human body odor formation and the development of novel formulations to fight against it was my main area of research. Coming from the depths of the human intestinal tract, my new work in human axilla was clearly an upgrade. I was amazed to see how much research was invested in products aimed at the prevention of body odor formation. With good reason! From a business point of view, the prevention of body odor formation by deodorants is clearly an attractive field. The global deodorant market for 2024 is estimated to be bigger than 90 billion US dollars[1].

 

Although also influenced by nutrition and underlying diseases affecting general metabolism, human body odor formation is mainly a microbiological phenomenon. Substances excreted by sweat glands onto the human skin are metabolized by the residential microbiota and converted into smaller, volatile compounds that finally find their way into our noses. Since this bacteriolysis takes some time, freshly excreted sweat does not smell. Everyone can prove this easily: directly after a strong work out, the fresh sweat does not smell. However, after one or two drinks at the sports bar (or approximately 30 minutes time), you and your towel will notably start smelling, indicating that it’s time to wash.

 

Sweat glands can be categorized into eccrine and apocrine glands. Eccrine sweat glands are the most abundant sweat glands. On average, 100 to 200 glands per cm2 are distributed virtually all over the body surface, excreting hundreds of milliliters of sweat per day. Their main functions are thermoregulation and prevention of bacterial colonization and growth through salting and acidification of the skin surface. Eccrine sweat is clear and composed of mainly water containing sodium and potassium salts as well as some organic compounds. Apocrine glands release their secret into hair canals, and are consequently limited to hairy body surfaces. They are activated during puberty and excrete only a few microliters per day. Apocrine sweat appears milky and viscous, and is known to comprise lipids, lactates, nitrogen, electrolytes, steroids, proteins, vitamins, ions and presumably also pheromones. However, the exact composition of apocrine secretion is still not clear due to the lack of pure samples. The same holds true for the biological function of apocrine sweat glands, which might involve emotional sweating and non-verbal communication[2].

 

Regarding odor formation, the human axilla is the hot spot of the human body. Here, the highest numbers of bacteria across the whole body surface are found. Cell densities frequently grow up to million cells per cm2. The reasons are obvious: The human axilla represents a sheltered environment with a huge (hairy) surface that offers warmth, wetness and a wealth of nutrients. The latter two are provided by sweat glands. While the eccrine sweat glands rather indirectly contribute to human body odor formation by providing wetness, which is pivotal for microbial life and activity, the apocrine sweat contains the substances that can be transformed into odorous molecules by bacteriolysis2.

 

Among all skin organisms, bacteria are best investigated and regarded most important for body odor formation. Classical cultivation-dependent, as well as modern molecular analysis, concurrently showed that the human axilla is colonized by a moderately diverse bacterial community, dominated by a few, Gram-positive genera, in particular, Corynebacterium, Staphylococcus, Cutibacterium (formerly known as Propionibacterium), as well as Micrococcus, Brevibacterium and Anaerococcus[3] 2. There is a clear, positive correlation between the bacterial load of the adult axilla, in particular with corynebacteria, and the strength of body odor. Despite all technological advancement in the field of odor analysis, the human nose is still the gold standard in (mal)odor analysis, and many studies in this field are performed by so-called sniffer panels, i.e. groups of experts specially trained in the multidimensional evaluation of odors.

 

“Compared to deodorants, antiperspirants are much more effective in preventing malodor production than deodorants”.

 

Chemical analysis proved a very complex composition of bacterially fermented human sweat, and a few lead compounds have been identified, among them 3M2H (3-methyl-2-hexanoic acid), which represents THE typical human sweat (mal)odor molecule. A bacterial enzyme named aminoacylase releases this 3M2H from a precursor compound by cleaving of an amino acid residue, turning the water-soluble precursor into the volatile 3M2H molecule. In particular, corynebacteria carry this enzyme activity, and many hopes are being put onto the development of specific enzyme inhibitors for cosmetic purposes[4]. However, inhibitors suitable for use in mass-market products are hard to find, mainly due to cost or toxicological reason or because they negatively interfere with the product matrix, i.e. by changing color or smell.

 

So, the daily battle against body odor is still dominated by two classical strategies, which – based on personal discussions with many of my students – are astonishingly unknown by many people. So-called deodorants sensu strictu or body sprays simply kill bacteria by alcohol (ethanol) and cover malodor by perfume. In contrast, so-called antiperspirants clog sweat pores by the protein-denaturing activity of ACH (aluminium chlorohydrate), thereby depriving the bacteria in the axilla of malodor precursors and water. In addition, antiperspirants are strong antimicrobials, which prevents the formation of nasty sweat patches on your clothes and create a feeling of dryness under your axilla. Compared to deodorants, antiperspirants are much more effective in preventing malodor production than deodorants. However, because aluminum ions are toxic, the use of ACH in antiperspirants is under an ongoing debate. To the best of my knowledge, no state-of-the-art study has so far proven that the long-term use of ACH in antiperspirants has any negative health effects. Nevertheless, an alternative to ACH (ideally being as effective and cheap) is desperately sought by the cosmetics industry.

 

I want to end this contribution with some knowledge long seen as “nerdy” or “useless”. In the late 1930s, a Japanese researcher published a paper in the German Journal of Racial Science, (which was very popular at that time), about a surprising association he had observed: While Asian people have whitish, crumbly ear wax and very weak body odor, Europeans have yellowish, oily ear wax and a much stronger body odor. It took more than 70 years to unravel the biological reason for this association. Many Asians carry a mutation in a transporter protein of their apocrine glands, which delivers wax into the human ear as well as sweat onto human skin. The mutated protein is significantly hampered in its transportation activity, leading to the observed changes in ear wax consistency and body odor. Since strong body odor is a big taboo in many Asian regions, carriers of the mutation probably had more reproductive success, so that this mutation could spread rapidly in Asian populations. Although led by a researcher of Beiersdorf, a competitor company to Henkel, the underlying publication[5] holds a top position in my all-time top ten of research publications (similar to “Smells like teen spirit” in my music top ten) because it nicely shows that the activity of human skin bacteria can strongly affect even human behavior and evolution.

 

[1] Trefis.com. n.d. Size of the global antiperspirant and deodorant market from 2012 to 2024 (in billion U.S. dollars)*. Statista. Accessed May 24, 2019. Available from https://www.statista.com/statistics/254668/size-of-the-global-antiperspirant-and-deodorant-market/.

[2] Egert M, Simmering R (2016). The Microbiota of the Human Skin. Adv Exp Med Biol. 902:61-81. doi: 10.1007/978-3-319-31248-4_5

[3] Fredrich et al. (2013). Daily battle against body odor: towards the activity of the axillary microbiota. Trends Microbiol. 21(6):305-12. doi: 10.1016/j.tim.2013.03.002

[4] Natsch A et al. (2005). Isolation of a bacterial enzyme releasing axillary malodor and its use as a screening target for novel deodorant formulations. Int J Cosmet Sci. 27(2):115-22. doi: 10.1111/j.1467-2494.2004.00255.x

[5] Martin A et al. (2010). A functional ABCC11 allele is essential in the biochemical formation of human axillary odor. J Invest Dermatol. 130(2):529-40. doi: 10.1038/jid.2009.254

 

 

 

 

 

 

 

 

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