A rose in full bloom, a fresh pot of brewed coffee, and a raging bonfire are only three of the millions of scents that the human olfactory system can detect. While humans are able to distinguish between many different types of odors, not all are as pleasant as a rose in full bloom. Take for instance, bromhidrosis (i.e., body odor) – a common yet often unpleasant phenomenon in post-pubertal individuals. Human body odor is caused by many factors including sex, diet, health, and disease; however, the main contributor to body odor is due to the activity of bacteria on sweat glands1-2.
Humans have three types of sweat glands: eccrine, apocrine and apoeccrine sweat glands, as well as sebaceous glands which influence the composition of sweat1-3. Although eccrine sweat glands are the most ubiquitous and are responsible for the largest volume of sweat excretion, bromhidrosis is due to the interaction of apocrine sweat glands with various microorganisms2-4. Unlike eccrine glands, apocrine sweat glands are restricted mainly to the axillary region, ear canals, the wings of nostrils and the perineal region. Apocrine glands produce an odorless fluid composed of proteins, lipids, fatty acids, branched-chain amino acids, vitamins, and steroids (i.e., sweat) that is discharged into the canals of hair follicles2-4. It's not until the apocrine sweat is metabolized by axillary microorganisms such as Micrococcaceae, Propionibacteria, Staphylococcus and nondiptheiroid Corynebacterium species does it develop its characteristic odor4-6.
A major odorant responsible for the onion-like malodor is generated mainly by Staphylococcus hominis, and is composed of volatile sulfur compounds such as 3-Methyl-3-sulfanylhexanol3-6. These volatile sulfur compounds are secreted by apocrine glands as glycine-cysteine conjugates and are enzymatically cleaved by bacterial dipeptidases and C-S lyases which release odoriferous mercaptoalcohols3-6. Other malodors include the volatile fatty acid 3-methyl-2-hexenoic acid (3M2H), which has a goat-like odor, and 3-methyl-3-hydroxy-hexanoic acid, which has a cumin like odor3-6. Studies indicate that 3M2H is released from the skin surface after interacting with Corynebacterium striatum and Corynebacterium bovis3-6. Another common malodor that is characterized by its vinegar-like smell is caused by Propionibacterium metabolizing glycerol and lactic acid, leading to the production of acetic and propionic acid3-6.
The composition of the skin microbiota is diverse and can be functionally distinct between individuals. The skin microbiome and the volatile odorants produced are also dynamic and have been shown to shift in certain environments. Additionally, gastrointestinal ailments, various metabolic disorders, or even malaria can cause a change in the skin microbiome and odorants produced3-6. It is therefore important to characterize the composition of the skin microbiome and the production of axillary odorants in health and disease, as it may lead to the development of novel diagnostic tools and therapeutic treatments.
References:
- Kurosumi, K., Shibasaki, S., & Ito, T. (1984). Cytology of the Secretion in Mammalian Sweat Glands. International Review of Cytology, 253–329. doi:10.1016/s0074-7696(08)62445-6
- Wilke K, Martin A, Terstegen L, Biel SS. A short history of sweat gland biology. Int J Cosmet Sci. 2007 Jun;29(3):169-79. doi: 10.1111/j.1467-2494.2007.00387.x.
- Fredrich E, Barzantny H, Brune I, Tauch A. Daily battle against body odor: towards the activity of the axillary microbiota. Trends Microbiol. 2013 Jun;21(6):305-12. doi: 10.1016/j.tim.2013.03.002.
- Mogilnicka I, Bogucki P, Ufnal M. Microbiota and Malodor-Etiology and Management. Int J Mol Sci. 2020;21(8):2886. doi:10.3390/ijms21082886
- Byrd, A., Belkaid, Y. & Segre, J. The human skin microbiome. Nat Rev Microbiol 16, 143–155 (2018). https://doi.org/10.1038/nrmicro.2017.157
- “Microbial Origins of Body Odor.” ASM.org,https://asm.org/Articles/2021/December/ Microbial-Origins-of-Body-Odor.
