Why your armpits smell bad

Armpit
tldr; = your armpits = lunch for bacteria

 

The human skin exhibits a diversity of ecological niches varying in moisture, the availability of nutrients, and the presence of host- and bacteria-derived antimicrobial peptides. In general, skin regions can be classified as dry, sebaceous, or moist environments with specifically adapted organisms establishing a distinct microbial profile characteristic for each topographical region. 16S rDNA profiling of the skin microbiota in the course of the Human Microbiome Project shed the first light on the community structure at different skin sites independent of microbial culture.

In moist areas such as the armpit (in medispeak, the “axilla”), Corynebacterium and Staphylococcus dominate the bacterial flora, which was demonstrated by culture-based approaches. The axillary region is a specific habitat and differs significantly from other body parts because it harbors hair follicles with sebaceous glands and a high density of sweat glands. In this environment nutrients are readily available, which allows for dense bacterial colonization reaching up to 106 cells per cm2. Different types of sweat glands are present including eccrine and apoeccrine glands which are responsible for sweating when you’re hot. Apocrine glands, so-called scent glands, contribute to the high density of nutrients. They secret a milky odorless fluid consisting of electrolytes, steroids, proteins, vitamins, and a variety of lipid compounds. Despite the fact that the exact composition of apocrine sweat is currently unknown due to insufficient availability of pure samples, the microbial biotransformation of these secreted nutrients in the human axilla undoubtedly leads to the development of characteristic and individual odor profiles. Perception and pleasantness of specific odor compounds have been reported to correlate with gender, sexual orientation, and the use of oral contraceptives.

It is of particular interest for the cosmetic industry to understand body-odor formation mechanisms to be able to design deodorants which specifically target them. The classic functional mechanism of deodorants is the depletion of cutaneous bacteria employing unspecific antimicrobial agents. This can cause skin irritations upon very frequent usage not only due to the direct topical action of alcoholic or organic substances. Also, disruption of the integrity of the skin microbiota may have negative effects on the host in terms of health because symbiotic and commensal bacteria participate in immune defense against pathogens. This article gives an overview of the biochemical origin of human axillary odor and the taxonomic composition of the axillary microbiota based on recent data generated with next-generation sequencing techniques.

 

Daily battle against body odor: towards the activity of the axillary microbiota. Trends in Microbiology. 05 April 2013 doi: 10.1016/j.tim.2013.03.002
The microbial community of the human axilla plays a key role in the formation of axillary odor by biotransformation of odorless natural secretions into volatile odorous molecules. Culture-based microbiological and biochemical studies have allowed the characterization of the axillary microbiota, but the advent of next-generation culture-independent DNA sequencing approaches has provided an unprecedented depth of data regarding the taxonomic composition of the axillary microbiota and intra- and interindividual variation. However, the physiological activity of the microbiota of an individual and its variation under different environmental conditions remains largely unknown. Thus, metatranscriptomics represents a promising technique to identify specific metabolic activities in the axillary microbiota linked to individual differences in body odor.

 

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