- 1 The Armpit
- 2 Overview
- 3 Detailed Environmental Description
- 4 Overview of Microbial Ecology as it is known
- 5 Expansion Topic 1: Implications to Smell
- 6 Expansion Topic 2: Implications to Disease/Rashes
- 7 Expansion Topic 3: Armpit Bacteria Transference
- 8 Key Microbial Players
- 9 Conclusion
- 10 References
By Andie Gomez-Patron
The armpit, or axilla to be precise, is home to a rich and diverse community of microorganisms. Armpits have similar communities to other moist areas of the human body, but, include a few special microbes that cause a pungent odor to be released. This page will describe why this is and how microbes interact with the human armpit environment.
Detailed Environmental Description
Physically, this environment is located under the arms of human beings. It is composed of human skin and provides a damp and humid environment, shielded from the sun for the majority of the time. Geochemically, this is independent of any earthly environments so it is not affected by soil or water but instead dictated by humidity, temperature, and pH. This particular environment can be described as rich in organic matter, moist due to sweat produced from regional glands, dark, and warm, which is ideal for microorganisms. A description of the specifics of the human skin with hair follicle environment is depicted in the image to the left. When looking at pH levels in our armpits, researchers can see that men report ~5.8 and women ~6.0 . Moreover, the environment is home to prominent hair follicles as well.
When distinguishing the armpit environment from general human skin microbiomes, scientists have distinguished three general environments in the body that populate similar microbes due to their similar environments. The categories are the oily locations, the moist locations, and the dry locations. The armpit falls into the middle category of moist locations which tends to see Corynebacterium. 
This environment is easily accessed and incredibly common, making it a perfect region to be studied. The human skin environmental ecology has been studied at great lengths, in the past and currently by microbiologists and dermatologists alike.  This research is ongoing and has no shortage of studies focusing on armpit microbial communities. Currently, one study is focusing on understanding how deodorant affects the microbial diversity in armpit populations. These scientists have already created cultures from found microbes and stated their analysis on these cultures, which we can see in the image to the right. Soon they will serve to show how chemical changes to the environment can affect the microbial ecology in this region, in a very detailed manner. 
Overview of Microbial Ecology as it is known
There is a large symbiosis and many commensals as well  between human skin and the microbes that inhabit it’s areas. Various parts of the skin house different kinds of microorganisms. On the surface of the skin we see mites inhabiting as well as bacteria and fungi, around the sweat glands we see the capability for virus housing, and around the hair follicles and entryways we see potential for viruses and prevalent fungi. 
However, different areas of the human body are more suited for different microbial communities and interact with each other differently. The armpits in particular, have one notable behavior that sets it aside from just any area of the body: sweat. Microbes within the armpit use the organic matter that comes out of the regional sweat gland as sustenance. The bacteria then break down the molecules from the sweat, and release them as volatile acids, which produce a particular smell. Interestingly, the armpit condition is variant due to a number of habits and external variables that happen to the human armpit such as clothing, outside temperature, stress levels, and more. This change in diversity abundances depending on different conditions, is depicted in the image on the top right.
In terms of alpha and beta diversity within this armpit system, we see the alpha as incredibly varied ranging from fungi, to mites, to bacteria, to viruses, not including the specifics of each that can arise. Therefore, we see an incredible richness of species diversity. In terms of beta diversity, we see that the community abundances can change greatly depending on the different conditions of the armpit such as hygiene, moisture content, odor represents, and more.
Bacteria that produce unwanted odors, such as those found the armpit, are found in these regions due to the particular kind of sweat glands in the area, called apocrine glands. These are the most prevalent glands in the armpits, as well as other regions like the bellybutton. These glands make unwanted smells because they secret sweat that is rich in protein. This protein rich sweat is easier for the regional bacteria to breakdown, which causes more smell production once the bacteria has finished its digestion of sorts. 
Looking at the two different main bacterial sources for bad odor in the armpit region, the diversity of the microbes is different in generalized men and women. Besides their different pH levels, males tend to secrete a larger amount of fatty substances when they sweat and tend to have thicker skin in comparison to females. This causes women to have more staphylococci, while male armpits usually have more lipophilic corynebacteria. Both of which are tied to bad odors in human armpits.
Finally, when considering the ecology of the armpit, it is necessary to look at the effect that the shirt being worn over the armpit has. In bottom right image within this section, we can see that microbial diversity and abundance of the region are somewhat affected by the different materials of the shirt being worn by the person being studied. As we can see, Staphylococcus has higher abundance with cotton and Corynebacterium has highest abundance in wool, nylon, and cotton. Additionally, Corynebacteria is one of the main contributors to pungent odor in armpits and is not prevalent in textiles. This means that Corynebacteria needs to grow on armpit environments that are more anaerobic. 
Expansion Topic 1: Implications to Smell
One recent study did a test comparing deodorant user armpits versus all natural armpits and found that those that do not use any kind of odor preventing substance such as antiperspirants or deodorants, have most prevalent Corynebacterium while people that have been using deodorants and antiperspirants for the majority of their lives, but stopped for a couple of days for this study, presented microbiomes that were heavy in Staphylococcaceae.  The presence of Staphylococcus hominis  is one of the leading and dominant factors contributing to this particular region’s pungent smell, along with Corynebacteria and Anaerococcus. The image to the left shows human habits that correlate with varying abundances of these three microbes.
This study also shows us that the most prevalent three microbes in the armpit are Staphylococcus, Corynebacterium, Anaerococcus, along with three other microbes that are prevalent in the armpit depending on human hygiene and habit which are Alicyclobacillus, Finegoldia, Campylobacter, Porphyromonas. This specific distribution is outlined in the table to the right, and shows which combination of grooming habit produces which kind of microbial diversity and abundance.
These bacteria eat sweat and produce pungent odors because their emissions are composed of volatile compunds. Specifically, there are unsavory smelling acids that are found in unwashed armpits that are associated with glutamine residue which comes from axilla sweat emissions. Moreover, Corynebacteria in particular, are confirmed to emit acids, separate form the axilla environment. Logically, if the abundance of bacteria growth is reduced, then there will be less unsavory odor emitted. This is because, with reduced numbers of beacterial enzymes, then less of the apocrine emissions are taken by the bacteria and turned into volatile compounds.
Expansion Topic 2: Implications to Disease/Rashes
Various diseases, infections, and rashes can occur due to the microbes living under the human arm. Some of the more prevalent and scary bacterial infections that can occur in the armpits are furuncles, also known as boils.  We could also see Erythrasma, which is caused by Corynebacterium minutissimum, a type of bacteria. This presents itself as lesions and scales of various irritated colors.
Moreover, scabies is an infection that can occur in various parts of the human body including the armpits. This forms from mites that live on the skin, specifically scabies mites, which cause a reaction since the skin is not used to the feces and proteins of the scabies mite.
Finally, we can also see fungal infections in the armpit region. One example is ringworm, which is caused by Trichophyton, Microsporum, and Epidermophyton, which belong to a fungi family called dermatophyte., and more specifically, Trichophyton rubrum. 
Expansion Topic 3: Armpit Bacteria Transference
Similar to recent studies being done on gut microbe transfers in order to alter weight, there are other scientists researching how to give someone who is more prone to pungent odor in their armpits, naturally less odor without the need for deodorant or antiperspirants. Researchers and scientists have been doing studies recently on introducing a healthy microbiome to the armpit from someone who is naturally non-odorous to replace a more odorous armpit microbiome.
The way that this transference works is that a close relative does not wash their armpit for three to our days and the person who will be receiving the new set of microbes will scrub and wash their armpit using using various antibiotics, special washes, and soap.  Once the antibiotics and antibacterials remove the host microbiome, the scientists use cotton swabs to apply the donor bacteria to the acceptor's armpit. Then they use incubation techniques in order to make sure there are no additional variables inctroduced into the new armpit ecology. Additionally, they apply cultured probiotics on the area as an extra measure.  The image to the right outlines the microbial transference process.
The results have been logged as short term and long term which are one month and three months respectively. As of now, with eighteen human trials done, both short term and long term results are very promising. 
Key Microbial Players
The skin has an incredible array of different microorganisms at play all over the body. Some of the most important microbial players across skin are Staphylococcus, Micrococcus, Corynebacterium, Brevibacterium, Dermabacter, and Malasezzia.
When looking at the microbes in the human armpit, it is important to understand that each area of the human body provides slightly different moistures, exposure to light, pH levels, gland secretion, and other environmental factors. Therefore, while there will be some underlying similarities in some of the communities on the body, that armpit will have its own very specific key microbes. The image to the left, shows that for the armpit (labeled Axillary Vault) in particular, there is a large majority of Proteobacteria, and a smaller collection of Bacteroidetes as well as Corynebacteriaceae. This paper and figure correlate with the data found in the armpit odor test done in the section, Expansion Topic 1: Implications to Smell, above.
What is incredible is that within the armpit scientists find microorganisms that fall into categories all over the tree of life ranging from fungi, to bacteria, to viruses, and to incredibly insects.
Many studies and research over the years have made cultures of these microbes to study them and their roles in this specific region. In summation, researchers see that some of the most important microbial players, Staphylococcus and Corynebacterium, are the cause of body odor in the armpit.
Looking at the three most prominent bacteria present in the armpit:
- Staphylococcus is known to exist in large abundance on human skin, warm-blooded mammal skin, and mucous membranes. They are gram-positive and do not require oxygen, also known as anaerobes. Moreover, they do not form spores and show little to no movement. Furthermore, they are cicrular or spherical nd cluster together as if they are grapes on vines as can be seen in the top image to the right in this section.
- Corynebacterium are generally or more popularly known as the bacteria that causes unwanted smells in regions of the human body such as the armpit. They, like the Staphylococcus tend to show up as gram-positive and do not form any spores. In terms of morphology, as depicted in the middle image to the right, they are shaped like irregular rods and also tend to clump together. Moreover, they respond positive to the catalase test. Since they fair better in more anaerobic environments, this tells us that they are facultative anaerobes that use oxygen as their terminal electron acceptor. 
- Anaerococcus, intuitively, is also found commonly on the human skin, as well as other areas of the human body such as within the mouth, in the human gut, in vaginal discharge, and the lungs when they contain abscesses. The bottom right image shows that it's morphology is similar to that of Staphylococcus in that it is spherical to rod-like and clumps together. Moreover, like Staphylococcus and Corynebacterium it is gram-positive. It also tests negative on the indole test to see if it is able to take tryptophan and turn it into indole. Finally, like the other two microbes, it is anaerobic. 
In summation, the armpit has a complex assortment of microorganisms that encompasses many different types of life on the phylogenetic tree. It is dominated by proteobacteriaand some bacteroidetes, but can contain viruses, fungi, many bacterium, and even mites. These microbes, specifically the Staphylococcus and Corynebacterium, interact with the sweat produced within this moist environment to feed themselves and, in turn, secrete a strong odor.
- “Armpit Microbes.” Public Science Lab, 21 Nov. 2016,
- Altor, Anne.: “Armpit Ecology!One Earth Body Care.” One Earth Body Care, 5 Aug. 2017.
- Grice E.A. and Segre J.A. 2011. The skin microbiome. Nature Reviews Microbiology 9:244–253.
- Akst, Jef.: “Microbes of the Skin.” The Scientist Magazine®.
- “Microorganisms Found on the Skin.” Microorganisms Found on the Skin | DermNet NZ.
- Urban, Julie, et al. “The Effect of Habitual and Experimental Antiperspirant and Deodorant Product Use on the Armpit Microbiome.”
- “Body Odor: Causes, Prevention, and Treatments.” Medical News Today, MediLexicon International
- “Armpit Body Odor Research.” Doctor Armpit, 8 Apr. 2020.
- Callewaert, Chris, et al. “Towards a Bacterial Treatment for Armpit Malodour.” Experimental Dermatology, vol. 26, no. 5, 2017, pp. 388–391., doi:10.1111/exd.13259.
- Rutsch, Poncie. “Meet The Bacteria That Make A Stink In Your Pits.” NPR, NPR, 31 Mar. 2015
- Natsch, A., et al. “Isolation of a Bacterial Enzyme Releasing Axillary Malodor and Its Use as a Screening Target for Novel Deodorant formulations1.” International Journal of Cosmetic Science, vol. 27, no. 2, 2005, pp. 115–122., doi:10.1111/j.1467-2494.2004.00255.x.
- Brannon, Heather L. “9 Bacterial Skin Infections You Should Know About.” Verywell Health, Verywell Health, 25 Feb. 2020
- Olesen, Jacob. “Ringworm in Armpit: Causes, Symptoms and Treatment of Armpit Fungus.” Make Life Natural, 5 Mar. 2019
- Society, Microbiology. “Microbes and Disease: Microbes and the Human Body.” Microbes and the Human Body | Microbiology Society
- The Editors of Encyclopaedia Britannica. “Staphylococcus.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 20 Nov. 2017.
- Collins, Matthew D., et al. “Corynebacterium Caspium Sp. Nov., from a Caspian Seal (Phoca Caspica).” International Journal of Systematic and Evolutionary Microbiology, vol. 54, no. 3, 2004, pp. 925–928., doi:10.1099/ijs.0.02950-0.
- Catalase Test - Virtual Interactive Bacteriology Laboratory
- “Anaerococcus Prevotii DSM 20548.” BacMap.
- Aryal, Sagar, et al. “Indole Test- Principle, Reagents, Procedure, Result Interpretation and Limitations.” Microbiology Info.com, 26 Apr. 2019.
- Fujii, Takayoshi, et al. “A Newly DiscoveredAnaerococcusstrain Responsible for Axillary Odor and a New Axillary Odor Inhibitor, Pentagalloyl Glucose.” FEMS Microbiology Ecology, vol. 89, no. 1, 2014, pp. 198–207., doi:10.1111/1574-6941.12347.