Your skin is crawling with microbes, and your armpits are no exception. The warm, moist pit environment is a haven for bacteria. However, pit-dwelling microbes can be quite rude; they make themselves at home then stink up the place, leaving us to deal with the smelly, sometimes embarrassing, consequences.
Since the 1950s, scientists have known that bacteria are the culprits behind armpit stank. While the armpit microbiome consists of various types of bacteria, it is dominated in part by Corynebacterium and Staphylococcus species, which are also key contributors to body odor, or BO. These microbes break down odorless molecules secreted from armpit sweat glands to release malodorous byproducts.
There is now experimental evidence to explain something our noses have always known: Not everyone's funk is the same. Factors including genetics, sex, and age are associated with individual differences in armpit microbiome composition and odor formation. As scientists gain greater insight into the microbial underpinnings of BO, they are coming up with new strategies that manage pit stink by targeting the bacteria that cause it.
Pit stink chemistry divides into two major classes: volatile fatty acids (VFAs) and thioalcohols. VFAs are responsible for the acidic twang of BO. Thioalcohols are sulfur-containing compounds that come in various shades of reek; some can have a meaty-oniony aroma while others are fruitier and less offensive. To make VFAs and thioalcohols, bacteria use special enzymes that hack off the stinky parts of sweat molecules, which evaporate from the skin to create the nose-wrinkling funk that is BO.
The presence and mix of VFAs and thioalcohols that make up people's pit scent profiles can depend on their genes. For example, scientists have found that one single gene is essential for development of underarm stench. The gene, called ABCC11, encodes a protein that pumps odor precursors to the armpit's surface, which are then metabolized by bacteria. A mutation in ABCC11 cuts the stink. Interestingly, this ABCC11 mutation is more prevalent in East Asian populations compared to those of European and African origin. These findings may point to geographical differences in the type and severity of armpit odor in people across the world, and raise questions about the evolutionary basis for BO.
Sex matters too. One study of 24 Caucasian participants illustrated that, compared to female study participants, male participants had higher bacterial abundances and a more intense armpit odor characterized by a "fatty and acid-spicy" odor profile. Generally, the women stunk less and their odor profiles had some "sulfury cat urine" notes. These sex differences are likely tied to variations in armpit microbiome composition. For example, men had more types of Corneyobacterium, which are prominent VFA and thiolacohol producers, than women. The researchers only observed sex variations in armpit odor in participants who did not use antiperspirants. Male and female participants who used antiperspirants had similar amounts of bacteria in their pits to each other, and comparable odor profiles and intensities.
Age is also an important factor. A recent study examining the armpit microbiome of individuals across different age groups found that people 55 years and older have more armpit bacteria than those between 18-30 and 35-50 years old. They also also had greater diversity of Corynebacterium bacteria. The authors speculate this may have something to do with development of the musty "old people smell" commonly associated with elderly people, though more research is required to determine whether this is the case.
Thanks to deodorants and anti-perspirants, we are not entirely at the mercy of our armpit bacteria. Still, these products are not entirely effective. Although deodorants and anti-perspirants decrease the total number of armpit bacteria, they can make it easier for odor-producing Actinobacteria to grow, which may be hardier than their non-odor producing counterparts. This is a problem with psychological consequences — unmanageable smelliness is hard to deal with. In some cases people take pills that prevent stimulation of sweat glands to deal with excessive sweating. Others undergo more invasive means of managing sweat, and thus their pit microbiome, by permanently destroying sweat glands with laser treatment.
But it doesn't have to be this way. Backed by years of research about pit microbes and their malodorous byproducts, scientists are creating novel means of managing underarm odor by targeting the microbes responsible for the stench.
Armpit microbiome transplants, championed by Christopher Callewaert, a scientist appropriately known as "Dr. Armpit," are one such method. The procedure is simple: Someone with troubling BO depletes their pit microbiome with antibiotics. A scientist then swabs the armpit of a less stinky donor and smears those microbes onto the recipient, who then skips washing their pits for a week to let the new bacteria settle in. Eighteen people have undergone transplants, with improvements in their BO that lasted for at least a month. However, there are risks associated with the procedure, including the transfer of potentially pathogenic bacteria from donors to recipients. All in all, Callewaert doesn't think armpit microbiome transplants are a mainstream solution to underarm stink, but may be best reserved for severe cases of BO.
Another tactic involves applying specific bacteria or compounds to the armpit that outcompete or eliminate odor-producing species, respectively. This method is used by several products currently on the market. Some of these newer deodorants contain "good" bacteria that outcompete odor-causing species. To this end, scientists recently showed that an armpit cream containing live Lactobacillus bacteria reduced the abundance of odor-forming Corynebacteria in study participants' armpits. Other products incorporate compounds, like acids, into deodorant formulations, to lower armpit pH, making the environment more hostile to bacteria.
Ultimately, a deeper understanding of the armpit microbiome, and the stink it emanates, has practical implications for pits everywhere. It also provides a little peace of mind: It's not you, it's your pit microbes.