Despite the explosion of research into the human microbiome, relatively little is known about the microbiome of human feet and its relation to foot odor formation. This is particularly apparent when compared to the wealth of knowledge that is available for body (axillary) odor formation.
While the lack of research may be due to commercial reasons, the research available to date is intriguing. Notably, in 2006, Dutch researcher Bart Knols was awarded the Ig Noble Prize* [1] for showing that the female malaria mosquito Anopheles gambiae is attracted equally to the smell of limburger cheese and to the smell of human feet.
Yet, foot odor plagues many of us – whether your own or that kindly shared by loved ones. This article delves into the microbial community residing on the feet and the current research landscape around foot odor.
Despite the explosion of research into the human microbiome, relatively little is known about the microbiome of human feet and its relation to foot odor formation.
The feet: An oasis in the skin microbiome
Microbial life flourishes under warm, wet and nutrient rich conditions. That is why kitchen sponges [2] and intestinal tracts are among the most densely colonized places by microbes on earth. In contrast, the human skin is a bit less attractive for microbes. Being relatively cold (less than 37°C), dry and salty (due to salt in sweat), the skin appears like a desert [3], while kitchen sponges and intestinal tracts seem more like luscious rain forests.
However, as it should be for a real desert, there are also some oases, where effervescent water allows life to flourish. In the case of human skin, water is delivered onto it via so-called eccrine sweat glands.
Eccrine sweat glands are the most abundant sweat glands on the human body. On average, 100 to 200 glands per cm2 are distributed virtually all over the body surface, excreting hundreds of milliliters of sweat per day. The palms and soles exhibit the highest densities of about 600 glands per cm 2, while lips and nail beds are depleted of eccrine glands.
Their main functions are thermoregulation and prevention of bacterial colonization and growth through salting and acidification of the skin surface. While temperature is the major stimulus for eccrine glands, these glands are also activated by pain, stress, fear, and anxiety, resulting in so-called emotional sweating [3].
Who has not experienced that: while waiting for an important job interview or before giving a presentation in front of a big crowd, your hands (and maybe also your feet?) get a little clammy and moist. From an evolutionary point of view, the sweat on your hands and palms is thought to increase the grip of these surfaces under stress conditions.
The true cause of that whiff
Body odor formation is mainly caused by the bacterial fermentation of special excretion products from apocrine sweat glands [4]. However, these do not occur on feet. Foot odor is thought to mainly originate from the microbial degradation of dead skin cells and skin proteins. The level of odor is strictly regulated by the wetness level of the feet.
One of the most comprehensive microbiome studies on the spatial distribution of bacteria on human feet and their relation to odor formation was published by Stevens and colleagues [5]. They investigated the feet of 16 healthy subjects (8 men and 8 women) and determined that the highest cell numbers (up to 105 CFU per cm2) were found on the soles of the feet, which is close to cell densities in the human axilla (or armpit) [2][3]. The research found that the dorsal parts of the feet were significantly less colonized with 102 CFU per cm2.
The bacterial community they found on the feet was typical for the human skin, comprising well-known genera such as Staphylococcus, Corynebacterium and Acinetobacter. Interestingly, the soles were clearly dominated by staphylococci, while the dorsal parts were more diverse [5]. Staphylococci are very typical skin bacteria, which are well-adapted to life on skin, e.g. due to a high salt-tolerance [6].
Foot odor is thought to mainly originate from the microbial degradation of dead skin cells and skin proteins.
Why so cheesy?
Foot odor has a distinct cheesy, acidic note that has particularly been attributed to a blend of volatile fatty acids. In particular, isovaleric acid is regarded as the lead component in foot odor [5].
Staphylococci, isolated from human feet, have been shown to convert branched aliphatic amino acids, such as L-leucine, to short-chain methylbranched volatile, fatty acids, including isovaleric acid. Stevens and colleagues [5] exclusively detected this acid on the soles of the investigated feet and concluded that foot odor is mainly caused by high numbers of staphylococci forming isovaleric (and other) acids particularly at the soles of the feet.
Based on their findings, they question the (prominent) role of other taxa previously associated with foot malodor production, such as Kytococcus sendentarius, Brevibacterium and Micrococcus. This is a bit sad, because a strong link between Brevibacterium species and foot odor would have been such a nice basis for story-telling: Brevibacteria, such as B. linens, play an important role in the production of strong-smelling red-smear cheese, e.g. Limburger cheese.
Nevertheless, since (to the best of my knowledge) microbiome studies with subjects suffering from very strong foot odor are still missing, there is some hope that a significant role of other bacteria than staphylococci just awaits to be unraveled. Until then, we can be thankful that despite staphylococci being abundant throughout the bodily microbiome, it is only the feet that have the distinct warm and wet conditions that provide the perfect environment for odor to take hold. Equally, we shouldn’t forget that our feet host up to 100 different genera of fungi compared to only a few genera dominating the core body [7][8].
*A satirical scientific prize celebrating research and achievements that first make people laugh, then make people think.
[1] https://www.improbable.com/
[2]https://thesecretlifeofskin.com/2019/11/20/the-kitchen-sponge-a-welcome-host-to-germs/
[3] Egert M and Simmering R (2016). The Microbiota of the human skin. Adv Exp Med Biol. 2016;902:61-81. doi: 10.1007/978-3-319-31248-4_5.
[4] https://thesecretlifeofskin.com/2019/07/03/bacteria-basics-smell-like-body-odor/
[5] Stevens D et al. (2015). Spatial variations in the microbial community structure and diversity of the human foot is associated with the production of odorous volatiles. FEMS Microbiol Ecol. 91(1):1-11. doi: 10.1093/femsec/fiu018
[6] https://thesecretlifeofskin.com/2019/03/19/the-staphylococci/
[7] https://www.nationalgeographic.com/news/2013/5/130522-fungus-feet-health-body-science-weird-gross/
[8] Findley K et al. (2013). Topographic diversity of fungal and bacterial communities in human skin. Nature 498(7454):367-70. doi: 10.1038/nature12171.
