Your skin normally has bacteria living on it; we co-exist with thousands of microorganisms and never give them any thought, until things go wrong -- or smelly.
Staphylococcus epidermidis and Bacillus subtilis, for example, are two types of bacteria that naturally live on the skin of your feet. Yes, they like living in the moist, warm environment your sock provides, but they also like life on a sweaty bare foot -- and they really like to eat an amino acid, leucine, found in the sweat the eccrine glands in your feet produce. When these bacteria eat leucine they produce their own gassy byproduct: isovaleric acid. This isovaleric acid is what gives both stinky cheese and stinky feet their malodorous scent. Foot odor can be particularly pungent if you happen to be a person who has a lot of B. subtilis living on your feet; as it turns out, they're naturally a bit stinkier (with a hint of vinegar) than other bacteria living off your leucine [source: Ara]. Different bacteria produce different odors, though, and if your feet tend to have a sulfuric or ammonia-like aroma, you can blame a different microorganism: Meet Brevibacterium linens.
B. linens is the bacteria that gives the notoriously stinky Limburger cheese its odor, and it's also the culprit behind most foot odor.B. linens flourish in environments that are salty, and that are about 70 to 85 degrees Fahrenheit (20 to 30 degrees Celsius) [source: Kenyon College]. Sound a bit, perhaps, like a sweaty foot? It's pretty close, but while B. linens like this climate, they don't really care so much about your sweat. B. linens like to eat the dry, dead skin cells on the soles of your feet -- they don't care if your soles are sweaty or not -- and as they digest your skin they produce rotten-egg odors (sulfur), a byproduct that happens as they convert amino acids in your skin into methanethiol [source: Hilo].
Author's Note: Why do feet stink?
No two feet are alike; they don't look alike, they won't sweat alike, and as it turns out, most people have one foot that's larger than the other. And if you were a butterfly, you'd have sensors in your feet for tasting your food.
More Great Links
- American College of Foot and Ankle Surgeons - Foot Health Facts. "Food Odor." (May 23, 2014) http://www.foothealthfacts.org/what-is/ns_foot-odor.htm
- Ara, K.; Hama, M.; Akiba, S.; Koike, K.; Okisaka, K.; Haqura, T.; Kamiya, T.; and F. Tomita. "Foot odor due to microbial metabolism and its control." Canadian Journal of Microbiology. Vol. 52, no. 4. Pages 357-364. April 2006. (May 23, 2014) http://www.ncbi.nlm.nih.gov/pubmed/16699586
- Brawley, Otis. "Expert Q&A: Why do my boyfriend's feet sweat?" CNN. July 13, 2011. (May 23, 2014) http://www.cnn.com/2011/HEALTH/expert.q.a/07/13/sweaty.feet.brawley/
- Dunn, Rob. "Biologist Spending Way Too Much Time Thinking about Discovery He Made on Jon Stewart's Body." Scientific American. Aug. 15, 2011. (May 23, 2014) http://blogs.scientificamerican.com/guest-blog/2011/08/15/biologist-finds-himself-spending-way-too-much-time-thinking-about-a-discovery-he-might-have-made-on-jon-stewarts-body/
- Gross, Liza. "A Genetic Basis for Hypersensitivity to 'Sweaty' Odors in Humans." PLOS Biology. Vol. 5, no. 11. November 2007. (May 23, 2014) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2043049/
- Hilo, Jessica. "Smelliot." Pacific Standard. Oct. 20, 2010. (May 23, 2014) http://www.psmag.com/navigation/nature-and-technology/smelliot-24427/
- Kenyon College. "Microbe Wiki: Brevibacterium linens." May 18, 2013. (May 23, 2014) http://microbewiki.kenyon.edu/index.php/Brevibacterium_linens
- Mayo Clinic. "Sweating and body odor." Jan. 25, 2014. (May 23, 2014) http://www.mayoclinic.org/diseases-conditions/sweating-and-body-odor/basics/causes/con-20014438
- Rehmus, Wingfield. "Bromhidrosis." Medscape. Jan. 17, 2012. (May 23, 2014) http://emedicine.medscape.com/article/1072342-overview
- Royal Society of Chemistry. "Substance: 3-Methylbutanoic Acid." Aug. 30, 2011. (May 23, 2014) http://www.rsc.org/learn-chemistry/wiki/Substance:3-Methylbutanoic_acid
- Yuhas, Daisy. "Human Sexual Responses Boosted by Bodily Scents." Scientific American. May 1, 2014. (May 23, 2014) http://www.scientificamerican.com/article/human-sexual-responses-boosted-by-bodily-scents/