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].