Sweat can be the cause of smelly feet, but sweat alone is not usually the cause of odor. The interactions of the bacteria Staphylococcus epidermidis, Bacillus subtilis and Brevibacterium linens on the foot are the main reasons behind bad-smelling feet. The foot can smell from the interactions of one of the bacteria or all three.
Odor is the result of the natural bacteria present on the foot internalizing the sweat that comes from one of the sweatiest parts of the body. The Staphylococcus epidermidis and Bacillus subtilis reside on feet to eat leucine, which is an amino acid found in the sweat of feet. As the bacteria digests the amino acid, isovaleric acid results from the interaction, which secretes a gassy odor. Odor from the Staphylococcus epidermidis and Bacillus subtilis is described as vinegary.
As opposed to the appetites of Staphylococcus epidermidis and Bacillus subtilis to sweat, the Brevibacterium linens eat the dry and flaky dead skin on the foot. This bacteria lives in hot and humid environments, but eats the dead skin on the foot no matter if the foot is sweaty or not. A malodorous gassy sulfur results from the bacteria turning dead skin cells into methanethiol, which has a rotten egg or cabbage smell.