Bacteria really go wild for foot sweat. Foot odor is often pungent, and shoes and socks provide a delectable feeding ground for bacteria that chow down on the sweat and waste matter emitted by the sole of the foot.
Scientists who study foot odor (and there are many) have found high levels of fatty acids in those with very smelly feet. The main culprit seems to be isovaleric acid, a short-chain fatty acid usually found in cheese and beer. In general, sweat also contains propionic acid, another fatty acid that smells strongly of vinegar [source: Yamazaki et al.].