The skin is the primary external coating of the human body. In adults, skin occupies approximately 2.4 square yards (approximately two square meters). Because it is exposed to the environment, the skin is inhabited by a number of bacteria. Over much of the body there are hundreds of bacteria per square inch of skin. In more moisture-laden regions, such as the armpit, groin, and in between the toes, bacteria can number upwards of one hundred thousand per square inch.

The majority of the skin microbes are found in the first few layers of the epidermis (the outermost layer of skin) and in the upper regions of the hair follicles. The bacteria found here are mostly Staphylococcus epidermidis and species of Corynebacteria, Micrococcus, Mycobacterium, and Pityrosporum. These species are described as being commensal; that is, the association is beneficial for one organism...