The skin is the largest and most accessible of all organs. Its major layers are the epidermis, an outer layer of dead cells continually being sloughed off and replaced, and the dermis, which lies atop the subcutaneous layer of tissue (figure 1a). Depending on its location, skin also contains hair follicles and several types of glands; and the outermost surface is covered with a protective, waxy cuticle that can help microbes adhere. The normal microbiota reside only in or on the dead cell layers, and except for areas around follicles and glands, it does not extend into the dermis or subcutaneous levels. The nature of the population varies according to site. Oily, moist skin supports a more prolific microbiota than dry skin. Humidity, occupational exposure, and clothing also influence its character. Transition zones where the skin joins with the mucous membranes of the nose, mouth, and external genitalia harbor a particularly rich set of microbial colonists.
Fig1. The landscape of the skin. (a) The epidermis, along with associated glands and follicles, provides rich and diverse habitats. The dermis and fatty tissue are free of microbes. (b) A highly magnified view (7,500×) of skin layers reveals rod- and coccus-shaped bacteria adhering to skin cells. (a): Don Bliss/National Cancer Institute (NCI); (b): Janice Haney Carr/CDC
Ordinarily there are two cutaneous populations. The transients cling to the skin surface but do not ordinarily grow there. They are acquired by routine contact and vary markedly from per son to person and over time. The transients include any microbe a person has picked up, often species that do not ordinarily live on the body, and are greatly influenced by the hygiene of the individual.
The resident populations live and multiply in deeper layers of the epidermis and in glands and follicles (figure 1b). The composition of the resident microbiota is more stable, predictable, and less influenced by hygiene than are the transients. The normal skin residents consist primarily of bacteria (notably Staphylococcus, Corynebacterium, and Propionibacterium) and yeasts. Moist skin folds, especially between the toes, tend to harbor fungi, whereas lipophilic mycobacteria and staphylococci are prominent in sebaceous1 secretions of the axilla, external genitalia, and external ear canal. One species, Mycobacterium smegmatis, lives in the waxy secretion, or smegma, on the external genitalia.
