Introduction

The skin is a physical and chemical barrier against microorganisms.

Moist areas of the skin (such as the armpit) support larger populations of bacteria than dry areas (such as the scalp).

Structure and Function of the Skin

The outer portion of the skin, called the epidermis, contains keratin-filled keratinocytes with a glycolipid intercellular "sealant", which provides a waterproof coating.

The inner portion of the skin, the dermis, contains hair follicles, sweat ducts, and oil glands that provide passageways for microorganisms.

Sebum and perspiration are secretions of the skin that can inhibit the growth of microorganisms.

Sebum and perspiration provide nutrients for some microorganisms.

Body cavities are lined with epithelial cells. When these cells secrete mucus, they constitute the mucous membrane.

Normal Microbiota of the Skin

Microorganisms that live on skin are resistant to desiccation and high concentrations of salt.

Gram-positive cocci such as staphylococci (S. epidermidis, S. aureus) and micrococci predominate on the skin. Diphtheroids (gram-positive pleomorphic rods), such as Propionibacterium acnes and Corynebacterium xerosis, are also present, as well as Pseudomonas aeruginosa, Peptococcus, and yeasts, such as Candida albicans and Pityrosporum ovale.

The normal skin microbiota are not completely removed by washing.

Members of the genus Propionibacterium metabolize oil from the oil glands and colonize hair follicles.

Pityrosporum ovale yeast grows on oily secretions and may be the cause of dandruff.