VII Fungal, Bacterial, and Viral Infections of the Skin
Jan V. Hirschmann, MD
Despite its large surface area and constant exposure to the environment, the skin resists infection well. The most important protective factor is an intact stratum corneum, the tough barrier of protein and lipid formed on the cutaneous surface by the underlying epidermis. 1 This barricade impedes invasion by environmental pathogens, and its dryness discourages colonization and growth of the many organisms that require moisture to survive, such as gramnegative bacilli. Furthermore, the constant shedding of epidermal cells impedes most microbes from establishing permanent residence.
Some organisms, however, can attach to skin cells and reproduce there; the normal cutaneous flora comprises primarily aerobic, gram-positive cocci and bacilli in densities ranging from about 1022 on dry skin to 107 organisms/cm2 in moist areas, such as the axilla. 2 This resident population inhibits harmful organisms from colonizing cutaneous surfaces by occupying binding sites on the epidermal cells, competing for nutrients, producing antimicrobial substances, and maintaining a low skin pH (about 5.5). Anaerobes are sparse except in areas with abundant sebaceous glands, such as the face and chest; in the deeper portions of these sites, as well as in hair follicles, anaerobes reach concentrations of 104 to 106 organisms/cm2 . organisms/cm
Cutaneous infections occur when the skin’s protective mechanisms fail, especially when trauma, inflammation, maceration from excessive moisture, or other factors disrupt the stratum corneum. The organisms causing infection may originate from the victim’s own resident flora, either on the skin or on adjacent mucous membranes, but many come from other people, animals, or the environment.
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Jan V. Hirschmann, MD
Despite its large surface area and constant exposure to the environment, the skin resists infection well. The most important protective factor is an intact stratum corneum, the tough barrier of protein and lipid formed on the cutaneous surface by the underlying epidermis. 1 This barricade impedes invasion by environmental pathogens, and its dryness discourages colonization and growth of the many organisms that require moisture to survive, such as gramnegative bacilli. Furthermore, the constant shedding of epidermal cells impedes most microbes from establishing permanent residence.
Some organisms, however, can attach to skin cells and reproduce there; the normal cutaneous flora comprises primarily aerobic, gram-positive cocci and bacilli in densities ranging from about 1022 on dry skin to 107 organisms/cm2 in moist areas, such as the axilla. 2 This resident population inhibits harmful organisms from colonizing cutaneous surfaces by occupying binding sites on the epidermal cells, competing for nutrients, producing antimicrobial substances, and maintaining a low skin pH (about 5.5). Anaerobes are sparse except in areas with abundant sebaceous glands, such as the face and chest; in the deeper portions of these sites, as well as in hair follicles, anaerobes reach concentrations of 104 to 106 organisms/cm2 . organisms/cm
Cutaneous infections occur when the skin’s protective mechanisms fail, especially when trauma, inflammation, maceration from excessive moisture, or other factors disrupt the stratum corneum. The organisms causing infection may originate from the victim’s own resident flora, either on the skin or on adjacent mucous membranes, but many come from other people, animals, or the environment.
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