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The microbes that live on or in the human body are collectively referred to as the human microbiome. Some microbes of the human microbiome cause disease and others do not (commensals). The skin microbiome is a complex and diverse population of organisms that includes many bacteria, both commensals and pathogenic. The Human Microbiome Project includes recent work using metagenomic sequencing that describes the skin microbiome in previously inconceivable detail. The effects of multiple factors, including sebum secretion, body location, lipid content, pH, and sweat production significantly influence the growth of bacteria on the skin.1 The effects of skin diseases such as psoriasis also dictate the composition of the skin microbiome.2 It is clear there are many more bacteria normally living on our skin than previously imagined. Understanding the factors that contribute to healthy skin and skin disease will lead to improved treatment and prevention of skin infections and perhaps many noninfectious skin diseases.
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The relationship of bacteria and the skin may be considered in four major categories1: (1) primary skin infections,2 (2) secondary infection of a primary skin disease (e.g., infected atopic dermatitis),3 (3) the skin lesions as manifestations of primary infection in some other organ system, usually the blood, and4 (4) reactive skin conditions resulting from bacterial infection elsewhere (e.g., erythema nodosum due to streptococcal pharyngitis). Thus, the balance of host immunity and the growth of the skin bacteria determine the disease state of the skin. Controlling a disease state, like atopic dermatitis, clearly reduces the number of skin infections that arise in the broken skin barrier. Conversely, controlling skin infections with dilute bleach baths and mupirocin ointment can lead to decreased atopic dermatitis flare as well.3
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When considering the patient, it is important to remember that not all skin infections are suppurative but may present as reactive responses (e.g., erythema nodosum). Equally important, not all suppurative skin problems are primary skin infections (e.g., hidradenitis suppurativa). Notably, many erythematous skin lesions are not infectious at all (e.g., ...