Flora Normal

Normal Microbiota of the Skin

Because of its constant exposure to and contact with the environment, the skin is particularly apt to contain transient microorganisms. Nevertheless, there is aconstant and well-defined resident flora, modified in different anatomic areas by secretions, habitual wearing of clothing, or proximity to mucous membranes (mouth, nose, and perineal areas).

Thepredominant resident microorganisms of the skin are aerobic and anaerobic diphtheroid bacilli (eg, Corynebacterium, Propionibacterium); nonhemolytic aerobic and anaerobic staphylococci (S epidermidis andother coagulase-negative staphylococci, occasionally S aureus, and Peptostreptococcus species); gram-positive, aerobic, spore-forming bacilli that are ubiquitous in air, water, and soil; -hemolyticstreptococci (viridans streptococci) and enterococci (Enterococcus species); and gram-negative coliform bacilli and Acinetobacter. Fungi and yeasts are often present in skin folds; acid-fast,nonpathogenic mycobacteria occur in areas rich in sebaceous secretions (genitalia, external ear).

Among the factors that may be important in eliminating nonresident microorganisms from the skin are the low pH,the fatty acids in sebaceous secretions, and the presence of lysozyme. Neither profuse sweating nor washing and bathing can eliminate or significantly modify the normal resident flora. The number ofsuperficial microorganisms may be diminished by vigorous daily scrubbing with soap containing hexachlorophene or other disinfectants, but the flora is rapidly replenished from sebaceous and sweatglands even when contact with other skin areas or with the environment is completely excluded. Placement of an occlusive dressing on skin tends to result in a large increase in the total microbialpopulation and may also produce qualitative alterations in the flora.

Anaerobes and aerobic bacteria often join to form synergistic infections (gangrene, necrotizing fasciitis, cellulitis) of skin and...