It has long been known that the availability of water is the primary rate-limiting factor for growth of bacteria on skin. The largest populations of microorganisms are found in those regions where high humidity results in high skin hydration, for example, perineum, axillae, and between the toes. The primary source of water on the skin is eccrine sweat. Transepidermal water loss (TEWL) also can contribute to skin hydration, particularly if the skin is occluded to limit evaporation. Studies have shown that TEWL is higher on labia majora skin than on forearm skin (1) or inner thigh skin adjacent to the vulva (2). Other sources of moisture unique to the vulvar area include vaginal secretions and urine. Increased skin hydration has been shown to result in both increases in microbial density and changes in the relative ratios of microorganisms (3,4). Adult forearms occluded tightly with plastic film showed increases in microbial populations from a baseline of approximately 102/cm2 to almost 108/cm2 over the course of several days. The relative population of micrococci decreased, Gram-negative rods emerged, and lipophilic diphtheroids became the dominant microflora. Although occlusion of the vulvar area resulting from tight-fitting clothing or nonbreathable fabrics is unlikely to approach the level provided by plastic film, it is readily apparent that increased moisture availability can have a dramatic effect on the quantitative and qualitative nature of microbial populations on skin.
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