Microbial Barrier

Normal stratum corneum, taken in its entirety, is a dense molecular continuum penetrable only by molecular diffusion. It is virtually an absolute barrier to microbes, preventing them from reaching the viable tissues and an environment suitable for their growth. The outermost stratum corneum is continuously being shed in the form of microscopic scales (natural desquamation) and, to a limited depth, is laced with tiny crevices. Many microorganisms, pathogens and harmless forms alike, are found in these rifts. The microorganisms residing on and in the skin can and do initiate infections if seeded into living tissues as a result of abrasive or disease-induced stratum corneum damage. Consequently, antiseptics and antibiotics are widely used to chemically sanitize wounds.

Beyond physical barrier protection, several natural processes lead to skin surface conditions unfavorable to microbial growth. Both sebaceous and eccrine secretions are acidic, lowering the surface pH of the skin below that welcomed by most pathogens. This acid mantle (pH « 5) (15) is moderately bacteriostatic. Sebum also contains a number of short-chain fungistatic and bacteriostatic fatty acids, including propanoic, butanoic, hexanoic, and heptanoic acids (16). That the skin's surface is dry also offers a level of protection. Glandular orifices provide possible entry points for microbes. The duct of the eccrine sweat gland is tiny and generally evacuated. Experience tells us that this is not an easy portal of entry, although localized infection is seen occasionally in infants suffering prickly heat. Pilosebaceous glands seem more susceptible to infection, particularly those on the forehead, face, and upper back referred to as sebaceous follicles. Sebaceous gland infections are usually localized. However, if the infected gland ruptures and spews it contents internally, deep infection is possible. For example, the destruction of cystic acne is deep, so much so that facial scarring is associated with it.

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