Although pathogenic organisms constantly alight on the skin, they find it a
very unfavorable environment and, in the absence of injury, have great
difficulty colonizing it. This “self-sterilizing” capacity of the skin results
from the tendency of all well-developed ecosystems toward homeostasis, or the
maintenance of the status quo.
Species that typically live in soil, water, and elsewhere rarely multiply
on the skin. Undamaged skin is also unfavorable to most human pathogens. The
skin is too acid and too arid for some species. The constant shedding of the
surface skin layers further hinders the establishment of invaders. The most
interesting defense mechanism, however, results from the metabolic activities
of the resident flora. Unsaturated fatty acids, an important component
of the lipids in sebum collected from the skin surface, inhibit the growth of
several bacterial and fungal cutaneous pathogens. These acids are a metabolic
product of certain gram-positive members of the cutaneous community, which
break down the more complex lipids in freshly secreted sebum.
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