GRE Reading Comprehension
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Source: NO3
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.
The "resident flora" mentioned in lines 21-22 refer to
- A Unsaturated fatty acids (line 22)
- B sebum collected from the skin surface (lines 24-25)
- C bacterial and fungal cutaneous pathogens ( lines 26-27)
- D certain gram-positive members of the cutaneous community (lines 28-29)
- E more complex lipids (lines 30-31)