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掌握了gre阅读里的长难句，到了实战演习的时候了。gre阅读练习每日一篇帮助gre考生循序渐进地进行练习和总结。希望gre考生在进行gre阅读练习时，也按着考试时候的时间规定自己的练习，这样才能有效果。 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. 17. The primary purpose of the passage is to | 
