试根据微生物学的特点,谈谈为什么说微生物既是人类的敌人又是人类的朋友?
推荐回答（2个）
微生物与人类的关系是十分密切而不可分割的。微生物（microorganism）是一类肉眼不能直接看见，必须借助光学显微镜或电子显微镜放大几百倍或几万倍后才能观察到的微小生物。其特点有八：（1）体积小；（2）结构简单；（3）种类繁多；（4）繁殖速度快；（5）数量庞大；（6）代谢旺盛 ；（7）营养源广泛；（8）易发生变异。绝大多数微生物对人类和动植物的生存是有益的且是必需的。自然界中N、C、S等多种元素循环靠微生物的代谢活动来进行。例如，空气中的大量氮气只有依靠微生物的转氮作用才能被植物吸收，土壤中的微生物能将动植物蛋白质转化为无机含氮化合物，以供植物生长的需要，而植物又为人类和动物所利用。因此，没有微生物，植物就不能进行新陈代谢，而人类和动物也将无法生存。许多寄生在人类和动物腔道中的微生物，在正常情况下对宿主是无害的，而且有的还具有阻止外来细菌的侵袭和定居，以及提供机体必须的营养物质（如多种维生素和氨基酸等）的作用。而有一部分微生物能引起人类或动植物的病害，或者在特定的条件下引起疾病。
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