Microorganisms are the richest in biological diversity on the earth， which is a large biological population， including bacteria， virus， fungi， some small protists and microscopic algaes. They are the earliest residents on the earth. There are soil microorganisms， space microorganisms， air microorganisms， enteric microorganisms and marine microorganisms according to different environments. They are very small but play important roles in maintaining the biosphere and providing people plenty of undeveloped resourses.
　　The modern medicine after the theory of bacteria made microorganisms show their true colours. This oldest life has different kinds of relationships with the biotic environment， such as the alternation relationship between the free living nitrogen fixing bacteria and the cellulose decomposing bacteria， the symbiotic relationship between the fungus and cyanobacteria， which combine to form the lichen， the parasitic relationship between different kinds of phytopathogens and their host plants， the antagonistic relationship between antibiotics bacteria and sensitive microorganisms， etc. As a result of microorganisms’ micro characteristics and the limitation of research means， the kinds of microorganisms people have found till now account for only a small fraction of the nature.
　　In order to survive and breed， people have to adapt to the external macro-ecological environment and the internal micro-ecological environment. In the long-term evolutionary process， the dynamically physiologic combination of normal microorganisms and the human bodies formed the microecological balance of the internal human body environment， they jointly maintain the body’s local stability of internal environment. People’s survival can not be separated from microorganisms even for a moment. Microorganisms have formed some kind of mutualistic symbiosis or alternation relations with human， they help people in digestion and provide people necessary vitamins， they eliminate the waste of human bodies， participate in the evolution of life. Therefore， the microbial genome is known as human’s second genome.
　　In daily life， people have been familiar with the using of some beneficial microorganisms for a long time. People use them to make bread， steamed bun， beer， yoghourt， cheese， fermented bean curd， condiment （monosodium glutamate）， etc. In modern medicine， the English bacteriologist Alexander Fleming founded penicillin as the penicillium stopped the growing of other bacteria. As people’s understanding about microorganisms becomes more profound， they begin to play important roles in bio-pharmaceuticals， energy， environmental protection， food， industry and other fields. Microorganisms are important catabolism class group in the biosphere. In the process of environmental pollution control， microorganisms can be used to degrade organic matters such as plastics and methylbenzene， they can also be used to dispose of the phosphates and sulfur-containing flue gases in the industrial wastewater. In the natural agriculture， the microecosystem with multiple functions consisted of microorganisms could restrain the actions of harmful microbes， especially the pathogenic bacteria and spoilage organisms， therefore promoting the plant growth and soil improvement. The microbial enhanced oil recovery reduced the viscosity of crude oil， increased its mobility and greatly increased the recovery efficiency， reduced the environmental pollution. Today， as ecological balance is always emphasized， as long as we completely understand and know microorganisms， discover and use their characteristics， we human will learn to live together with them in peace， and let them be partners who benefit the human society.