IntroductionMicrobes are a diverse group of organisms that can be divided into the viruses, unicellular groups (Archaea, Eubacteria, protista, some fungi and some chlorophyta) and a small number of organisms with a simple multicellular structure (the large fungi and chlorophyta).Microbiology: the science (logos) of small (micro) life (bios). The study of living things so small that they cannot be seen with the naked eye.The three-domain system is a biological classification introduced by Carl Woese that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria (now Bacteria) and Archaebacteria (now Archaea).Microorganisms were first visualized by Antoni van Leeuwenhoek (1632–1723), a Dutch cloth merchant and an expert lens grinder.The first proof of the involvement of bacteria in disease and the definitive proof of the germ theory of disease came from the German Robert Koch.Koch’s po stulates:1, The microorganism must be present in every instance of the disease and absent from healthy individuals.2, The microorganism must be capable of being isolated and grown in pure culture.3 ,When the microorganism is inoculated into a healthy host, the same disease condition must result.4, The same microorganism must be re-isolated from the experimentally infected host. Chapter oneFungi include yeasts, molds, and mushrooms.Viruses are noncellular organisms; they are intra-cellular parasites of animals, plants, or bacteria.1, The Prokaryotes can be divided into two kingdoms, Bacteria and Archaea. The classification was first proposed by Woese based on the differences in 16S rRNA sequence.2, Prokaryotic microorganism（原核微生物）: It is an single-c elled organism which doesn’t have a nuclear membrane outside the cell nucleus and only has the bare DNA called as the nuclear area. 3, Many bacteria are shaped like long rods twisted into spirals or helices; they are called spirilla （螺旋菌）if rigid and spirochetes (螺旋体) when flexible.4, Capsules (荚膜) and slime layers (粘液层) usually are composed of polysaccharides (多糖), but they may be constructed of other materials.5, Gram staining procedure(革兰氏染色的步骤)In the first step of the Gram-staining procedure, the smear is stained with the basic dye crystal violet, the primary stain. It is followed by treatment with an iodine solution functioning as a mordant. That is, the iodine increases the interaction between the cell and the dye so that the cell is stained more strongly. The smear is next decolorized by washing with ethanol or acetone. This step generates the differential aspect of the Gram stain; gram-positive bacteria retain the crystal violet, whereas gram-negative bacteria lose their crystal violet and become colorless. Finally, the smear is counterstained with a simple, basic dye different in color from crystal violet. Safranin, the most common counterstain, colors gram-negative bacteria pink to red and leaves gram-positive bacteria dark purple.6, The Mechanism of Gram Staining （革兰氏染色的机制）Although several explanations have been given for the Gramstain reaction results, it seems likely that the difference between gram-positive and gram-negative bacteria is due to the physical nature of their cell walls. If the cell wall is removed from grampositive bacteria, they become gram negative. The peptidoglycan itself is not stained; instead it seems to act as a permeability barrier preventing loss of crystal violet. During the procedure the bacteria are first stained with crystal violet and next treated with iodine to promote dye retention. When gram-positive bacteria then are decolorized with ethanol, the alcohol is thought to shrink the pores of the thick peptidoglycan. Thus the dye-iodine complex is retained during the short decolorization step and the bacteria remain purple. In contrast, gram-negative peptidoglycan is very thin, not as highly cross-linked, and has larger pores. Alcohol treatment also may extract enough lipid from the gramnegative wall to increase its porosity further. For these reasons, alcohol more readily removes the purple crystal violet-iodine complex from gram-negative bacteria.简而言之，不同细菌的染色差异（G+或G-）是由于细胞壁化学成分的差异而引起了物理特性（脱色能力）的不同。
