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Any colored, organic compound, usually called dye, used to stain tissues, cells, cell components, or cell contents. The dye may be natural or synthetic. The object stained is called the substrate. The small size and transparency of microorganisms make them difficult to see even with the aid of a high-power microscope. Staining facilitates the observation of a substrate by introducing differences in optical density or in light absorption between the substrate and its surround or between different parts of the same substrate. In electron microscopy, and sometimes in light microscopy (as in the silver impregnation technique of staining flagella or capsules), staining is accomplished by depositing on the substrate ultraphotoscopic particles of a metal such as chromium or gold (the so-called shadowing process); or staining is done by treating the substrate with solutions of metallic compounds such as uranyl acetate or phosphotungstic acid. Stains may be classified according to their molecular structure. They may also be classified according to their chemical behavior into acid, basic, neutral, and indifferent. This classification is of more practical value to the biologist. See Medical bacteriology