XX staining method was invented by Danish doctor Christain Gram in 1884, it is a widely used identification staining method in bacteriology, and is also a kind of double staining method. Unstained bacteria, due to the refractive index difference with the surrounding environment is very small, so it is extremely difficult to observe under the microscope, after staining the bacteria and the environment to form a sharp contrast, you can clearly observe the bacteria's morphology, arrangement and some structural features, used to classify and identify. Through this method of staining bacteria can be identified as gram-positive bacteria (G+) and gram-negative bacteria (G-) two categories, the different color reaction of bacteria is due to the difference in the permeability of the cell wall to ethanol and the ability to resist decolorization, mainly peptidoglycan layer thickness and structure of the decision, the crystal violet-stained cells treated with iodine solution to form an insoluble complex, ethanol can make it decolorized.

XX staining solution adopts non-classical Gram staining formula, using red and green dyes, making the color more rich, clean background, strong contrast of cytoplasm of cytosol and nucleus, with typical characteristics of bacterial staining, which is mostly used for staining of tissue sections.