Gram stain was invented by Danish doctor Christain Gram in 1884, it is a widely used identification staining method in bacteriology, and also a double staining method. Unstained bacteria, due to its 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.

Sand yellow re-staining solution is mainly composed of sand yellow, mostly used in conjunction with crystal violet and iodine solution to detect Gram-positive bacteria and Gram-negative bacteria.