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.

Compound red re-staining solution is mainly composed of alkaline magenta (i.e., compound red), mostly used in conjunction with crystal violet, iodine solution, etc., to detect Gram-positive and Gram-negative bacteria, and the staining effect is better than that of sand yellow re-staining solution.