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a method for the differential staining of bacteria, proposed in 1884 by the Danish physician H. C. Gram.
In Gram’s method the bacteria are stained with basic dyes, including crystal or methyl violet, which are then fixed in an iodine solution. After rinsing the dyed preparation in alcohol some types of bacteria remain thoroughly stained (the so-called gram-positive bacteria), and others (gram-negative) are decolorized. Such a division is of great importance in the classification of bacteria and in the microbiological diagnosis of infectious diseases.