Definition of Gram stain
- a staining technique for the preliminary identification of bacteria, in which a violet dye is applied, followed by a decolorizing agent and then a red dye. The cell walls of certain bacteria (denoted Gram-positive) retain the first dye and appear violet, while those that lose it (denoted Gram-negative) appear red. Also called Gram’s method.
late 19th century: named after Hans C. J. Gram (1853–1938), the Danish physician who devised the method