Gram stain was devised empirically by Christian Gram, a Danish physician, in 1884. When bacteria are stained with certain basic dyes (such as the crystal violet in Rapid Gram Stain), those cells with high peptidoglycan contents (Gram-positive) in their cell wall will resist decolorization with organic solvents such as ethanol or acetone, while the Gram-negative species (with a thin layer of peptidoglycan in the cell wall) can be easily decolorized.