Staining technique for the initial identification of bacteria , devised in 1884 by the Danish physician Hans Christian Gram (1853–1938).
The stain reveals basic differences in the biochemical and structural properties of a living cell. A slide containing a smear of bacteria is treated with a purple dye; the slide is then dipped in an iodine solution, followed by an organic solvent (such as alcohol) that can dissolve the dye. Gram-positive bacteria remain purple because they have a thick cell wall that the solvent cannot easily penetrate; gram-negative bacteria lose their colour because they have thin cell walls that allow the solvent to penetrate and remove the dye.