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.
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