Gram's stain

a technique described by H.C.J. Gram, the Danish bacteriologist, in 1884, which involves using a stain to differentiate between certain BACTERIA . Bacteria on a microscope slide are first stained with a violet dye and iodine, then rinsed in ethanol to decolorize and a second red stain added. Gram-positive bacteria keep the first stain and appear violet when examined under the microscope, while Gram-negative forms lose the first but take up the second stain, thus appearing red. The difference in staining is because of the structure of the bacteria cell walls.