a widely used microbiological staining technique that greatly aids in the identification and characterization of bacteria. It was devised by a Danish physician, Hans Christian Gram, in 1884. The Gram reaction reflects fundamental differences in the biochemical and structural properties of bacteria. A slide containing a heat-fixed smear of bacterial cells is treated with crystal-violet stain (a basic dye), during which the cells turn purple. The slide is then flushed with an iodine solution, followed by an organic solvent (such as alcohol or acetone). Gram-positive bacteria remain purple because they have a single thick cell wall that is not easily penetrated by the solvent; gram-negative bacteria, however, are decolorized because they have cell walls with much thinner layers that allow removal of the dye by the solvent. In a final step, a counterstain, such as safranin, is added and stains the gram-negative cells red.