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Gram's Method. A method for the differential staining of bacteria (q.v.), developed by the Danish physician Hans C.J. Gram. Smears of bacteria are stained with gentian violet, a plant dye,then treated with Gram's solution that consists of iodine, potassium iodide and water, decolorized with alcohol (q.v.) and washed with water. Gram-positive bacteria retain the stain, whereas Gram-negative bacteria are decolorized.