Gram's stain

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Gram's stain (gramz) n. a method of staining bacterial cells, used as a primary means of identification. The bacterial cells are stained with a violet dye, treated with decolorizer (e.g. alcohol), and then counterstained with red dye. Gram-negative bacteria lose the initial stain but take up the counterstain, so that they appear red microscopically. Gram-positive bacteria retain the initial stain, appearing violet microscopically. [ H. C. J. Gram (1853–1938), Danish physician]

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