Gram's stain A staining method used to differentiate bacteria. The bacterial sample is smeared on a microscope slide, stained with a violet dye, treated with acetone-alcohol (a decolourizer), and finally counterstained with a red dye. Gram-positive bacteria retain the first dye, appearing blue-black under the microscope; such bacteria have a thick layer of peptidoglycan in their cell walls. In Gram-negative bacteria, the acetone-alcohol washes out the violet dye and the counterstain is taken up, the cells appearing red. The cell walls of these bacteria have an outer layer of lipoprotein overlying a thin layer of peptidoglycan. The stain is named after the Danish bacteriologist H. C. J. Gram (1853–1938), who first described the technique (since modified) in 1884.

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]

Gram's stain The staining procedure used to determine the Gram reaction of bacteria.