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Gram reaction A reaction obtained when bacteria are subjected, in the laboratory, to a certain staining procedure called the Gram stain or Gram's stain (after the Danish scientist Christian Gram (1853–1938) who first devised the technique in 1884). The bacteria are killed and stained, for example with crystal violet; the stained cells are then treated with an organic solvent such as acetone or ethanol. Bacteria fall into two categories: those that are readily decolorized under these conditions, and those that retain the stain. Cells of the former type are said to be Gram-negative; those of the latter type are said to be Gram-positive. The difference in reaction reflects a fundamental difference in the structure of the cell wall in the two types of bacteria.