a method of staining bacteria for the purpose of classification, involving treatment with crystal violet, an iodine solution, and alcohol: the violet stain is retained by Gram-positive bacteria or lost by Gram-negative bacteria when the alcohol is applied
after Hans C. J. Gram (1853-1938), Dan physician
See Gram's method in American Heritage Dictionary 4
A staining technique used to classify bacteria in which a bacterial specimen is first stained with crystal violet, then treated with an iodine solution, decolorized with alcohol, and counterstained with safranine. Gram-positive bacteria retain the violet stain; gram-negative bacteria do not.
Origin: After Hans Christian Joachim Gram (1853-1938), Danish physician