Microbiology. Named after Hans Gram, 1853-1938. A staining procedure used to differentiate bacteria into gram-positive and gram-negative bacteria. A heat fixed bacterial smear is stained with crystal violet, treated with iodine solution, by washing with alcohol the blue colour is washed out of the gram-negative bacteria, they are stained red after counterstaining with carbol-fuchsin. Those microorganisms that retain the crystal violet stain are said to be gram-positive. See also gram-positive and gram-negative.