| Noun | 1. | Gram stain - a staining technique used to classify bacteria; bacteria are stained with gentian violet and then treated with Gram's solution; after being decolorized with alcohol and treated with safranine and washed in water, those that retain the gentian violet are gram-positive and those that do not retain it are gram-negativestaining - (histology) the use of a dye to color specimens for microscopic study | |