Gram staining

Gram staining, also called Gram's method, is a method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative). The name comes from its inventor, Hans Christian Gram.

In a Gram stain test, Gram-positive bacteria retain the crystal violet dye, while a counterstain (commonly safranin or fuchsine) added after the crystal violet gives all Gram-negative bacteria a red or pink coloring.

The permeability hypothesis states that the Gram-positive cells stain so because of the thickness of their cell wall, and not because of he murein in the cell. This is supported by the experiments that stained yeasts and some archaea Gram-positive.