<P> Gram stain or Gram staining, also called Gram's method, is a method of staining used to distinguish and classify bacterial species into two large groups (gram - positive and gram - negative). The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique . </P> <P> Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, which is present in the cell wall of Gram - positive bacteria . Gram - negative cells also contain peptidoglycan, but a very small layer of it that is dissolved when the alcohol is added . This is why the cell loses its initial colour from the primary stain . Gram - positive bacteria retain the crystal violet dye, and thus are stained violet, while the Gram - negative bacteria do not; after washing, a counterstain is added (commonly safranin or fuchsine) that will stain these Gram - negative bacteria a pink color . Both Gram - positive bacteria and Gram - negative bacteria pick up the counterstain . The counterstain, however, is unseen on Gram - positive bacteria because of the darker crystal violet stain . </P> <P> The Gram stain is almost always the first step in the preliminary identification of a bacterial organism . While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique . This gives rise to gram - variable and gram - indeterminate groups . </P>

What is the colour of gram positive bacteria