<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> <P> The method is named after its inventor, the Danish scientist Hans Christian Gram (1853--1938), who developed the technique while working with Carl Friedländer in the morgue of the city hospital in Berlin in 1884 . Gram devised his technique not for the purpose of distinguishing one type of bacterium from another but to make bacteria more visible in stained sections of lung tissue . He published his method in 1884, and included in his short report the observation that the typhus bacillus did not retain the stain . </P> <P> Gram staining is a bacteriological laboratory technique used to differentiate bacterial species into two large groups (gram - positive and gram - negative) based on the physical properties of their cell walls . Gram staining is not used to classify archaea, formerly archaeabacteria, since these microorganisms yield widely varying responses that do not follow their phylogenetic groups . </P>

What is the purpose of crystal violet used in the gram stain procedure