<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 color 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>

What is the principle of the gram stain
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