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Gram stain or Gram staining, also called Gram's method, is a method of staining used to distinguish and classify bacterial species (Bacterium) into two large groups: GramPositive bacteria and GramNegative bacteria. Gram staining differentiates bacteria by the chemical and physical properties of their cell walls. Gram-positive cells have a thick layer of peptidoglycan in the cell wall that retains the primary stain, crystal violet. Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of Ethanol. They are stained pink or red by the counterstain commonly safranin or fuchsine. Lugol's iodine solution is always added after addition of crystal violet to strengthen the bonds of the stain with the cell membrane. [from Wikipedia]
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