Gram Stain
Purpose: The Gram stain is a differential stain which distinguishes Gram-positive from Gram-negative bacteria.
How it works: Gram+ bacteria have a thick layer of peptidoglycan in their cell wall. Gram- bacteria have a thin layer of peptidoglycan in their cell wall, plus an outer membrane. Both types of cells take up the primary stain, crystal violet. Iodine acts as a mordant to enhance the uptake of the stain. When treated with ethanol, Gram+ cells are able to retain the crystal violet, while Gram- cells are decolorized. Applying the counterstain safranin stains the decolorized Gram- cells. At the end of the staining process, Gram+ cells are purple and Gram- cells are pink. (Note: Gram stains are performed on smears that have been heat-fixed.)
Overview of Gram-staining process:
Results:
