Gram staining is a differential staining procedure in which bacteria are classified as Gram-negative or Gram-positive, depending on whether they retain or lose the primary stain when subject to treatment with a decolorizing agent. The procedure is named after a Danish physician, Hans Christian Gram. The staining procedure reflects the underlying structural differences in the bacterial cell of Gram-negative and Gram-positive bacteria. Staining has considerable medical value because the cell walls not only determine the staining difference, but also the behavior and antibiotic sensitivity of the bacteria. In Gram staining, heat fixed bacteria are first stained with crystal violet and then with iodine solution, which is followed by an alcohol or acetone rinse. The Gram- positive bacteria become bright purple, and the Gram- negative bacteria become decolorized on staining.
