Gram staining is the most widely used staining method in medical bacteriology. It is used to stain bacteria and distinguish them on direct examination by their ability to take up gentian violet (Gram +) or fuschin (Gram -). This stain is used to provide important medical information quickly.
Gram staining is based on the successive action of an aniline dye, crystal violet, iodine, and then a mixture of alcohol and acetone. The stain initially enters the wall and cytoplasm. Iodine reacts with the stain in the second stage and makes it insoluble. Gram negative bacteria are de-stained because of their greater permeability to alcohol. Gram positive bacteria remain stained violet or mauve coloured. A counter-stain (for example, pink) can be used to revisualise the cell bodies of Gram negative bacteria.