Gram stain is widely used in medical bacteriology, it can stain bacteria and distinguish them by their ability to fix gentian violet (Gram +) or fuchsin (Gram -). The bacteria can be grouped into 2 categories according to the Gram staining method.

The Gram stain is based on the successive action of an aniline dye, crystal violet, iodine and then a mixture of alcohol and acetone. At first, the dye enters the wall and the cytoplasm. In a second time, the iodine reacts with the dye and makes it insoluble.

The next step involves an alcohol solution. Because of their thicker wall structure and particular chemical composition, Gram + bacteria keeps the color violet. Gram- bacteria, with a thinner wall and more permeable to fading, lose the purple color.

The greater permeability of Gram-negative bacteria to alcohol allows discoloration. Gram-positive bacteria remain colored purple or purple. A counterstain (for example in pink) makes it possible to visualize again the cell bodies of Gram-negative bacteria.