The size, shape and arrangement of the organisms can also be determined from a stained specimen. Primary stain - cover the smear with crystal violet for one minute . All bacteria will take up this dye and appear purple. Rinse off the excess dye with water. Mordant - Gram's iodine is added to interact for one minute with the crystal violet.
Excess iodine is rinsed off with water.
Counterstain - Safranin is added for 20 seconds to dye any decolorized cells. It will not change the color of the cells that retain the crystal violet. It is believed that the difference that distinguishes Gram positive and gram negative cells is related to the nature of the cell wall. The value of knowing the Gram reaction therefore tells something about the chemistry of the cell. In clinical laboratories this information is available very quickly and can help in selecting a drug to fight an infecting organism.