The bacteria are stained with gram stain (weakly alkaline solution of crystal violet or gentian violet). The stained (blue or purple) bacteria are then treated with 0.5% iodine (k1) solution. They are then washed with water and finally with alcohol or acetone. The bacteria which retain the stain are termed as Gram positive bacteria wherea, the bacteria which lose the stain are termed as Gram negative bacteria. Variation is due to difference in the structure of the cell wall. The thick cell wall of Gram psitive bacteria does not let the alcohol or acetone solvents to penetrate.
