Bacteria that retain the primary crystal violet stain first applied as part of the staining process are classified as gram-positive bacteria. The stain is retained because the cell wall of a gram-positive bacteria is not as porous as the cell wall of a gram-negative bacteria. Therefore, when the decolorizer is applied to the smear, the crystal violet will be retained if the bacteria is gram-positive and the bacteria will appear blue or purple. Before adding the decolorizer, iodine is applied to the smear. The iodine fixes the crystal violet stain to cellular components of gram-positive bacteria, further preventing the decolorization of the gram-positive organisms. The bacteria indicated by the arrows in Image 1 are gram-positive bacteria.
Image 1
For gram-negative bacteria: The decolorizer will pull the crystal violet stain out of the cell walls, allowing the bacteria to be stained with the counterstain so gram-negative bacteria appear red or pink. The arrows in Image 2 point to gram-negative bacteria.
Image 2
