How are Gram positive bacteria usually stained after decolorization?

Gram positive bacteria are typically stained with a crystal violet stain after the decolorization step in the Gram staining procedure. This is due to the structure of their cell walls, which have a thick peptidoglycan layer that retains the initial crystal violet dye even after the application of a decolorizing agent, such as alcohol or acetone.

The primary stain, crystal violet, binds to the peptidoglycan, making Gram positive cells appear purple under a microscope. This retention occurs because the decolorizer disrupts the outer membrane of Gram negative bacteria, allowing the crystal violet to wash out and leading to their subsequent staining with a counterstain, like safranin, which turns them pink. Therefore, for Gram positive bacteria, the staining process with crystal violet demonstrates their ability to retain the dye, affirming their classification in the Gram staining differential method.