Gram Negative bacteria will go pink after this dyeing, whereas Gram Positive bacteria will remain purple, from the original purple dye. This technique is also referred to as acid-fast staining . This technique is necessary because some bacteria, notably all Mycobacteria , have waxy coats on their cell walls that prevent them taking in the dye from the Gram Staining procedure. As a part of the acid-fast staining process, detergents are applied which remove this waxy coat. The bacteria are stained with hot Carbol-Fuchsin, a red dye which contains detergents.
All bacteria are then stained red.
The bacteria are washed with acid alcohol. Those bacteria that retain the red dye are known as acid-fast bacteria. The bacteria are then stained with Methylene Blue, a blue dye. Those bacteria that retain the red dye from the original stain are known as acid-fast bacteria, all others go blue. Thus, this procedure cannot be used to differentiate between species of Mycobacteria.