While most bacteria can be differentially stained by the Grams method, some bacteria are resistant to the Grams reagents and an acid-fast stain must be used to visualize them. These bacteria possess a thick waxy lipoidal cell wall that can't be removed by the acid alcohol in this procedure and thus they are called acid-fast organisms. Three different reagents are used in an acid-fast stain:

i. Carbol fuchsin - a red stain, unlike crystal violet, that is
soluble in the lipoidal material that is the major
portion of the myocbacterial cell wall. Penetration of
the stain is usually aided by the application of heat.

ii. Acid Alchol - this serves as a decolorizing agent which re-
moves the red stain from the non-acid-fast bacteria.

iii. Methylene blue - this serves as a counterstain which stains
all non-acid-fast cells blue.

INTERPRETING TEST RESULTS:
ACID- FAST = red stained bacteria
NON-ACID-FAST = blue stained bacteria