In: Biology
If the cells from the mixed smear were observed after each step of the acid-fast or cold acid-fast procedure, predict the colors of the cells at each step.
Principle of Acid-Fast Stain
When the smear is stained with carbol fuchsin, it solubilizes the lipoidal material present in the Mycobacterial cell wall but by the application of heat, carbol fuchsin further penetrates through lipoidal wall and enters into the cytoplasm. Then after all cell appears red. Then the smear is decolorized with the decolorizing agent (3% HCL in 95% alcohol) but the acid fast cells are resistant due to the presence of large amount of lipoidal material in their cell wall which prevents the penetration of decolorizing solution. The non-acid fast organism lack the lipoidal material in their cell wall due to which they are easily decolorized, leaving the cells colorless. Then the smear is stained with counterstain, methylene blue. Only decolorized cells absorb the counter stain and take its color and appears blue while acid-fast cells retain the red color.
Summary of Acid-Fast Stain
Application of Reagent Cell colour
Acid fast Non-acid fast
Primary dye Carbol fuchsin Red Red
Decolorizer Acid alcohol Red Colorless
Counter stain Methylene blue Red Blue