Question

Acid fast staining method is used for detection of mycobacteria and bacterial spores. Arrange the steps according to the appropriate procedure for Acid fast staining.
(A). Primary staining by Strong Carbol fuchsin
(B). Use mordant by steaming the slide with a Bunsen burner over the sink
(C). Decolorise by Acid Alcohol
(D). Counterstain by flooding the slide with Methylene Blue
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D)
• B. (A), (C), (B), (D)
• C. (B), (A), (D), (C)
• D. (C), (B), (D), (A)

Hint:

Remember the diagnostic colors after completing the Ziehl-Neelsen stain:
- Acid-fast bacteria (e.g., Mycobacterium tuberculosis) appear Red/Pink.
- Non-acid-fast bacteria and background tissue cells appear Blue.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Acid-fast staining, primarily performed using the classic Ziehl-Neelsen technique, is a key differential staining procedure used in microbiology.
This method is specifically designed to distinguish acid-fast organisms, such as Mycobacterium species, from non-acid-fast bacteria.
The cell wall of acid-fast bacteria contains exceptionally high concentrations of lipids, particularly mycolic acids, which create a waxy, hydrophobic barrier.
This waxy barrier resists standard water-soluble stains like Gram stain, necessitating a specialized process involving a primary stain, a mordant (heat), a decolorizing agent, and a counterstain.

Step 2: Detailed Explanation:
Let us analyze and arrange the stages of the acid-fast staining protocol in the correct operational sequence:
-

Step 1: Application of the Primary Stain (A):
The first step involves flooding the prepared and heat-fixed bacterial smear with the primary stain, Strong Carbol Fuchsin.
Carbol fuchsin is a lipid-soluble phenolic stain that can dissolve in and penetrate the lipid-rich mycolic acid layer of the bacterial cell wall.
- Application of Heat as a Mordant (B):
After applying the primary stain, the slide is gently heated (steamed) using a Bunsen burner over a sink for about 5 minutes.
The heat acts as a physical mordant, softening the waxy mycolic acids and enabling the carbol fuchsin dye to penetrate deep into the cytoplasm.
Once the slide cools down, the waxy lipids solidify again, trapping the red carbol fuchsin dye inside the cell.
-

Step 2: Decolorization (C):
The slide is washed with water and then treated with a decolorizing agent, typically Acid Alcohol (contains \(3%\) hydrochloric acid in \(95%\) ethanol).
Acid-fast bacteria resist decolorization because the primary dye remains locked within the solidified mycolic acid layer.
Non-acid-fast bacteria, which lack this protective lipid barrier, are rapidly decolorized and become completely colorless.
-

Step 3: Counterstaining (D):
The slide is washed again and flooded with a counterstain, typically Methylene Blue, for about 1 minute.
This basic dye stains the colorless non-acid-fast cells blue, providing a sharp contrast.
The acid-fast bacteria do not take up the counterstain and remain bright red or pink.
Thus, the correct operational sequence of the steps is:
\[ \text{(A) Primary Stain} \rightarrow \text{(B) Heat Mordant} \rightarrow \text{(C) Decolorization} \rightarrow \text{(D) Counterstain} \] This matches the sequence (A), (B), (C), (D), which corresponds to Option (A).

Step 3: Final Answer:
The correct sequential order of the steps is (A), (B), (C), (D).
Therefore, the correct choice is Option (A).