What does Acid-Fast Staining do?

DSSSB -28 August 2019 (Shift-2)

Applied Microbiology & Infection control

Easy

Appeared in: DSSSB -28 August 2019 (Shift-2)

Explanation

Acid-fast staining is a differential stain used to distinguish bacteria based on their cell wall properties.
Acid-fast bacteria possess a high concentration of mycolic acid, a waxy substance, in their cell walls.
This waxy wall allows the bacteria to retain the primary red stain (carbol fuchsin) even when treated with an acid decolorizer.
Non-acid-fast bacteria lack this waxy wall, are decolorized by the acid, and subsequently take up the blue counterstain (methylene blue).

Option B: This option is incorrect on two counts. First, the stain's primary purpose is to differentiate bacteria, which this option denies. Second, it incorrectly assigns red to non-acid-fast and blue to acid-fast bacteria.
Option C: This option correctly states that the stain differentiates but reverses the color results. Acid-fast bacteria are red, and non-acid-fast bacteria are blue.
Option D: This option is incorrect because the primary function of a differential stain like the acid-fast stain is precisely to differentiate between bacterial types.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Principle and interpretation of Acid-Fast Staining (Ziehl-Neelsen method) as background academic context rather than a clinical decision trigger.
This stain is crucial for the presumptive diagnosis of tuberculosis (TB), caused by Mycobacterium tuberculosis, a significant public health issue in India and worldwide.
Nurses play a key role in patient education and collection of adequate sputum samples for Acid-Fast Bacilli (AFB) testing, which often requires 2-3 early morning samples on consecutive days for higher yield.
A positive AFB smear result necessitates immediate implementation of airborne infection control precautions, including placing the patient in a negative-pressure room and ensuring healthcare staff use N95 respirators.

How to Approach the Question

Identify the question type: This is a factual recall question about a fundamental microbiology procedure.
Focus on the term 'Acid-Fast'. This implies a resistance ('fastness') to a treatment with acid.
Recall the purpose of differential stains: to tell different types of cells apart. This makes any option with 'Cannot differentiate' highly suspect and likely incorrect.
Remember the color scheme for this specific stain. The primary stain is a strong red (carbol fuchsin). The bacteria that are 'fast' (resistant) hold onto this red color.
The 'non-fast' bacteria are washed out by the acid and then counterstained, typically with blue (methylene blue).
Evaluate the options based on this logic: The correct option must state that the stain differentiates and must assign red to acid-fast and blue to non-acid-fast bacteria.

Concept Tested & Keywords

Concept Tested: Principle and interpretation of Acid-Fast Staining (Ziehl-Neelsen method).
Stem keywords: Acid-Fast Staining
Lead-in keywords: What does