During gram staining process dilute carbol fuchsin is used for?

UPRVUNL - 2021

Applied Microbiology & Infection control

Easy

Appeared in: UPRVUNL - 2021

Explanation

In the Gram staining process, a counterstain is applied as the final step to color bacteria that have been decolorized.
Dilute carbol fuchsin serves as a secondary stain, imparting a pink or red color to Gram-negative bacteria.
Gram-negative bacteria have a thin cell wall that does not retain the primary crystal violet-iodine complex after the alcohol wash, making them available to be stained by the counterstain.
This allows for the visual differentiation between purple Gram-positive bacteria and pink/red Gram-negative bacteria under a microscope.

Option A: Drying, specifically air-drying and heat-fixing, is a physical process done at the beginning to adhere the bacterial smear to the slide. It is not a chemical staining step and does not involve carbol fuchsin.
Option C: Decolorizing is the step that removes the primary stain (crystal violet) from Gram-negative bacteria. This is typically done using alcohol or an acetone-alcohol mixture, not carbol fuchsin.
Option D: Washing the smear is done with water between staining steps to remove excess reagent. It is a rinsing step and not the function of a dye like carbol fuchsin.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Gram staining procedure and reagents as background academic context rather than a clinical decision trigger.
Gram staining is a critical first step in microbiology for identifying bacteria from clinical specimens (e.g., blood, urine, sputum).
The result (Gram-positive or Gram-negative) provides rapid, preliminary information that helps guide the initial choice of empirical antibiotic therapy, often before culture results are available.
What if? If the counterstain step was skipped, Gram-negative bacteria would remain colorless after decolorization and would be invisible under a light microscope, leading to a misinterpretation that only Gram-positive bacteria (if any) are present in the sample.

How to Approach the Question

First, identify the core subject of the question, which is the 'Gram staining process' and a specific reagent, 'dilute carbol fuchsin'.
This is a factual recall question from microbiology. Access your knowledge of the standard steps in Gram staining.
Recall the four main steps in order: 1. Primary stain (Crystal Violet), 2. Mordant (Iodine), 3. Decolorizer (Alcohol/Acetone), 4. Counterstain (Safranin or dilute Carbol Fuchsin).
Match the reagent 'dilute carbol fuchsin' to its function in the sequence. It is used in the final step to stain the decolorized cells.
Evaluate the given options: 'Drying', 'Decolorizing', and 'Washing' are all parts of the overall slide preparation and staining process, but they are distinct actions performed by different means (heat, alcohol, water). 'Counter staining' is the specific function of safranin or dilute carbol fuchsin.
Therefore, conclude that the correct role of dilute carbol fuchsin is counterstaining.

Concept Tested & Keywords

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E6 Microbiology Essentials Sastry 4e Part 1 pp. 46-48, 45-47