Which of the following stains is used to differentiate between Gram-positive and Gram-negative bacteria?

Appeared in: BTSC 3 August 2025

Explanation

Gram stain is the primary differential stain used to categorize bacteria based on their cell wall structure.
Gram-positive bacteria have a thick peptidoglycan cell wall that retains the primary stain (crystal violet) and appear purple.
Gram-negative bacteria have a thin peptidoglycan layer and an outer lipopolysaccharide membrane. They are decolorized by alcohol and take up the counterstain (safranin), appearing pink or red.
This differentiation is a crucial first step in bacterial identification and informs preliminary treatment decisions.

Option A: Ziehl-Neelsen stain is not used for Gram differentiation. It identifies acid-fastness.
Option C: Albert's stain is a special stain for identifying specific intracellular structures, not for general bacterial classification like Gram staining.
Option D: Acid-fast stain is another name for the Ziehl-Neelsen stain. It differentiates bacteria based on their ability to resist decolorization by acid, not based on their Gram reaction.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Microbiological Staining Techniques as background academic context rather than a clinical decision trigger.
The result of a Gram stain from a clinical specimen (e.g., blood, urine, CSF) is one of the fastest and most important pieces of information for a clinician.
It allows for the initiation of empirical antibiotic therapy. For example, if Gram-positive cocci are seen, an antibiotic targeting these organisms is chosen. If Gram-negative rods are seen, a different class of antibiotics is selected, pending final culture and sensitivity results.
What if? If a lab technician over-decolorizes the smear during Gram staining, Gram-positive bacteria can lose the primary stain and incorrectly appear pink (Gram-negative). This could lead to the wrong antibiotic choice and treatment failure.

How to Approach the Question

Identify the core task in the question: The question asks for a method to 'differentiate between Gram-positive and Gram-negative bacteria'.
Analyze the options provided. Each option is a different staining technique.
Recall the specific purpose of each stain. Associate 'Gram stain' with 'Gram-positive/Gram-negative'.
Associate 'Ziehl-Neelsen' or 'Acid-fast' with 'Mycobacterium/tuberculosis'.
Associate 'Albert's stain' with 'Corynebacterium diphtheriae/metachromatic granules'.
Select the option that directly matches the task described in the question stem.

Concept Tested & Keywords

QZr7SlXRNIogcVGH04_NJw

E6 Microbiology Essentials Sastry 4e Part 1 p. 45-47

E6 Microbiology Applied AnjuDhir 3e Part 1 p. 109-111

Practise the full BTSC 3 August 2025

Attempt every question from this paper in a timed mock, then review the full solution for each one.