The following gram-staining, gram-positive organisms take up ---colour

RUHS, Jaipur, M.Sc Nursing Entrance Exam-2022

Applied Microbiology & Infection control

Easy

Appeared in: RUHS, Jaipur, M.Sc Nursing Entrance Exam-2022

Explanation

Gram-positive bacteria are characterized by a thick peptidoglycan layer in their cell wall.
During the Gram staining process, this thick wall retains the primary stain, crystal violet, even after the decolorization step.
The crystal violet, complexed with iodine (mordant), is trapped within the cell, giving the bacteria a distinct purple or violet color under the microscope.
This is in contrast to Gram-negative bacteria, which have a thin peptidoglycan layer and are decolorized, subsequently taking up the pink/red counterstain (safranin).

Option A: Pink is the color taken up by Gram-negative bacteria, not Gram-positive. Their thin peptidoglycan wall cannot retain the initial crystal violet stain after decolorization, so they absorb the pink counterstain, safranin.
Option C: Yellow is not a color produced by any of the standard reagents (crystal violet, iodine, safranin) used in the Gram staining procedure.
Option D: Green is the color associated with endospore staining, where malachite green is used as the primary stain to visualize bacterial spores. It is not used in Gram staining.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Gram Staining Procedure and Bacterial Cell Wall Differences as background academic context rather than a clinical decision trigger.
Gram staining is often the first and most crucial step in identifying a bacterial pathogen from a clinical sample (e.g., blood, sputum, urine).
The result (Gram-positive vs. Gram-negative) and morphology (cocci vs. bacilli) provides rapid, preliminary information that helps clinicians choose an appropriate initial or empirical antibiotic, as many antibiotics are more effective against one group than the other.
What if the sample is from an older culture? If a Gram-positive culture ages, the cell walls can degrade. This may cause them to lose the ability to retain the crystal violet stain, leading to a false Gram-negative (pink) or Gram-variable result. Therefore, using young, fresh cultures is critical for accurate results.

How to Approach the Question

Identify the key terms in the question: 'gram-positive organisms' and 'colour'.
This is a factual recall question based on a fundamental microbiology procedure.
Recall the steps of the Gram stain: primary stain (crystal violet), mordant (iodine), decolorizer (alcohol), and counterstain (safranin).
Remember the key difference: Gram-positive bacteria have a thick peptidoglycan wall, while Gram-negative bacteria have a thin one.
Connect the wall structure to the outcome: The thick wall of Gram-positive bacteria traps the initial purple dye (crystal violet).
Therefore, Gram-positive organisms will appear purple. Select the option that matches this fact.

Concept Tested & Keywords

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

E6 Microbiology Applied AnjuDhir 3e Part 1 p. 106-108