Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Assertion [a]: Gram negative bacteria show staining with saffranin.
Reason [r]: Gram negative bacteria have an outer membrane with lipopolysaccharides.
Options:
(A) Both [a] and [r] are true and [r] is the correct reason for [a].
(B) Both [a] and [r] are true but [r] is not the correct reason for [a].
(C) Both [a] and [r] are false.
(D) [a] is true but [r] is false.
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.10)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Microbiology (Gram Staining Mechanism & Cell Wall Architecture)
**2. Detailed Step-by-Step Analysis**:
- **Assertion [a]**: *“Gram negative bacteria show staining with saffranin.”*
- **TRUE**: In the Gram stain protocol, cells are first stained with crystal violet and treated with Gram's iodine (forming a CV-I complex). When treated with a decolorizer (alcohol/acetone), the outer membrane of Gram-negative bacteria is dissolved, and their thin peptidoglycan layer ($2 - 7\text{ nm}$) fails to retain the CV-I complex, leaving them colorless. The pink/red counterstain **safranin** is then taken up, staining Gram-negative cells pink/red.
- **Reason [r]**: *“Gram negative bacteria have an outer membrane with lipopolysaccharides.”*
- **TRUE**: Gram-negative cell envelopes uniquely possess an outer membrane whose outer leaflet is composed of lipopolysaccharide (LPS).
- Alcohol wash dissolves this lipid-rich outer membrane and increases permeability across the thin cell wall, allowing the primary stain to wash out completely, which directly enables the counterstain (safranin) to stain the cell. Thus, the presence of the lipid-rich outer membrane and thin peptidoglycan layer is the direct physical cause for decolorization and subsequent safranin staining.
- Therefore, **both [a] and [r] are true and [r] is the correct reason for [a]**.
- **Option-by-Option Evaluation**:
- **Option (A) [CORRECT]**: Both statements are true, and the outer membrane lipid architecture is the biological reason why Gram-negative cells decolorize and take up safranin.
- **Option (B) [INCORRECT]**: [r] directly explains the mechanism of Gram-negative staining.
- **Option (C) [INCORRECT]**: Both statements are well-established facts.
- **Option (D) [INCORRECT]**: [r] is a foundational microbiological fact.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **A**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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