Question

Read the following statements on Gram staining:
A. Gram negative cells retain primary stain crystal violet because of thick peptidoglycan layer.
B. Iodine is used as mordant in Gram stain to retain CV-I complexes formed.
C. Safranin is the counter stain used.
D. Gram negative cells allow crystal violet stain to wash out, on addition of ethanol due to thin peptidoglycan layer.
E. Crystal violet is the counter stain used whereas safranin is the primary stain used in Gram staining.
Choose the correct answer from the options given below:

• A. A, B, C and E only
• B. C, D and E only
• C. B, C and D only
• D. A, B and C only

Hint:

Remember: Gram positive = purple (crystal violet), Gram negative = pink/red (safranin).

Answer 1

The Correct Option is C

Step 1: Gram Staining Principle Review.
Gram staining categorizes bacteria as Gram-positive or Gram-negative based on cell wall composition.
- Gram-positive: Possesses a thick peptidoglycan layer, which retains the crystal violet stain.
- Gram-negative: Features a thin peptidoglycan layer and an outer lipid membrane. This structure causes the crystal violet stain to be lost upon decolorization with ethanol, necessitating a counterstain.
Step 2: Statement Analysis.
- (A) Incorrect: Gram-positive bacteria, not Gram-negative, retain crystal violet due to their thick peptidoglycan.
- (B) Correct: Iodine functions as a mordant, forming stable crystal violet-iodine (CV-I) complexes.
- (C) Correct: Safranin serves as the counterstain.
- (D) Correct: Gram-negative bacteria decolorize and lose crystal violet following alcohol treatment.
- (E) Incorrect: Crystal violet is the primary stain, and safranin is the counterstain.
Step 3: Correct Statement Identification.
Statements B, C, and D are accurate.
Step 4: Final Determination.
The correct answer is (C), indicating statements B, C, and D only.