Question

Gram-negative bacteria are characterized by:

• A. Thick peptidoglycan layer
• B. Outer lipopolysaccharide membrane
• C. Retention of crystal violet stain
• D. Absence of cytoplasmic membrane

Hint:

Gram-negative = thin wall + outer LPS membrane + pink stain. Gram-positive = thick wall + purple stain.

Answer 1

The Correct Option is B

Concept: Gram staining is a differential staining technique used to classify bacteria into Gram-positive and Gram-negative based on their cell wall structure. Gram-negative bacteria have a unique cell wall architecture that distinguishes them from Gram-positive bacteria.

Step 1: Structure of Gram-negative bacteria.
Gram-negative bacteria possess:
• Thin peptidoglycan layer
• Outer membrane containing lipopolysaccharides (LPS)
• Periplasmic space
• Inner cytoplasmic membrane

Step 2: Role of outer membrane.
The outer membrane:
• Contains lipopolysaccharides (LPS)
• Acts as a barrier to harmful substances
• Contributes to pathogenicity (endotoxins)

Step 3: Gram staining behavior.
During Gram staining:
• Gram-negative bacteria do NOT retain crystal violet
• They appear pink/red after counterstaining (safranin)

Step 4: Evaluating options.
• Option (A): Thick peptidoglycan layer — Characteristic of Gram-positive bacteria.
• Option (B): Outer lipopolysaccharide membrane — Defining feature of Gram-negative bacteria. Correct.
• Option (C): Retention of crystal violet stain — Gram-positive property.
• Option (D): Absence of cytoplasmic membrane — Incorrect, all bacteria have cytoplasmic membrane. Final Conclusion:
Gram-negative bacteria are characterized by the presence of an outer lipopolysaccharide membrane. Hence, the correct answer is option (2).