Which feature is present in Gram-negative bacteria but absent in Gram-positive bacteria?

• A. Outer membrane
• B. Teichoic acids
• C. Peptidoglycan
• D. Capsule

Answer: (A)

The test focuses on the structure of the bacterial cell envelope that distinguishes Gram-negative from Gram-positive organisms. Gram-negative bacteria have an outer membrane that surrounds a thin peptidoglycan layer, with a periplasmic space between membranes. Gram-positive bacteria lack this outer membrane and instead have a much thicker peptidoglycan layer, often with teichoic acids embedded in it.

This outer membrane presence in Gram-negative bacteria is what sets them apart from Gram-positive ones, making it the feature that is present in the former and absent in the latter.

Teichoic acids are characteristic of Gram-positive walls, not Gram-negative. Peptidoglycan exists in both types, though its thickness differs. Capsule can be present in both depending on the strain, so it isn’t a reliable distinguishing feature.