Which feature is characteristic of Gram-negative bacteria but not Gram-positive bacteria?

• A. Outer membrane
• B. Teichoic acid
• C. Thick peptidoglycan wall
• D. LPS absence

Answer: (A)

Understanding the differences in bacterial cell envelopes is what this item tests. The feature that distinguishes Gram-negative bacteria from Gram-positive bacteria is the presence of an outer membrane that sits outside a thin peptidoglycan layer, forming a separate outer barrier with a periplasmic space in between. This outer membrane is a defining trait of Gram-negative cells and contains lipopolysaccharide, which contributes to their unique interactions with antibiotics and the immune system. Gram-positive bacteria, by contrast, lack this outer membrane and have a much thicker peptidoglycan wall rich in teichoic acids. Because of the outer membrane, Gram-negative cells behave differently in Gram staining, often losing the crystal violet-iodine complex during decolorization and so appearing pink, while Gram-positive cells retain the stain due to their thick peptidoglycan layer. Thus, the outer membrane is the feature that sets Gram-negative apart from Gram-positive.