Which component is characteristic of Gram-negative bacteria?

• A. Teichoic acids
• B. Peptidoglycan
• C. Mycolic acid
• D. Lipopolysaccharide

Answer: (D)

A key feature of Gram-negative bacteria is the outer membrane that contains lipopolysaccharide. This LPS layer sits outside a relatively thin peptidoglycan wall and forms a protective barrier; it also carries lipid A as an endotoxin and an O-antigen that contributes to serotype diversity. Because of this outer membrane and LPS, Gram-negative cells stain differently and respond to antibiotics in distinct ways compared to Gram-positive bacteria.

Teichoic acids are characteristic of Gram-positive cell walls, which lack an outer membrane. Mycolic acid is a hallmark of acid-fast organisms like Mycobacterium and is not typical of Gram-negative bacteria. Peptidoglycan is present in both groups, but in Gram-negative bacteria it is thinner and located within the periplasm, not the defining outer layer.