Lipopolysaccharide (LPS) is a component of the outer membrane of which bacteria?

• A. Gram-positive
• B. Archaea
• C. Gram-negative
• D. Fungi

Answer: (C)

Lipopolysaccharide is a hallmark of the outer membrane of Gram-negative bacteria. In these organisms, the outer membrane sits outside a relatively thin peptidoglycan layer and is packed with LPS, which anchors in the membrane via lipid A and includes a core polysaccharide plus an O-antigen extending outward. This feature is not present in Gram-positive bacteria, which lack an outer membrane, nor in Archaea, which have different envelope components, or in fungi, which have cell walls made of chitin and other polymers. Because LPS defines the outer membrane composition of Gram-negative bacteria, it is the best indicator of that group.