Which of the following statements correctly describes the difference between Lipid A and O antigen in the structure of Gram-negative bacterial cell walls?

• A. Lipid A is a polysaccharide, while O antigen is a lipid.
• B. Both Lipid A and O antigen are found only in Gram-positive bacteria.
• C. O antigen is responsible for toxicity, while Lipid A determines antigenic specificity.
• D. Lipid A is the toxic component of lipopolysaccharide, while O antigen is the variable, antigenic polysaccharide portion.

Answer: (D)

Verified step by step guidance

1. Understand the structure of the Gram-negative bacterial outer membrane, which contains lipopolysaccharide (LPS) molecules composed of three main parts: Lipid A, core polysaccharide, and O antigen.
2. Recognize that Lipid A is a lipid component embedded in the outer membrane and is responsible for the toxic effects (endotoxin activity) of LPS.
3. Know that the O antigen is a polysaccharide chain extending outward from the core polysaccharide and varies among bacterial strains, providing antigenic specificity.
4. Distinguish that Lipid A is not a polysaccharide but a lipid, and O antigen is not a lipid but a polysaccharide, which helps clarify the incorrect statements.
5. Conclude that the correct description is: Lipid A is the toxic component of LPS, while O antigen is the variable polysaccharide portion responsible for antigenic specificity.