What is LPS?

• A. Endotoxin released when bacteria are destroyed
• B. Exotoxin released into the surrounding environment
• C. A protein component of the cytoplasmic membrane
• D. A capsule polysaccharide

Answer: (A)

LPS stands for lipopolysaccharide, a molecule embedded in the outer membrane of Gram-negative bacteria. It functions as an endotoxin because the lipid A portion can trigger strong immune responses, especially when the bacteria are lysed and LPS is released into the surrounding environment. LPS is not a secreted protein (that would be an exotoxin), nor is it a component of the cytoplasmic (inner) membrane. It’s also not a capsule polysaccharide; capsules are separate polysaccharide layers outside the cell. The outer-m membrane location and the fact that it is released upon bacterial destruction are what make LPS a classic endotoxin.