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An Introduction to Infection Quiz
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Which statement best describes an endotoxin?
- A A bacterial adhesion molecule that allows attachment to epithelial cells You picked ✕
- B A lipopolysaccharide component of the Gram-negative outer membrane that can trigger inflammation You picked ✓
- C An enzyme that degrades host immunoglobulins to evade adaptive immunity You picked ✕
- D A bacterial membrane-damaging factor that directly lyses host cells You picked ✕
- E A secreted protein toxin that is highly immunogenic and neutralised by antibodies You picked ✕
Correct!
Incorrect
The answer is B: A lipopolysaccharide component of the Gram-negative outer membrane that can trigger inflammation
Explanation
Why B is correct
See Virulence and Toxins. Option B is correct because an endotoxin is lipopolysaccharide (LPS), a component of the outer membrane of Gram-negative bacteria that can trigger inflammation. Gram-negative bacteria have an outer membrane (an extra outer layer of the cell wall) that is rich in LPS. LPS has three parts: lipid A, which is the part that behaves as endotoxin, a core sugar chain and an outer sugar chain called the O-antigen. When endotoxin reaches the bloodstream it triggers a massive immune response with widespread release of cytokines (chemical messengers of inflammation). This causes widespread vasodilation (widening of blood vessels) and a dramatic fall in blood pressure, which is how Gram-negative infection can lead to sepsis. Endotoxin is part of the bacterial structure, so it is not a secreted protein, which sets it apart from exotoxins. Memory aid: endotoxin is on the Inside of the Gram-negative bacterium's wall, while exotoxins are Exported.
- Structure: endotoxin is lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria.
- Active part: lipid A behaves as the endotoxin.
- Effect: triggers release of cytokines, vasodilation and a fall in blood pressure, which can lead to sepsis.
Why the other options are incorrect
- A. A bacterial adhesion molecule that allows attachment to epithelial cells: This describes an adhesin, which helps bacteria stick to host cells. An adhesin is not a toxin, whereas an endotoxin harms the host by triggering inflammation. Your answer
- C. An enzyme that degrades host immunoglobulins to evade adaptive immunity: This describes an immune evasion enzyme, a protease that breaks down antibodies. It is a secreted protein, unlike an endotoxin, which is part of the bacterial outer membrane. Your answer
- D. A bacterial membrane-damaging factor that directly lyses host cells: This describes cytolytic factors such as haemolysins, which punch holes in host cell membranes. An endotoxin does not directly lyse host cells; it acts by stimulating the immune system. Your answer
- E. A secreted protein toxin that is highly immunogenic and neutralised by antibodies: This describes an exotoxin. Exotoxins are proteins released by the bacterium, and antibodies can neutralise them, which is why some are used in vaccines. An endotoxin is a structural part of the cell wall, not a secreted protein. Your answer
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