A Gram-negative bacterium is lysed by the immune system. The patient's fever worsens dramatically immediately after. Which virulence factor is most directly responsible?
AAdhesins released from the broken cell
BExotoxins that were being actively secreted
CLipopolysaccharide (endotoxin) released from the cell wall
DCapsule material that floods the bloodstream
Endotoxin (LPS) is a structural component of Gram-negative outer membranes. When the bacterium is lysed, LPS is released in large quantities, triggering a strong inflammatory response (fever, sepsis). This is why antibiotic treatment of Gram-negative sepsis can paradoxically worsen symptoms initially — killing the bacteria releases endotoxin. Exotoxins are secreted proteins, not structural components released upon lysis.
Question 2 True / False
Natural selection consistently favors pathogens that are as virulent as possible, since killing the host quickly maximizes damage and replication.
TTrue
FFalse
Answer: False
There is a virulence-transmission tradeoff: a pathogen that kills its host before transmission can occur leaves no offspring. Natural selection often favors intermediate virulence — enough to replicate efficiently, but not so lethal that the host dies (or is bedridden and isolated) before the pathogen can spread. Pathogens with alternate transmission routes (vectors, environmental persistence) can evolve higher virulence because they are less dependent on a mobile, living host.
Question 3 Short Answer
Mycobacterium tuberculosis survives inside macrophages despite being engulfed. What category of immune evasion strategy does this represent, and why is it particularly difficult for the immune system to counter?
Think about your answer, then reveal below.
Model answer: Intracellular survival. By hiding inside host immune cells, M. tuberculosis avoids antibodies (which cannot penetrate cells) and other extracellular defenses. Eliminating the bacteria requires cell-mediated immunity (cytotoxic T cells killing infected macrophages), but M. tuberculosis can also inhibit phagolysosome fusion, disarming the macrophage's own killing mechanisms.
This question tests understanding of why different immune evasion strategies pose different challenges — not just a list of strategies. Intracellular pathogens exploit the fact that the adaptive immune system's most powerful extracellular weapon (antibodies) cannot follow them inside cells.