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Difficulty: MediumInfectious Disease Mechanisms

Match each patient's clinical presentation with the primary molecular mechanism by which the causative pathogen survives within host cells.

  • A 65-year-old man develops severe pneumonia and confusion after attending a convention. Sputum Gram stain shows numerous neutrophils but no organisms.Inhibits phagosome-lysosome fusion via a Type IV secretion system, establishing a replicative vacuole surrounded by host ribosomes.
  • A 24-year-old traveler presents with a step-wise fever and rose spots on his abdomen. Blood cultures grow H2S-producing, motile bacteria.Employs a Type III secretion system to remodel the containing vacuole, protecting the organism from reactive oxygen species.
  • A 40-year-old sheep farmer presents with high fever, headache, and atypical pneumonia; transaminases are elevated.Actively relies on the fusion of the phagosome with lysosomes, requiring a highly acidic environment for metabolic activation.
  • A 70-year-old woman develops meningitis after eating unpasteurized cheese. Cerebrospinal fluid grows Gram-positive rods with tumbling motility.Produces a pore-forming toxin that lyses the phagosomal membrane, enabling the organism to escape into the cytosol.

Answer

Legionella pneumophila (convention pneumonia) inhibits phagosome-lysosome fusion. Salmonella typhi (typhoid fever) remodels its vacuole using a Type III secretion system. Coxiella burnetii (Q fever) requires the highly acidic environment of the fused phagolysosome. Listeria monocytogenes (cheese-associated meningitis) escapes the phagosome by producing a pore-forming toxin.
Each correct pair accurately links a classic clinical presentation to the specific, distinct molecular strategy used by that causative pathogen to survive and replicate within host cells.

Step-by-Step Solution

1
Analyze each clinical vignette to identify the most likely causative pathogen based on demographics, exposures, and microbiological findings.
Pathogens identified: Legionella pneumophila, Salmonella typhi, Coxiella burnetii, and Listeria monocytogenes.
Accurate identification of the pathogen is the necessary first step to determine its specific pathogenesis.
2
Recall the distinct molecular strategy each identified intracellular pathogen uses to survive within host macrophages.
Legionella prevents fusion; Salmonella modifies the vacuole; Coxiella utilizes the acidic pH; Listeria escapes into the cytosol.
Understanding molecular pathogenesis is essential for mastering infectious disease mechanisms and cellular immunology.

Key Concept

Intracellular pathogens utilize distinct molecular mechanisms to survive within host phagocytes, including preventing phagosome-lysosome fusion, escaping into the cytosol, remodeling the enclosing vacuole, or adapting to the highly acidic environment of the mature phagolysosome.
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Topics

Infectious Disease Mechanisms

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