A patient has chronic cough and weight loss. Sputum shows red bacilli on Ziehl–Neelsen staining. Which feature is most responsible for persistence of this pathogen inside macrophages?

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BlackTom AI · Accepted Answer

First, let's restate the scenario to ground our analysis: a patient with chronic cough and weight loss, Ziehl–Neelsen staining shows red bacilli, which points to a mycobacterial infection such as Mycobacterium tuberculosis. Now we evaluate each option in turn.
Option 1: Pneumolysin-mediated lysis of host cells. This mechanism is associated with Streptococcus pneumoniae, not Mycobacterium tuberculosis. Pneumolysin is a pore-forming toxin that damages host cells and can contribute to tissue injury, but it does not explain intracellular persistence within macrophages for mycobacteria. Therefore this option is incorrect for the given pathogen.
Option 2: M protein preventing opsonisation. M protein is a virulence factor of Streptococcus pyogenes (Group A strep) that helps the bacterium resist phagocytosis, but it is not involved in the intracellular survival strategy of mycobacteria. Since the question concerns persistence inside macrophages of a mycobacterial organism, this choice does not fit and is incorrect.
Option 3: Elastase expression aiding tissue invasion. Elastases are enzymes used by certain bacteria to degrade elastin and promote tissue invasion, seen in pathogens like Pseudomonas or Staphylococcus aureus in some contexts, but they do not account for the key intracellular niche of mycobacteria within macrophages. This option thus does not explain the persistence mechanism in this scenario and is incorrect.
Option 4: Waxy mycolic acid cell wall. Mycobacteria possess a unique, thick, waxy cell wall rich in mycolic acids, which makes them resistant to many chemical and physical stresses and impedes phagosome-lysosome fusion within macrophages. This structural feature explains the intracellular survival and persistence of the pathogen inside macrophages, aligning with the Ziehl–Neelsen red-staining characteristic and the clinical picture. Therefore, this option is correct.
In summary, the first three options describe virulence factors or mechanisms relevant to other bacteria and tissues, but they do not account for the intracellular persistence mechanism characteristic of mycobacteria. The waxy mycolic acid-containing cell wall uniquely accounts for the pathogen’s ability to persist inside macrophages, explaining the clinical scenario and staining findings.

MIMI2002 (ND) Tute-shop Quiz 3

A patient has chronic cough and weight loss. Sputum shows red bacilli on Ziehl–Neelsen staining. Which feature is most responsible for persistence of this pathogen inside macrophages?

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