Mycobacterium marinum infection — DTM&H MCQ

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ModerateTropical Bacterial InfectionsMycobacterium marinum infectionDTM&H

A 40-year-old fisherman in Brazil presents with a chronic nodular lesion on his arm that has developed satellite lesions along the lymphatic drainage. He sustained a fish-related puncture wound 3 weeks ago. The lesions do not respond to antibacterial therapy. What is the most likely causative organism?

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Correct answer: D — Mycobacterium marinum

The correct answer is D, Mycobacterium marinum. This atypical (non-tuberculous) mycobacterium is the classic cause of a chronic nodular skin lesion with sporotrichoid spread (satellite nodules tracking along lymphatics) following a puncture injury from fish, shellfish, or aquarium/fish-tank water, giving it the name fish tank granuloma. The organism grows optimally at cooler skin temperatures (30 to 32 degrees C), which explains its predilection for exposed extremities such as the hand and forearm, and why it fails to respond to standard antibacterial antibiotics that target common skin pathogens. Diagnosis is often clinical, supported by biopsy showing granulomatous inflammation, with confirmation by mycobacterial culture; treatment requires prolonged combination antimycobacterial therapy, typically clarithromycin plus ethambutol (or rifampicin), for several months. Why the other options are wrong: C. Staphylococcus aureus: Causes acute pyogenic infections (abscesses, cellulitis) that typically respond promptly to conventional antibacterial therapy, not a chronic indolent sporotrichoid nodular pattern. E. Sporothrix schenckii: Produces an identical sporotrichoid lymphangitic pattern but follows exposure to soil, thorns, or plant matter (classically rose gardeners), not fish or aquatic puncture wounds. A. Nocardia brasiliensis: Causes mycetoma or cutaneous nocardiosis after soil contamination of wounds, with filamentous branching organisms on staining, not linked to fish exposure. B. Leishmania braziliensis: Causes cutaneous or mucocutaneous leishmaniasis transmitted by sandfly bites in Latin America, presenting as ulcerating skin lesions rather than a lesion following a fish puncture injury. Key point: A non-healing nodular lesion with lymphatic satellite spread after a fish or aquarium related puncture wound, unresponsive to antibacterials, is Mycobacterium marinum until proven otherwise.

Reference: British Journal of Dermatology / North Bristol NHS Trust case data and international clinical review: Mycobacterium marinum, non-tuberculous mycobacterial fish tank granuloma, diagnosis and management with clarithromycin and ethambutol combination therapy. PMC10384600, https://pmc.ncbi.nlm.nih.gov/articles/PMC10384600/