Bacterium
Mycobacterium marinum
Photochromogen of fish-tank granuloma
my-koh-bak-TEER-ee-um muh-RY-num
High-yield clue
A nodule on the hand or arm after aquarium or fish exposure (fish-tank granuloma) is the classic study clue.
Overview
A slow-growing nontuberculous mycobacterium (Runyon group I photochromogen) found in fresh and salt water. It causes localized skin and soft-tissue nodules known as fish-tank or swimming-pool granuloma.
Classification
- Acid-fast bacillus
- Nontuberculous mycobacterium
- Runyon group I photochromogen
- Slow-growing
- Prefers cooler ~30 C temperatures
Lab & identification clues
- Acid-fast staining vocabulary
- Yellow pigment produced after light exposure (photochromogen)
- Grows better at ~30 C than 37 C
- Lesions favor cooler distal skin
Associations
- Aquarium and fish-handling exposure vocabulary
- Swimming-pool contact epidemiology
- Sporotrichoid nodule spread along lymphatics
- Environmental water reservoir framing
Commonly confused with
- Sporothrix schenckii (sporotrichoid nodules)
- Mycobacterium tuberculosis
Your notes
Original student-study summary. Sources checked: OpenStax Microbiology 2e, NCBI Bookshelf Medical Microbiology, and CDC topic pages where applicable; reviewed 2026-06. Educational only; no diagnosis, treatment, dosing, or specimen-handling guidance.