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
Frequently asked questions
What is Mycobacterium marinum?
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.
What are the lab clues for Mycobacterium marinum?
Lab clues for Mycobacterium marinum include Acid-fast staining vocabulary, Yellow pigment produced after light exposure (photochromogen), Grows better at ~30 C than 37 C, and Lesions favor cooler distal skin.
What is Mycobacterium marinum commonly confused with?
Mycobacterium marinum is commonly confused with Sporothrix schenckii (sporotrichoid nodules) and Mycobacterium tuberculosis.
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.