Morphology: Acid-fast bacillus. Slow-growing photochromogen (yellow pigment with light exposure). Optimal growth at **30°C** (not 37°C) — explains cutaneous tropism. Environmental — fresh + salt water, fish tanks, aquariums.
Typical drugs
- #1Doxycycline— **100 mg PO BID** — preferred for localized cutaneous disease. Single-agent acceptable for limited disease.
- #2Trimethoprim-sulfamethoxazole— 1 DS BID alternative monotherapy. Clarithromycin (not seeded) 500 mg PO BID is an alternative.
- #3Rifampin— + ethambutol for deep / disseminated / refractory disease — combo therapy.
- #4Ethambutol— Add for deep / tenosynovitis / disseminated disease — combo with rifampin.
Empiric therapy when resistant
Generally susceptible. Deep / refractory disease → rifampin + ethambutol ± clarithromycin combo. Surgical debridement for tenosynovitis / deep tissue disease.
Resistance notes
Resistance uncommon to first-line agents.
Pearls
"Fish tank granuloma" — classic derm vignette. History: hobbyist with home aquarium, fish handler, fisherman, contact with brackish/salt water → papulonodular skin lesion on hand or arm 2–6 weeks later, sometimes sporotrichoid spread proximally (mimics sporotrichosis — distinguish with AFB stain + culture). Pathognomonic for ID + derm boards. Treatment: 1–2 antibiotics × 3–4 months minimum (until 1–2 months after clinical resolution). Doxycycline monotherapy for localized cutaneous; rifampin + ethambutol + clarithromycin for deep tenosynovitis / disseminated / refractory. Surgical debridement for deep involvement. Don't culture at 37°C — request 30°C mycobacterial culture.
References
- ATS/ERS/ESCMID/IDSA Nontuberculous Mycobacterial Guidelines (2020)
- Aubry — M. marinum Cutaneous Infection (2002)