Atypical mycobacterial skin infection
Non-tuberculous mycobacterial skin infection; NTM skin infection; fishtank granuloma; swimming-pool granuloma; Buruli ulcer (M. ulcerans)
Atypical (non-tuberculous) mycobacterial skin infection is an under-recognised cause of non-healing nodules, ulcers and sporotrichoid lymphocutaneous spread — frequently biopsied as a presumed cSCC, keratoacanthoma or other malignancy before the correct diagnosis is reached. M. marinum (fishtank / swimming-pool granuloma) is the commonest UK organism, contracted from contaminated aquarium / sea water. M. ulcerans causes Buruli ulcer in tropical regions but rarely in UK travellers. The rapidly-growing mycobacteria (M. fortuitum, chelonae, abscessus) are increasingly recognised after cosmetic procedures, tattoos and injections. Diagnosis requires acid-fast staining, prolonged Lowenstein-Jensen / liquid culture (4–8 weeks) and PCR; empirical antibiotics often required while awaiting confirmation.
Common UK organisms
- M. marinum — fishtank / swimming-pool granuloma. Aquarium / sea-water exposure; nodular hand / forearm lesion 2–4 weeks post-inoculation; sporotrichoid spread.
- M. ulcerans — Buruli ulcer; tropical / sub-Saharan Africa; painless undermined ulcer. UK only in returned travellers.
- M. fortuitum / chelonae / abscessus (rapidly-growing mycobacteria) — post-procedural infections (cosmetic injections, tattoos, surgery), inflammatory nodules and abscesses; immunosuppressed at higher risk.
- M. avium-intracellulare complex (MAC) — disseminated cutaneous disease in HIV / immunosuppressed; rare in immunocompetent.
- M. haemophilum — immunosuppressed; characteristic temperature sensitivity (requires 30 °C culture).
- M. kansasii — pulmonary disease primarily; occasional cutaneous involvement.
Clinical features
- Painless or mildly painful nodule, plaque or ulcer at site of inoculation — typically hand, forearm, knee for M. marinum.
- Sporotrichoid lymphocutaneous spread — proximal nodules along lymphatic drainage; differential includes sporotrichosis, leishmaniasis, atypical mycobacteria, nocardiosis.
- Incubation 2–8 weeks; chronic course often months before diagnosis.
- Common misdiagnosis as cSCC, KA, foreign-body reaction or simple cellulitis.
- Constitutional symptoms uncommon in immunocompetent hosts.
Diagnosis
- Take a thorough exposure history — aquarium maintenance, sea-water injury, occupation, recent cosmetic procedures, travel.
- Skin biopsy (deep punch or excisional) with split tissue:
- Histopathology — granulomatous inflammation, sometimes neutrophilic, occasionally caseating; AFB stain positive in only 30–50%.
- Microbiology — Ziehl-Neelsen / auramine-rhodamine stain; mycobacterial culture (Lowenstein-Jensen / MGIT). Cultivate at both 30 °C (for M. marinum, M. haemophilum) and 37 °C.
- PCR for mycobacterial DNA / species identification (16S rRNA sequencing).
- Cultures take 4–8 weeks; empirical antibiotics often initiated while awaiting confirmation.
- Susceptibility testing where culture-positive — sensitivity varies between rapidly-growing species.
Management
- M. marinum — clarithromycin + ethambutol (or doxycycline + rifampicin) for 3–6 months until 1–2 months after clinical resolution.
- Rapidly-growing mycobacteria — combination therapy based on sensitivities; often clarithromycin + amikacin + cefoxitin or imipenem; 6–12 months total.
- Buruli ulcer (M. ulcerans) — rifampicin + clarithromycin (modern WHO regimen) for 8 weeks; surgical debridement for selected cases.
- Surgical excision rarely curative alone; antimicrobial therapy is the cornerstone.
- Immunosuppressed host — extend duration; multidisciplinary infectious-disease input.
- Multidisciplinary input — dermatology, infectious diseases, microbiology, plastics for wound care.
Pitfalls and learning points
- Any non-healing nodular skin lesion with relevant exposure history (aquarium, fish handling, sea-water injury) should prompt mycobacterial biopsy and culture before assuming cSCC.
- Acid-fast stains are insensitive — negative AFB does not exclude mycobacterial infection; rely on culture and PCR.
- Treatment is prolonged (minimum 3 months); patient counselling about treatment duration is essential.
- Cosmetic / surgical procedure-associated outbreaks (M. chelonae, M. abscessus) are increasingly reported in UK private clinics — relevant exposure history matters.
- Surveillance for ophthalmic / pulmonary disease in disseminated infection.
References
- Griffith DE et al. An official ATS/IDSA statement: diagnosis, treatment, and prevention of nontuberculous mycobacterial diseases. Am J Respir Crit Care Med; 2007.
- World Health Organization. Treatment of Mycobacterium ulcerans disease (Buruli ulcer): guidance for health workers. Geneva: WHO; 2012; and WHO Buruli ulcer health topic page, accessed 18 May 2026.
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