Atypical (nontuberculous) mycobacterial infections

[1]

Meet the patient

A 34-year-old aquarium worker presents with a painless violaceous nodule on his right index finger and a chain of similar nodules creeping up his forearm over six weeks. He cleans tropical fish tanks bare-handed. Standard antibiotics have done nothing, and a routine wound swab is growing nothing.[3]

The two questions that decide his next month are the two every NTM stem turns on: what is the organism, and what temperature does it grow at? (M. marinum, cultured at 30 to 32 degrees C, not 37) and what two drugs will actually work — and is RHZE among them? (it is not). Hold those two and the whole topic lines up.[1]

Slow or fast — the growth-rate split that runs the topic

Classify NTM by growth rate in culture, because that single split points to the species, the exposure, and the regimen. Slow growers (more than 7 days to positivity) include the dermatology headliners; rapid growers (fewer than 7 days, often 3 to 7) cluster after procedures and trauma.[1][2][9]

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Slow growers. M. marinum is the commonest cutaneous NTM, grows at 30 to 32 degrees C, and follows aquatic exposure. M. ulcerans causes Buruli ulcer via its mycolactone toxin. MAC (M. avium-intracellulare complex) is usually pulmonary and cutaneous only in the immunosuppressed. M. kansasii is a photochromogen, mostly pulmonary. M. haemophilum needs iron-supplemented media and 30 degrees C and hits the immunosuppressed.[1]

Rapid growers. M. abscessus complex (subspecies abscessus, massiliense, bolletii) is post-procedure with intrinsic macrolide resistance via the erm41 gene. M. fortuitum group follows trauma and wounds and is more antibiotic-susceptible. M. chelonae disseminates in the corticosteroid-immunosuppressed.[6][8]

Pathophysiology — inoculation, and the one toxin that changes everything

NTM are ubiquitous in the environment — water, soil, dust, and biofilms in plumbing. Infection arrives by direct inoculation through broken skin (minor trauma, abrasions, bites, tattoos, injections, surgery), aquatic exposure (fish tanks, pools, marine or brackish water), iatrogenic contamination (instruments, injectable solutions, mesotherapy, liposuction, catheter sites), or disseminated disease in the immunosuppressed (HIV, transplant, anti-TNF).[1][2]

The immune response is suppurative and/or granulomatous, caseating or non-caseating. M. ulcerans is the exception that defines the rule: its mycolactone toxin causes direct tissue necrosis and local immune suppression, which is why the Buruli ulcer is painless, destructive, and undermined. A painless ulcer that should hurt is the single most important clinical clue in this topic.[2]

[1] [1]

Clinical features by species

Mycobacterium marinum — fish-tank granuloma

M. marinum is the commonest cutaneous NTM. The exposure is aquatic — fish tanks, aquariums, pools, marine or brackish water, fish handling — plus minor trauma, and the incubation is 2 to 3 weeks (range 1 to 8 weeks).[3][5]

The lesion is a solitary verrucous nodule or plaque on a finger or hand (the dominant hand) that may ulcerate, with sporotrichoid (lymphocutaneous) spread — a linear chain of nodules along lymphatic drainage up the arm in 25 to 50 percent of cases. Deep inoculation reaches tenosynovitis, osteomyelitis, or arthritis. The laboratory clue that earns the mark: cultures grow at 30 to 32 degrees C and are usually negative at 37 degrees C. Treatment is clarithromycin plus ethambutol for 3 to 4 months; alternatives include doxycycline, minocycline, rifampicin plus ethambutol, and TMP-SMX. Surgical excision or debridement is reserved for deep tenosynovitis or osteomyelitis.[4][5]

Mycobacterium ulcerans — Buruli (Bairnsdale, Daintree) ulcer

Endemic in West Africa (Benin, Ghana, Côte d'Ivoire), Australia (Victoria), Papua New Guinea, and parts of Southeast Asia. The pathogenesis is unique: mycolactone, a polyketide-derived macrolide toxin that causes tissue necrosis and suppresses local immunity (macrophage apoptosis, cytokine suppression) — which is exactly why the ulcer is painless.[2]

The lesion begins as a painless subcutaneous nodule, papule, plaque, or oedema, then progresses to a large ulcer with characteristic undermined edges — necrosis extends subcutaneously, larger than the surface opening. Ulcers can be huge and destructive through skin, fat, and even bone, yet systemic symptoms are absent. Stages run pre-ulcerative, ulcerative, then fibrosis with contracture and scarring. WHO first-line is rifampicin plus clarithromycin for 8 weeks (or rifampicin plus streptomycin or amikacin), with surgery (debridement, skin grafting) for advanced or fibrotic lesions; antibiotics have replaced surgery as primary treatment. BCG gives partial protection epidemiologically.[2]

Mycobacterium abscessus complex — the post-procedure nightmare

A rapid grower (culture positive in 3 to 7 days) that appears after injections, mesotherapy, liposuction, surgery, tattoos, piercing, or catheters. The presentation is multiple erythematous nodules, papules, pustules, abscesses, or draining sinus tracts weeks to months after the procedure, often clustered along the procedural track.[6][8]

The defining problem is intrinsic antibiotic resistance: the erm(41) gene confers inducible macrolide resistance in M. abscessus subspecies abscessus, but NOT in subspecies massiliense, which carries a non-functional erm41 — so subspecies identification is essential for predicting macrolide susceptibility. Treatment is amikacin plus imipenem or cefoxitin plus a macrolide (if susceptible, based on erm41 or subspecies), prolonged for 6 months or more, with surgical excision often needed for localised disease. M. abscessus is also a significant pulmonary pathogen in cystic fibrosis and chronic lung disease.[7][8]

Mycobacterium fortuitum, chelonae, and the others

M. fortuitum group is a rapid grower after trauma, catheters, surgery, or tattoos — similar to abscessus but with smaller abscesses and better antibiotic susceptibility, treated with amikacin plus a fluoroquinolone (ciprofloxacin, moxifloxacin) or doxycycline or TMP-SMX for 4 to 6 months. M. chelonae disseminates as multiple violaceous nodules or abscesses on the extremities, especially in the corticosteroid-immunosuppressed, treated with tobramycin plus clarithromycin plus imipenem or linezolid, with surgery for localised disease.[6]

M. haemophilum needs iron-supplemented media (hemin or ferric ammonium citrate) and 30 degrees C incubation — easily missed if not specifically requested — and hits the immunosuppressed (transplant, HIV, anti-TNF) with tender violaceous papules, nodules, or ulcers on the extremities. MAC is primarily pulmonary; cutaneous disease is rare and occurs in advanced HIV with CD4 less than 50 or in transplant recipients, with azithromycin prophylaxis when CD4 is under 50.[1][2]

Histopathology and diagnosis — culture at two temperatures, then speciate

Histology shows mixed suppurative and granulomatous inflammation — neutrophils plus epithelioid granulomas with or without necrosis. AFB stain (Ziehl-Neelsen or Fite) may reveal organisms, but sensitivity is variable because organisms can be scant; M. ulcerans lesions show extensive necrosis with clumps of extracellular AFB in the fat. Histology alone cannot speciate NTM — culture and molecular methods do that.[2][10]

Tissue biopsy (deep, including subcutaneous fat) drives everything: AFB stain, culture at multiple temperatures (30 degrees C and 37 degrees C) on Lowenstein-Jensen or Middlebrook agar with an explicit NTM request, histopathology, and molecular speciation by PCR, line probe assay, or 16S rRNA gene sequencing (the gold standard). M. marinum must be cultured at 30 to 32 degrees C; M. haemophilum needs iron-supplemented media at 30 degrees C; M. abscessus subspecies identification runs through erm(41) gene sequencing. Antibiotic susceptibility testing is essential for all NTM — resistance patterns differ markedly from M. tuberculosis and between NTM species.[1][3][8]

Differential — the granulomatous and ulcerative mimics

The sporotrichoid pattern of M. marinum is indistinguishable clinically from sporotrichosis (Sporothrix schenckii, often cat-related thorn injury) — only culture and histology separate them. Cutaneous tuberculosis (lupus vulgaris, scrofuloderma, warty TB) shows caseating granulomas with a positive Mantoux or IGRA and a response to RHZE. Leishmaniasis shows amastigotes on Giemsa with a travel history. Staphylococcal or streptococcal abscesses are acute with a positive Gram stain.[10]

Foreign-body granuloma (suture, silica, tattoo ink — polarised light), sarcoidosis (non-caseating naked granulomas with systemic features), and pyoderma gangrenosum (undermined ulcer, rapidly progressive, associated systemic disease) complete the list. The Buruli ulcer's painless undermined edge must be separated from pyoderma gangrenosum, tropical ulcer, cutaneous leishmaniasis, and squamous cell carcinoma.[2]

Management — species-specific, susceptibility-guided, never monotherapy

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The five principles: identify the species, guide by susceptibility, combine at least two active drugs, treat for months, and add surgery for deep disease. The recurring trainee error is monotherapy or standard anti-TB therapy — both fail.[1][7]

• Species-specific — always identify the organism by culture, PCR, or 16S sequencing.[1]
• Susceptibility-guided — base choices on in vitro susceptibility; NTM resistance differs from TB and between species.[7]
• Combination therapy — at least two active drugs to prevent resistance; monotherapy is never acceptable.[7]
• Duration — typically 3 to 6 months or more, depending on species, severity, and immune status.
• Surgery — excision or debridement for localised, deep, or treatment-resistant lesions (tenosynovitis, osteomyelitis).
• NTM does NOT respond to standard anti-TB therapy (RHZE) — a common exam and clinical pitfall.[1][7]

Species-specific regimens

Prognosis and the species that fights back

Most cutaneous NTM infections respond well to appropriate therapy when the species is correctly identified and treatment is susceptibility-guided. M. abscessus is the hardest to treat — intrinsic resistance, prolonged therapy, surgical excision often required — and M. ulcerans causes significant morbidity (contracture, disability) when treatment is delayed. Disseminated disease in the immunosuppressed carries the worst prognosis.[2][8]

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The mnemonic expands to FISH-MAC across the species that matter: Fortuitum (rapid, post-trauma, more susceptible), Immunocompromised specialists (M. abscessus and M. chelonae; abscessus carries erm41, massiliense does not), Slow growers (M. marinum at 30 to 32 degrees C, MAC, M. kansasii), Healed by scarring (M. ulcerans via mycolactone; WHO 8 weeks of rifampicin plus clarithromycin), M. marinum (fish-tank granuloma, the commonest cutaneous NTM; clarithromycin plus ethambutol for 3 to 4 months), Atypical means non-tuberculous and environmental (RHZE fails; treat by species and susceptibility), and Clarithromycin is the macrolide backbone (except in erm41-positive M. abscessus).[1]

Special populations — the immunosuppressed carry the heaviest burden

Tumour necrosis factor-alpha inhibitors, rituximab, methotrexate, systemic corticosteroids, JAK inhibitors, and calcineurin inhibitors all raise NTM susceptibility. Patients starting biologics should be screened for latent TB and NTM risk factors; active NTM is a relative contraindication to starting anti-TNF therapy. In solid-organ and stem-cell transplant recipients, cutaneous M. chelonae or M. abscesses appear at catheter sites or surgical wounds.[2]

Disseminated MAC is an AIDS-defining illness in HIV with CD4 under 50 cells per microlitre; prophylaxis with azithromycin 1200 mg weekly is indicated when CD4 is below 50 and no active MAC is present. Children may develop cervical lymphadenitis from MAC, often treated with surgical excision plus clarithromycin-based therapy. In pregnancy, avoid aminoglycosides and fluoroquinolones; rifampicin and ethambutol are generally acceptable.[1]

Pitfalls — RHZE, monotherapy, and the wrong temperature

The classic trap: treating NTM with standard anti-TB RHZE. Most NTM are resistant; treatment failure is inevitable. Always identify the species and guide therapy by susceptibility testing.[1]

Monotherapy causes resistance. NTM treatment always combines at least two active drugs; clarithromycin alone for M. marinum or a single agent for M. abscessus selects resistance within weeks. The wrong temperature — culturing M. marinum or M. haemophilum only at 37 degrees C returns a false negative; always request culture at 30 degrees C and 37 degrees C with iron-supplemented media when NTM is suspected.[1][3]

Missing the post-procedure cluster — multiple nodules or abscesses weeks to months after a cosmetic procedure, tattoo, or injection should trigger NTM suspicion and an AFB-requested biopsy, not another course of flucloxacillin. And in the immunosuppressed, new cutaneous nodules mean disseminated disease until proven otherwise — assess for systemic involvement.[6]

Evidence and where regions differ

The dermatology NTM literature is review-based rather than trial-based: the Gonzalez-Santiago and Drage overview, the Franco-Paredes cutaneous mycobacteria review,[1][2] species-focused work on M. marinum (Aubry; Medel-Plaza; Tsiolakkis),[3][4][5] the rapidly growing mycobacteria reviews (Sepulcri; Cristancho-Rojas),[6][8] the Lange consensus on pulmonary NTM,[7] and the cutaneous granulomatosis overview (Terziroli Beretta-Piccoli).[10] The WHO 8-week rifampicin plus clarithromycin regimen for Buruli ulcer displaced the older streptomycin-based course on toxicity grounds; M. abscessus subspecies-level management via erm41 sequencing is the major recent advance.[8]

Regional differences track endemic exposure. Buruli ulcer clusters in West Africa and coastal Australia; M. marinum follows aquarium and marine exposure everywhere; post-procedure M. abscessus outbreaks track mesotherapy and cosmetic surgery practice patterns. Access to 16S sequencing, erm41 testing, and reference-lab susceptibility varies — in low-resource settings the clinical picture plus AFB and culture at two temperatures remain the backbone.[1]

Exam pearls

[1]

Ward-round test

An aquarium worker has sporotrichoid nodules up his forearm and standard antibiotics have failed. Organism, temperature, and regimen?

Mycobacterium marinum — the commonest cutaneous NTM. Culture the biopsy at 30 to 32 degrees C (cultures at 37 degrees C may be falsely negative). Treat with clarithromycin plus ethambutol for 3 to 4 months; add surgical debridement for deep tenosynovitis or osteomyelitis. The clue is aquatic exposure plus a sporotrichoid pattern that ignores routine antibiotics.

[3] [4]

A returned traveller from West Africa has a large painless ulcer with undermined edges. Organism, toxin, and WHO regimen?

Mycobacterium ulcerans (Buruli ulcer). Its mycolactone toxin causes tissue necrosis and local immune suppression, which is exactly why the ulcer is painless despite its size. WHO first-line is rifampicin plus clarithromycin for 8 weeks, with surgical debridement and skin grafting for advanced or fibrotic lesions.

[2]

Three weeks after mesotherapy, a patient develops multiple erythematous abscesses along the injection tracks. What organism, and why is the macrolide answer uncertain?

Mycobacterium abscessus complex — a rapid grower, post-procedure. The erm(41) gene confers inducible macrolide resistance in subspecies abscessus but NOT in subspecies massiliense, so subspecies identification is essential before relying on a macrolide. Treatment is susceptibility-guided combination therapy (amikacin plus imipenem or cefoxitin plus macrolide if susceptible) for 6 months or more, often with surgical excision.

[6] [8]

A registrar plans to treat a culture-confirmed M. marinum infection with standard anti-TB RHZE. Why is this wrong?

Because NTM do not respond to standard anti-TB therapy — most are resistant, and treatment failure is inevitable. NTM treatment is species-specific, susceptibility-guided, and always combines at least two active drugs. For M. marinum that means clarithromycin plus ethambutol for 3 to 4 months, not RHZE.

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References

1. [1]Gonzalez-Santiago TM, Drage LA. Nontuberculous Mycobacteria: Skin and Soft Tissue Infections Dermatol Clin, 2015.PMID 26143432
2. [2]Franco-Paredes C, Marcos LA, Henao-Martínez AF, et al. Cutaneous Mycobacterial Infections Clin Microbiol Rev, 2018.PMID 30429139
3. [3]Aubry A, Mougari F, Reibel F, et al. Mycobacterium marinum Microbiol Spectr, 2017.PMID 28387180
4. [4]Medel-Plaza M, Esteban J. Current treatment options for Mycobacterium marinum cutaneous infections Expert Opin Pharmacother, 2023.PMID 37145964
5. [5]Tsiolakkis G, Liontos A, Filippas-Ntekouan S, et al. Mycobacterium marinum: A Case-Based Narrative Review of Diagnosis and Management Microorganisms, 2023.PMID 37512971
6. [6]Sepulcri C, Vena A, Bassetti M. Skin and soft tissue infections due to rapidly growing mycobacteria Curr Opin Infect Dis, 2023.PMID 36718980
7. [7]Lange C, Böttger EC, Cambau E, et al. Consensus management recommendations for less common non-tuberculous mycobacterial pulmonary diseases Lancet Infect Dis, 2022.PMID 35090639
8. [8]Cristancho-Rojas C, Varley CD, Lara SC, et al. Epidemiology of Mycobacterium abscessus Clin Microbiol Infect, 2024.PMID 37778416
9. [9]Emmerich K, Fabri M. [Nontuberculous mycobacteria] Hautarzt, 2017.PMID 28331949
10. [10]Terziroli Beretta-Piccoli B, Mainetti C, Peeters MA, et al. Cutaneous Granulomatosis: a Comprehensive Review Clin Rev Allergy Immunol, 2018.PMID 29352388