Nontuberculous Mycobacteria Infections
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Direct answer
Runyon's 1959 classification still opens nontuberculous mycobacteria — environmental mycobacteria other than the M. tuberculosis complex and M. leprae — sorted by pigment and growth speed. Group I photochromogens (M. kansasii, M. marinum) make pigment only on light exposure; group II scotochromogens (M. scrofulaceum, M. gordonae) pigment in the dark; group III non-chromogens include M. avium complex, the disseminated pathogen of advanced HIV; group IV rapid growers (M. fortuitum, M. chelonae, M. abscessus) produce colonies within seven days and cause post-injection and post-surgical abscesses. The bench signature: an AFB-positive smear with CBNAAT reporting "MTB not detected" means NTM until proven otherwise — and species identification by MALDI-TOF, line-probe assay or sequencing changes therapy, because NTM disease does not respond to standard antitubercular therapy.
What you must remember
- Runyon groups with species: I photochromogens — kansasii (chronic lung disease, HIV), marinum (swimming pool or fish tank granuloma, grows best at 30 degrees); II scotochromogens — scrofulaceum (historically cervical adenitis), gordonae (the tap water contaminant, almost never a pathogen); III non-chromogens — avium complex (M. avium and M. intracellulare), xenopi, simiae; IV rapid growers — fortuitum, chelonae, abscessus.
- Sources and biofilms: tap water, plumbing and soil; NTM form biofilms on hospital water lines, bronchoscopes and multidose vials — the origin of pseudo-outbreaks and genuine post-procedure infections.
- Clinical syndromes: chronic fibrocavitary lung disease in COPD and bronchiectasis; nodular bronchiectatic disease (the "Lady Windermere" pattern of middle-aged slender women); cervical lymphadenitis in children (MAC now leading); skin and soft tissue disease after trauma, injections and surgery; disseminated MAC bacteraemia when CD4 counts fall below about 50.
- The laboratory signature: AFB smear positive with CBNAAT or Truenat "MTB not detected"; growth on Lowenstein-Jensen medium in days (rapid grower) or weeks; identification by MALDI-TOF, line-probe assay, HPLC of mycolic acids or sequencing.
- Colonisation trap: a single NTM isolate from sputum may be colonisation or contamination; accepted criteria require compatible clinical features, radiology and repeated positive cultures with exclusion of tuberculosis before committing to months of therapy.
- Therapy principle: species-specific regimens, not standard ATT — MAC: clarithromycin or azithromycin plus ethambutol with rifabutin; kansasii: rifampicin-based regimens respond well; abscessus: amikacin, tigecycline, imipenem with macrolides, noting that an erm gene confers inducible macrolide resistance.
- Indian context to quote: post-injection and post-surgical abscess clusters from rapid growers, reported repeatedly in India; M. simiae in Indian tap water series; and misdirected ATT for AFB-positive, CBNAAT-negative specimens — the costliest practical error.
AFB-positive sputum, CBNAAT negative — what next?
Run the decision the way a referral laboratory should. A bronchiectasis patient's sputum is AFB smear positive, but CBNAAT reports MTB not detected and rifampicin resistance not applicable. First, exclude old treated tuberculosis and sample contamination. Repeat sputum mycobacterial cultures — two further positive cultures with the same NTM species, alongside compatible symptoms and radiology, meet disease criteria; one isolate alone does not. Second, identify the species: MALDI-TOF from solid culture or a line-probe assay returns the answer in hours to days; rapid growth within a week already assigns Runyon IV and points toward fortuitum or abscessus. Third, tailor therapy and expect duration: abscessus lung disease may need months of intravenous agents and even resection; MAC needs a macrolide-ethambutol backbone for a year beyond culture conversion. Transfer the algorithm to a cluster of post-injection cold abscesses: rapid growers from skin and environmental samples, genomically linked, confirm a common source — and the fix (single-use vials, sterile water) is operational.
Overlay the NTEP reality: "AFB positive, MTB negative" is the programme's own signal to seek NTM identification at intermediate reference laboratories.
Where the exam sets traps
Group names get swapped — photochromogen needs light, scotochromogen pigments in the dark, and examiners exploit exactly this inversion in matching questions. Second, M. gordonae: the candidate who calls a gordonae isolate from bronchoscopy material "disease" fails the pseudo-outbreak question — it is the tap water bacillus, and its recovery from instrument-rinsed specimens signals bronchoscope contamination, not patient infection. Third, prescribing ATT for any AFB-positive report fails: NTM disease under rifampicin-isoniazid regimens does not respond, breeds resistance and loses months. Fourth, disseminated MAC belongs to advanced immunosuppression (CD4 below roughly 50) — azithromycin prophylaxis was part of advanced-HIV care before widespread antiretroviral therapy, a NACO-era point that earns viva credit. Finally, remember M. marinum's temperature preference (30 degrees, not 37) — the reason skin and cooler tissues, not lungs, host it.
Frequently asked questions
What does an AFB-positive smear with CBNAAT MTB-not-detected indicate?
Nontuberculous mycobacteria until proven otherwise, since CBNAAT detects M. tuberculosis complex specifically; the next step is repeat cultures and species identification by MALDI-TOF or line-probe assay.
How do the four Runyon groups differ?
By pigment and speed: photochromogens pigment on light exposure, scotochromogens in the dark, non-chromogens have no pigment, and rapid growers produce colonies within seven days regardless of pigment.
Which NTM causes disseminated disease in advanced HIV?
Mycobacterium avium complex, typically when CD4 counts fall below about 50 cells per microlitre, producing fever, weight loss, anaemia and positive blood cultures.
Why do rapid growers cause post-injection abscesses?
M. abscessus, M. chelonae and M. fortuitum survive in water and biofilms, contaminating multidose vials, diluents and instruments; inoculated subcutaneously, they form chronic draining abscesses weeks later.
Is a single NTM isolate from sputum enough to start treatment?
No — disease requires compatible clinical features, radiographic abnormalities and repeated positive cultures of the same species, because colonisation and environmental contamination are common.