Non-Tuberculous Mycobacteria
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Direct answer
Non-tuberculous mycobacteria are environmental mycobacteria other than the Mycobacterium tuberculosis complex and Mycobacterium leprae. Runyon classified them into four groups by pigment production and growth rate, which guides species identification. Mycobacterium avium complex is a frequent cause of chronic lung disease, and rapidly growing species are often resistant to standard tuberculosis drugs.
What you must remember
- Runyon I: photochromogens that form pigment on exposure to light, with Mycobacterium kansasii and M. marinum as examples.
- Runyon II: scotochromogens that form pigment even in the dark, such as M. gordonae.
- Runyon III: nonphotochromogens with little or no pigment, of which Mycobacterium avium complex is the main pathogen.
- Runyon IV: rapid growers that form visible colonies within seven days, including M. abscessus, M. fortuitum and M. chelonae.
- MAC lung disease: typically affects older women with nodular bronchiectasis, while disseminated disease occurs in advanced HIV with very low CD4 counts.
- M. marinum: causes skin granulomas after exposure to fish tanks or swimming pools, often with sporotrichoid lymphangitis.
- Significance: these organisms are environmental, so a single positive culture from a non-sterile source may be a contaminant, and clinical criteria with repeated cultures are needed.
The Runyon classification in practice
A sputum culture from an older woman with bronchiectasis grows a slow-growing, non-pigmented mycobacterium that is not M. tuberculosis. The table sorts such isolates by pigment behaviour and growth rate, which points the identification in the right direction.
| Runyon group | Pigment behaviour | Growth rate | Examples and clinical notes |
|---|---|---|---|
| I | Photochromogen, pigments on light exposure | Slow, several weeks | M. kansasii causes pulmonary disease; M. marinum causes skin granulomas |
| II | Scotochromogen, pigments in the dark | Slow, several weeks | M. gordonae and M. scrofulaceum, often low-virulence or contaminant isolates |
| III | Nonphotochromogen, little or no pigment | Slow, several weeks | M. avium complex is the main group III pathogen causing chronic lung disease |
| IV | Variable pigment | Rapid, within seven days | M. abscessus, M. fortuitum and M. chelonae cause skin, soft tissue and lung disease |
Rapid growers are often resistant to standard antituberculous drugs and need specialist susceptibility testing. Species-level identification, usually by molecular methods, then decides whether an isolate is significant and how it should be treated.
Where the exam tries to catch you
Non-tuberculous mycobacteria questions test classification, clinical associations and the fact that these organisms are not treated like tuberculosis by default.
- Trap: treating every positive NTM culture as infection Correct: NTM are environmental organisms that often contaminate samples, so clinical criteria and repeated cultures are needed.
- Trap: starting the standard four-drug tuberculosis regimen for M. marinum skin disease Correct: cutaneous disease is commonly treated with clarithromycin or rifampicin with ethambutol, guided by exposure history and susceptibility.
- Trap: expecting M. abscessus to respond to standard oral tuberculosis drugs Correct: it is intrinsically resistant to many drugs and usually needs prolonged multidrug therapy with specialist input.
Frequently asked questions
What is the Runyon classification of non-tuberculous mycobacteria?
Runyon classified non-tuberculous mycobacteria into four groups: photochromogens, scotochromogens, nonphotochromogens and rapid growers. The grouping uses pigment production and growth rate, and it helps guide species identification in the laboratory.
Which non-tuberculous mycobacterium causes skin granulomas after fish tank exposure?
Mycobacterium marinum causes skin granulomas after exposure to fish tanks or swimming pools. It is a photochromogenic Runyon group I organism, and it often produces sporotrichoid lymphangitis, a chain of nodules along the lymphatics.
Which patients get Mycobacterium avium complex disease?
Mycobacterium avium complex mainly causes chronic lung disease in older women with nodular bronchiectasis. It also causes disseminated disease in people with advanced HIV, typically when CD4 counts are very low.
Why are rapidly growing mycobacteria hard to treat?
Rapidly growing mycobacteria such as Mycobacterium abscessus are often resistant to many antibiotics, including several first-line tuberculosis drugs. Treatment therefore usually needs prolonged multidrug and sometimes parenteral therapy guided by susceptibility testing.
How do clinicians decide that an isolate of non-tuberculous mycobacteria is significant?
Clinicians decide significance from clinical features, imaging and repeated positive cultures, because non-tuberculous mycobacteria are often environmental contaminants. A single positive culture from a non-sterile source does not by itself confirm disease.