Biological properties, epidemiology, and pathogenesis
Non-tuberculous mycobacteria belong to the genus Mycobacterium. In terms of their basic biological characteristics, they closely resemble classic tuberculosis pathogens, yet they have several crucial clinical and epidemiological differences. First, they possess intrinsic multidrug resistance. Second, the infection is virtually incapable of being transmitted from an infected person to a healthy one.
In their natural environment, these bacteria are typical saprophytes. Their reservoir is broad: they freely inhabit soil, water, and household dust. Moreover, certain species (e.g., M. smegmatis) can be part of the normal human microflora.
Infection occurs via three main routes:
- Airborne: through the inhalation of infected dust or airborne droplets.
- Contact: the pathogen enters through wounds and micro-lesions of the skin.
- Alimentary: through swallowing bacteria along with contaminated food or water.
Upon entering the body, NTM cause mycobacterioses — diseases whose clinical presentation closely resembles tuberculosis. Specific granulomas form in the affected tissues. The primary risk factor triggering the development of pathology is the presence of immunodeficient states, including those induced by immunosuppressant therapy.
Runyon classification
In practical microbiology, the Runyon classification is used to systematize NTM. It divides all atypical mycobacteria into four major groups. The basic criteria for this categorization are the colony growth rate on artificial nutrient media, the ability to produce pigment depending on lighting, and morphological and biochemical properties.
| Group | Pigmentation features | Growth rate | Typical representatives |
|---|---|---|---|
| 1. Photochromogens | Produce a yellow-orange (carotenoid) pigment exclusively in the light. | Slow-growing | M. kansasii, M. marinum |
| 2. Scotochromogens | Synthesize yellow pigment in the dark. Upon exposure to light, the color intensifies to orange-red. | Slow-growing | M. scrofulaceum, M. gordonae |
| 3. Non-chromogens | Pigment is completely absent; colonies remain colorless under any conditions. | Slow-growing | M. avium complex |
| 4. Rapid growers | May exhibit properties of both photo- and scotochromogens. | Rapid growth (from 1–2 up to 7 days) | M. fortuitum, M. smegmatis, M. chelonei |
Microbiological diagnostic algorithm
The diagnosis of mycobacterioses is built on algorithms similar to tuberculosis detection, but laboratories face specific differential challenges. The key and most informative method is bacteriological (culture) inoculation.
Work with the isolated pure culture is divided into two global stages:
- Differentiation: specialists must accurately determine whether they are dealing with the true tuberculosis pathogen or a non-tuberculous mycobacterium.
- Identification: if the presence of NTM is confirmed, it is necessary to determine the specific species, assess its virulence, and establish its Runyon group affiliation.
The recognition process has several levels. Basic anti-tuberculosis dispensary laboratories perform a primary evaluation: analyzing colony growth rate, morphology, pigment production, and temperature preferences. Final verification is carried out exclusively in specialized laboratories. There, biochemical tests are performed and animal inoculation (in laboratory animals—rats, mice, chicks) is conducted. Determining the antimicrobial susceptibility of the culture is a mandatory step.
Auxiliary methods include smear microscopy and serological reactions (antibody detection). Skin allergy tests are of little diagnostic value in mycobacterioses.
Principles of treatment and prevention
Treatment of mycobacterioses is a complex task, as NTM possess intrinsic multidrug resistance (MDR). Standard antituberculosis regimens often prove ineffective. As drugs of choice, physicians use medications to which the microorganisms retain susceptibility—for example, cycloserine, ethambutol, and rifampicin.
Specific prophylaxis for mycobacterioses has not been developed to date, and effective vaccines are lacking.
Briefly: answers to topical questions
What specific biochemical tests are used to differentiate non-tuberculous mycobacteria from M. tuberculosis?
The cited sources indicate that biochemical tests are used at the final identification stage of non-tuberculous mycobacteria, but the specific set of tests for differentiating non-tuberculous mycobacteria from M. tuberculosis is not listed. The niacin test characterizes the ability of M. tuberculosis to synthesize large amounts of niacin. Nitrate reductase and pyrazinimidase tests are provided in the source for differentiating M. tuberculosis and M. bovis, rather than non-tuberculous mycobacteria.
What groups of antibacterial drugs are used to treat mycobacterioses?
For the treatment of mycobacterioses, the sources list the following drugs and groups: macrolides — azithromycin, clarithromycin; aminoglycoside — amikacin; rifampicin, ethambutol, cycloserine, tigecycline, imipenem with cilastatin, cefoxitin, minocycline, moxifloxacin, and linezolid are also used. For MAC therapy, a combination of a macrolide, rifampicin, and ethambutol is indicated. The choice of drugs is made taking into account drug susceptibility test results. Therapy may be ineffective due to the intrinsic multidrug resistance of NTM.
What risk factors, apart from immunodeficiency, contribute to the development of mycobacterioses?
In addition to immunodeficient states, the development of mycobacterioses is associated with the intake of immunosuppressants.
To easily remember the Runyon groups, pay attention to the word roots: PHOTOchromogens require light to synthesize pigment, SKOTOchromogens (from Greek skotos — darkness) do not require light, NON-chromogens always remain pale, and the fourth group stands out exclusively for its high growth rate.
Class and exam questions
How do mycobacterioses fundamentally differ from tuberculosis in terms of epidemiology?
The main difference is the lack of contagiousness. Nontuberculous mycobacteria are acquired from the environmental source; transmission of the pathogen from a sick person to a healthy one does not occur.
Why do standard antituberculosis regimens fail in the treatment of mycobacterioses?
Atypical mycobacteria exhibit pronounced intrinsic resistance to many major antituberculosis drugs, requiring personalized selection of antibiotics (e.g., cycloserine or rifampicin).
Which diagnostic method for mycobacterioses is the primary one?
The main method is bacteriological (culture) inoculation. It allows not only the isolation of the pathogen, but also the determination of its species, Runyon group, and antibiotic susceptibility.
What else to study on this topic
- Features of specific granuloma formation in affected tissues.
- Mechanisms of multidrug resistance formation in non-tuberculous mycobacteria.
- The role of immunosuppressants in the development of opportunistic infections.
- Technique of performance and interpretation of animal inoculation in laboratory animals.
- Cultural and biochemical properties of Mycobacterium avium complex representatives.
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Non-tuberculous mycobacteria" and another 4,383 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in the section "Irregular Gram-positive rods (Diphtheria, Mycobacteria/tuberculosis and leprosy, Actinomycetes, Nocardia, Bifidobacteria, Gardnerella, etc.)"
Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.