Biological Properties, Epidemiology, and Pathogenesis
Nontuberculous mycobacteria belong to the genus Mycobacterium. In their basic biological characteristics, they closely resemble classic tuberculosis pathogens, yet they possess several crucial clinical and epidemiological differences. First, they exhibit intrinsic multidrug resistance. Second, infection is virtually incapable of spreading from a sick 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.
Transmission occurs via three main routes:
- Aerogenic: by inhaling infected dust or airborne droplets.
- Contact: the pathogen enters through wounds and minor skin lesions.
- Alimentary: by ingesting bacteria along with contaminated food or water.
Upon entering the body, NTM cause mycobacterioses—diseases whose clinical presentation closely mimics tuberculosis. Specific granulomas form in the affected tissues. The primary risk factor triggering the pathology is the presence of immunodeficiency states, including those induced by immunosuppressive therapy.
Runyon Classification
In practical microbiology, the Runyon classification is used to systematize NTM. It divides all atypical mycobacteria into four major groups based on colony growth rate on artificial media, pigment production relative to exposure to light, as well as morphological and biochemical properties.
| Group | Pigmentation Characteristics | Growth Rate | Typical Representatives |
|---|---|---|---|
| 1. Photochromogens | Form a yellow-orange (carotene) pigment exclusively upon light exposure. | Slow-growing | M. kansasii, M. marinum |
| 2. Scotochromogens | Synthesize yellow pigment in the dark. Exposure to light intensifies the color to orange-red. | Slow-growing | M. scrofulaceum, M. gordonae |
| 3. Nonchromogens | Pigment is entirely absent; colonies remain colorless under any conditions. | Slow-growing | M. avium complex |
| 4. Rapid growers | May possess properties of photo- or scotochromogens. | Rapid growth (from 1–2 to 7 days) | M. fortuitum, M. smegmatis, M. chelonei |
Microbiological Diagnostics Algorithm
The diagnosis of mycobacterioses follows algorithms similar to those for tuberculosis detection, though laboratories face specific differential challenges. The key and most informative method is bacteriological (culture) isolation.
Working with the isolated pure culture involves two major stages:
- Differentiation: specialists must precisely determine whether the isolate is a true tuberculosis pathogen or a nontuberculous mycobacterium.
- Identification: once NTM presence is confirmed, the specific species must be determined, its virulence assessed, and its Runyon group established.
Identification involves several levels. Basic tuberculosis dispensary laboratories perform initial evaluations: analyzing colony growth rate, morphology, pigment production, and temperature preferences. Final verification is carried out exclusively in specialized reference laboratories, where biochemical tests and bioassays (inoculation of laboratory animals such as rats, mice, or chicks) are performed. Drug susceptibility testing of the culture is mandatory.
Auxiliary methods include smear microscopy and serological tests (antibody detection). Skin-allergy tests hold little diagnostic value in mycobacterioses.
Principles of Treatment and Prevention
Treating mycobacterioses is a complex task because NTM exhibit intrinsic multidrug resistance (MDR). Standard antituberculous regimens are frequently ineffective. As drugs of choice, clinicians use agents to which the microorganisms retain susceptibility—for example, cycloserine, ethambutol, and rifampin.
Specific prophylaxis for mycobacterioses has not been developed to date, and effective vaccines are lacking.