Morphology and cell wall
Tuberculosis pathogens belong to the family Mycobacteriaceae. These are Gram-positive, non-motile, non-spore-forming rods prone to pronounced polymorphism (e.g., M. tuberculosis are long and thin, whereas M. bovis are short and thick).
Their most important feature is a complex cell wall rich in lipids (up to 40% of dry weight), which includes mycolic acid, peptidoglycan, and lipoarabinomannan. Due to this "waxy armor," the bacteria do not take up standard aniline dyes well. Ziehl–Neelsen staining is used to detect them, confirming the acid-fastness of the microbes. Under unfavorable conditions, the pathogen can transition into L-forms, ensuring prolonged persistence.
Cultural and biochemical properties
Mycobacteria are extremely fastidious and grow very slowly. Enrichment media (Lowenstein–Jensen, Finn-2) supplemented with growth stimulants (lecithin, vitamins) and substances that neutralize their own toxic metabolites (albumin, charcoal) are used for cultivation.
- M. tuberculosis is an obligate aerobe. On liquid media, it forms a wrinkled pellicle after 5–7 days, and on solid media, a dry, cauliflower-like R-form growth.
- M. bovis is a microaerophile; it grows even slower and forms smooth S-form colonies.
Biochemically, the pathogens exhibit high catalase activity. The human species synthesizes a large amount of niacin (positive Conno's test) and also reduces nitrates to nitrites, which distinguishes it from the bovine species.
Pathogenicity factors and immunity
Mycobacteria do not produce classic exotoxins. Their pathogenicity is due to their ability to survive and multiply inside pulmonary macrophages.
Mechanisms of defense against phagocytosis include the synthesis of sulfolipids and cord factor (trehalose dimycolate), which together block phagosome-lysosome fusion. In addition, bacteria produce ammonia to alkalize the medium and neutralize oxygen radicals. Tissue destruction in tuberculosis is largely related not to the bacteria themselves, but to the immune system's response—delayed-type hypersensitivity (type IV), which leads to granuloma formation and caseous necrosis.
Epidemiology and stability
The main source of infection is a human patient with the pulmonary form, and the main route of transmission is aerogenic (airborne and dust-borne).
Due to the lipid cell wall, the pathogen is extremely stable in the external environment, resistant to drying and many disinfectants (requires activated solutions with an exposure of several hours). However, bacteria are rapidly killed by UV radiation (in 2–3 minutes) and boiling (in 5 minutes). The disease develops in only 10% of infected individuals; in the rest, the process is limited to the formation of an asymptomatic Ghon focus.
Laboratory diagnostics
A set of methods is used to detect the pathogen:
- Bacterioscopy: Ziehl–Neelsen staining of sputum smears or fluorescence microscopy (allows the detection of mycobacteria even in small quantities, sometimes using flotation).
- Bacteriological method: The "gold standard," possessing the highest sensitivity. Due to slow growth, classic culture takes 3–6 weeks, which is why automated systems (Bactec) are now actively used to speed up results.
- Molecular-genetic methods (PCR): Provide rapid results and are used alongside microscopy for rapid diagnostics.
- Tuberculin skin testing: The Mantoux test and Diaskintest are used to detect infection (specific sensitization), especially in children.
Briefly: answers to topical questions
Which mycobacterial species are part of the Mycobacterium tuberculosis complex (MTC)?
The Mycobacterium tuberculosis complex includes the following genetically related mycobacterial species:
- Human species (M. tuberculosis) — causes the overwhelming majority of disease cases.
- Bovine species (M. bovis) — also includes the vaccine strain M. bovis BCG.
- Intermediate species (M. africanum).
- Rare species (M. microti, M. canettii, M. caprae, M. pinnipedii).
Which cells form a specific tuberculous granuloma?
A specific tuberculous granuloma is formed by several types of cells arranged in layers around a central zone of caseous necrosis. Its cellular composition includes:
- Epithelioid cells — transformed macrophages forming an inner palisade-like wall.
- Giant multinucleated cells (Langhans giant cells) — formed by the fusion of epithelioid cells.
- Macrophages and sensitized T lymphocytes — form the outer cellular wall.
- Fibroblasts — constitute the outer layer (boundary) of the granuloma.
Which specific recombinant protein antigens are used in the Diaskintest?
The Diaskintest utilizes specific ESAT6 and CFP10 protein antigens. These secretory proteins are encoded by the RD1 region of the M. tuberculosis genome and induce a strong immune response, which is important for diagnosing tuberculous infection.
Which drugs belong to first-line anti-tuberculosis antibiotics?
First-line (core) anti-tuberculosis drugs include the following antibiotics and chemotherapeutic agents:
- Isoniazid;
- Rifampicin;
- Pyrazinamide;
- Ethambutol;
- Streptomycin.
These drugs form the basis of standard tuberculosis treatment regimens and are prescribed in combinations during the intensive and continuation phases of therapy.
To remember the components of the primary tuberculosis complex (Ghon focus), use the rule: Focus (pneumonic focus) — Lymphangitis (vessel inflammation) — Lymphadenitis (regional node involvement).
Class and exam questions
Why do tuberculosis mycobacteria not stain with Gram stain?
Due to the high content of waxes and lipids in the cell wall, they poorly take up standard water-soluble stains. The Ziehl-Neelsen staining method involving heating is used to detect them.
What is a cord factor?
This is a virulence factor (trehalose dimycolate) that clumps mycobacteria into cords or "ropes." It blocks phagocytosis and exerts a toxic effect on host cells.
Why are serum or activated charcoal added to nutrient media for mycobacteria?
During metabolism, bacteria release toxic fatty acids that can halt their own growth. Additives act as sorbents, neutralizing these substances.
What else to study on this topic
- Mechanisms of intracellular survival and phagolysosome blockade
- Secretory proteins ESAT6 and CFP10: functions and role in the Diaskintest
- Differential diagnostics of species within the M. tuberculosis complex
- Pathogenesis of tuberculous granuloma formation
- Methods of sample concentration in bacterioscopy (flotation)
- Principles of Mantoux test interpretation and tuberculin skin test conversion
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