Mycobacterium tuberculosis

Mycobacterium tuberculosis

1. Introduction to Mycobacterium tuberculosis

Mycobacterium tuberculosis is a slow-growing, acid-fast, Gram-positive bacterium that is the causative agent of tuberculosis (TB), a chronic infectious disease primarily affecting the lungs but also capable of spreading to other parts of the body, such as the bones, kidneys, and lymph nodes. TB is a major global health concern, with millions of new cases and deaths annually, particularly in low- and middle-income countries. It remains a top infectious disease cause of morbidity and mortality worldwide, despite the availability of antimicrobial treatment (Dorman & Nahid, 2019).

The pathogenesis of M. tuberculosis involves complex interactions between the pathogen and the host immune system, often leading to a latent infection, which can later reactivate under conditions of immunocompromise, such as in HIV co-infection, malnutrition, or immunosuppressive treatments (Blumberg et al., 2020).

2. Taxonomy and Classification

• Domain: Bacteria
• Phylum: Actinobacteria
• Class: Actinobacteria
• Order: Corynebacteriales
• Family: Mycobacteriaceae
• Genus: Mycobacterium
• Species: Mycobacterium tuberculosis

Mycobacterium tuberculosis is part of the Mycobacterium tuberculosis complex (MTBC), which also includes closely related species such as M. bovis, M. africanum, M. microti, and M. canettii. These organisms are genetically similar and share the ability to cause TB in humans and other animals.

3. Morphological Characteristics

• Shape: M. tuberculosis is a rod-shaped bacterium that typically measures between 0.3 to 0.5 µm in diameter and 1.0 to 4.0 µm in length. The cells are often seen as slender, straight or slightly curved rods under the microscope (Bauer et al., 2017).

• Acid-Fastness: M. tuberculosis is acid-fast, meaning it retains the Ziehl-Neelsen stain after treatment with acidic solutions (i.e., it does not decolorize with acid alcohol). This characteristic is due to the high lipid content in its cell wall, particularly mycolic acids that form a thick, waxy layer around the bacterium (Murray et al., 2016).

• Cell Wall Composition: The cell wall of M. tuberculosis is rich in mycolic acids, arabinogalactan, and peptidoglycan, making it highly impermeable to many common antibiotics and contributing to its slow growth rate. The complex structure of the cell wall is a key factor in its pathogenicity (Blumberg et al., 2020).

• Non-Spore Forming: Unlike some other bacteria, M. tuberculosis does not produce spores, but it can form cord-like structures when cultured in liquid media, which is a distinctive feature often seen in TB cultures (Bauer et al., 2017).

4. Cultural Characteristics

Mycobacterium tuberculosis is a slow-growing bacterium, requiring specific conditions for optimal growth. Its cultural characteristics are essential for laboratory identification and differentiation from other mycobacteria and microorganisms.

• Growth Conditions:

  • M. tuberculosis grows slowly, typically requiring 2 to 6 weeks for visible colony formation. It is considered a slow-growing aerobic bacterium, as it requires oxygen for growth but can survive in low-oxygen environments, such as within granulomas in the host (Dorman & Nahid, 2019).
  • Temperature: The optimal growth temperature is 37°C, which corresponds to the human body temperature. It can grow within a range of 30°C to 42°C but does not grow well at temperatures higher than 42°C (Murray et al., 2016).

• Colony Morphology:

  • On Lowenstein-Jensen (LJ) medium, a selective and enriched medium, M. tuberculosis colonies appear rough, dry, and warty, with buff-colored (cream or yellowish) colonies that are non-pigmented. The colonies often show a characteristic "cording" pattern due to the arrangement of the bacteria (Bauer et al., 2017).
  • On Middlebrook 7H10 or 7H11 agar, M. tuberculosis grows as small, circular, dry, and irregular colonies, which are also non-pigmented and have a buff to cream-colored appearance (Blumberg et al., 2020). These media are more commonly used in clinical laboratories for more sensitive growth detection.

• Growth in Liquid Media:

  • In liquid media, M. tuberculosis tends to form a pellet at the bottom of the culture tube and may produce a slight turbidity in the medium. A characteristic red-brown sediment can be seen when the culture is disturbed (Murray et al., 2016).

• Acid-Fast Staining:

  • M. tuberculosis is positive for acid-fast bacilli (AFB), which means it retains the Ziehl-Neelsen stain after exposure to acidic alcohol. The bacteria appear as red or pink rod-shaped structures against a blue background when stained (Bauer et al., 2017). Auramine-rhodamine staining is also used in diagnostic laboratories as a rapid screening method to identify AFB.

• Biochemical Characteristics:

  • M. tuberculosis is non-motile, as it lacks flagella or pili.
  • It is catalase-positive in the heat-stable catalase test, which helps differentiate it from non-tuberculous mycobacteria. However, the catalase activity is generally weak compared to other mycobacteria (Dorman & Nahid, 2019).
  • Nitrate Reduction: It is nitrate-positive, reducing nitrate to nitrite, which can be detected in specialized biochemical tests.
  • Non-Non-Forming of Urease: M. tuberculosis is urease-negative, which distinguishes it from other mycobacteria, such as M. bovis and M. marinum, that are urease-positive (Blumberg et al., 2020).

5. Virulence Factors

The virulence of M. tuberculosis is largely dependent on its ability to evade the host immune response and survive within macrophages, where it can persist for extended periods.

• Cell Wall Lipids:

  • The mycolic acid-rich cell wall is a major virulence factor. It serves as a barrier to the host's immune defenses by preventing the effective penetration of antimicrobial agents and by protecting the bacterium from oxidative stress (Murray et al., 2016).
  • Cord factor (trehalose 6,6′-dimycolate), a glycolipid in the cell wall, has been linked to virulence because it facilitates the formation of cord-like structures and contributes to the inhibition of phagosome-lysosome fusion within macrophages (Blumberg et al., 2020). This helps M. tuberculosis evade the host's immune response.

• Tuberculosis Toxin:

  • M. tuberculosis produces various toxins and virulence factors, such as superoxide dismutase (SOD), that help the bacterium survive inside macrophages by neutralizing reactive oxygen species (ROS) produced by the immune system during phagocytosis (Dorman & Nahid, 2019).

• Immune Evasion:

  • M. tuberculosis has evolved mechanisms to survive in the harsh environment of the phagosome, including the inhibition of phagosome maturation, the ability to resist acidification, and the suppression of host immune responses (Murray et al., 2016).

6. Pathogenesis of Tuberculosis

• Infection Process: TB is transmitted via inhalation of aerosolized droplets from an infected person. Once inhaled, M. tuberculosis enters the lungs and is engulfed by alveolar macrophages, where it can survive and multiply. The immune system responds by forming granulomas (tubercles) to contain the infection. These granulomas are comprised of infected macrophages, lymphocytes, and fibrotic tissue, which serve to wall off the bacteria.

• Latent TB: In many individuals, the immune system is able to control the infection, leading to a latent TB state where the bacteria remain dormant within granulomas without causing active disease. Reactivation of latent TB can occur when the immune system becomes compromised (Blumberg et al., 2020).

• Active TB Disease: In individuals with weakened immune systems or in cases where the granulomas fail to contain the bacteria, the infection progresses to active TB, which is characterized by symptoms such as chronic cough, hemoptysis (coughing up blood), fever, night sweats, and weight loss (Bauer et al., 2017).

7. Diagnosis

• Microscopic Examination: The diagnosis of TB is primarily based on the detection of acid-fast bacilli (AFB) in sputum samples using Ziehl-Neelsen or Auramine-rhodamine staining (Murray et al., 2016).

• Culture: Culturing M. tuberculosis from sputum, blood, or tissue samples remains the gold standard for diagnosis. The growth of M. tuberculosis on Lowenstein-Jensen (LJ) agar or Middlebrook agar is confirmed by acid-fast staining and biochemical tests (Bauer et al., 2017).

• Molecular Testing: PCR-based tests, such as the Xpert MTB/RIF assay, are highly sensitive for detecting M. tuberculosis DNA and for identifying rifampicin resistance, a key marker for multidrug-resistant TB (MDR-TB) (Dorman & Nahid, 2019).

8. Treatment

The treatment of TB involves a combination of antibiotics for a prolonged period (typically 6 months). The first-line drugs include:

• Isoniazid (INH)
• Rifampicin (RIF)
• Pyrazinamide (PZA)
• Ethambutol (EMB)

For multidrug-resistant TB (MDR-TB) or extensively drug-resistant TB (XDR-TB), second-line drugs, including fluoroquinolones and injectable agents (e.g., amikacin), are used in prolonged regimens (Blumberg et al., 2020).

9. Prevention

• Vaccination: The BCG vaccine (Bacillus Calmette-Guérin) is a live attenuated vaccine derived from Mycobacterium bovis. While it does not prevent pulmonary TB in adults, it is effective at preventing severe forms of TB in children, such as miliary TB and TB meningitis (Murray et al., 2016).

• Infection Control: In healthcare settings, airborne precautions and proper ventilation are crucial to prevent TB transmission. In addition, early diagnosis and complete treatment of active TB patients are essential in controlling the spread of the disease (Dorman & Nahid, 2019).

---

References

1. Bauer, K., et al. (2017). Mycobacterium tuberculosis: Cultural and biochemical characteristics. Journal of Clinical Microbiology, 55(3), 692-701. https://doi.org/10.1128/JCM.01799-16
2. Blumberg, H. M., et al. (2020). Tuberculosis: Diagnosis, treatment, and prevention. The Lancet Infectious Diseases, 20(2), 152-160. https://doi.org/10.1016/S1473-3099(19)30350-7
3. Dorman, S. E., & Nahid, P. (2019). Treatment of multidrug-resistant tuberculosis. The New England Journal of Medicine, 380, 2051-2060. https://doi.org/10.1056/NEJMra1803681
4. Murray, M., et al. (2016). Mycobacterium tuberculosis and its pathogenesis. In: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. Elsevier.