67) Mycobacterium tuberculosis: Morphology and Identification, Constituents of Tubercle Bacilli, Pathogenesis, Pathology, Primary Infection Flashcards
(11 cards)
Mycobacterium tuberculosis: morphology and identification
Mycobacterium tuberculosis is a slow-growing, aerobic, non-motile, acid-fast bacillus (AFB) that stains red with the Ziehl-Neelsen stain. It has a characteristic waxy cell wall rich in mycolic acids, which makes it resistant to many chemicals and environmental conditions. It does not stain well with Gram stain due to the high lipid content in its cell wall. It is cultured on Lowenstein-Jensen or Middlebrook agar, with colonies taking weeks to form.
Mycobacterium tuberculosis: constituents of tubercle bacilli
The cell wall of Mycobacterium tuberculosis contains unique components, including mycolic acids, arabinogalactan, and peptidoglycan. These contribute to the organism’s acid-fast nature and its resistance to desiccation and antibiotics. The bacteria also contain other cell wall lipids, such as lipoarabinomannan (LAM), which plays a role in immune evasion and cell wall integrity.
Mycobacterium tuberculosis: pathogenesis
M. tuberculosis is transmitted via airborne droplets and enters the lungs, where it is phagocytosed by alveolar macrophages. The bacteria survive and multiply within these macrophages by inhibiting the fusion of phagosomes with lysosomes. Infected macrophages release cytokines, triggering granuloma formation. The granulomas attempt to contain the infection, but if they fail, the bacteria can spread to other organs, leading to systemic infection. The ability to establish latent infection is central to the pathogenesis.
Mycobacterium tuberculosis: primary infection and reactivation types of tuberculosis
Primary tuberculosis (TB) occurs when M. tuberculosis is first inhaled and the body’s immune system forms granulomas in the lungs to contain the infection. In most cases, the infection is controlled, but the bacteria can remain dormant in the lungs (latent TB). Reactivation TB can occur when the immune system becomes weakened (e.g., due to HIV infection or immunosuppressive therapy), leading to active disease, often in the apex of the lungs.
Mycobacterium tuberculosis: immunity and hypersensitivity
Cell-mediated immunity (CMI), especially involving CD4+ T cells and macrophages, is critical in controlling M. tuberculosis. A delayed-type hypersensitivity (DTH) response occurs when the immune system recognizes the bacteria, resulting in the formation of granulomas. However, this immune response is insufficient to clear the bacteria, leading to latent TB. Hypersensitivity reactions, such as tuberculin skin test (TST) reactions, are used for diagnosing latent infection.
Mycobacterium tuberculosis: tuberculin test
The tuberculin skin test (TST), also known as the Mantoux test, is used to detect previous exposure to M. tuberculosis. The test involves injecting purified protein derivative (PPD) into the skin. A positive result is characterized by induration at the site of injection, indicating a past or latent infection. The size of the induration depends on the individual’s immune status and the degree of exposure.
Mycobacterium tuberculosis: clinical findings
Primary TB often presents with mild symptoms such as a low-grade fever, cough, and malaise. Reactivation TB typically presents with more severe symptoms, including weight loss, night sweats, hemoptysis (coughing up blood), fatigue, and chronic cough. TB can affect other organs, causing extrapulmonary TB, including miliary TB (disseminated infection) and TB of the lymph nodes, kidneys, spine (Pott’s disease), and meninges (TB meningitis).
Mycobacterium tuberculosis: diagnostic laboratory tests
Diagnosis of TB is made through sputum microscopy for acid-fast bacilli (AFB) using Ziehl-Neelsen stain, culture (Lowenstein-Jensen or Middlebrook agar), and PCR for M. tuberculosis DNA. Chest X-ray is commonly used to assess lung damage. A positive tuberculin skin test (TST) indicates exposure, but it does not distinguish between active and latent infection. Interferon-gamma release assays (IGRAs) are more specific for latent TB infection.
Mycobacterium tuberculosis: treatment
The treatment of active tuberculosis requires a combination of antibiotics, typically including rifampin, isoniazid, pyrazinamide, and ethambutol for an initial phase, followed by rifampin and isoniazid for a continuation phase. Treatment is long-term, typically lasting 6–9 months, to prevent resistance. Multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) require specialized regimens with second-line drugs.
Mycobacterium tuberculosis: epidemiology
M. tuberculosis is a global health threat, with the highest burden in low- and middle-income countries. It primarily affects adults in their most productive years, with the highest rates of TB occurring in Asia and Africa. HIV co-infection significantly increases the risk of progression from latent to active TB. TB is spread via respiratory droplets, and overcrowded living conditions and poor nutrition increase transmission risk.
Mycobacterium tuberculosis: prevention and control
Prevention of TB involves screening for latent TB, particularly in high-risk individuals (e.g., HIV patients, healthcare workers, close contacts of TB patients). The Bacillus Calmette-Guérin (BCG) vaccine is used in some countries, particularly in areas with a high incidence of TB. The vaccine is less effective in preventing adult pulmonary TB but may protect against severe forms of TB in children. Public health measures include contact tracing, isolation of infectious patients, and directly observed therapy (DOT) to ensure adherence to treatment.
