Mycobacterium - Leprosy Flashcards
What are the general features of Mycobacterium tuberculosis?
Mycobacterium tuberculosis is an aerobic, acid-fast bacillus with a high lipid content (approximately 60%) in its cell wall. It is neither Gram-positive nor Gram-negative and is virtually the only bacteria that is acid-fast.
What is the significance of the term ‘acid-fast’ in mycobacteria?
The term ‘acid-fast’ refers to an organism’s ability to retain the carbol fuchsin stain despite subsequent treatment with an ethanol-hydrochloric acid mixture.
What are the medically important mycobacteria and the diseases they cause?
- Mycobacterium tuberculosis – Tuberculosis.
- Mycobacterium leprae – Leprosy.
- Atypical mycobacteria – Tuberculosis-like disease (e.g., M. avium-intracellular complex and M. kansasii).
- Rapidly growing mycobacteria (e.g., M. chelonae) – Occasionally cause human disease in immunocompromised patients.
How is Mycobacterium tuberculosis transmitted?
M. tuberculosis is transmitted from person to person by respiratory aerosol. The initial site of infection is the lung, and it resides chiefly within reticuloendothelial cells (e.g., macrophages).
What are the symptoms of tuberculosis?
Symptoms include fever, fatigue, weight loss, persistent cough, blood in cough, and night sweats.
What is the pathogenesis of Mycobacterium tuberculosis?
M. tuberculosis enters via inhalation, infects macrophages, and survives within a phagosome. It can escape lysosomal degradation and form granulomas, leading to latent or active TB. The bacteria can remain dormant for years.
What are the two types of lesions in tuberculosis?
- Exudative lesions: Acute inflammatory response, usually in the lungs at the initial site of infection.
- Granulomatous lesions: Central area of giant cells (Langhans’ giant cells) containing tubercle bacilli, surrounded by epithelioid cells.
What is the difference between latent TB and active TB?
• Latent TB: Asymptomatic, non-contagious, positive blood and skin test.
• Active TB: Symptomatic, contagious, positive blood and skin test.
What is miliary tuberculosis?
Miliary tuberculosis is a severe form of TB where the bacteria spread throughout the body via the bloodstream, causing numerous tiny lesions resembling millet seeds. It can affect multiple organs, including the lungs, liver, spleen, and brain.
How is tuberculosis diagnosed using the tuberculin skin test?
The tuberculin skin test involves injecting a small amount of purified protein derivative (PPD) tuberculin into the skin. A positive reaction (induration of 10 mm or more) indicates previous exposure to M. tuberculosis, but it does not necessarily indicate active disease.
What is the IGRA test, and how does it work?
The IGRA (Interferon-Gamma Release Assay) test checks for TB infection by measuring the immune response to M. tuberculosis antigens. It detects interferon-gamma released by sensitized CD4 T cells in response to TB-specific antigens.
What are the methods used for laboratory diagnosis of tuberculosis?
Methods include sputum specimen collection, AFB staining (Ziehl-Neelsen method), cultivation on Lowenstein-Jensen medium, and rapid molecular diagnostic tests like the Xpert MTB/RIF test.
What is the treatment for tuberculosis?
Treatment involves long-term therapy (6-9 months) with three drugs: Isoniazid, Rifampin, and Pyrazinamide. A fourth drug, Ethambutol, is used in severe cases or in immunocompromised patients.
What is the BCG vaccine, and how effective is it?
The BCG vaccine contains a live, attenuated strain of M. bovis and is used to induce partial resistance to tuberculosis. It is effective in preventing clinical disease, especially in children, with protection ranging from 60% to 90%.
What are the characteristics of Mycobacterium leprae?
M. leprae causes leprosy (Hansen’s disease), affects the skin and peripheral nerves, and grows very slowly with a doubling time of 14 days. It cannot be grown in the laboratory but can be grown in experimental animals like mice and armadillos.
How is leprosy transmitted?
Leprosy is transmitted through prolonged contact with patients who have lepromatous leprosy, particularly through nasal secretions and skin lesions.
What are the two forms of leprosy, and how do they differ?
- Tuberculoid leprosy: Few lesions, low tissue destruction, few acid-fast bacilli, low likelihood of transmission, and a positive lepromin skin test.
- Lepromatous leprosy: Many lesions, marked tissue destruction, many acid-fast bacilli, high likelihood of transmission, and a negative lepromin skin test.
What is the treatment for leprosy?
Treatment involves Dapsone plus Rifampin for at least 2 years. There is no vaccine available for leprosy.
What are atypical mycobacteria, and how do they differ from M. tuberculosis?
Atypical mycobacteria are found in the environment (water, soil, milk, food, animals) and do not cause tuberculosis or leprosy. They are subdivided into slow growers and rapid growers based on colony formation time.
What is the significance of the Xpert MTB/RIF test?
The Xpert MTB/RIF test is a rapid diagnostic test that identifies M. tuberculosis DNA and resistance to rifampicin. It has high diagnostic accuracy and improves early detection of TB and drug-resistant TB.
