Mycobacterium Tuberculosis: General Notes

Mycobacterium Tuberculosis

• Causes 2nd most frequent infectious deaths in the world, with 1/3 of the world's population (2-3 billion) infected, and 9-10 million new active cases every year, resulting in 1-2 million deaths yearly.

Characteristics

• Aerobic, non-motile, acid-fast intracellular bacilli.
• Sensitive to UV, alcohol, and aldehydes.
• Resistant to alkaline acid and ammonia.

Transmission

• Mainly through aerosol inhalation (rarely via skin or ingestion).
• High risk for immunocompromised individuals, especially those with HIV, and those with multi-drug resistant (MDR) strains.

Major Human Pathogens

• M. tuberculosis (most common in humans)
• M. africanum
• M. bovis (common in cows, transmitted to humans through dairy products)
• M. microti (smooth growth on culture, with a cotton-like appearance)

Growth Medium

• Lowenstein-Jensen (LJ) medium: egg-glycerol enhances MTB growth, malachite green suppresses other bacteria, with a 4-8 week growth period, and a characteristic white, dry, rough growth (cauliflower-like).
• Liquid medium: detects O2 consumption and CO2 production in 8-14 days, allowing for earlier detection.

Pathogenesis

• Transmission to lungs through aerosol inhalation requires at least 10 bacilli in one micro-droplet.
• Primary Tuberculosis (TB) typically occurs in the lungs, skin, intestine, and lymph nodes, usually asymptomatic.
• Macrophages engulf the bacteria, forming a granulomatous lesion (tubercle), which is then attacked by macrophages, leading to a granuloma (Gohn focus).
• The infection can progress to lung infection or become necrotic (caseation) and calcify, leading to latent TB, with a 5-10% reactivation chance in healthy individuals.

Clinical Picture

• Classic features of active pulmonary TB include:
  • Cough, fever, weight loss, and night sweats
  • Hemoptysis (coughing blood)
  • Chest pain

Diagnosis

• History and examination are crucial in diagnosis.
• Laboratory tests include:
  • Sputum/tissue acid-fast stain (Ziehl-Neelsen stain or Kinyoun): 80% of cases are positive, with a 10-20% false negative rate.
  • Culture (gold standard, but requires at least 78 days).
  • PCR for diagnosis and drug resistance detection.
  • Purified protein derivative (PPD) test (Mantoux test): detects delayed hypersensitivity reaction in the skin, indicating active infection, vaccine, or environmental TB.