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.

This quiz covers the general notes on Mycobacterium Tuberculosis, including its transmission, characteristics, and impact on global health.