Mycobacterium tuberculosis: Overview

Mycobacterium tuberculosis Overview

• Acid fast bacilli
  • TB is the classic acid fast bacilli of the Mycobacteria family
  • Retains stain due to the high mycolic acid content of their cell walls
• Virulence factors
  • Cord factor
    • Creates a “serpentine cord” appearance in virulent M tuberculosis strains
    • Activates macrophages
      • promotes granuloma formation
      • Induces release of TNF-alpha
  • Sulfatides
    • Surface glycolipids that inhibit phagolysosome fusion
      • Enable survival and intracellular replication in macrophages
• Transmission
  • respiratory droplets are inhaled
    • This is why TB patients are placed in negative pressure isolation rooms for containment
• Pathophysiology
  • TB phagocytosed by and replicates intracellularly in macrophages
    • Activation of macrophages induces phagocytosis by cord factor
    • Survives due to failure of phagolysosome fusion due to sulfatides
  • Forms granulomas with caseous (cheese-like) necrosis
    • Composed of activated macrophages, Th1 T-cells (helper T-cells)
      • Langhans giant cells (conglomerates of macrophages) may also be seen
    • Functions to contain the bacterial infection
    • Granulomas can cause PTH-independent conversion of vitamin D, leading to hypervitaminosis D and ultimately hypercalcemia
    • Caseation can help differentiate vs. sarcoidosis