Medicine & USMLE

Mycobacterium tuberculosis: Overview

Bacteria - Gram Indeterminate
  1. Mycobacterium tuberculosis: Overview
  2. Mycobacterium tuberculosis: Disease (TB)
  3. Mycobacterium avium
  4. Mycobacterium scrofulaceum
  5. Mycobacterium leprae
  6. Gardnerella vaginalis (Bacterial Vaginosis)
  7. Mycoplasma pneumoniae

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