Microbiology · MU Sofia

70. Family Mycobacteriaceae: Mycobacterium tuberculosis, etc. causative agents of tuberculosis

Covers Mycobacterium tuberculosis and related species: the general features of these acid-fast bacilli and their staining, then the virulence factors listed in a table. It follows the pathogenesis from inhalation through granuloma formation to reactivation, then the primary, latent, secondary and extrapulmonary stages. Clinical features and laboratory diagnosis are set out, including specimen types, microscopy, culture, molecular tests, skin and blood tests. Treatment gives a RIPE regimen table, drug resistance categories, and prevention with BCG vaccination and public health measures.

Inside: 12 sections · 8 labelled pictures · 2 tables · 15 flashcards · 4 min

Topic 70 of 113 in the MU Sofia syllabus for Microbiology (Dental Medicine, Year 2). The free AceMed note covers it in 12 sections, about 462 words (4 min of reading), with 15 flashcards.

In this note

  1. Family Mycobacteriaceae: Mycobacterium tuberculosis and Related Species
  2. GENERAL CHARACTERISTICS
  3. VIRULENCE FACTORS
  4. PATHOGENESIS
  5. STAGES OF TUBERCULOSIS
  6. CLINICAL FEATURES
  7. LABORATORY DIAGNOSIS
  8. TREATMENT
  9. Drug Resistance
  10. PREVENTION
  11. BCG Vaccine
  12. Public Health Measures

GENERAL CHARACTERISTICS

  • Shape: Straight or slightly curved bacilli, slender rods
  • Gram stain: Weakly Gram-positive (does not stain reliably); acid-fast bacilli (AFB) due to mycolic acid
  • Staining methods:
  • Ziehl–Neelsen (ZN) stain: Bright red rods on a blue background
  • Auramine–rhodamine fluorescent stain: Yellow-green under UV — more sensitive
  • Oxygen: Obligate aerobe
  • Motility, spores, capsule: Non-motile, non-spore-forming, no capsule
  • Growth rate: Slow-growing, doubling time ~18–24 hrs
  • Colony characteristics: Buff-colored, rough, dry colonies on Lowenstein–Jensen (LJ) medium

PATHOGENESIS

  1. 1. Inhalation of droplets from an active TB patient → enters alveoli
  2. 2. Phagocytosed by alveolar macrophages → M. tuberculosis survives inside due to sulfolipids
  3. 3. Multiplies within macrophages → triggers pro-inflammatory cytokines (e.g., TNF-α)
  4. 4. Recruitment of immune cells → formation of a granuloma (tubercle) to contain infection
  5. 5. Caseous necrosis develops at center of granuloma → latent or active TB
  6. 6. Reactivation occurs when immune defenses weaken → leads to progressive disease and cavitation

STAGES OF TUBERCULOSIS

1. Primary TB

  • Occurs on first exposure (usually in childhood)
  • Infects lower part of upper lobe or upper part of lower lobe
  • Ghon focus: Granuloma with central necrosis
  • Ghon complex: Ghon focus + hilar lymph node involvement
  • Often asymptomatic or mild flu-like symptoms

2. Latent TB

  • Bacilli walled off in granulomas → no symptoms
  • Not contagious
  • May persist for life or reactivate

3. Secondary (Reactivation) TB

  • Occurs in immunocompromised or elderly
  • Infection reactivates in apical lobes of lung
  • Causes cavitary lesions, severe symptoms

4. Extrapulmonary TB

  • CNS: TB meningitis
  • Spine: Pott’s disease
  • Lymph nodes: Scrofula
  • Renal: Sterile pyuria
  • Adrenal: Addison’s disease

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