Lecture 8: Mycobacteria

1. Taxonomy

• Class: Actinomycetes

• Order: Actinomycetales

• Family: Mycobacteriaceae

• Genus: Mycobacterium

2. Acid-Fast Cell Wall Structure

• Peptidoglycan (inner layer)

• Arabinogalactan (links to mycolic acid)

• Mycolic Acid (rich fatty acids, making bacteria acid-fast)

• Free Lipids: Glycolipids, phosphatidylinositol mannosides (PIM)

• Porins: Transport hydrophilic molecules

• Surface Proteins: Vary among species

3. Growth Classification

• Rapidly Growing Mycobacteria (RGM): Colonies appear in < 7 days

• Slow Growing Mycobacteria: Colonies take > 7 days (e.g., M. tuberculosis)

4. Mycobacterium Tuberculosis Complex

• Includes: M. tuberculosis, M. bovis, M. africanum, M. canetti, M. orygis, etc.

• Epidemiology:

  • ⅓ of the world’s population is infected

  • 10 million new cases yearly

  • 2.9 million deaths

  • Strong link with HIV co-infection

Pathogenesis

• Latent TB:

  • Dormant bacteria in high-oxygen areas (e.g., lungs)

  • No symptoms, not infectious

• Active TB:

  • Reactivation of latent infection → Secondary TB

  • Granuloma formation → Caseous necrosis (cheese-like tissue destruction)

  • Disseminated TB: Spreads in immunocompromised patients (miliary TB)

5. Diagnosis

• Tuberculin Skin Test (PPD Test):

  • Injected intradermally (0.1 mL)

  • Read 48-72 hours later

  • Positive: Induration (not erythema)

• Acid-Fast Staining (AFB Smear): Ziehl-Neelsen, Kinyoun, Fluorochrome staining

• Culture:

  • Solid Media: Lowenstein-Jensen, Middlebrook 7H10

  • Liquid Media: More rapid growth

• Molecular Tests:

  • Xpert MTB/RIF (detects rifampicin resistance)

  • QuantiFERON-TB Gold (measures interferon-gamma release)

6. Nontuberculous Mycobacteria (NTM)

Runyon Classification

Key NTMs

• M. kansasii: Pulmonary & skin infections

• M. marinum: "Fish tank granuloma" from water exposure

• M. avium-intracellulare (MAC): Affects HIV/AIDS patients

• M. abscessus: Wound infections, catheter-related infections

7. Mycobacterium leprae (Leprosy / Hansen’s Disease)

• Cannot be cultured in vitro

• Transmission: Inhalation of nasal secretions

• Types:

  • Tuberculoid (Paucibacillary): Mild, localized, strong immunity

  • Lepromatous (Multibacillary): Severe, immune defect, widespread lesions

  • Leonine facies: Thickened skin on the face

  • Madarosis: Eyebrow loss

• Treatment: Dapsone

8. Laboratory Diagnosis

Specimen Collection

• Pulmonary TB: Sputum, bronchoscopy, gastric lavage

• Extrapulmonary TB: CSF, pleural fluid, urine, wounds, skin

• NTM & Leprosy: Skin biopsies, blood

Microscopy

• Fluorochrome Staining: Most sensitive (auramine-rhodamine)

• Ziehl-Neelsen Staining: Heat-based AFB staining (Carbol fuchsin)

• Kinyoun Staining: Cold acid-fast stain (no heat)

Culture Media

• Solid: Lowenstein-Jensen (egg-based), Middlebrook 7H10 (agar-based)

• Liquid: Faster growth (used for MDR-TB detection)

9. Biochemical Identification

• Niacin Test: M. tuberculosis = (+) positive

• Nitrate Reduction: M. tuberculosis, M. kansasii (+) positive

• Catalase: Most mycobacteria are catalase positive

• Tellurite Reduction: MAC (+) positive

• Arylsulfatase: M. fortuitum, M. marinum (+) positive

10. Treatment

First-Line Drugs for TB

• Isoniazid (INH)

• Rifampin (R)

• Pyrazinamide (Z)

• Ethambutol (E)

Standard TB Regimens

1. 2HRZE / 4HR (6 months total)

   • Intensive Phase: 2 months HRZE

   • Continuation Phase: 4 months HR

2. 2HRZE / 10HR (12 months total) (Severe cases)

NTM Treatment

• Varies per species (often multiple antibiotics)

Leprosy Treatment

• Dapsone (main drug)

Key Takeaways

✔ M. tuberculosis = Primary cause of TB, acid-fast, slow-growing
✔ TB Pathogenesis: Latent (dormant) → Active (infectious) → Disseminated TB
✔ NTMs: Classified by pigmentation & growth rate (Runyon I-IV)
✔ M. leprae: Leprosy, unculturable, person-to-person spread
✔ Diagnosis: Skin test (PPD), AFB smears, culture, PCR (Xpert MTB/RIF)
✔ Treatment: Standard TB drugs (HRZE), individualized for NTMs & Leprosy