Mycobacteria

• Mycobacterium tuberculosis


  Bacterium that causes tuberculosis
• Tuberculosis


  Infectious disease caused by Mycobacterium tuberculosis
• Löwenstein–Jensen (LJ) medium


  • Culture medium used to grow Mycobacterium tuberculosis
• Induration in 48-72 hrs POSITIVE

  Indicates a positive tuberculin skin test
• Non-tuberculous Mycobacteria


  Mycobacteria that do not cause tuberculosis
• Mycobacterium leprae


  Bacterium that causes leprosy
• Mycobacterium tuberculosis


  Bacterium that causes tuberculosis
• Tuberculosis


  Infectious disease caused by Mycobacterium tuberculosis
• The Ziehl-Nielsen stain is used to identify mycobacteria.
• “Myco” 🡪 fungus (mold-like growth)
• Mycobacteria


  Slender, curved / straight rods (bacilli) that are acid fast* and resistant to acids, alkalis, and dehydration
• Cannot be Gram stained 🡪 ATYPICAL
• Mycobacteria cell wall

  • Thick cell wall contains complex waxes, glycolipids, hydrophobic mycolic acid
  • Outer membrane & Capsule
• Mycobacteria


  • Highly resistant to disinfectants, heat & drying
  • Aerobic
  • Bacilli
• Multiplication on enriched media (Blood agar)

  1. Very slow, with doubling times of 18 to 24 hours

  2. Clinical isolates may require 4 to 6 weeks to grow
• Basis for classifying mycobacteria

  • Growth rate
  • Catalase production
  • Niacin production
  • Pigmentation in light or dark
• Mycobacterium species

  • Mycobacterium tuberculosis complex (9 members)
  • Non-tuberculous species (M. leprae)
• Gene probe technology now facilitates distinction between mycobacterium species
• Mycobacterium tuberculosis

  • M. tb Discovered by Robert Koch (1882) 🡪 Noble prize 1905
  • Koch’s bacillus
  • Tubercle bacillus
  • Gram variable
  • Mycolic acid waxy coating
  • Acid fast stains- Ziehl-Neelsen- fluorescent stain (auramine)
  • Part of a 9-member complex M. tuberculosis sensu stricto, M. africanum, M. canetti, M. bovis, M. caprae, M. microti, M. pinnipedii, M. mungi, and M. orygis.
  • Catalase (+)
  • Niacin (+)
  • Highly aerobic
  • Non-spore former
  • Non-motile
  • Primarily infect mammalian respiratory system
  • Tuberculosis Koch’s infection
  • Deadliest infectious disease worldwide (2 billion)
• Tuberculosis

  Infectious disease caused by tuberculous mycobacteria
• Tuberculosis transmission

  1. Tuberculous mycobacteria enter the alveoli by airborne transmission

  2. Inhaled & phagocytosed by alveolar macrophages

  3. M. tb resist destruction and multiply, forming the primary lesion or tubercle

  4. Spread to regional lymph nodes, enter the circulation, and reseed the lungs
• Tuberculosis

  • Tissue destruction results from cell-mediated hypersensitivity
  • Susceptibility is influenced by genetic and ethnic factors
  • Acquired resistance is mediated by T lymphocytes, which lyse infected macrophages directly or activate them via soluble mediators (e.g., gamma interferon) to destroy intracellular bacilli
  • Antibodies play no protective role
• M tuberculosis is contagious, but only 5–10 percent of infected normal individuals develop active disease
• Groups most susceptible to tuberculosis

  • Elderly
  • Poor
  • Malnourished
  • Immunocompromised, especially persons infected with human immunodeficiency virus (HIV)
• Persistent infection

  May reactivate after decades owing to deterioration of immune status; exogenous re-infection also occurs
• Primary symptoms of pulmonary tuberculosis

  • Chronic productive cough
  • Low-grade fever
  • Night sweats
  • Weight loss
• Extrapulmonary symptoms of tuberculosis (15-20%)

  • Pleura (tb pleurisy)
  • CNS (tb meningitis)
  • Lymphatic system (scrofula)
  • GUT (urogenital tb)
  • Bones & joints (Pott's)
  • Miliary tb → disseminated tb
• Diagnostic methods for active tuberculosis

  • Clinical manifestations
  • Abnormal CXR
  • Acid-fast bacilli in sputum or bronchoscopic specimens and recovery of the organism
  • Mantoux test / Pirquet test / PPD (Purified Protein Derivative)
• Löwenstein–Jensen (LJ) medium


  M. tb grows as brown, granular colonies
• Mantoux skin test

  Recent infection results in conversion to a positive test with purified protein derivative (PPD)<|>Induration in 48-72 hrs POSITIVE >5mm in HIV px, recent contact w/ a TB px, (+) TB on CXR, organ transplants & immunocompromised pxs >10mm
• Tuberculosis treatment

  1. 6 to 9 month course of isoniazid, rifampin, pyrazinamide and ethambutol

  2. Additional drugs may be used if drug resistance is suspected

  3. If the patient is HIV-positive, treatment for longer periods (9–12 months) is recommended

  4. PPD conversion without other signs or symptoms may warrant prophylactic isoniazid therapy for 6 months
• BCG ( Bacillus Calmette Guerien) vaccine

  Used in more than 120 countries, but its efficacy is controversial<|>Although not used routinely in the U.S., the current epidemic has prompted a reevaluation of its use, especially in high-risk populations
• Non-tuberculous Mycobacteria


  Resemble other mycobacteria
• Multiplication


  Similar to that of other mycobacteria, except for rapidly growing strains (e.g. M. fortuitum)
• Classification

  By pigmentation in the light or dark and by growth rate
• Species


  Several contain many serotypes, based upon lipooligosaccharides or peptidoglycolipid
• Pathogenesis

  • Similar to that of other mycobacteria
  • May involve granuloma formation and delayed hypersensitivity
• The infection is not transmissible between humans, but is acquired from natural sources (e.g., soil and water)
• Opportunistic pathogens


  Important in immunocompromised patients (e.g., patients infected with HIV)
• Diagnosis


  Requires culture and identification