TB

Question 1

Describe the composition of an acid-fast cell wall and indicate the advantages and disadvantages of this structure for the bacterium.

Answer 1

• Cell Envalope made from:
  
  • Waxes
  • Mycolic Acids
  • Polysaccharides
  • Peptidoglycan (murein)
  • Phenolic Glycolipid I (PGL-I) • Lipoarabinomannan (LAM)
• Cell Wall made of:
  
  • Lipoarabinomannan
  • Glycolipid
  • Mycolic acids
  • Arabinogalactan
  • Peptidoglycan: only a small amount/single layer
• Acid fast staining b/c of these structures-Don’t stain w/ gram stain
• These structures protect the bacterium from the environment

Question 2

Mode of Transmission

Answer 2

• Respiratory
  
  • Inhalation of droplet nuclei (< 10 μmdiameter)
  • man-to-man transmission
• Ingestion
  
  • Intestinal route via ingestion of M. bovis in contaminated milk and milk products
  • animal- to-man transmission

Question 3

Pulmonary TB – Stage 1

Answer 3

• Inhalation of droplet nuclei- seeding into mid to lower lung
• Bacteria multiply in alveolar spaces
• Bacteria are ingested by alveolar macrophages
• Bacteria are destroyed or
  
  • Bacteria grow in macrophages and kill them

Question 4

Pulmonary TB – Stage 2

Answer 4

• Logarithmic growth of bacteria inside macrophages
• Arrival of circulating macrophages
• Bacteria spread to arriving macrophages
• Dissemination via infected macrophages into lymphatics and blood stream
• No host defense during first few weeks after infection

Question 5

Pulmonary TB – Stage 3

Answer 5

• Bacteria growth in macrophages countered by immune response
• Activated macrophages ingest bacteria and destroy them
• Cytotoxic T-cells recognize infected macrophages and destroy
• Formation of granulomas and Ghon complexes (primary lesions) at site of primary infection

Question 6

Life Cycle:

Answer 6

• Droplets enter the respiratory system and M. tuberculosis taken up by alveolar macrophages
• Macrophage goes to blood
• Becomes a foamy macrophage w/ lipids and blood vessels and lymphocytes surround
• Fibrous cuff forms around the foamy macrophage and the infected macrophages
• Caseum forms in the center of the surrounded mass
• Mass bursts through the airway releasing necrotic granuloma and mycobacterium

Question 7

Describe the principle underlying the tuberculin test and how the results of the test should be interpreted in normal and AIDS patients.

Answer 7

-Tuberculin Test:
-Intradermal injection of 5 TU PPD-T in 0.1 ml of solution
-Produces wheal 6 mm to10mm diameter
-Reaction is usually read in 48-72 hrs
>5mm positive in:
-HIV infected patient
-Close contact to TB case
-Fibrotic changes on CXR consistent with old TB
-Patients immunosuppression
>10mm positive in:
-Recent immigration from countries w/ high incidence of TB
-IV drug users
-Residents and employees in high risk congregate settings
-Workers in mycobacteria labs
-Clinical Conditions putting at risk
-Children 15mm positive in:
-Persons w/ no know risk factors for TB
-False positive if
-Non- TB mycobacteria
-BCG vaccine
-False Negative
-Anergy
-Recent TB infection
-Very young <6m
-Live virus vaccine
-Overwhelming TB infection

Question 8

Explain the treatment of tuberculosis.

Answer 8

• Isoniazid
• Rifampin
• Pyrazinamide
• XDR TB = TB whose isolates are resistant to isoniazid and rifampin plus resistant to any fluoroquinolone and at least one of three injectable second-line drugs (i.e., amikacin, kanamycin, or capreomycin)

Question 9

Primary TB

Answer 9

• Infection in nonimmune host
• Presentation: hilar nodes
  
  • Gohn complex affects the lower lobe
• Results:
  
  • heal by fibrosis-Immunity and hypersensitivity (PPD +)
  • Progressive lung disease (HIV/malnutrition)-> death
  • Severe bactermia- miliary TB-> death
  • Preallergic lymphatic or hematogenous dissemination
    
    • dormant tubercle bacilli in several organs
    • Reactivation in adult life

Question 10

Secondary TB

Answer 10

• Reactivation TB
• Fibrocaseous cavitary lestion
• Usually affects upper lobe
• Extrapulmonary TB:
  
  • CNS (parenchymal tuberculoma or meningitis)
  • Vertebral body: Pott’s disease
  • Lymphadenitis
  • Renal
  • GI