Mycobacterium tuberculosis

Question 1

Incubation Period

Answer 1

2-12 weeks

Question 2

General Characteristics

Answer 2

Slender, rod-shaped bacilli, slightly bent

non-motile, obligate aerobes, grow in clusters, do not form spores.

Has lipid-rich cell walls that are resistant to penetration by gram stain. (weakly gram positive)

Acid-fast bacilli (Ziehl-Neelsen stain)

Question 3

Transmission

Answer 3

Respiratory droplets

Question 4

Pathogenesis

Answer 4

1.) inhaled microbe. Alveolar macrophage will try and to kill it.
2.) due to sulfolipids, microbe will inhibit that. It will thrive instead.
3.) More macrophages infiltrate area due to multiplying bacteria.
4.) Walling it off, stopping spread of organism but not killing it.
5.) Hard outer case calcifies, forming tubercle. Death occurs inside, aka liquefaction
6.) Liquefaction occurs until tubercle ruptures and organism spreads to other parts of body.

Question 5

Diagnosis

Answer 5

Acid-fast (positive) aka Ziehl-Nielson stain*
Gram stain (weakly positive)
Tuberculin skin test aka PPD test** Positive skin test does not mean active disease. just means past infection / vaccination.
Chest X-ray
Culture
Nucleic acid amplification test

Question 6

Isolation

Answer 6

Grow on Lowenstein-Jensen or Middlebrook Media

Question 7

Cell wall characteristics

Answer 7

Impervious to many chemical disinfectants, bacteriostatic agents and drying.

Viable for weeks in sputum, bedding, etc. due to clumped growth.

Question 8

Virulence Factors

Answer 8

Wax D: Induces delayed-type hypersensitivity reaction

Cord factor (trehalose mycolate): inhibits mitochondrial respiration, inhibits neutrophil metration, causes organism to grow as “Serpentine cords” in liquid medium.

Sulfolipids (Sulfatides): Inhibit phagosome-lysosome fusion allowing organism to survive intracellularly.

Question 9

Miliary TB

Answer 9

Occurs when microbe affects part of body other than lung.

Question 10

Treatment

Answer 10

Isoniazid (INH) - inhibits cell wall synthesis
Rifampin - inhibits bacterial RNA polymerase
Pyrazinamide - blocks CoA synthesis

Use these three, then one of the below:

Ethambutol - inhibits ccell wall synthesis
Streptomycin - inhibits protein synthesis.

Question 11

Prevention

Answer 11

Prophylactic Isoniazid for high risk groups (antibiotic)

BCG vaccine