Mycobacteria

Question 1

Mycobacteria

Answer 1

Unicellular, rod-shaped obligate aerobes.
Non-motile, non-capsulated, non-sporulating bacteria.
Have a high lipid content cell wall.

Question 2

Mycobacteria’s cell wall

Answer 2

60% lipid, rich in mycolic acid, LAM, PIMs, & cord factor.
High lipid content helps survival in host cells & provides resistance to many antimicrobials, heat, drying, and chemical agents.

Question 3

Ziehl-Neelsen (ZN) staining

Answer 3

1) Hot carbol fuchsin stains all bacteria pink.
2) After decolourization with acid-alcohol, only acid-fast Mycobacteria remain pink.
3) Counterstain with methylene blue so non-acid-fast cells appear blue, while Mycobacteria stay pink.

Question 4

Why are Mycobacteria called acid-fast?

Answer 4

They retain carbol fuchsin stain even after acid-alcohol decolorization due to their lipid-rich cell wall.

Question 5

Name major infections caused by Mycobacteria.

Answer 5

M. tuberculosis → Pulmonary TB
○pneumonia, weight loss, fever, & a cough with blood.
M. bovis → TB (from milk).
Leprosy → skin lesions, nerve damage, & disfiguration.

Question 6

Ghon focus

Answer 6

Healed primary lesion containing dormant MTB which can reactivate later.

Question 7

tubercle

Answer 7

A granuloma formed by immune response to MTB containing macrophages & T-cells.

Question 8

Stages of TB infection

Answer 8

1) M. tuberculosis transmitted via inhalation of airborne droplets released when infected person talks, coughs, or sneezes.
2) In lungs, MTB engulfed by alveolar macrophages but survives & multiplies within them.
3) Infected macrophages eventually burst, releasing bacteria & triggering further immune responses.
4) IL-12 activates T-cells → forms tubercle to contain infection.
5) Primary lesion may heal & become a dormant Ghon focus, but it can reactivate later when immune system becomes weakened.

Question 9

How is pulmonary TB diagnosed?

Answer 9

Sputum microscopy with ZN stain → reveals acid-fast bacilli.
Culture on Lowenstein-Jensen (LJ) medium → form yellow, buff-colored colonies.

Question 10

cord factor

Answer 10

Glycolipid found in cell wall of M. tuberculosis.
Causes bacteria to grow in serpentine cords.
Virulence factor that inhibits WBC migration.

Question 11

TB treatment

Answer 11

Combination therapy for 6 months: isoniazid, rifampicin, pyrazinamide, ethambutol.

Question 12

Progression of TB

Answer 12

~90% of infected individuals asymptomatic due to strong immune response.
In 10%, active TB develops.
TB may spread to other organs → meningitis, joint infections.
Reactivation can occur when immune function is compromised → HIV, cancer treatment, or old age.