Mycobacterium

Question 1

What are the general characteristics of Mycobacterium?

Answer 1

They are aerobic
non spore forming
not easily stained by gram method
resistant to decolouration by acid or alocohol so they are called acid fast bacteria
Can survive for long periods of time in dried material
M.Tuberculosis is UV and heat sensitive
2 hours of sunlight will kill it

Question 2

What are the MTB complex mycobacterium?

Answer 2

M.Tuberculosis
M.Bovis
M.Microti
M.Africana

Question 3

What are the MAC Mycobacterium?

Answer 3

M.Avium
M.intracellulare
M.paratuberculosis
M.lepraemurium

Question 4

What are the characteristics of MOTT?

Answer 4

Catalase positive
Lack guinea pig pathology
Isoniazid resistant

Question 5

What does mycolic acid on mycobacterium cell wall do?

Answer 5

It is complexed with peptidogllycan and causes granuloma

It prevents complement deposition

Prevents killing inside the macrophage

Question 6

What does cord factor do?

Answer 6

It inhibits migration of WBCs
Causes granuloma
Acts as adjuvants

Question 7

What is the function of the high lipid content of the mycobacterial cell ?

Answer 7

Makes it impermeable to dyes
Makes it resistant to many antibiotics
Makes it resistant to being killed by acid and alkali
Makes it resistant to osmotic lysis
Makes it resistant to being killed by macrophages

Question 8

How is Tuberculosis transmitted?

Answer 8

It is passed person to person, by inhaling droplets of M.Tuberculosis that are in the air. When someone coughs or sneezes, droplets are released into the air that can infect others. It can also be transmitted from skin to skin contact via flesh wounds, tattoos and non sterilized instruments but this is more rare

Question 9

Explain the pathogenesis of MTB.

Answer 9

It enters the body by inhalation or ingestion

Bacilli enters into alveolar macrophages

Move to bronchial lymph nodes

Macrophage presents the MTB bacilli to T cells

If no CMI develops, death follows

if DTH and CMI develops

Disease is contained and the bacterial will live but wont replicate.

The individual becomes Tuberculin positive

Question 10

What are the characteristics of MTB infection?

Answer 10

Caseation: Incomplete necrosis of tissue resulting in solid or semisolid acellular amourphous materials

Formation of Ghons complex following primary infection

Question 11

What is the Tuberculin test?

Answer 11

A hypersensitivity skin test that indicates hypersensitivity or cell mediated immune response to certain antigenic components(proteins) of Mycobacterium following infection with MTB

It is characterized by development of skin erythema and induration

A purified Protein Derivative is employed as the test antigen

5TU(tuberculin units) of PPD in a 0.1ml volume is intracutaneously injected in the forearm.

Formation of indurations is read at 48 or 72 hours

Question 12

How do you interpret the Tuberculin test?

Answer 12

10mm induration - 90% infected

15-20mm indurations - almost all are infected

5-10mm indurations- suspicious for MTB infection

False positive- infection with non-tuberculous mycobacteria

False negative:
-Normally 20% may be negative with active TB

• Protein malnutrition
• Viral infection
• Sarcoidosis
• Steroid
• Disease of reticuloendothelial system

Question 13

How is MTB diagnosed?

Answer 13

Specimens required are:

Sputum: 5-10ml early morning samples

Urine: 100ml early morning sample

Plural or peritoneal fluids: 100ml

Pus: as much as possible

Tissue: Adequate

3 consecutive samples are required

Question 14

How is TB bacilli detected?

Answer 14

By Culturing and acid fast bacili(AFB) or Ziehl-Neelsen staining

Question 15

What molecular methods can be used to detect MTB?

Answer 15

PCR or Probe/hybridization can be used to detect MTB specific nucleic acid

Question 16

What is a ghon focus and how is it formed?

Answer 16

After 3 weeks of infection, cell mediated immunity kicks in and forms a granuloma to wall off bacteria and prevent it from spreading.

The tissue in the middle dies, a process known as caseous necrosis and forms what is known as a “Ghon focus”

Question 17

What is a ghon complex and how is it formed?

Answer 17

TB can get to the hilar lymph nodes either carried by immune cells through the lymph or as a direct extension of the ghon focus infection and causes caseation there as well
Together this caseating tissue and associated lymph node make up the “Ghon complex”

They are usually sub-pleural and occur in the lower lobes of the lungs

Question 18

What are the treatments for patients with active infection?

Answer 18

Drug combinations of:
Isonazid
Rifampin
Ethambutol
Pyrazinamide

Question 19

What happens when Ghon focus is reactivated?

Answer 19

Reinfection can occur and can spread to both upper lobes of the lungs

This causes memory T cells to release cytokines and leads to more caseous necrosis

This time it may cavitate, allowing TB to spread through airways to other parts of the lungs causing pneumonia or in the vascular system infecting other tissue