Lecture 16

Question 1

Tuberculosis is the disease caused by

Answer 1

either Mycobacterium tuberculosis

or Mycobacterium bovis.

They cause slowly progressive chronic
disease.

Question 2

How do you test for TB?

Answer 2

M. tuberculosis and M. bovis are bacilli that
do not stain with Gram’s stain.

They can be grown on agar or in broth.

•  Organisms seen in sputum (or other

samples) with ZN stain (or other stains). •  Carbol fuchsin (red) in phenol driven into bacilli with heat. Background decolourised with acid/alcohol (doesn’t decolourise so AFB). Background counter-stained with methylene
blue.

•  Colonies grown on solid or in liquid media. Growth cycle = 18 hrs. Detection of colonies usually takes 2-6
weeks.

•   M. tuberculosis DNA detected by PCR
• Mantoux and Quantiferon tests

Question 3

What are the two main sites of tubercolosis?

Answer 3

•   Pneumonia (85%)

*   Lymph node (5-10%)

Question 4

What are the properties of TB?

Answer 4

•   Usually one organ involved
•   Occasionally disease in many organs

•  Gradual onset and slow progression
(months)

Question 5

What is the Epidemiology of tuberculosis?

Answer 5

•  Tuberculosis was responsible for
approximately 1/3 of all adult deaths in
Europe in 1800s

•   1/3 world population (= 2 billion) infected
•   16 million (= 0.8%) active tuberculosis
•   8 million new cases per year
•   2 million deaths per year

•  Increasing again in wealthy countries
- due to : immigration from 3rd world

immunosuppression due to HIV

Question 6

What is mycobacterium tuberculosis pathogenesis?

Answer 6

•  Exposure to infection
•  From a patient with pulmonary tuberculosis
who coughs

•  Inhalation of droplet nuclei

containing 1-3 bacilli

A macrophage is always present between alveolar cells to ingest the bacteria

Question 7

What happens once TB is ingested?

Answer 7

•  Blockage of fusion of
phagosome with lysosomes. Multiplication of M. tuberculosis within
phagosome. Escape of M. tuberculosis into cytoplasm. Multiplication
within cytoplasm. Death of infected macrophage once about 8 bacilli present in cell. Ingestion of released bacilli by other macrophages. Dissemination of infected macrophages
through body. Recruitment of T helper
(CD4) lymphocytes to sites of infection. Release of inflammatory
cytokines by T helper lymphocytes. Activation of macrophages to increase ingestion and
intracellular killing of bacilli.

Question 8

How does a granuloma form due to TB?

Answer 8

• a central region containing necrotic material from dead cells and bacilli. Surrounding macrophages with ingested bacilli and some giant cells. T helper lymphocytes at the edges.

Question 9

What happens after a granuloma forms in TB?

Answer 9

•  Establishment of an equilibrium:
- multiplication of bacilli

• ingestion and killing of bacilli

•  Leads to formation and maintenance

of many small, stable granulomata

•  In about 1% infected patients per year
the granulomas in some tissue (usually lung)
enlarge

due to the equilibrium shifting in favour of
the bacilli

Question 10

What are the consequences of a granuloma?

Answer 10

•  In about 1% infected patients per year
the granulomas in some tissue (usually lung) enlarge due to the equilibrium shifting in favour of the bacilli. If the enlarging granuloma in the lung erodes into a bronchiole then the increased oxygen supply to the bacilli shifts the equilibrium markedly in their favour. The bacteria multiply more quickly
in the liquified caseous necrosis. The bacilli can be more readily dispersed by coughing.

Question 11

What are the consequences of TB treatment?

Answer 11

•  Prevention of infection progressing to
disease is dependent on activation of
macrophages by T helper lymphocytes

•  Any process that impairs T helper
lymphocyte function will favour progression of infection to disease

Question 12

Mantoux and Quantiferon tests

Answer 12

Both detect release of Interferon by lymphocytes in response to mycobacterial antigens.

A positive result to either indicates the presence in blood of lymphocytes previously sensitised to mycobacterial antigens.
Neither distinguishes between latent infection and disease

Question 13

Mantoux test = Tuberculin Skin Test

Answer 13

1.  Inject into dermis 0.1ml tuberculin (purified protein derivative= PPD)
= cell wall extract of M. tuberculosis

2. In a person whose lymphocytes have previously been exposed to similar
   mycobacterial antigens they will migrate to the injection site and release Interferon and other cytokines that will result in capillary permeability and fluid extravasation.

A palpable swelling will develop over 2- 3 days.

3. Measure the diameter of induration at 2-3 days > 5mm = positive.

Question 14

Quantiferon Gold Test

Answer 14

Quantiferon Gold blood test tubes have M. tuberculosis specific antigens (not shared with M. bovis like the PPD antigens ) attached to the inside of the tube.

Lymphocytes sensitised by prior exposure to M tuberculosis release interferon

Interferon levels are measured after 3 days incubation of the blood tube.

Question 15

What are the differences between Mantoux and Quantiferon?

Answer 15

The PPD antigen used in the Mantoux test

is present in M. tuberculosis and in M. bovis.

The antigen used in the Quantiferon Gold test

is present only in M. tuberculosis.

Question 16

What is the live organism present in BCG vaccine?

Answer 16

M. bovis
People who have been vaccinated with BCG

will have a positive Mantoux test but a negative Quantiferon test

Question 17

How would you treat tuberculosis?

Answer 17

Use multiple drugs.
•  Routine situation where resistance of all
infecting bacilli is thought unlikely

eg. no prior treatment for Tb

•   rifampicin
•   isoniazid for months
•   pyrazinamide

Question 18

How do you prevent tb?

Answer 18

•  Isolation and effective treatment of infectious patients
ie. those with lung disease

•  Immunisation - BCG vaccine is of minimal
utility except in the prevention of neonatal
disease