Chapter 19: Pathogenic Gram- Positive Bacteria: Mycobacterium: Acid Fast Bacilli

Question 1

Describe the mode of transmission (MOT) of Mycobacterium tuberculosis

Answer 1

• It causes the diseases: tuberculosis
• MOT: continuous exposure to, and inhalation of the fine respiratory droplets—entry stage
• The infective dose is as small as 10 bacilli; one of the smallest ID’s for microorganisms
• Attach to proteins—attachment stage

Question 2

2. List the two antiphagocytic factors of M. tuberculosis and explain briefly the way in which they allow it to resist phagocytosis and dryness.

Answer 2

Colonization stage:
* Cord factor: inhibits migration of phagocytes and is toxic to mammal cells

• Mycolic acid (waxy lipid): enables bacteria to resist digestion by macrophages; it also resist dryness, mild acids and various disinfectants.

Question 3

List the three stages of tuberculosis and give the main events that occur in each one of them. During which stages of the disease are patients contagious?

Answer 3

Disease stage

• Primary tuberculosis(1): bacteria replicates within macrophages, gradually killing the white blood cells. Bacteria released by macrophages are phagocytized by other macrophages, which are also killed by the bacteria. Becomes dormant. Immune system keeps Microorganisms from spreading but does not kill them. patient becomes asymptomatic and non- contagious at this stage—person becomes a carrier
• Secondary (reactivated) tuberculosis: it occurs in the 5% of exposed people in which, the bacterium don’t become dormant, or becomes reactivated (in 10% of carriers) after a period of time. Classic signs and symptoms.
• Disseminated (extrapulmonary) tuberculosis: it occurs when macrophages carry the pathogens to other parts of the body, mainly:
   Regional lymph nodes
   The bone marrow of long bones and vertebrae
   The central nervous system
   The urinary system including the kidneys
   The genital tract
  Macrophages transport them—that’s how they spread because they are phagocytized.
  Macrophages think the bacteria is dead so it carries it out of the lungs

Question 4

List the 4 classic symptoms of pulmonary tuberculosis

Answer 4

• Fever- usually low grade accompanied by night sweats
• Cough- that sometimes ending with the ejection of blood tinged sputum
• Weight loss- often caused by lack of appetite
• Chest pain- accompanied by fatigued

Breathing difficulty (dyspnea) and fatigue may also be present

Question 5

List the 5 main diagnostic tests for tuberculosis. Which of these tests are for screening the population? Which tests indicate an active case of the disease?

Answer 5

1. Tuberculin Skin test (Monteaux): develop a red area of swelling. Positive skin test means exposure to the organism. It cannot distinguish a carrier state from an active infection. This is done when you introduce small amount of cell material from M. Tuberculosis.
2. Chest X- Rays: CT’s and MRI’s
3. AFB smears (on sputum and sometimes urine) – stage 3 out the lungs
4. AFB cultures on sputum (and sometimes urine) – stage 3 out the lungs
5. Gene probe test (DNA analysis in sputum)