antibiotics, modes of transmission, tuberculosis

Question 1

define infectious disease

Answer 1

disease caused by an infectious agent (pathogen) which can cause damage or injury to the host and is transmitted from one organism to another

Question 2

what is the virulence of a pathogen?

Answer 2

a quantitative measure of the ability of a pathogen to cause disease

Question 3

what do Gram-positive and Gram-negative bacteria mean?

Answer 3

Gram-positive=thick cell wall
Gram-negative=thin cell wall

Question 4

what are the defining characteristics of bacterial cells?

Answer 4

1. peptidoglycan cell wall
2. circular DNA
3. absence membrane-bound organelles

Question 5

what are the 2 types of antibiotics?

Answer 5

1. bacteriostatic: inhibit the growth/ cell division of bacteria
2. bactericidal: kills bacteria

Question 6

name the modes of action of bactericidal antibiotics (two)

Answer 6

1. inhibition of cell wall synthesis
2. disruption of cell membrane

Question 7

name the modes of action of bacteriostatic antibiotics (four)

Answer 7

1. inhibit DNA replication
2. inhibit transcription
3. inhibit translation
4. inhibit metabolic enzymes

Question 8

what type of antibiotic is Penicillin?

Answer 8

bactericidal antibiotic

Question 9

describe the mode of action of Penicillin

Answer 9

1. binds irreversibly to the active site of transpeptidase an enzyme which catalyses peptide cross-link formation between N-acetylmuramic acid (NAM) of different peptidoglycan chains
   =>penicillin prevents peptide cross-links formation between peptidoglycan chains in the cell wall of dividing bacteria cells
2. stimulates the release of autolysins that hydrolyse B-1,4-glycosidic bonds between peptidoglycan chin which creates pores in the existing cell wall
   => this also weakens the cell wall of dividing bacteria
   => as water enters the bacterium by osmosis, the cell wall cannot withstand the osmotic pressure and osmolysis takes place

Question 10

modes of transmission of pathogens

Answer 10

air-borne, food-borne, water-borne, soil-borne, vector-borne, (prolonged direct) contact, sexually transmitted infections

Question 11

what is the pathogen which causes tuberculosis (TB)?

Answer 11

Mycobacterium tuberculosis (M. tuberculosis)

Question 12

how is TB transmitted?

Answer 12

air-borne; inhalation of air-borne respiratory droplets containing the pathogen from an infected person when they sneeze/cough

Question 13

what is the site of infection for TB?

Answer 13

epithelial cells in the lower respiratory tract (alveolar macrophages)

Question 14

outline the infection and disease process of M. tuberculosis

Answer 14

1. Mycobacterium tuberculosis bacteria infect the alveoli of the lungs
2. inside the lungs, alveolar macrophage phagocytoses the bacteria via receptor-mediated endocytosis
3. inside the phagosome, M. tuberculosis inhibits the fusion of the phagosome with lysosome, hence phagolysosome cannot be formed and no lysosomal enzymes are available so the bacteria is not killed
4. M. tuberculosis also inhibits the proton pump hence acidification of the phagolysosome cannot take place, hence the bacteria is not killed
5. M. tuberculosis survives and reproduce within the macrophages by
   =>inhibiting lysosomal enzymes
   =>having the ability to escape out of the phagosomes into the cytosol
   =>becoming resistant to digestive enzymes
6. the infected macrophage releases cytokines which recruits other macrophages and other immune cells such as CD4 T cells, CD8 T cells, natural killer cells and neutrophils to the infected alveoli
7. this forms a tubercle which is a cluster of macrophages
8. then a granuloma forms which consist of many infected macrophages surrounded by immune cells such as activated T cells
9. as the immune cells attempt to kill the bacteria, the enzymes released by the immune cells cause permanent damages to the lung tissues
10. the centres of large tubercles/granulomas become deprived of oxygen and the macrophages in the centre die, forming a caseous centre
11. many bacteria remain dormant (latent TB) and the disease is arrested at this stage as the aerobic bacteria do not grow well in this low oxygen environment
12. over time, the caseous centre enlarges and M. tuberculosis released from dead macrophages reproduce in air-filled cavities
13. the tubercle ruptures, releasing the bacteria which spread through the lungs
14. as the lung tissues and blood vessels are damaged, infected individual often coughs out blood-stained sputum containing M. tuberculosis bacteria

Question 15

what is a tubercle?

Answer 15

a cluster of macrophages

Question 16

what are granulomas?

Answer 16

consists of many infected macrophages surrounded by immune cells such as activated T cells

Question 17

how is a caseous centre formed?

Answer 17

when large tubercles/granulomas are deprived of oxygen, the macrophages in the centre die forming a caseous centre

Question 18

why do people infected with TB cough out blood?

Answer 18

over time after infection, lungs and blood vessels become damaged causing infected individuals to cough out blood-stained sputum containing M. tuberculosis bacteria