Determinants of Infectious disease

Question 1

Virulence factors

Answer 1

Adhesion, invasion, evasion of host defence, obtaining nutrients from host and toxicity.

Question 2

Types of transmission

Answer 2

Direct (host-to-host transmission).

Indirect (host-to-host transmission through living of inanimate objects).

Question 3

Direct routes of transmission

Answer 3

Respiratory (M. tuberculosis), body contact (S. aureus), faecal-oral (S. enterica), body fluids and vertical.

Question 4

Indirect transmission types

Answer 4

Vehicles and vectors. Vehicles include: Soil, contaminated water (V. cholerae), contaminated food, fomites (C. difficile).
Vectors include: Anopheles spp (mosquito), warm-blooded animals (rabies) and rat flea.

Question 5

Adherence to surfaces depends on?

Answer 5

Adhesins

Question 6

Types of Adhesins?

Answer 6

Proteins (fimbrial and Afimbrial surface proteins) and polysaccharides (part of capsules, lipo/teichoic acid)

Question 7

How does E. coli adhere in UTIs?

Answer 7

P-pilli bind to sugar moieties (mannolose, globobiose) of glycolipids on epithelial cells lining urinary tract.

Question 8

Two types of Invasion?

Answer 8

Extracellular and intracellular.

Question 9

How is extracellular invasion achieved?

Answer 9

Production of enzymes that attack extracellular matrix, degrade carbohydrate-proteins complexes between cells, disrupt cell surface.

Question 10

5 steps process of phagocytosis.

Answer 10

1: WBC encounters bacterium that binds to cell membrane.
2: Phagocyte uses cytoskeleton to push cell membrane around the bacterium creating a phagosome.
3: Phagosome seperates from cell membrane and moves into cytoplasm.
4: Lysosomes fuse with phagosome.
5: Bacterium killed and digested.

Question 11

3 types of intracellular residence?

Answer 11

• Reside in phagolysosome.
• Reside in unfused phagosome.
• Destroy/escape phagosome and live in cytosol.

Question 12

Limiting nutrient for bacteria?

Answer 12

Iron.

Question 13

How is iron acquired?

Answer 13

Siderophores, these compounds (with catechol or hydroxymate group) have high affinity for iron and will compete for free or bound iron. Transport iron into cell.

Question 14

How do bacteria avoid host defence?

Answer 14

Evade complement, resist phagocytosis, reside intracellularly, evade host antibody response.

Question 15

Describe complement evasion.

Answer 15

Thick polysaccharide layer around cell and O-antigen (elongated O chains) prevents complement activation. LPS

Question 16

Methods of resisting phagocytosis.

Answer 16

Prevent effective contact with phagocyte (biofilm, capsules), Affect phagocyte migration, destroy phagocytes

Question 17

Describe the ‘evade host antibody response’.

Answer 17

Binds host proteins (S. pyogenes), surface protein which binds antibodies backwards (S. aureus) which prevents opsination (marked for elimination).

Question 18

Difference between endotoxin and exotoxin?

Answer 18

Exotoxin is actively secreted during growth, endotoxin is part of the bacteria that is released during bacteria lysis.

Question 19

Two types of transmission of exotoxins.

Answer 19

Ingestion of pre-formed exotonin. Colonisation followed by exotoxin production.

Question 20

What do superantigens cause and lead to?

Answer 20

Massive non-specific inflammatory response. Leads to endothelial damage, circulatory shock and multiple-organ failure.

Question 21

What does lipid A component cause?

Answer 21

Fever, initiates complement and clotting cascades can cause toxic shock.

Question 22

Describe Quorum sensing.

Answer 22

Cell-cell communication. When Autoinducer (AI) concentration gets high (when there is high cell density) R protein binds to AI and activates transcription of genes for virulence factors.