Chapter 13 Background to the infectious diseases

Question 1

What happens if the multiplication threshold of a disease is reached due to a delayed immune response?

Answer 1

disease or death

Question 2

What is the best-studied example of the appearance of a highly lethal pathogen in a host population that gradually settles down to a state of more balance pathogenicity, and what are two potential causes for this?

Answer 2

Myxomatosis is the best-studied example of the appearance of a highly lethal pathogen in a host population that gradually settles down to a state of more balance pathogenicity. Either the virus changes and becomes less pathogenic or the host becomes more resistant to the pathogen.

Question 3

In what three main ways can pathogens invade a healthy host?

Answer 3

1. microbial attachment/penetration mechanism
2. biting arthropod
3. skin wound
   Invasion may also occur if the host is immunosuppressed

Question 4

Who founded the ‘germ theory’ of disease, and what does this theory state?

Answer 4

Robert Koch’s ‘germ theory’ maintained that certain diseases were caused by a single species of microbe.

Question 5

What are Koch’s postulates?

Answer 5

• The microbe must be present in every case of the disease.
• The microbe must be isolated from the diseased host and grown in pure culture.
• The disease must be reproduced when a pure culture is introduced into a non-disease-susceptible host.
• The microbe must be recoverable from an experimentally infected host.

Question 6

Describe the ‘iceberg’ concept of infectious disease.

Answer 6

tip: classical disease picture
midway: less severe disease
below the surface: asymptomatic infection (individual infects others, seroconverts, resists re-infection)

Question 7

What obligatory steps must a successful microorganism take?

Answer 7

attachment +/- entry into body

• local or general spread in the body
• multiplication
• evasion of host defenses
• shedding from body (exit)
• cause damage to host (not strictly necessary but often occurs)

Question 8

How do microbes fight the mechanical host defense of being rinsed away from epithelial surface by host secretions (plus ciliary activity in respiratory tract)?`

Answer 8

• bind firmly to epithelial surface (surface molecule on microbe attaches to ‘receptor’ molecule on host epithelial cell
• interfere with ciliary activity (produce ciliotoxic/ciliostatic molecule)

Question 9

How do microbes fight the mechanical host defense of host cell membranes as a barrier to the pathogen?

Answer 9

• traverse host cell membrane (fusion protein in viral envelope)
• enter cell by active penetration (microbial enzymes mediate cell penetration)

Question 10

How do microbes fight the phagocytic and immediate host defense of being ingested and killed by the phagocyte?

Answer 10

• inhibit phagocytosis (microbial outer wall or capsule impedes phagocytosis)
• inhibit phagosome-lysosome fusion (sulphatides of M. tuerculosis inhibit fusion)
• interfere with signal transduction in macrophage (induction of SOCS protein)
• resist killing and multiply in phagocyte (exit from phagosome into cytoplasm -Listeria)

Question 11

How do microbes fight the phagocytic and immediate host defense of the host molecule restricting availability of free iron needed by microbe?

Answer 11

microbe competes with host for iron (microbe posses avidly iron-binding siderophores)

Question 12

How do microbes fight the phagocytic and immediate host defense of complement activation with antimicrobial effects?

Answer 12

• inactivate complement components (production of an elastase)
• interfere with complement-mediated phagocytosis (C3b receptor on microbe competes with that on phagocyte and complement access blocked)

Question 13

How do microbes fight the phagocytic and immediate host defense of infected host producing interferons to inhibit virus replication?

Answer 13

• induce poor interferon response (core antigen of hepatitis B suppresses IFNbeta production)
• insensitive to interferons (prevent activation of IFN-induced enzymes)

Question 14

What is the microbial answer to the infected host producing antibody?

Answer 14

• destroy antiboy (bacterium liberates IgA protease)

- display Fc receptor on microbial surface (antibody bound to microbe in upside-down position)

Question 15

What is the microbial answer to infected host producing antimicrobial cell-mediated immune response?

Answer 15

• invade T cell, and interfere with their function or kill them (virus envelope molecule binds to CD4 on helper T-cell surface)
• induce regulatory T cells (suppress beneficial immunity)

Question 16

What is the microbial answer to the antimicrobial immune response of recognizing infected cells and destroying them?

Answer 16

-microbe in cell fails to display microbial antigens on cell surface (viral antigens not synthesized; virus inhibits transport of MHC I molecules to cell surface thus avoiding recognition by CD8 T cell)

Question 17

What is the microbial answer to production of an effective immune response being produced?

Answer 17

-vary microbial antigens in individual host, or during spread in host immunity (switch on different surface antigens; mutation, genetic recombination)

Question 18

What happens in your if you become infected with Epstein-Barr virus in your teens if you had not been infected in childhood?

Answer 18

Develop mononucleosis

If infected in childhood, then immune against EBV