PMB: Pathogenesis

Question 1

What is required for a pathogen to be able to infect?

Answer 1

• Their must be interaction between host and pathogen
• The Pathogen must be able to carry out the attack - be virulent
• The host must be capable of being infected (associated with a degree of being compromised)

Question 2

Describe symptoms between people when infected with a pathogen/ infectious disease

Answer 2

• Vary in terms of severity between people
• Often vary based on tissue affected and organism

Question 3

1. What are the symptoms of botulism?
2. What organism causes it?

Answer 3

1. Chlostridium Botulinum
2. Inital tiredness, weakened eyes, trouble speaking
3. Followed by spread to weakening of limbs and chest muscles

Question 4

What does virulence mean?

Answer 4

The capacity of a pathogen to cause disease (LD50)

Question 5

What is infectivity?

Answer 5

The number of organisms required to cause infection

Question 6

What does severity mean?

Answer 6

The impact on the host. Can range from acute to causing death

Question 7

What are factors which affect virulence?

Answer 7

• Age of individual
• Immune status
• Presence of other conditions
• Enviromental conditions
  
  • Temp, time of year

Question 8

What does a pathogen need to do to infect?

Answer 8

Must gain entery to the body and reach the approproate tissue

Question 9

1. What are weakness where pathogens are likely to gain entery?
2. What are altenrative sites of entery?

Answer 9

1. Eyes, Ears, other parts of skin (e.g. piercings)
2. Cut and insect bites

Question 10

After entery to the body what are the next steps for the pathogen to cause succesful infection

Answer 10

Following exposure to a pathogen, the pathogen must:

• Adhere to the organism
• Invade the tissue
• Evade the defence mechanism
• Cause disease

Question 11

Describe the adhering process

Answer 11

• Different approaches
• Selective or non-selective
• Capsules and slime layers
• Fimbriae and pili provide cell surface attachment via selective proteins

Question 12

1. Inital point of conact is not enough, what else is required
2. What is needed for this?

Answer 12

1. The pathogen must multiply to have enough organisms to cause infection
2. For the pathogen to survive sufficent nutriets are required. The provision of iron is cruicial. MAny bacterial pathogens often secrete siderophages which have a high affinity for iron

Question 13

Where does the bacteria go once infection site reached?

Answer 13

• Some stay locally
• Others spread to other parts of the body
  
  • Bacteriaemia - spread of bacteria in the blood stream
  • Can result in blood-borne systemic infections (septicaemia)

Question 14

What do some pathogens produce to enhance infection?

Answer 14

• Virulence Factors (VFs)
• These enchance virulence either directly or indirecly

Question 15

What type of molecule are most Vfs?

Answer 15

Enzymes

Question 16

What are examples of virulence factors used by different pathogens?

Answer 16

1. Many Staphylococcus species produce the enzyme hyaluronidase. This degrades polysaccharide hyaluronic acid (intercellular ‘glue’)
2. Staphylococcus species also produce the enzyme coagulase. This promotes fibrin clot formation which protects against attack by the host’s immune system.

Question 17

What other to VFs do pathogens use to aid virulence?

Answer 17

Many pathogens secrete endotoxins.

3 Main types:

1. Cytotoxins
2. AB toxins
3. Superantigens

Question 18

What do cytotoxins do?

Answer 18

Cytolytic in nature

Break open cells by degrading the host membrane and causing cell lysis

Question 19

Give examples of specific cytotoxins and their mechanisms

Answer 19

1. Heamolysins - destroy the phospholipid bilayer in cells e.g. erythrocytes
2. a-toxin from chlostridium perfringens is a lecithinase. It cleaves the membrane lipids in myocytes (muscle cells) and causes myonecrosis (gangrene)

Question 20

What is the general mechanism of AB toxins

Answer 20

• B subunit promotes up take of A subunit. A subunit causes damage to host cell

Question 21

Give specific examples of AB toxin and their mechanisms

Answer 21

• Corynebacterium diphtheriae produces the diphtheria toxin. Inhibits protein synthesis
• B subunit is bound to the cell surface. Causes endocytosis. A sub-unit is released after cleavage event. A subunit inactivates the elongation factor EF-2. Involved in translocation during elongation phase
• Clostridium tetani produces an AB toxin which acts as a neurotoxin. Blocks release of neurotransmitters at neuromuscular junctions. Causing spastic paralysis
• Specifically binds to ganglioside lipids of inhibitory neurons. Blocks glycine release. Causes uncontrolled release of acetylcholine (Ach). Failure of intercostal muscle relaxation causes asphyxiation
• Clostridium botulinum AB toxin (botox) causes flaccid paralysis via prevention of acetylcholine release

Question 22

Describe Superantigen toxin mechanism

Answer 22

• Superantigen toxins stimulate immune response
• Causes extensive inflammation
• Stimulate immune T helper cells in absence of normal antigens
• Problems such as toxic shock syndrome