Micro-organisms in disease

Question 1

Define pathogenicity and its requirements

Answer 1

The capacity of a micro-organism to cause an infection

• Transmissibility
• Establishment in/on a host
• Harmful effects
• Persistent

Question 2

What are the steps in the chain of infection?

Answer 2

1) Pathogenic organism
2) Resevoir
3) Mode of exit
4) Mode of transmission
5) Portal of entry
6) Susceptible host

Question 3

Define virulence

Answer 3

The degree to which a micro-organism is able to cause disease

Question 4

Define infectivity

Answer 4

The ability of a micro-organism to become established in/on a host e.g microbial ligand, host cell surface receptor

Question 5

What are ligand-receptor interactions? Give examples

Answer 5

• E.Coli fimbriae= glycolipids on human uroepithelial cells
• Tiny hair-like structures that express specific adhesins
• S.Pyogenes= fibronectin
• Large multifunctional glycoprotein in connective tissue
• Influenza haemagglutinin= respiratory epithelial sialic acid receptors

Question 6

Describe endotoxin

Answer 6

• Virulence factor
• Component of gram -ve bacterial cell wall (E.Coli)
• Released from damaged/dead cells
• Active component= lipopolysaccharide
• Binds to a number of host cell receptors
• Induces a range of uncontrolled host responses

Question 7

What is the host response to endotoxin?

Answer 7

• Systemic inflammatory response syndrome (SIRS includes septic shock)
• Uncontrolled T-lymphocyte response= cytokine release, fever, rigors, hypotension, collapse, tachycardia, cardiac/renal failure
• Uncontrolled activation of clotting cascade= disseminated intravascular coagulation, depletion of clotting factors, bleeding tendency
• Uncontrolled activation of complement

Question 8

What are the effects of Neisseria Meningitidis

Answer 8

• Endotoxin mediated inc in vascular permeability
• causes loss of protein, fluid, plasma into tissue
• Pathological compensatory vasoconstriction

Question 9

Describe exotoxins

Answer 9

• Virulence factor
• Protein produced by living bacteria
• Usually have specific effects on host
• Botulism

Question 10

What is clinically presented with botulism?

Answer 10

• Diplopia
• Dysphagia
• Dry mouth
• Dysarthria
• Death=respiratory failure

Question 11

What is tetanus?

Answer 11

• Clostridium Tetani
• Toxin produced= Tetanospasmin
• Produced on germination of spores, binds to nerve synapses, inhibits release of inhibitory neurotransmitters in CNS
• Presents Opisthotonos
• Death by respiratory paralysis

Question 12

Name some other exotoxin mediated infections

Answer 12

• Cholera
• Diptheria
• C.Diff
• E.Coli
• Whooping cough
• Scarlet fever
• Scalded-skin syndrome

Question 13

What syndromes can be caused by Strep Pyogenes and how?

Answer 13

• Virulence factors Hyaluronidase & Streptokinase promote connective tissue breakdown & invasion
• Streptolysins lyse red & white blood cells & platelets
• C5a peptidase inactivates complement component C5a
• Strep throat
• Erysipelas
• Necrotizing fasciitis
• Scarlet fever (erythrogenic toxin)
• Toxic shock syndrome toxin

Question 14

How does immune evasion occur?

Answer 14

• Inhibition of phagocytosis= S.Pyogenes (M-protein binds fibrinogen & masks bacterial surface blocking opsonisation), S.Pneumoniae (inhibits opsonisation)
• Intracellular pathogens= mycobacterium TB, Salmonella, Listeria monocytogenes