L11&12- Micro-Organisms in Disease

Question 1

What is Virulence ?

Answer 1

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

E.g: S. aureus is more virulent than S. viridans, as it causes disease much more readily.

Question 2

Koch’s postulates (Do we need to know?)

Answer 2

Organism should be present in disease but not in health.

Organism should be isolated from the diseased animal and grown in pure culture

Organism should cause the same disease in a newly inoculated animal.

Organism should be re-isolated from the experimentally-infected animal.

Question 3

What is Infectivity ?

Answer 3

The ability of a micro-organism to become established on/in a host

Question 4

Virulence factor: Endotoxin …What is it?

Answer 4

Component of the Gram-negative bacterial cell wall
E.g: E. coli and other Gram-negative bacilli
Neisseria meningitidis

Active component is lipopolysaccharide - LPS

Induces a range of uncontrolled host responses

Question 5

Endotoxin: host response - What can happen?

Answer 5

Systemic inflammatory response syndrome (SIRS):
-Uncontrolled T-lymphocyte response

• Uncontrolled activation of the clotting cascade
• Uncontrolled activation of complement

Question 6

Botulinum toxin effects

Answer 6

• Neurotoxic protein produced by the bacterium Clostridium botulinum (obligate anaerobe)
• Affect Neuromuscular junction- prevents muscle contraction
• prevents the release of acetylcholine

Question 7

Clostridium tetani (obligate anaerobe) - toxin effects

Answer 7

-Toxin production
Tetanospasmin
      -Produced on germination of spores
      -Binds to nerve synapses
      -Inhibits release of inhibitory neurotransmitters (e.g.            gamma-amino butyric acid) in the central nervous system

-Death caused by respiratory paralysis

Question 8

Meaning of Opisthotonus/opisthotonos

Answer 8

State of severe hyperextension and spasticity in which an individual’s head, neck and spinal column enter into a complete “bridging” or “arching” position

(Happens with Clostridium tetani)

Question 9

Streptococcus pyogenes can cause conditions such as:

Answer 9

Streptococcal sore throat

Erysipelas

Necrotizing fasciitis

Scarlet fever

-Strep. pyogenes has virulence factors that promote connective tissue breakdown and invasion

Question 10

Examples of how some bacteria can evade the immune system…

Answer 10

S. Pyogenes: M-protein binds fibrinogen and masks bacterial surface, blocking complement binding and opsonisation

S. pneumoniae: Polysaccharide capsule inhibits opsonisation and therefore phagocytosis1

Intracellular pathogens
Mycobacterium tuberculosis
Salmonella typhi
Listeria monocytogenes

Question 11

Brief overview of viruses

Answer 11

Small particles consisting of up to three components:

1) Genome - RNA or DNA
2) Capsid - protein
3) (Envelope – lipid bilayer)

Whole particle is called a virion

Not capable of independent existence

Use host mechanisms to satisfy the requirements of growth and replication

Question 12

List the generic stages of a virus’s life cycle

Answer 12

1) Adsorption - Interaction between host and virus
2) Penetration - receptor mediated endocytosis
3) Uncoating - Nucleic acids released
4) Synthesis - Proteins synthesised by host&viral enzyme
5) Assembly - of viral components
6) Release - Complete Virus’ released