Microbial Virulence

Question 1

What is commensal?

Answer 1

humans contain indigenous biota that live on body surfaces, both internal and external. GI tract mainly. low pathogenic potential unless there is a breach or changes in environment.

Question 2

What is virulence?

Answer 2

relative capacity of a microbe to cause damage to host cell

Question 3

What is a pathogen?

Answer 3

microbe capable of causing host cell damage

Question 4

How can pathogens be categorised?

Answer 4

obligate pathogens are only found associated with disease (e.g mycobacterium tuberculosis)

typical pathogens can be present as part of normal biota but frequently cause disease e.g. haemophilus influenzae

opportunistic pathogens are only seen in immunocompromised patients, breaches of normal defences or congenital defects e.g S. epidermis

Question 5

What is the difference between endogenous and exogenous infections?

Answer 5

endogenous infections occur when normal microbiota multiply and develop due to disruption of normal defences

exogenous infections occur when there is contraction from an external source, e.g inoculation

Question 6

How do bacteria colonise a host tissue?

Answer 6

• entry aided by polysaccharide capsule (prevents phagocytosis)
• evade defences
• adherance via pili/fimbraie to colonise mucosa
• cell uptake by endocytosis or molecular syringe (enzymes or proteins)
• toxin release

Question 7

How do bacteria cause host/tissue damage?

Answer 7

degrading enzymes e.g. proteases, lipases, DNAases

toxins-endotoxin (outer membranes of gram -ve bacteria provoke inflammatory response)

exotoxins- disrupt function or kill target cell

superantigens- bind t cell receptors and mhc class 2 and mimic effects of antigen presentation (damage via stimulation of massive cytokine release)

Question 8

How does the cholera toxin cause diarrhoea?

Answer 8

The cholera toxin is composed of an A and 5 B subunits. The B subunit activates GM1-ganglioside receptor on the intestinal cell allowing for endocytosis of the toxin ,which is then cleaved. The A subunit will activate an adenylyl cyclase, which causes an increase in cAMP, which stimulates an efflux of of water. This results in severe dehydration and electrolyte imbalance.

Question 9

How do viruses cause cell damage?

Answer 9

direct viral cytopathogenesis- dna damage, alteration of cell membrane structure or cytotoxicity

viral immunopathogenesis- flu like symptoms, delayed hypersensitivity or immune complex diseases

Question 10

What mechanisms do bacteria use to evade host defences?

Answer 10

• polysaccharide capsule to prevent phagocytosis
• sheltering in protected zones e.g abscess
• producing enzymes to block chemotaxis
• blocking of opsonisation e.g M protein produced by S aureus and other gram positive bacteria
• mimicking of antigens