Pathogenicity II: Virulence & Determinants

• What are the 3 main toxic effects which cause damage?

  Endotoxin (LPS) effects

  Exotoxin effects

  Inappropriate immune response e.g cytokine storm
• What is endotoxin (LPS) produced by and what are its main features?

  Produced by Gram -ve bacteria

  Heat stable & poorly antigenic (doesn't trigger a good immune response)
• How does endotoxin (LPS) trigger its response?

  -Lipid A associates with LPS binding protein (LBP)

  
  

  -This binds to receptor CD14 in conjunction with TLR4

  
  

  -This triggers : cytokine storm, iNOS activation, inflammation
• Where is LPS found?

  outer membrane of gram negative bacteria

  
• What is the general structure of LPS?

  Lipid A followed by 3 polysaccharide regions

  
  

  Shorter the polysaccharide chain, the more potent (not always)

  
• Describe the PRR recognition of LPS and what happens.

  LPS binding protein combined with LPS is recognised by the TLR4/CD14/MD2 receptor complex.

  
  

  This causes recognition by macrophages and leads to massive production of pro-inflammatory cytokines

  
  

  Main driving agent of sepsis and septic shock in gram -ve blood

  
  

  Presence in the blood is called EDOTOXAEMIA and drives a cytokine storm

  
• What can exotoxins function as?

  Membrane-acting toxins (enzymatic effect leads to digestion or breakdown of membrane, pore formation)

  OR

  Intracellular toxins

  
• What will a pore-forming toxin associate with?


  A particular receptor in the surface of the membrane; can be a protein factor or cholesterol

  
• What happens after a pore-forming toxin associates with its target?

  Leads to polymerisation of the different subunits of the pore-forming toxin which can be anywhere from 7-30 subunits

  
  

  Then a conformational change occurs where they protrude into the membrane structure as a whole and pass through

  
• What can membrane-acting toxins do, what are they?

  
• Give an example of the small toxin and what they do.

  ST toxin is heat stable so can be boiled

  
  

  1. It binds to extracellular domain of gyanylylcyclase C

  
  

  2. activates intracellular catalytic domain to produce cGMP

  
  

  3.This causes exflux of Cl- and prevents influx of NA+ - CAUSING OSMOTIC DIARROHEA

  
• What do superantigens do?

  activate non-specific T cells and release cytokines; this causes toxic shock syndrome, fever, hypotension, shock, organ failure and death

  
  

  Also causes polyclonalactivation instead of monoclonalactivation leading to weak antiviral activity.

  
• What do intracellular toxins need to be able to do to work?

  Bind to cell surface

  Cross membrane

  Have enzymatic activity

  
• Describe the different types of AB toxins.

  AB toxins have 2 sub-units:

  
  

  -A subunit (has toxic affect of toxin)

  -B subunit (subunit that binds to the cell to gain entry)

  
• What are some examples of neurotoxins & what do they do?

  They attack signalling between nerve cells by attacking the vesicle docking in the synapse

  
  

  E.g Botulinum & tetanus toxin

  
• What is the mechanism of the tetanus toxin?

  Blocks release of inhibitory neurotransmitters (Glycine and GABA) to cause continued contraction

  
  

  Symptoms : Restlessness, headaches, muscle spasms

  
• What is the mechanism of the botulinum toxin (BOTOX)?

  Prevent the excitatory transmitter vesicles from fusing with synaptic membrane

  
  

  so release of excitatory transmitter is stopped and signal is never instigated in ongoing neurone

  
  

  Symptoms : muscle weakness (eye drooping), excess sweating, dry mouth
• What is the mechanism of the diphtheria toxin?


  It causes ADP-ribosylation of EF-2

  
  

  "A" unit makes ADP-ribosyl-EF-2 which the "B" unit ejects into the host cell

  
  

  This enzyme blocks protein synthesis by the ribosome, leading to epithelial cell damage and myocarditis

  
  

  Symptoms; inflamed throat

  
• What are some ADP-ribosylating toxins?

  Diphtheria toxin &P. aeruginosaETA-modify EF2 (blocks protein synthesis)

  Cholera & pertussis toxins &E. coliLT-modify signalling proteins
• What are the Type III/IV secretion systems & what have they evolved from?

  Needle-like structure enables bacteria to inject exotoxins into host cells

  
  

  Type III evolved from flagellum

  Type IV evolved from pili
• Show the Type III secretion in Yersinia.

  
• What is the Type VI secretion system?

  It's evolved from bacteriophages and is used to 'inject' both eukaryotic & prokaryotic cells e.g.Vibrio cholerae

  
• Why are toxins so potent?

  Target choice- key cellular components:

  Attack membrane

  Protein synthesis

  Signalling mechanisms

  Enzymatic action

  one molecule can kill a cell