Pathogenicity: Virulence Determinants Flashcards
What roles do toxins have in disease?
- Historical
- Developing World
- Food poisoning
- Cancer
- Periodontology
What are the 3 damages that toxins cause?
- Endotoxin (membrane structure that is associated with gram - organisms)
– Lipopolysaccharide
2. Exotoxin (toxins secreted into cell) – Pore-forming toxins – Surface acting toxins – Intracellular toxins • including type III and IV toxins
3.Inappropriate immune response – Cytokine storm – Superantigens – Mimicry (b haemolytic Streptococci) – Chronic cell mediated response (TB granuloma)
Explain what an endotoxin (LPS) is
- Toxin produced by gram - bacteria (in outer membrane)
- Heat stable and poorly antigenic (hard to get rid of)
- Leads to shock and fever
- Lipid A associates with LPS Binding Protein (LBP) and binds to receptor CD14 in conjunction with TLR4
- Triggers signalling: cytokine release (cytokine storm), iNOS activation (Causing release of nitric oxide associated with shock), inflammation
What is the structure of endotoxin?
Has a lipid core and then 3 polysaccharide regions.
3 regions: Has an O antigen which comes from the outer core (repeating polysaccharides) and then the two other polysaccharide regions on the inner and outer core.
Different polysaccharides lengths leads to different appearances of colonies.
A long O antigen leads to a smooth and glossy colony form. Short o antigen - rough and matt colonies. No O antigen leads to rough region.
This results in changes in toxicity. The shorter the chain tends to be a more toxic bacteria.
How does the body recognise endotoxin?
It is the typical pathogen associated molecular pattern.
It complexed with LPS binding protein.
Then recognised by TRL4, CD24, MD2 as a receptor complex.
Recognition by macrophages leads to their activation and massive production of proinflammatory cytokines.
Leads to a cytokine storm.
Humans are very sensitive to endotoxin, other animals are less sensitive to them.
Exotoxins:
These are either membrane acting toxins and intracellular toxins.
What 3 effects can membrane-acting toxins have?
- Causing an enzymatic effect causing breakdown of membrane. Main enzyme is phospholipase which associates with phospholipid tail in membrane and leads to lesions here.
- Pore formation - forms holes in membrane so contents leaks out or in. These toxins will associate with a membrane receptor and once associated, polymerisation of subunits of pore forming toxins occurs. Once associated, the subunits undergo a conformational change to pass the whole way through the membrane.
- Membrane acting toxins which perturbs signalling - Rather than impacting the membrane itself, these exotoxins can work to affect signals coming into the cell also.
Give some examples of membrane acting toxins
- Small toxins (raises cyclic AMP levels)
- Specific proteases (cleave of cell receptors)
- Superantigens (activates a sub-class of the variable region of the T cell receptor
How does small toxin work?
They are small cysteine rich proteins which are heat stable.
They bind to extracellular domains and have molecular mimicry (similar to natural ligand).
This activates the intracellular domain to produce cAMP and cGMP.
This causes an exflux of chloride ions and prevents influx of sodium ions leading to diarrhoea.
How do superantigens work?
Super antigens do not have to bind to a specific receptor.
They associate with MHC2 receptor on APC and cross link it to a V beta region for the T cell receptor of a non specific T cell. This means that rather than getting specific activation of a T cell though its recognition of peptides and MHC together, we get non-specific activation.
Either leads to an increase in immune system and the other is enabling the microbe to evade the immune response by making sure the immune response is weak. T cell that will be activated will not be specific to peptide giving a weaker response.
How can intracellular toxins affect a cell?
- Can affect the membrane proteins
- Can be directly injected into cell to have effect
What are intracellular toxins classfied?
- Enzymatic activity
they bind to cell surface, cross membrane and then have their enzymatic effect
Explain the AB toxin which is an exotoxin
The toxin has 2 subunits. A is the activity of the toxic affect and the B part binds to the cell and gains entry.
Can get these with different structures e.g. more A or more B subunits.
They have a site of proteolytic cleavage to get inside cell.
How do neurotoxins work?
These attack the signalling between nerve cells.
Normally we see the production of signal down the neuron, reaching the synaptic knob causing vesicles to be released to the synaptic knob to release neurotransmitter across the space to be detected by the next neuron.
Toxins prevent vesicle containing the neurotransmitter to fuse with synaptic knob so no transmission occurs.
Common we see zinc proteases associated with the toxins affecting the functioning of the cytoskeleton to prevent the vesicle from fusing.
Explain how these two neurotoxins work?
- Tetanus toxin
- Botulinum toxin
What are the symptoms?
Tetanus - normal signal down neuron, get the single to the synaptic knob, release of neurotransmitters and then neurones release inhibitory transmitters to stop the response. The tetanus toxin stops the production of the inhibitory neurotransmitter so signal is nerve turned off so the signal is no longer short lived.
Symptoms = Restlessness, head-aches, irritability
BoTox - prevents the neurotransmitter vesicle from binding to the synaptic knob membrane so the signal is no longer instigated. Signal stops in the cell affected by this toxin.
Symptoms = muscle weakness, excess sweating, dry mouth, parkinsons
How does the toxin which causes diphtheria affect protein synthesis?
Toxin of diphtheria has the impact by blocking EF2 elongation. This is part of the protein synthesis pathway which enables the extension of a protein as the ribosome moves along. As the ribosome moves along, the amino acids are catalysed to bind to the next amino acid by EF2.
The toxin adds ADP ribose to inactivate EF2 so protein extension does not occur.
Structure = A and B region (active and binding).
Has an endosome insertion region which consists of alpha helixes which negatively charged loops between.
When in an endosome, conditions are acidic so H+ get associated with the negatively charged loops. This means the alpha helixes become closely accepted with membrane and can eventually penetrate through.
