Pathogenicity and virulence II Flashcards
what are the three main ways a microbe can cause damage?
endotoxin
exotoxin
inappropriate immune response
what bacteria produces endotoxin (LPS)?
- Produced by Gram negative bacteria and found on the outer membrane
what is the structure of LPS?
Lipid A core followed by three polysaccharide regions. Inner and outer core region and an O-Antigen which is a repeating motif of polysaccharides. Difference length of the chains of polysaccharides will lead to different appearances of colonies.
The shorter the polysaccharide chain sometimes the more potent the molecule is although this is not always the case.
What is the process of recognition of LPS?
- LPS is the prototypical PAMP
- Complexes with LPS Binding Protein (LBP)
- Recognised by TLR4/CD14/MD2 receptor complex
- Recognition by macrophages and monocytes leads to their activation and massive production of pro-inflammatory cytokines
- One of the main driving agents of sepsis and septic shock in gram-ve blood infections (bacteraemia)
- Presence in the blood is called endotoxaemia and drives a ‘cytokine storm
Different species demonstrates different experience to endotoxins.
What are exotoxins?
Exotoxins are Membrane damaging toxins they do this by enzymatic process, pore formation.
Enzymatic effect lead to the breakdown of the constituents of the membrane. Phospholipase (C. perfringens, Listeria, C.albicans etc).
explain how pore forming exotoxins leads to damage?
Pore formation – large pores or small pores 7 - ~30 subunits. This is seen by E. coli α-haemolysin, S. pyogenes streptolysin O, S.pneumoniae pneumolysin. Pore forming toxins will associate cholesterol this then leading to the polymerisation of the pore forming structure. Then undergo a confirmational change and protrude into the membrane structure.
There is a third process where a hydrophobic protein which will associate with a hydrophobic tail causing regions of the membrane to dissociate leading to lesions in the membrane.
give examples of perturbs signalling (surface/membrane acting signaling)?
- small toxins – ST (stable toxin) activates guanylin receptor to raise cyclic GMP
- superantigens – activates a sub-class of the vb regions of the T Cell receptor
- specific proteases – cleave of receptors form the surface of the cell.
what is a ST toxin?
- Small, cysteine rich peptide. Very Heat stable hence can be boiled.
- Produced by Enterotoxigenic E. coli (ETEC)
- It has its impact by Binding to the extracellular domain of guanylylcyclase C
- It does this through a process called molecular mimicry where it mimic natural ligand guanylin. Which leads to the Activation of the intracellular catalytic domain to produce cGMP
- High levels of this will lead to the exflux of Cl- and prevents influx of Na+, leading to osmotic diarrhoea.
what is a superantigen?
They bind to the MHC class II on the antigen presenting cells and cross link to region on the T cell receptor on a nonspecific T cell. Nonspecific activation.
Causes Toxic Shock Syndrome. Due to excessive activation of T cell immune response.
How do intracellular toxins attack cells ?
The intoxication of cells can be divided into three steps:
1 – Cell binding
2 – Membrane translocation
3 – Enzymatic action
give examples of intracellular toxins?
Exotoxins can be intracellular toxins.
* neurotoxins
* enterotoxins
* others
Generally classified by enzymatic activity like ADP-ribosylation, proteases, etc
For these toxins to work they need to be able to bind to cell surface then cross membrane and finally have enzymatic activity.
what are neurotoxins?
We usually see this in botulinum and tetanus toxin.
These toxins attack signalling between nerve cells. These toxins prevent the fusion of these vesicles with these neurotransmitters. Zinc proteases attack vesicle docking in synapse
what are the mechanisms of neurotoxin tetanus anf botulinum?
Tetanus toxins – STOPS the inhibitory transmitters so the signal across the synapse is not stopped, get a much prolonged signal.
Botulinum Toxin - prevents the excitatory transmitters vesicles from fusing with the synaptic membrane. Signal is never instigated.
what are the effects of neurotoxin tetanus toxin?
- Restlessness
- Head-aches
- Irritability
- Muscle spasms – ‘Lock Jaw
what are the effects of neurotoxin botulinum toxin?
- Muscle weakness : involuntary eye lid drooping, paralysis
- Excess sweating
- Dry Mouth
- Parkinson’s
what is the Clinical use of botulinum toxin?
- wrinkles: frown lines, “crow’s feet” etc - 1 million procedures last year in UK
- Associated with Aesthetic Dentistry
- Botox parties
what is the symptoms of diphtheria?
inflamed throat, diphtheric membrane in throat, fever.
what is the current treatment of diphtheria?
Prevention: vaccine
what is the mechanisms of diphtheria toxin?
Toxin has its impact by blocking EF-2 elongation factor. EF 2 is a major part of the protein synthesis pathway, Involved in peptide extension. Diphtheria toxin adds a ADPribose to EF2, inactivating it so no longer peptide extension hence no proteins.
what is the structure diphtheria?
The endosome insertion region consists of a series of alpha helices with these negatively charged loops between hydrophobic helices. When its taken up in the endosome the conditions become acidic which means we get hydrogen ions associating with these negatively charged loops. Alpha helices can be associated with the membrane.
what the mechanism of toxin in non toxin diseases?
These microbes do produce toxins but they need to find away for the toxins to get in the cell. They uses Type III and type IV secretion systems.
how do type III and type IV secretion systems work?
- Type III secretion system evolved from Flagellum
- Type IV secretion system evolved from Pili
Type III secretion system:
A complex of protein contact a eukaryotic cell trigger a compression which leads to the protrusion of this complex into the target cell.
Type IV secretion system:
Evolved from bacteriophages
Used to ‘inject’ both eukaryotic and prokaryotic cells
why are toxins so potent?
Target choice – picking key cellular components such as protein synthesis means they can have a massive impact.
* many toxins attack membrane
* a few toxins attack protein synthesis, cell can longer function and dies.
* many toxins signalling mechanisms, modulate cellular activity such as causes cells to produce a large number of nutrients.
Enzymatic action - one molecule can kill a cell
