How Bacterial Toxins Contribute to Disease and Dissemination Flashcards
What are Bacterial Toxins
Substances that are released/secreted from
the bacterium that causes damage to the host
(generally) regarded as critical in disease progression
What do bacterial toxins play a significant role in
Nutrient acquisition
Transmission
Immune evasion
Competition
What are Endotoxins
They are lipopolysaccharides (LPS)
They are relatively non-specific and trigger inflammatory responses which result in:
- fever
- septic shock
- organ failure
- death
What are LPS
Components of the Gram-negative cell wall that are vital to the structure and functional integrity of the bacterial outer-membrane
Can be released during the division or death of a bacterial cell (following antibiotic or immune system exposure)
What are the LPS components
- O-antigen: highly variable (serological specificity, exposed to the immune system)
- Lipid A: Disaccharide backbone which is highly conserved. Acyl chain length/substitution pattern is the primary determinant of its endotoxicity
What do Endotoxins trigger
Toll-like receptor engagement and activation
Inflammasome activation
Complement activation
What is the contribution of LPS in disease
Dysregulation/manipulation of the immune system
Our bodies recognise PAMP - evolved to be super-sensitive to LPS and therefore induced hyper-inflammatory conditions to eliminate bacteria from the bloodstream
What are exotoxins
They are actively secreted from the bacterium
They are often classified based on their binding/target site, mode of action, structure or generic function
What are exotoxin’s mode of action
ADP-ribosyltransferase (Cholera toxin)
Beta-barrel pore forming toxins e.g., S. aureus alpha haemolysin
What are exotoxins target/binding site
Cholesterol e.g., streptolysin O
Elongation factor 2 e.g., diphtheria toxin
Where/what does the exotoxin act on
Cytolytic, dermonecrotic, enterotoxin, leucocidin, neurotrotoxin, superantigen
What is the structure of Bacterial Exotoxins
Can be single proteins or organised into oligomeric protein structures.
Organised with distinct AB structure-function properties
What does the A domain of an Exotoxin include
Encodes an active (catalytic) activity
Examples:
- Ribosylation
- Glucosylation
- Proteolysis
- Non-covalent modification of host proteins
- modification through direct binding of host proteins.
What does the B domain of Exotoxins include
2 functional domains:
1) Receptor binding domain
2) Translocation domain
What is the structure of a Binary toxin
2 independent polypeptide chains: non-associated AB toxins (anthrax toxin)
What is the structure of an oligomeric toxin
Multimeric complex consisting of two or more non-covalently linked subunits/domains (cholera toxin)
What is the structure of a protoxin
Secreted in an inactive form (proenzyme) which can be converted into an active form by proteolytic enzyme (Iota toxin of Clostridium perfringens)
What is the structure of a pore-forming toxin
monomeric or bi-component molecules which bind to specific cells and oligomerise on he surface resulting in a cell membrane pore
What is Cholera
Bacteria: Gram-negative Vibrio cholerae
Mode of transmission: faecal-oral route
Symptom: voluminous diarrhoea (rice water stool)
Causes of death:
- dehydration
- acidosis
- hypovolemic shock
What is the cholera toxin (CTX)
- Classical AB(5) structure
Heterodimeric subunit composed of 2 polypeptide chains (CTX-A1 and CTX-A2) and a pentameric CTX-B subunit composed of 5 identical polypeptide chains. - CTX-B specifically binds to receptors on host cells (intestinal epithelial cells)
- Receptors: GM1 gangliosides (Primary) or histo-blood group antigens (Secondary) - binding promotes the endocytosis of the toxin
What is the CTX internalisation process
1) Endosomal compartment - trafficked to the Golgi and then to the ER
2) In the ER CTX dissociates, CTX-A1 is exported out of the cytosol and activated by the ADP ribosylation factor 6 (ARF6)
3) ARF6-CTX-A1 activates adenyl cyclase (AC) by catalysing ADP ribosylation of a G protein-coupled receptor (GPCR)
4) AC catalyses the conversion of ATP to cyclic adenosine monophosphate (cAMP), increasing the intracellular cAMP conc.
5) Activation of protein kinase A (PKA) phosphorylates the CFTR chloride channel proteins, ultimately resulting in the release of electrolytes (Cl-, HCO3-, Na+, K+) and water into the intestinal lumen, causing the secretory diarrhoea characteristic of cholera.
What is the Botulinum toxin
Bacteria: Gram-positive, anaerobic, spore-forming Clostridium botulinum
It is a neurotoxin
Prevalent in soli and marine sediments
Botulism can occur as a result of food contamination (canned homemade, preserved, or fermented)
What is the structure of the Botulinum toxin
Binary AB toxin: N-terminal is a zinc dependent metalloprotease which is linked to the C-terminal B fragments by a disulphide bond
How does the BoNT interfere with normal neurotransmitter release
Blocks the release of acetylcholine at the neuromuscular junction causing flaccid paralysis
What are the two specific receptors found on nerve cells that BoNT binds to:
- Polysialoogangliosides (complex sugar molecules on nerve cells)
- Synaptic vesicle proteins (Synaptotagmin & Synaptic Glycoprotein 2)
What happens when BoNT binds to the nerve cells
It is taken up via endocytosis and the light chain of the BoNT cleaves the SNARE proteins (which are needed for neurotransmitter release)
How does BoNT cause paralysis
By preventing the SNARE proteins from working, BoNT blocks the release of acetylcholine, a neurotransmitter essential for muscle contraction
Without acetylcholine, muscles cannot contract, leading to flaccid paralysis
How do normal neurotransmitters work
1) Acetylcholine is released at the neuromuscular junction via the assembly of the SNARE complex
2) Acetylcholine is released into the synaptic cleft and then binds receptors on the muscle cell
3) Acetylcholine is the primary neurotransmitter of the parasympathetic nervous system that enable smooth muscle contraction
How does the BoNT help Clostridium botulinum
There is the consumption of infectious spores while grazing
The spores would germinate and cause fulminant infection resulting in animal death
This results in release of nutrients and spores that can inoculate the soil and potentially promote plant growth and lead to transmission to a new host
BoNT may have some specific activity in the soil that is undiscovered - it would constitute the selective pressure to retain such a lethal factor
What are Leukotoxins: Leukocidins
Produced prominently by S. aureus
Responsible for a wide range of infections which differ in severity
How do Leukotoxins help the bacteria
- Releases nutrients from cells (haemoglobin from erythrocytes
- Target and kill immune cells
- Transmission: pus
What is the Enterotoxin: Shiga toxin
Released by: Gram-negative Escherichia coli common commensal in the bovine gut
Responsible for the morbidity and mortality in humans (Enterohaemorrhagic E. coli (EHEC))
Possesses Shiga toxin-encoding prophage - key role in disease progression
EHEC transmitted to humans from ruminants - contaminated foods, water animals, infected persons, contaminated surfaces.
What is the structure of the Shiga toxin
AB5 toxin - enzymatically active monomeric A subunit non-covalently attached to pentamer of identical B subunits
How does the shiga toxin it interact with the host cell
It is iron regulated - restricted at high Fe concentrations and therefore toxin production occurs in the distal small intestine and colon
Interacts with cell membrane glycolipid globotriaocylceramide (Gb3) - initiates endocytosis and internalisation
What is the process of internalisation of the shiga toxin
1) The toxin localises to early and recycling endosome
2) Retrograde trafficking machinery shuttles toxin from endosomal compartment through the Golgi and ER
3) The transfer requires clathrin and retromer proteins that are important in generating membrane curvatures and retrograde tubules
What is retro-translocation
This is a host cell intracellular pathway that translocates A subunit from the ER into the cytosol.
Here, furin cleaves the C terminal A domain (endosome) but they remain linked by disulphide bonds
In the ER lumen, disulphide bonds are reduced and the A domain is liberated
What does the Shiga toxin target
It facilitates the endohydrolysis of the N-glycosidic bonds at one specific adenosine residue at position 4324
It targets the 28S rRNA of eukaryotic ribosomes
It inhibits aminoacyl tRNA binding to the A site of the 60S ribosomal subunit - prevents the peptide elongation, protein synthesis and results in cell death.
What are the alternative roles of the shiga toxin
- Bacteria evolve toxins to compete or establish a niche, outside human hosts
- These ‘virulence factors’ provide a competitive advantage only in this niche and are therefore positively selected for.
What is the shiga toxin’s natural environment
Soil, water and bovine gastrointestinal tract
