type 3 toxins - A-B toxins and examples of diseases caused by A-B toxins Flashcards
cholera
it’s a disease that’s transmitted via contaminated water.
- causative agent: vibrio cholerae
gram-negative, curved rod, motile (single polar flagellum)
- capable of respiratory and fermentative metabolism
reservoirs of cholera
infected humans especially asymptomatic carriers
cholera toxin disease
disease due to the secretion of enterotoxin: cholera toxin
- highly fatal (50-60% mortality)
- symptoms - sudden onset of diarrhea
leads to dehydration, anuria, acidosis,, and shock
loss of potassium ions leads to cardiac complications and circulatory failure ->Death
prevention: clean water supply, effective sewage treatment
cholerae epidemiology - pandemics
several recorded pandemics and these date back to 1817.
Start in the Indian subcontinent and then they get spread throughout the world.
So in this second pandemic, one of the key events here was that Lord Snow discovered
that actually this was a waterborne disease and it was the first evidence of transmission
of an infectious disease. So he was able to show that cases of Cholera in London
were all Associated with people using the same water pump
vibrio cholerae major virulence factors - motility
you ingest the bacteria And then they colonise the intestinal tract
- key to its colonization are that its got flagellum that helps with motility and that’s been shown to help it get through the mucosal layer above the epithelial cells to access tepithelial cells.
- There’s also what’s called the accessory colonization Factor genes that have been shown to regulate motility and again that will help Crossing that barrier of mucus to get to the epithelia.
vibrio cholerae major virulence factors - neuraminidase
It also produces a neuraminidase which again might help with penetration of the mucin
vibrio cholerae major virulence factors - adherence
- toxin co regulated pili
- clumped at one pole of the cell
- Tcp genes are clustered on a pathogenicity island
- Absolutely essential for colonization.
○ If you get rid of the genes that encode the pilli you lose colonization.
non fimbrial adhesins: haemagglutinins
evolution of pandemic strains of cholera toxin
So in terms of evolution of these pandemic strains There’s been two key events involving the acquisition of large chunks of DNA
- first of all, You’ve got the ancestral strain that doesn’t produce a toxin and that acquired
the pathogenicity island that encodes the TCP pilli - Then the pilli act as a receptor for CTX phage that then brought in the cholera toxin.
So they had to have acquired this pathogenicity island before they could acquire the phage encoding the toxins
pathogenisis of cholera
- first of all, you ingest the bacteria in contaminated water or food
-the bacteria adhere to and colonise small intestinal mucosa. They don’t invade they just stay localized within the intestinal tract, but they’re able to colonize and multiply - Important in the colonization- the flagella for motility and the TCP pilli for adherence
- So once the bacteria colonise, they start to produce the Toxin and the key toxin is cholera toxin
- the cholera toxin acts on the mucosal cells. It doesn’t kill them. It doesn’t disrupt them. But it Alters their function in that now the cells Drive the movement of water ions out of the body into the gut Lumen.
-So then you get this massive fluid and ion loss
what changes does cholera toxin cause in movement of ions and water across intestinal mucosa
Normally water can pass freely across bacterial membranes.
the movement of water is controlled by the transport of ions such as sodium, chloride potassium.
- So you’ve got pumps on cells that transport ions across the cell membrane.
- in the gut Lumen, the epithelial cells you have a net uptake of sodium into the epithelial cells and that brings with it water and then that will go into the Blood and Tissue.
what happens with cholera toxin is it blocks that sodium uptake and transports chloride ions out of the epithelial cells.
- So as chloride ions go Out it brings water with it and then that brings ions and water from the bloodstream and the tissues
structure of cholera toxin
- cholera toxin is a good example of a compound A-B Toxin and there’s five B subunits to every a chain.
- the toxin is encoded by two different genes CTX a and CTX b
- the CTX a encodes the a chain- the catalytic component of the toxin
- Whereas CTX b encodes the B chain
- the code transcribe these two genes but you obviously will end up with two different polypeptides.
- the ribosome binding site for CTX B is slightly stronger than that for CTX a so you get slightly more of B protein being produced and then what happens is that they come together in a five to one ratio.
2 domains within the A chain of cholera toxin
- Within the a chain there is actually two domains for cholera toxin
○ There’s a part that encodes the catalytic component, but there’s also a second part A2 and they get held together by a disulfide bond and this gets cleaved just like a simple A-B toxin. - the receptor binding part is the B chain.
cholera toxin assembly
The A chain and the B chain get synthesised. they get exported to the bacterial periplasm where they assemble in a five to one ratio.
- Then the a chain gets cleaved into A1 and A2 and those two bits get held together by a disulfide bond and it’s the A1 which is where you’ve got the catalytic enzymatic activity.
what does cholera toxin do
- it Alters the activity of the enzyme called adenylate cyclase.
- adenylate cyclase is responsible for cyclic AMP production and cholera toxin locks adenylate cyclase into an on position. So it produces too much cyclic AMP
- And that affects chloride channels in the cell leading to the loss of chloride ions.
how does cholera toxin get into cells
- the B subunits bind to gm1 gangliosides on the surface of epithelial cells.
-and that gets internalized into an endosome. - it gets retrograde trafficked to the endoplasmic reticulum and it may also pass through the Golgi as an intermediate, but it ends up fusing with the endoplasmic reticulum.
-the cholera toxin now is associated with the membrane of the ER, so the B subunits stay bound to that membrane. - Then a disulfide isomerase comes along and break that disulfide bond releasing the A1 chain - that gets secreted out of the ER by sec 61, which is a protein export system.
- it gets unfolded, Goes into the host cell cytosol and that’s where it reforms its shape and carries out its toxic activity.
