helicobacter pylori Flashcards
discovery of helicobacter pylori
-1979 - Robin Warren began to note spiral bacterial in gastric biopsies from ulcer patients
- 1983 - bacteria were cultured by Warrens colleage barry marshall
- considerable scepticism over role as ulcer causing as stomach considered a sterile site
- marshall drank H.pylori in order to fulfill Kochs postulates - he developed mild illness with gastritis
- H.pylori now known to commonly colonise and persist in the human stomach mucous layer and cause ulcers
the bacterium
- spiral, gram-negative, distantly related to E.coli and salmonella
- tuft of 4-7 polar sheathed flagella
- microareophilic - grows best with around 5% oxygen in the environment
- very urease positive
H.pylori disease
- infections generally asymptomatic but in 20% cause some form of gastric pathology
- gastritis
- gastric ulcers
- gastric cancer
- only bacterium to be classified by WHO as type 1 carcinogen and h. pylori accounts for 95% of gastric cancers
- however, infection may also be beneficial
treatment
ulcers now routinely treated with antibiotic based triple or quadruple therapy - omeprazole or lansoprazole
- proton pump inhibitors that suppresses gastric acid secretion by specific inhibition of the H++/ k+ATPase in the gastric parietal cell
- clarithromycin
- amoxicillin and/ or metronidazole
7 day treatment regimen
- 80-90% effective in eradicating it
routes of infection and global infection rates
- no environmental or animal reservoir - spreads form person to person
- infection is by faecal - oral (water borne) or oral-oral (person to person) route
- higher incidence of infection among institutionalised children and adults
- also clustering of H.pylori infection within families
helicobacter colonising the stomach
there is a very thick layer of mucin that overlays the gastric cells, Largely protecting them against the Lumen environment
- The lumen environment is very dangerous because the pH is incredibly low
- it has to survive this acid environment and Lots of bacterial cells are unable to do that.
○ So it will initially likely be exposed to that acid environment and even once it’s colonized it may be exposed transiently to it subsequently
- and it needs to escape from this and find a replicative niche in the mucin layer where the pH is much more favorable.
And to do that it produces a whole host of virulence factors
H.pylori virulence factor to move into the mucin layer
- So it needs to be able to move so it has flagella
- it needs to protect itself against acid. So it has Urease
- it needs to enter this mucin layer - flagella - and it needs to adhere to cells and it needs to interact with cells and it needs to Control the immune response over the period of colonization.
- has a cork screw cell shape to burrow through dense matrix
how does H.pylori adhere to gastric epithelial cells
via Hop family outer membrane proteins
- BabA binds to Lewis B blood group antigen
- SabA binds to sialyl Lewis X
urease
for survival in the acid environment of the stomach
- stomach pH as low as 1 may rise to 3/4 on ingestion of food
- pH 7 optimal for bacterial growth
- acidic pH damages periplasmic and cytoplasmic proteins
- survival at acidic pH requires presence of urea
- to maintain cytoplasmic pH homeostasis H.pylori produces a cytoplasmic urease
- urease is a nickel - requiring metalloenzyme - six copeos of 2 subunits ureA and Urea
UreI
Inner membrane transporter for urea
when helicobacter finds itself in the stomach or in a low PH environment as a marker of the stomach environment Then the pH drops and this transporter will open its pores
- once the pH is low urea is taken up into the cell
- And then it’s degraded or broken down by the urease enzyme into carbon dioxide and ammonia.
- So the carbon dioxide is excreted and the ammonium can combine with protons
- Then the ammonium ion is then excreted out of the cell.
So as protons from a low PH environment are leaking into the cell. They are being mopped up by the ammonia that’s produced by ureI. So a very neat mechanism for maintaining pH homeostasis.
so this allows pylori to survive in this low PH environment.
diagnostic urea breath test
it involves feeding carbon 13 labelled urea to the patient in a liquid So it’s harmless.
- And then this labeled urea Will Be cleaved by the urease enzyme giving ammonia and carbon dioxide.
- So if your H pylori positive you’ll get this reaction occurring And you’ll get labeled carbon dioxide produced.
- That label carbon dioxide will then enter your bloodstream go to your lungs and then we’ll be excreted in your exhaled breath and after that you’re asked to breathe into a bag
- and then that bag is counted in a machine trying to identify the levels of radioactive carbon here.
- So elevated levels of carbon 13 indicate positive status for h. Pylori.
type 4 secretion systems
three functional types:
- conjugation - transfer of DNA into the host e.g. Agrobacterium tumefaciens
- transformation - DNA uptake e..g helicobacter pylori ComB system
- protein transfer - e,g. bordetella pertussis, legionella pneumophila, helicobacter pylori CagA
Cag pathogenicity island
- large pieces of DNA with multiple genes And it contains 29 genes
- for the most part they’re involved in making the type 4 secretion system in the bacterium.
- 1 of these proteins that are significant is the cag C pillin subunit.
. And then finally on the pathogenicity island is the single secreted effector protein. - So the presence of the CAG pathogenicity island makes a strain more likely to cause overt disease (some have it some dont)
CagA effect on cytoskeleton and cell shape
- delivery of CagA into host cell cytosol results in the activation of genes implicated in maintaining the host cell cytoskeleton
- this results in dramatic changes in cell shape
- Integrity of the monolayer is broken down
- that Integrity of the gastric epithelium
monolayer is critical For maintaining an appropriately structured gastric environment.
how is this dramatic change in cell architecture done ?
- it’s done by the activity of this single protein CagA, but it has multiple effects in the host cell,
- the first of these involves interacting with the host cell kinases which phosphorylate CagA
- this pathway is dependent on these host cell enzymes (Src and Abl) And this correlation occurs on tyrosine residues Within These motifs in the cagA protein itself.
- So cagA is injected by the Type 4 secretion system and then its phosphorylated by the host and that’s all designed by the bacteria
○ and then in a separate pathway Which is phosphorylation independent, the cagA protein will dimerize within the host cell and go on to have an effect.
○ not dependent at all on host cell enzymes.
