16: Microbes Causing Human Disease II - Escherichia Coli O157:H7

Question 1

Why was it initially difficult for scientists to identify the causal agent of HUS?

Answer 1

• Initially, microbiology and biochemical tests were not consistent with known E.coli
  pathogens.
• Only evidence that 0157:H7 was causative was that it was a very rare subtype of E.coli
  and it was associated with the cases - otherwise it didn’t resemble known E.coli pathogens

Question 2

What major mobile elements has O157:H7 acquired since its divergence from K12?

Answer 2

• LEE pathogenicity island - attachment and effacement
• 2 stx phages - makes them EHEC
• P0157 colonisation

Question 3

Why is the LEE island called the Locus of enterocyte effacement?

Answer 3

• responsible for attaching and effacing the intestinal epithelium of mammalian hosts

Question 4

Explain the system encoded by LEE that EPEC and EHEC strains use to attach to enterocytes

Answer 4

• T3SS, secretion of flagellin

Question 5

What class of toxin is Stx?

Answer 5

• Classical AB toxin. A subunit is toxin. B subunit involved in attachment. B subunit binds
  to Stx1 and Stx2. A subunit binds and damages the 23S rRNA.
• This and other activities of A subunit trigger stress, MAPK kinase pathway and apoptosis.

Question 6

What is the major target of toxicity of Stx?

Answer 6

The 23S rRNA. Stx cleaves 23S rRNA which prevents translation, triggers stress, MAPK kinase pathway and apoptosis

Question 7

How does Stx get into the endoplasmic reticulum?

Answer 7

• Shiga toxin exploits a process called retrograde transport whereby eukaryotic cells transport proteins backward through the secretion route
• Stx is transported from the membrane to golgi and back to endoplasmic reticulum. It then gets actively exported into the cytoskeleton and is able to act on targets

Question 8

Why are humans, pigs and rabbits affected by Stx, but not cattle?

Answer 8

Cattle have the GB3 receptor but don’t express it in gastrointestinal tract. Stx binds GB3 in rabbit and pigs, and causes pathogenisis

Question 9

What contribution does the pO157 plasmid make to EHEC?

Answer 9

• The p0157 plasmid carries a large no. of toxin genes including TOXB protein which is
  involved in adherence to epithelium. P0157 stimulates increased EPS and also carries genes involved in biofilm formation.

Question 10

Why is it distressing that antibiotics are still routinely used as a prophylactics and growth enhancers in agriculture?

Answer 10

• In the case of 0157:H7, the Stx phage that causes HUS in humans is a lambdoid phage. This means that its release is stimulated by the SOS response.
• The SOS response is caused by stress such as the administration of antibiotics.
• This results in cleavage of lambda C1 protein, excision of lambda genome, production of
  more phage particles, release of more toxin, and the subsequent infection of all other bacteria in the gut, making them pathogenic