0220 - Infectious Diarrhoea

Question 1

What is the difference between intoxication and infection followed by intoxication (diahrroea)?

Answer 1

Intoxication results from the ingestion of a toxin that has been produced outside the body - the presence of viable bacteria is not required.
Infection followed by intoxication results when a pathogen colonises the body and releases toxins which cause the disease.

Question 2

Give 2 examples of diarrhoea by intoxication.

Answer 2

Botulism Toxin (Clostridium Botulinum)
Staph Endotoxin (SEA/B/C etc) - Staph. Aureus.

Question 3

Give 4 examples of diarrhoea by infection followed by intoxication.

Answer 3

Vibrio Cholerae - Cholera Toxin
Enterotoxigenic Escherichia Coli (ETEC) - Heat-labile and heat-stable toxins
Enterohaemorrhagic Escherichia Coli (EHEC) - Shiga Toxin
Clostridium Difficile - Toxin A and Toxin B

Question 4

Describe Cholera Toxin. How does it cause diarrhoea?

Answer 4

Toxin produced by Vibrio Cholerae - Gram-negative, facultative anaerobic, highly motile rod bacteria that spreads through contaminated water/food. Adheres in small intestine, and has 7 toxins that together result in severe diarrhoea.
CT is an AB5 toxin, A bound to pentamer of B, who deliver A to host cell. Causes increased Cl secretion, decreased Na absorption (increased NaCl in lumen). Catalyses ATP to cAMP, increasing activity of CFTR and decreasing activity of NHE2/3 transporters.

Question 5

What are the 6 mechanisms of causing diarrhoea?

Answer 5

All work either on transporters or lateral spacings between cells.
1 - CFTR/CLCA - Increase Cl- secretion (channel)
2 - NHE3 - Reduce Na+ Absorption (Na/H exchanger)
3 - Tight Junctions - Interfere to increase paracellular permeability.
4 - DRA - Reduce Cl- absorption (Cl/HCO3- exchanger)
5 - Aquaporins - Reduce H2O absorption
6 - SGLT1 - Reduce Na+ and glucose absorption (Na/GLU cotransporter).

Question 6

Describe Enterotoxigenic Escherichia Coli (ETEC) and its toxins. How do they cause diarrhoea?

Answer 6

Gram-negative, facultative anaerobic, motile rod bacteria which colonises the small intestine and releases toxins that directly cause watery diarrhoea. Heat Labile Toxin (LT) and Heat Stable Toxin (ST)
LT - Similar to Cholera toxin in structure (AB5) and function.
ST - Cysteine-rich peptides, catalyse conversion of GTP to cGMP, increasing activity of CFTR (Cl-), and decreasing activity of NHE3 transporters. This leads to increased NaCl in the lumen.

Question 7

Describe Enterohaemorrhagic Escherichia Coli (EHEC) and the diarrhoea it causes.

Answer 7

Gram-negative, facultative anaerobe rod. Spread via raw meat/milk and faecal/oral contamination. Requires Intimin and Tir to colonise. Toxins are not solely responsible for causing diarrhoea.
Intimin (adhesin) and Tir (receptor secreted through type III secretion system) mediate attaching/effacing lesions - destroy microvilli, decrease surface area causing increased osmolarity and malabsorption.
Shiga Toxin 1&2 - Antigenically distinct AB5 toxins. STxA inactivate ribosomes, inducing apoptosis. Both 1 and 2 inhibit absorption of water from lumen, and are responsible for progression to haemolytic uraemic syndrome (causes renal failure).

Question 8

Describe Clostridium Difficile

Answer 8

Gram-positive, anaerobic, spore-forming rod. Multi-drug resistant and spreads easily due to spores which aren’t killed by much except bleach.
Part of normal flora, but symptoms present post-antibiotics.
Requires colonisation to produce toxins that directly and indirectly cause diarrhoea.

Question 9

What toxins does Clostridium Difficile contain?

Answer 9

All very big (150-300kDa)
Toxin A (TcdA) - directly increases transepithelial permeability (barrier), while increasing chloride secretion and decreasing sodium absorption (increased NaCl in lumen)
Toxin B (TcdB) - accesses basolateral side of cell after tight junction disruption 
Binary Toxin (CDT) - Destabilises cytoskeleton, potentiating toxicity of TcdA and TcdB.

Question 10

How do TcdA and TcdB work? (C. Diff toxins)

Answer 10

Hit both tight junctions and ion transport.
Glycosylate GTPase protein Rho, disrupting cystoskeletal integrity, leading to apoptosis.
Initiate inflammatory cascade, causing increased damage and fluid release.
Activate enteric nerves, and TcdA enhances production/release of neuropeptides - elicitying chloride secretion.

Question 11

What is the pathogenesis of C. Difficile diarrhoea?

Answer 11

Enterotoxins trigger intestinal secretion, impair tight junctions with necroinflammation, activating mast cells, nerves, vascular endothelium and immune cells.