Bacterial Intestinal Tract Infections

Question 1

What is included in the GI tract?

Answer 1

Stomach, large and small intestines

Question 2

What are the 3 main bacteria associated with GI tract infections and where do they act and what do they cause?

Answer 2

1) Helicobacter pylori (stomach) - gastritis, ulcers, stomach cancer
2) Vibrio cholerae (small intestine) - Cholera
3) The enterobacteriaceae, including E.coli and Salmonella (large intestine)
- E.coli includes variety of pathotypes including EHEC (enterohemorrhagic E.coli)
> Diarrhoea, haemolytic uremic syndrome

Question 3

What is H.pylori?

Answer 3

Small, spiral, gram-negative bacillus that inhabits the mucous layer overlaying gastric epithelial cells in humans

Question 4

How does H.pylori use ammonia to cause tissue damage?

Answer 4

• Produces a potent urease, which, by producing ammonia, neutralises gastric acid
  Ammonia has toxic effect on epithelial cells

Question 5

What % of the worlds population is affected with H.pylori and what % of infected are asymptomatic?

Answer 5

~50% infected

~80% of infected are asymptomatic

Question 6

How do pathogenicity islands work in more pathogenic strains of H.pylori?

Answer 6

• Have pathogenicity island (PI) in genome
• The PI encodes type IV secretion system which injects which injects the protein CagA into cytoplasm of host cells
• CagA interacts with numerous proteins within gastric epithelial cells

Question 7

What are the 3 main effects of CagA?

Answer 7

1) Perturbation (system deviation) of intracellular actin trafficking
2) Stimulation of inflammatory responses
3) Disruption of cellular tight junctions

Question 8

What 2 types of infection can E.coli cause?

Answer 8

Intestinal infections (IPEC)
Extra-intestinal infections (ExPEC)
ExPEC's include:
- Bloodstream (sepsis)
- Brain (meningitis)
- Urinary tract (UPEC strains) (common)

Question 9

How does Vibrio cholerae cause cholera?

Answer 9

• Bacterium colonises the small intestine, using type IV pilus (toxin coregulated pilus or TCP) that attaches to specific host receptors
• Diarrhoea is caused primarily as a consequence of the action of cholera toxin (CT), another AB type toxin (A1B5) on host cells
• CT secreted from bacterial cell using bacterial type II secretion system

Question 10

How does cholera toxin work on a molecular level? (WARNING: VERY COMPLICATED)

Answer 10

• B subunit binds to GM1-ganglioside receptor on colonic mucosa and the A subunit is internalised by endocytosis
• A subunit is an ADP ribosylating enzyme that leads to disruption to ion flow across mucosa and hence severe diarrhoea
• A1 fragment activates the G protein Gs alpha to stimulate adenylate cyclase to produce cAMP
• High cAMP levels activate cystic fibrosis transmembrane conductance regulator (CFTR), causing a dramatic effluc of ions of water from infected erythrocytes, leading to watery diarrhoea

Question 11

What does commensal mean?

Answer 11

Living with
Opposed to pathogenic
E.coli can be either

Question 12

Is enterohaemorrhagic E.coli (EHEC) commensal or pathogenic in humans?

Answer 12

Pathogenic

Question 13

What syndrome is EHEC associated with??

Answer 13

Associated with development of haemolytic uraemic syndrome (HUS)
- E.coli O157:H7 most commonly associated with HUS

Question 14

Is EHEC invasive or non invasive?

Answer 14

Non invasive (colonises the intestinal mucosa - does not spread further)

Question 15

What is the range of HUS symptoms?

Answer 15

In men, can range from none life threatening to life threatening, associated with microangiopathic haemolytic anaemia (destruction of erythrocytes)
Common symptoms are severe abdominal cramps and severe bloody diarrhea

Question 16

What % of EHEC cases result in HUS, and what % is fatal?

Answer 16

HUS appears in around 10-15% of cases, with higher incidence in children
Fatal in around 4% of cases

Question 17

When was E.coli O157:H7 thought to have evolved?

Answer 17

In the last 40 years

Question 18

What is Bundle forming pilus (BFP)?

Answer 18

A type IV pilus

• needed for initial attachment of EHEC
• forms dynamic fibres that promote the formation of aggregates

Question 19

Explain how EHEC invades the epithelial cells on the mucosa surface?

Answer 19

Promotes formation of attaching and effacing structures on the mucosal surface with disruption of the microvilli in the small intestine
The involves:
> loss of microvillus
> formation of actin rich structures on the cell surface

Question 20

What gene is required for attachment and effacement?

Answer 20

• Found on pathogenicity island
• Called Locus enterocyte effacement (LEE)
• Includes a type III secretion system (TSS)

Question 21

What proteins are secreted by the type III secretion system and what do they cause?

Answer 21

• Tir, which is localised to the host cell membrane and acts as the receptor for non-fimbrial adhesin Intimin
• Thus, EHEC changes the host to display an adhesin to which the bacteria can bind
• Also secretes proteins that stimulate WASP (Wiscott-Aldrich syndrome proteins) and AP2/3 promoting actin polymerisation and attaching and effacing lesions

Question 22

The EHEC ancestor sis not possess the LEE but acquired the attributes common to O157:H7 in a step wise manner. What are these steps?

Answer 22

1) Acquired Shiga toxin 2 gene
> These genes encoded on a lambdoid bacteriophage
2) Switch from O55 to O157
3) Acquired large virulence plasmid pO157
4) Ability to produce Shiga toxin 1 (also bacteriophage encoded)

Question 23

What evidence suggests that organisms with Shiga toxin 2 gene do not need LEE to cause HUS?

Answer 23

Closely related pathogens that cause HUS do not encode the LEE

Question 24

How does Shiga toxin cause disease?

Answer 24

Binds to Gb3 on specific cells in kidney, leading to acute onset renal impairment with microangiopathic haemolytic anaemia
- Effecr of Stx requires it to be internalised by clatherin-dependent endocytosis

Question 25

How does the Stx2 cause disease?

Answer 25

Causes depurination of the 28s subunit of the rRNA leading to inhibition of protein synthesis

• This causes ribotoxic stress, cytokine release and apoptosis
• This causes damage to vascular endothelium and is associated with thrombotic lesions and disseminated intravascular coagulation (DIC)
• Renal failure most significant health issue

Question 26

What are the common causes of EHEC transmission?

Answer 26

Ruminants, especially sheep and cattle, are common source of infection

• Person to person spread is around 20% of cases
• Infection in animals is sporadic and asymptomatic
• Meat, diary products, veg and apple juice most common sources

Question 27

What is carriage in animals in relation to EHEC?

Answer 27

Carriage = transport/transmission
Carriage highly variable in aminals and sporadic
- Carriage does not necessarily mean to animal is shedding the bacterial
- Carriage higher in warmer months

Question 28

What is the source sink model?

Answer 28

A local demographic surplus arises in good quality habitats (source), and a local demographic deficit occurs in habitats of poor quality (sink). Within a landscape, a permanent migration of individuals from source to sink habitats may lead to a stabilization of the overall demographic system.

Question 29

How is EHEC transmittion related to source sink?

Answer 29

EHEC pathogenesis in humans fits a source sink model

• ruminant are source
• humans are sink

Question 30

What are the most common preventative measures against EHEC spread?

Answer 30

• Cooking food
• Water chlorination
• Prevention of food cross contaminating
• Hand washing
• Infected individuals can be isolated

Question 31

How is renal failure associated with HUS prevented?

Answer 31

Administration of intravenous fluid

Question 32

Why are vaccines and antibiotics not commonly used?

Answer 32

Vaccination of animals is costly

Antibiotics costly and likely to increase resistance

Question 33

What is a new suggestion that may decrease carriage in cattle?

Answer 33

Feeding them hay rather than grain