Environmental Pathogens Lecture 2- E.coli

Question 1

Describe E.coli

Answer 1

• Gram negative
• Facultative anaerobe
• Non-sporulating
• Rod shaped
• Can live on a variety of substrates
• Mixed acid fermentation in anaerobic conditions- ideal for lower gut
• Optimal growth at 27 degrees, some ok at 49.
• Divides every 20 mins at optimum temperature

Question 2

What is a strain?

Answer 2

A subgroup within a species that has unique characteristics from other strains (molecular level and growth differences)

Question 3

Why can we source track E.coli

Answer 3

Different strains of E.coli are host specific, making source tracking possible

Question 4

Virulence determinants of E.coli

Answer 4

Adhesins- Type 1 fimbrae
Invasins- Haemolysin, Shigella
Motility- Flagella
Toxins- Shiga toxins, LPS
Antiphagocytic surface properties- capsules (K antigens), LPS
Defence against immune response= antigenic variation, capsules, LPS
Genetic attributes- genetic exchange, transmissible plasmids, drug resistance plasmids, siderophores (Fe+ uptakes from RBCS, iron is locked up in human body), pathogenicity islands, toxin and virulence plasmids.

Question 5

Name the most common pathogenic serotype of E.coli

Answer 5

E.coli O157:H7

There are over 700 serotypes of E.coli based off O, H and K antigen

Question 6

Describe the F antigen in E.coli

Answer 6

• F antigen- Fimbrae
• Shorter than flagella ~1000/cell
• Enhances bacterial ability to attach to host and cause disease (Virulence factor)

Question 7

Describe the H antigen in E.coli

Answer 7

• H antigen- Flagellum- Bacterial motor
• Driven by ATP, whip like structure, sensory perception and pathogenicity.
• 3 domains, N terminal, C terminal (which are conserved) and a middle variable region
• Surface exposed variable domain for flagellin is antigenically diverse
• Motility adhesion secretion of Virulence factors

Question 8

Describe the K antigen in E.coli

Answer 8

• K-antigen = capsule

- Makes the bacterium larger in size- elevated volume so macrophage can’t get hold of it.

Question 9

List the 5 pathogenic groups of E.coli

Answer 9

• Enterotoxigenic (ETEC)
• Enteroaggregative (EAEC)
• Enteroinvasive (EIEC)
• Enteropathogenic (EPEC)
• Enterohaemorrhagic (EHEC)- usually specified by their toxin producing capabilities e.g., Shiga-like producing (STEC)

Question 10

Describe Enterotoxigenic E.coli (ETEC)

Answer 10

Hosts: Causative agent of diahorrea (without fever) in pigs, humans, cattle, goats, sheep, dogs and horses

> Uses fimbrial adhesions to bind to enterocyte cells in the small intestine
Can produce 2 exotoxins
LT enterotoxin, similar to cholera toxin (structure/function)
ST enterotoxin causes cGMP accumulation and a subsequent secretion of fluid and electrolytes into the intestinal lumen
Non-invasive- they don’t leave the intestinal lumen
-ETEC is leading bacteria cause of diahorrea in the developing world with 200 million cases per year and 380,000 deaths.

Question 11

Describe Enteroaggregative E coli (EAEC)

Answer 11

Hosts= found only in humans

> Fimbrae which aggregate to tissue culture cells
EAEC bind to the intestinal mucosa to cause watery diahorrea without fever.
EAEC are not invasive.
They produce haemolysin and an ST enterotoxin similar to that of ETEC.

Question 12

Describe Enteroinvasive E. coli (EIEC)

Answer 12

Hosts= Found only in humans

> Infection causes a syndrome similar to Shigellosis with profuse diahorrea and fever.
Invasion by M cells
Phagocytosis by macrophages but EIEC can escape the phagosome and induce apoptosis of macrophages
Genes necessary for invasiveness carried by a plasmid
Prominent among those genes are mxi and spa genes that encode T3SSS
Ip (secreted) proteins IpA acts on host causing actin rearrangement and membrane ruffling resulting in bacterial internalisation (into cytoplasm)
IpB degrades vacuole that bacteria are inside within the cell and therefore release bacteria into cytosol
Outer membrane protein VirG are critical for triggering actin polymerisation and binding cytosol components

Question 13

Describe Enteropathogenic E Coli (EPEC)

Answer 13

Hosts= Causative agent of diahorrea in humans, rabbits, dogs, cats and horses
> Use of adherin (intimin) to bind to intestinal cells
> Lack fimbrae ST and LT toxins
> But has an array of virulence factors similar to those found in Shigella and may possess Shiga toxin.
> Adherence of EPEC to intestinal mucosa causes a rearrangement of actin in the host cell, causing significant deformation.
>Changes in intestinal cell ultrastructure is the cause of diahorrea
> Moderately invasive (i.e., they enter host cells) and elicit an inflammatory response.

Question 14

Describe Enterohaemorrhagic E.coli (EHEC)

Answer 14

Hosts= found in humans, cattle and goats.
> Most infamous virotype is E.coli O157:H7
> Causes bloody diahorrea and no fever
> Can cause haemolytic uremic syndrome (HUS)- haemolytic anaemia, blood clotting and sudden renal failure.
> Bacterial fimbrae for attachment (E.coli common plus, ECP).
> Moderately invasive
>Possess a phage- encoded Shiga toxin that can elicit intense inflammatory response.
> EHEC referred to by their toxin e.g., Shiga like producing EC (STEC).

Question 15

E.coli O157:H7 is not usually present in humans, so how does it transfer to them?

Answer 15

• Can survive 15 weeks in soil
• Not usually found in humans, but found in 1% of healthy cattle and meat therefore be contaminated.
• Water may also be contaminated by agricultural runoff and passed to humans via contaminated water
• Spread in petting zoos with ruminants
• Raw/unwashed vegetables
• Failing to wash hands

Question 16

E.Coli Signs and Symptoms

Answer 16

Define symptoms: Bloody diahorrea and vomiting.
Potential signs and symptoms: Fever

Complications: HUS and Thrombotic Thrombocytopenia purpura (TTP)

Question 17

What is HUS?

Answer 17

Haemolytic Uremic syndrome

• Acute complication that causes renal failure, mostly in children
• Characterised by haemolytic anaemia (RBCs are destroyed quicker than they are made).
• Low platelet count
• Causes significant blood clotting in the capillaries- as RBCs pass through clotted capillaries they are torn apart.
• Kidney failure is also seen where waste can’t be filtered

Question 18

What is TTP?

Answer 18

TTP= Thrombotic Thrombocytopenia Purpura

• Characterised by the same features as HUS (blood clots, low platelet count, haemolytic anaemia, kidney failure).
• But can involve fever and include CNS
• May have a more gradual onset than HUS

Question 19

Progression of E.coli O157:H7 in children

Answer 19

> -3 days before symptoms children ingest contaminated food/water
Day 0 symptoms including diahorrea, vomiting, abdominal pain and fever present
Day 2 bloody diahorrea (~9-%)
Day 3 stool specimen
Day 5 positive culture of E.coli O157
Day 7 diahorrea improves
In 85% of people spontaneous resolution occurs, but 15% of children develop HUS

Question 20

E.coli O157:H7 Scout case study

Answer 20

• Aberdeenshire: Agricultural ground where sheep has been grazing on land before the scout camp (sheep were asymptomatic).
• 337 campers, 20 became ill, no deaths
• No suggestion that food was primary source, instead was contaminated hands

Question 21

E. coli O157:H7 Germany Case Study

Answer 21

• Germany (2011) outbreak affected thousands of people
• Initially though to be STEC, but later confirmed EAEC-shiga
• 4000 cases, 54 deaths
• Fenugreek (from Egypt) was contaminated food source

Question 22

E.coli O157: H7 Lanarkshire Case Study

Answer 22

• Lanarkshire 1996
• Church lunch- supplied from an unhygenic butcher- contaminated beef stew was not properly cooked. 87 fell ill, 8 died.
• Residential home- cooked meats purchased from a second butcher who supplied from the first butcher. 8 deaths
• Individual cases: 8 deaths from infection- consumption of meat
  RESULT= changes in practices related to raw and cooked meat preparation and handling

Question 23

E.coli O157: H7 Shiga Toxin producing E.coli, UK

Answer 23

• UK 2013
• Bagged salads were the source of infection (watercress)
• 19 cases, 13 interviewees of which has purchased watercress from one retailer
• 0157 PT2 Stx 2 infection
• PT2= phage type 2, Stx 2= Shiga toxin type 2

Question 24

Explain how Shiga toxins cause disease

Answer 24

> Shiga toxins damage RBCs and cause blockages of capillaries
After ingestion of contaminated water the E.coli colonise the gut and produce the toxin which crosses the GI barrier to enter systemic circulation and reach kidney/other organs.