Lecture 1 - Escherichia coli Flashcards
Microbial pathogens
16 million deaths from infectious diseases
1 in 12 live with viral hepatitis
50M new chlamydial cases per year
Clostridium botulinum - 3g could kill UK population, 400g could kill world population
1400 known species of microbial pathogens
Human pathogens: 1% of total microbial species on planet
E. coli
E. coli is not a pathogen per se
Gram negative, facultative anaerobe and non-sporulating
rod shaped, (2um x 0.5um with 0.6-0.7um3 volume)
Lives on wide variety of substrates - mixed-acid fermentation in anaerobic conditions - ideal for the lower gut
Optimal growth of E. coli occurs at 37 degrees, some ok at 49 degrees
Strains that possess flagella are motile - mostly peritrichous
HGT - Possess the ability to transfer DNA via bacterial conjugation, transduction or transformation, which allows genetic material to spread horizontally through an existing population, can act as donor or recipient (see later lecture).
Traits of E. coli
Very high degree of genetic and phenotypic diversity (only 20% of genome is common to all strains)
Many strains: A strain (phylotype) is a sub-group within the species that has unique characteristics that distinguish it from other strains (molecular level; growth differences)
Different strains of E. coli are often host-specific, making it possible to determine the source of faecal contamination in environmental samples.
e.g. Whether water contamination originated from a human, another mammal or a bird.
Strains are part of the normal flora of the gut,
and can benefit their hosts by producing vitamin K2 and B12
prevent (with others) the establishment of pathogenic bacteria within the gut.
E. coli constitute about 0.1% of gut flora.
E. coli normally colonizes an infant’s gastrointestinal tract within 40 hours of birth, arriving with food or water or with the individuals handling the child.
In the bowel, it adheres to the mucus of the large intestine.
As long as these bacteria do not acquire genetic elements encoding for virulence factors, they remain benign commensals.
E. coli turns bad - diseases
Causes in humans - Gastroenteritis, UTIs, neonatal meningitis
Rarer cases: Haemolytic-uremic syndrome (HUS)
Peritonitis (peritoneum)
Mastitis
Septicaemia
Gram-negative pneumonia
Virulence Determinants of pathogenic E. coli
Adhesins e.g. P fimbriae, S fimbriae, CFAI/CFAII
Invasins e.g. haemolysin
Motiltiy/chemotaxis e.g. flagella
Toxins e.g. LT toxin (heat liable), ST (heat stable), Shiga
Antiphagocytic surface proteins e.g. capsules, K antigens, Lipopolysaccharides
Defence against serum bactericidal reactions e.g. LPS, K antigens
Defense against immune responses e.g. capsules, K antigens, LPS
Genetic attributes - genetic exchange by transduction/conjugation
Plasmids
Classification of Pathogenic E.coli
Over 700 antigenic types (serotypes) of E. coli are recognised based on O, H and K antigens
O antigen - part of lipopolysaccharide layer
K antigen - Capsule
H antigen - flagellin
F antigen - MR fimbriae (rare)
Escherichia coli O157:H7
All elicit an immune response in animals
Flagellum: H antigen
, sensory perception and pathogenicity
composed of over 20 distinct structural proteins to form the flagellar basal body, hook, and filament.
All bacterial flagellin proteins can be divided into three distinct subdomains:
the N and C termini (conserved within a given species) and a middle variable region
The surface-exposed variable domain of the
flagellin is antigenically diverse –exploited for identification
Flagella are often involved in pathogenesis, with roles in motility adhesion and, in some cases, the secretion of virulence factors
Pathogenic E. coli
Have acquired virulence genes (HGT)
Six recognized categories of diarrhoeagenic E. coli (phylotypes) each have unique features in their interaction with eukaryotic cells
Pathogentic types of E. coli
EnteroPathogenic E. coli
EnteroHaemorrhagic E. coli
EnteroToxigenic E. coli
EnteroAggressive E. coli
Diffusely Adherent E. coli
EnteroInvasive E. coli
EPEC
EnteroPathogenic EC adhere to small bowel enterocytes
destroys the normal microvillar architecture
Cytoskeletal cellular derangements are accompanied by an inflammatory response and diarrhoea.
1.Initial adhesion
2. Protein translocation by type III secretion
3. Pedestal formation
EHEC
EnteroHaemorrhagic EC induce the attaching and effacing lesion, but in the colon.
The distinguishing feature of EHEC:
- is the secretion of Shiga toxin (Stx),
systemic absorption
- leads to potentially life-threatening complications.
- inflammatory response and diarrhoea.
ETEC
Similar to EHEC, but EnteroToxigenic EC adhere to small bowel Enterocytes
induces watery diarrhoea by the secretion of heat-labile (LT) and/or heat-stable (ST) enterotoxins.
EAEC
EnteroAggressive EC adheres to small and large bowel epithelia in a thick biofilm
releases secretory enterotoxins and cytotoxins
DAEC
Diffusely Adherent EC elicits:
- a characteristic signal transduction effect in small bowel enterocytes
- manifests as the growth of long finger-like cellular projections, which wrap around the bacteria.
EIEC
EnteroInvasive EC invades the colonic epithelial cell
- lyses the phagosome
- moves through the cell by nucleating actin microfilaments.
The bacteria might move laterally through the epithelium by direct cell-to-cell spread or might exit and re-enter the baso-lateral plasma membrane
Model of pathogenesis induced by EIEC
EIEC invade the epithelium from the intestinal lumen through M-cells.
They invade epithelial cells and are phagocytosed by resident macrophages.
EIEC escape the phagosome and replicate within epithelial cells - induce apoptosis in macrophages.
Bacteria are released may:
invade the epithelial cells from the baso-lateral side,
move into the cytoplasm by triggering actin polymerization
spread to adjacent cells
Genes necessary for invasiveness are carried 140-MDa plasmid in EIEC (pInv)
Genes: mxi and spa loci
Type III secretion systems (T3SSs) which are central virulence factors
used to inject protein effectors of virulence into eukaryotic host cells.
The Ipa proteins (IpaA to IpaD) are secreted proteins
causing actin rearrangement
membrane ruffling resulting in bacterial internalization
Once inside a vacuole in the cell
the IpaB protein degrades the vacuole,
releasing the bacteria into the cytosol
An outer-membrane protein VirG (IcsA)
triggers actin polymerization by binding cytosol components which propels the organism through the cell into neighboring cells
VirR gene controls expression of virulence genes
chromosomally encoded
affected by temperature
Bacteria which are invasive at 37oC become non-invasive at 30oC
EHEC
Most infamous member is O157:H7
Bloody diarrhoea and no fever
Can cause hemolytic-uremic syndrome and sudden kidney failure
Bacterial fimbriae for attachment (E. coli common pilus, ECP)
Moderately invasive and possesses a phage-encoded shiga toxin can can elicit an intense inflammatory responses
EHEC - referred to by their toxin producing capabilities, verocytotoxin, producing E. coli (VTEC) or Shiga-like toxin producing E. coli
E. coli O157:H7 (EHEC)
Relatively recent origin (30 years).
Not normally found inhuman intestinal flora but is present in 1% of healthycattle
Other meat may become contaminated during animal slaughter and processing.
Can be spread from ruminants to humans by way of undercooked meat, unpasteurized milk, and contaminated water (drinking and/or bathing).
Spread to humans in contact with recently
Petting Zoos with ruminants.
contaminated fields
Consuming contaminated raw vegetables.
failing to wash their hands.
E. coli O157:H7:Pathophysiology
The organism is swallowed and moves through the gastrointestinal tract where it adheres to the mucosa and colonizes.
Becomes resistant and tolerant to the stomach acid and then proliferates
The body reacts with watery and bloody diarrhea to flush the bacterium out of the body
The shiga toxins enter the bloodstream and begin translocation and damages kidneys
E. coli O157:H7: signs and symptoms
Definite Signs and Symptoms
Bloody diarrhoea
Vomiting
Fever not always present
Potential Signs and Symptoms
Bloody diarrhea , vomiting, and no or low fever
Hemolytic Uremic Syndrome (HUS)
Thrombotic Thrombocytopenic Purpura (TTP)
E. coli O157:H7: Complications
Hemolytic Uremic Syndrome (HUS)
Acute complication that causes renal failure, mostly in children
It is characterized by hemolytic anemia, acute renal failure, and low platelet count
Causes significant blood clotting in the capillaries
As the red blood cells pass through the clotted capillaries, they are torn apart
Kidney failure is also seen, in which urea and other waste products build up in the blood stream since it can not be filtered by the kidneys
Thrombotic Thrombocytopenic Purpura (TTP)
Characterized by the same features as HUS, but can include central nervous system involvement and fever and may have a more gradual onset
Progression of E.coli O157 infection in children
-3 Days - Ingestion
0 days - Diarrhoea, abdominal pain, fever, vomiting
2.5 days - Bloody diarrhoea (~90%)
3 days - Stool specimen
4.5 days - Positive culture
7 days - Diarrhoea improves
After 7 days (HUS ~15%, Spontaneous resolution ~85%)
E. coli O157:H7 - Scouts
In May 2000 a scout camp at the agricultural showground in Aberdeenshire.
Before camping event: 300 sheep/lambs had been grazing on the site for 6d.
337 campers at the event (233 scouts and 104 adults).
The event was abandoned early because of heavy rainfall.
20 campers who became ill between 28 May and 3 June. NO DEATHS.
All were confirmed as having E. coli O157 infection.
No suggestion that food was the primary source.
Subsequent investigations supported the hypothesis that transmission of E. coli O157 was from the environment to cases by contaminated hands, either directly from hand to mouth, or via food.
Asymptomatic animals excreting E .coli O157 in their faeces
The farmer or landowner was not aware and not compelled to check
E.coli O157 spread to people by direct contact and indirectly via faeces on land.
E.coli O157 was subsequently isolated from the sheep droppings, tents, climbing equipment, soil, and from clothing and footwear, indicating that this was an outbreak caused by environmental contamination.
E. coli O157:H7 can survive in soil in excess of 15 weeks.
Lanarkshire, 1996
Local Butcher 1 (E. coli O1577H7 infected meat) -> Church lunch who incorrectly cooked stewed beef (87 people: 45 infected - 8 died)
Local Butcher 1 (E. coli O1577H7 infected meat) -> Local Butcher 2
E. coli O157H7 infected meat -> Residential home cooked turkey for sandwiches
5 deaths
Overall - 490 infected and 21 deaths
