E.coli Flashcards
(35 cards)
E.coli organism
-rod-shaped, non spore-forming gram-negative, facultative anaerobe
- belongs to the family enterobacteriaceae
- serotyped using: O antigens of LPS, H antigens of flagellin protein
- Produces pilli - the smaller surface projections
And produces peritricus flagella
why is E.coli a model organism
-Can be easily genetically manipulated
- it grows quickly on range of media
- genetic systems well developed
- metabolically versatile
dual aspect of E.coli
E.coli has a dual aspect of being both a commensal organism and a pathogen
e.coli - commensal organism
- colonises GI tract a few hours after birth
- most abundant facultative anaerobe of human intestinal microflora
E.coli - pathogen
– UTI
- Diarrhoea
- haemolytic uraemic syndrome
- septicaemia, pneumonia, meningitis
pathotypes of E.coli
pathotypes - groups of strains from within a single species that cause a specific disease using a common set of virulence factors
- extra-intestinal E.coli
- diarrheagenic pathotypes
- extra-intestinal E.coli
- neonatal meningitis
- uropathogenic
- avian pathogenic
diarrheagenic pathotypes
- enteropathogenic (EPEC)
-enterohaemorrhagic (EHEC)
-enterotoxigenic (ETEC)
-enteroaggregative (EAEC)
-enteroinvasive (EIEC)
site of pathogenic E. coli colonisation
These pathotypes cause different diseases through colonising different sites on the body
- brain: NMEC
-bloodstream: UPEC and NMEC - large bowel: EHEC, EIEC, and EAEC
-kidney: UPEC - small bowel: EPEC, ETEC, DAEC, and EAEC
- bladder: UPEC
site of diaarhoeagenic E.coli colonisation - large intestine
The role of the 7 metre long intestine is to both absorb nutrients but also to form a barrier to toxic substance and bacteria entering the human body: has undulating ridges and villi to increase SA
- Diaryogenic bacteria must colonise this environment, often directly interacting with these polarised epithelial cells
enteropathogenic E.coli
-important cause of infant diarrhoea in developing countries
diarrhoea results from multiple mechanisms:
- active ion secretion
- increased intestinal permeability
- increased inflammation
- a loss of absorptive microvilli
The most characteristic function of EPEC infection is the intimate interaction it can set up with host cells termed the ‘attaching and effacing’ phenotype
pathogenic mechanisms of enteropathogenic E.coli
-EPEC is an ectracellular pathogen initially binding to the host cell surface using bundle forming pili
-following binding EPEC injects a variety of effector proteins into the host cell using a type 3 secretion system - these hijack the cells for the benefit of the bacterium
- the most prominent change in cell structure is the formation of attaching and effacing structures on epithelial cells
what is effacement
effacement is the destruction of the microvilli with production of the pedestal by T3SS effector protein mediated cytoskeletal changes involving acrin polymerisation
enterohaemorrhagic E.coli
EHEC causes: - non bloody or bloody diarrhoea, haemolytic uraemic syndrome
-similar attaching and effacing activity to EPEC
- importantly produces shiga toxin (Stx)
- outbreaks - particularly associated with beef/beef products and farm visits by children
- serotype 0157:H7 most important in US/UK
EHEC Stx as a virulence factor
- an A1B5 type toxin - Stx or veryoctyotoxin
- EHEC Stx is closely related to Shiga toxin produced by shigella species responsible for dysentery
- receptor for B subunit of toxin is glycolipid globotriaosylceramide
- intracellular target of toxin A subunit is ribosomal RNA that is cleaved leading to cessation of protein synthesis
EHEC STX toxin mechanism of action
-Stx A subunit contains the active site glutamic acid at position 167/293 of the AI subunit
- cleavage at trypsin-sensitive region results in an A1 subunit and A2 peptide linked via a disulphide bond
- the b pentamer allows the toxin to bind to cells that express Gb3 glycolipid on their surface
- toxin is taken up by endocytosis, transported in a retrogade manner through the Golgi/ ER and the AI domain released into the cytoplasm
- toxin cleaves 28srNA component of the ribosome halting protein synthesis
EHEC symptoms
in the colon, Stx can cause bloody diarrhoea, hemorrhagic colitis, necrosis and intestinal perforation
- however, Stx can travel in the bloodstream to the kidney
- receptor is found on kidney cells and toxin binding leads to inflammation and potentially HUS with symptoms:
- haemolytic anaemia
- thrombocytopenia
- ultimately acute renal failure
EHEC causing Haemolytic uraemic syndrome (HUS)
- EHEC is the most common cause of HUS
- classic symptoms of HUS present within 2-12 days following onset of diarrhoea in approximately 15% of cases of EHEC - associated gastroentiritis
- children under 5 and the elderly are more likely to progress to HUS
epidemiology of EHEC in the UK in 2018
in 2018, 607 confirmed cases of EHEC O157 were reported in england and wales
- almost 1/3 of cases were hospitalised
- 612 non- O157 EHEC cases
- four EHEC outbreaks reported, involving 55 cases
EPEC/N EHEC pathogenicity island
- Contains 41 genes
This locus is necessary and sufficient for the production of the attaching and effacing phenotype by an ecoli strain - Thus the EPEC lee can be transferred to an ecoli K-12 lab strain and this will now interact with host cells in exactly the same way
○ To remove microvilli and produce the predestal structure
-The 41 lee genes are arranged into 7 transcripitonal units or operons containing mulitple co regulated genes- LEE1 through to LEE7
There are also 4 genes that have their own individual promoters
- LEE1 through to LEE7
locus of enterocyte effacement - secreted translocator proteins
Form the needle and translocon at the Tip of the machinery, interacting with the host
cell
locus of enterocyte effacement - chaperones
Ensure the appropriate effector Proteins are secreted
locus of enterocyte effacement - transcriptional regulators
Ensuring that this machinery is only produced in the appropriate environment
locus of enterocyte effacement - intimin
Present on the bacterial cell surface and acts as an adhesin binding to one of the
Secreted effector proteins Tir
EPEC injects the receptor TIR, into the host cell, that then localises to the cell membrane whilst EPEC also produces the adhesin intimin, which specifically binds to this receptor
