This document provides a history and overview of Escherichia coli (E. coli) and its pathogenic strains. It discusses the genus and species definitions of E. coli and outlines both its commensal and pathogenic roles. The major pathogenic strains are described including enterotoxigenic E. coli (ETEC), enterohemorrhagic E. coli (EHEC), enteroinvasive E. coli (EIEC), enteroaggregative E. coli (EAEC), and diffusely adherent E. coli (DAEC). For each pathotype, the key virulence factors and mechanisms of pathogenesis are summarized. In closing, the document notes that while research has enhanced understanding of diarrhe

1. PRESENTED BY : DR. KUMAR VIKRAM
MD (MICROBIOLOGY)
IGIMS, PATNA

2. HISTORY:Escherich’s legacy to
Science
• German pediatrician Theodore Escherich
described the organism in 1885 as Bacterium
coli commune.
1885
• Castellani and Chalmers defined the genus
Escherichia and established the type species
E. coli (Castellani and Chalmers 1919).
1919
• Bray established the causative role of a
specific type of E.coli during a hospital
outbreak of childhood diarrhea in London.
1945

3. GENUS AND SPECIES DEFINITION
GENUS DEFINITION
• gram-negative,
• rod-shaped bacteria,
• oxidase-negative,
• usually motile by peritrichous flagella
• do not produce spores.
• facultatively anaerobic;
• gas is usually produced from
fermentable carbohydrates.
• They are methyl red-positive and
Voges–Proskauer-negative.
• Many strains produce polysaccharide
capsules or microcapsules
SPECIES DEFINITION(Escherichia Coli)
• Most strains of this species promptly
ferment lactose or give a positive o-
nitrophenyl-b-D-galactopyranoside
(ONPG) reaction.
• They produce indole,
• fail to hydrolyze urea.
• H2S production is not detectable on
triple sugar iron (TSI) agar
• phenylalanine is not deaminated,
• Gelatin is not liquefied.
• Most strains decarboxylate lysine.
• they do not grow on Simmons’ citrate
agar

4. E. Coli : The Good

5. E.coli: The Enemy within
Despite the fact that Escherichia coli as a commensal bacteria
can be found in intestinal microflora of a variety of animals
including man, not all the strains are harmless, and some can
cause debilitating and sometimes fatal diseases in humans.
WHO (April 2013):
• Diarrhoeal disease is the second leading cause of death in children
under five years old.
• Each year diarrhoea kills around 760 000 children under five.
• Rotavirus and Escherichia coli are the two most common etiological
agents of diarrhoea in developing countries.

6. • INTESTINAL
PATHOGENS(ENTEROVIRULENT
E.COLI)
• Enteropathogenic E.coli
• Enterohaemorrhagic E.coli
• Enterotoxigenic E.coli
• Enteroaggregative E.coli
• Enteroinvasive E.coli
• Diffusely adherent E.coli
• EXTRAINTESTINAL PATHOGENS
• Uropathogenic E.coli
• Meningitis
• septicemia
PATHOGENIC E.COLI

7. Common themes in E. Coli virulence
• Like most mucosal pathogens, E. coli can be said to follow a
requisite strategy of infection:
(i) colonization of a mucosal site,
(ii) evasion of host defenses,
(iii) multiplication, and
(iv) host damage.
• The most highly conserved feature of diarrheagenic E. coli
strains is their ability to colonize the intestinal mucosal
surface despite peristalsis and competition for nutrients by
the indigenous flora of the gut.
• Diarrheagenic E. coli strains possess specific fimbrial
antigens that enhance their intestinal colonizing ability and
allow adherence to the small bowel mucosa.

8. • Once colonization is established, E. Coli may
cause diarrhea by three general paradigms:
(i) enterotoxin production (ETEC and EAEC),
(ii) invasion (EIEC), and/or
(iii) intimate adherence with membrane
signalling (EPEC and EHEC).

11. • First recognized in the late 1960s as a cause of
cholera-like diarrhoea in india.
• Resemble vibrio cholerae in that they induce
profuse,watery diarrhea by elaboration of toxins
that act on the mucosal cells.
• Adhere to the small intestinal mucosa, but do
not invade.

12. • no apparent histological changes and little
inflammation.
• a common cause of dehydrating diarrhea in
children in developing countries, particularly
when they are weaned.
• considered to be the leading cause of
travelers’ diarrhea accounting upto 75% of
these cases

13. VIRULENCE FACTORS
2 classes:
Enterotoxins
Heat labile
toxin(LT)
LT I
LT II
Heat Stable
Toxin(ST)
ST I
ST II
adhesive factors termed
colonization factor antigens
(CFA) or coli surface (CS)
associated antigens

14. • Oligomeric toxins that are closely related in
structure and function to the cholera enterotoxin
(CT) expressed by vibrio cholerae.
• Heat-labile toxin (lt) is inactivated by incubation
at 100°c for 30 min.
• 2 major serogroups :
– LT I
– LT II

15. • LT-I is expressed by E. coli
strains that are pathogenic
for both humans and
animals.
• Composed of :
– One A subunit
• The A1 domain constitutes
the active toxin and is linked
to the A2 domain via a
disulfide bond
• The A2 fragment is the
helical portion of the
molecule and anchors the A
subunit to the B pentamer
– Five B subunits
• Bind to ganglioside GM 1
receptor.

16. MECHANISM OF ACTION
endocytosis and translocation of toxin through the cell in a
process involving trans-Golgi vesicular transport.
The A1 peptide has an ADP-ribosyltransferase activity and acts by
transferring an ADP-ribosyl moiety from NAD to the alpha subunit of
the GTP-binding protein, GS.
permanent activation of adenylate cyclase
increased levels of intracellular cyclic AMP
(cAMP).
Activation of cAMP dependent protein
kinase (A kinase).

17. supranormal phosphorylation of chloride
channels located in the apical epithelial
cell membranes.
stimulation of Cl2 secretion from secretory
crypt cells and inhibition of NaCl
absorption by villus tip cells.
The increased luminal ion content draws
water passively through the paracellular
pathway, resulting in osmotic diarrhea.

18. Mechanism of Action

19. OTHER MECHANISMS
• Prostaglandins (PGE1 and PGE2): Stimulate electrolyte
transport and intestinal motility.
• Enteric nervous system: Serotonin and vasoactive
intestinal polypeptide, both of which can stimulate
intestinal epithelial cell secretion via the ENS, are
released into the human small bowel after treatment
with CT.
• Intestinal inflammatory response:CT has been reported
to stimulate production of the proinflammatory cytokine
interleukin-6 (IL-6), thereby activating the enteric
immune system and potentially generating arachidonic
acid metabolites that stimulate secretion.

20. • 55 to 57% identity to LT-I and CT in the A subunit.
• no homology to LT-I or CT in the B subunits.
• LT-II increases intracellular cAMP levels by similar
mechanisms to those involved with LT-I toxicity.
• But LT-II uses GD1 as its receptor rather than
GM1
• There is no evidence that LT-II is associated with
human or animal disease.

21. • Small, monomeric toxins that contain multiple
cysteine residues, whose disulfide bonds
account for the heat stability of these toxins.
• 2 classes:
– ST I (ST a) – soluble in methanol
• ST Ip (porcine)
• ST Ih (human)
– ST II (ST b) – insoluble in methanol

22. This activity leads ultimately to stimulation of chloride secretion and/or
inhibition of sodium chloride absorption, resulting in net intestinal fluid
secretion.
Binding of ST - I to GC-C stimulates GC activity, leading to increased intracellular
cGMP levels
The major receptor for ST- I is a membrane-spanning enzyme called
guanylate cyclase C(GC-C)

23. ST - II
• ST -II is associated primarily with ETEC strains
isolated from pigs.
• induces histologic damage in the intestinal
epithelium, consisting of loss of villus epithelial cells
and partial villus atrophy.
• stimulates the secretion of bicarbonate from
intestinal cells .
• stimulate increases in intracellular calcium levels
from extracellular sources .
• also stimulates the release of PGE2 and serotonin.

24. • A heterogenous group of proteinacious surface
structures
• Fimbrial, non-fimbrial or fibrillar.
• The more recent nomenclature refer to these
structures as coli surface (CS) antigen.
• At least 21 colonization factor antigens and coli
surface antigens specific for humans have been
defined.
• CFA/I and CFA/ II are the most commonly found
adhesins in human ETEC.

26. Vero cytotoxin-producing
E.coli(VTEC)

27. Vero cytotoxin-producing
E.coli(VTEC)
• Described first by Konowalchuk in 1977.
• In 1983, one particular serotype of E. Coli
(O157:H7) was identified as the causative
agent involved in two outbreaks of a distinctive
bloody diarrheal syndrome (Riley et al. 1983).
• Since then, these organisms have received
much attention as a cause of epidemic or
sporadic bloody and non-bloody diarrhea,
HUS, and thrombotic thrombocytopenic
purpura.

28. • Also called as shiga toxin producing E.
Coli(STEC). - Because toxins are closely related
to shiga toxin and that have a cytotoxic effect
on vero cells,
• The term enterohaemorrhagic E. Coli (EHEC) is
applied to those STEC serotypes that have the
same clinical, epidemiological and
pathogenetic features associated with the
prototype strain E. Coli O157:H7.

29. Virulence factors
• VT 1 and VT2
• Main virulence factor
• associated with HUS
Vero
cytotoxin
• Mediates attachmentIntimin
• EnterohemolysinEHEC
plasmid

30. Vero cytotoxin
• Encoded on Stx
bacteriophage
• Originally discovered in
Shigella dysenteriae (Stx1-
like)
• Multiple variants-VT1,
VT2 (VT2c, d, e, f, g)
• AB-5 toxin (5 B
components and one A
component)

31. The death of renal endothelial cells, intestinal epithelial cells, Vero or HeLa cells, or any
cells which possess the Gb3 (or Gb4 for VT2e)receptor.
A1 peptide has N-glycosidase activity that inhibits protein synthesis through cleavage
of 28S ribosomal RNA.
Translocates A subunit which is cleaved into an A1 peptide
5 B subunits bind to Gb3/Gb4 glycolipid receptor (Kidney epithelial cells and human
intestinal mucosa).
Toxin enters blood stream

32. Enterohemolysin
• Found in nearly all O157:H7 strains and is
widely distributed among non-O157 VT-
producing E. coli strains.
• Two other genetically distinct phage encoded
hemolysins, called Ehly1 and Ehly2, have been
reported to be produced by many VT-producing
E. coli strains.
• There are no data to suggest in vivo expression
or any role in pathogenesis for these
hemolysins.

35. • EPEC is an important category of
diarrheagenic E. Coli which has been linked to
infant diarrhea in the developing world.
• Most EPEC infections occur in the first 3 years
of life.
• EPEC infections show a marked seasonality
and are associated with warm season peaks.

36. Pathogenesis
• Attaching-and-effacing histopathology
– hallmark of infections due to EPEC
– characterized by effacement of
microvilli and intimate adherence
between the bacterium and
the epithelial cell membrane.
– Marked cytoskeletal changes,
including accumulation of polymerized actin, are seen
directly beneath the adherent bacteria; the bacteria
sometimes sit upon a pedestal-like structure

37. Pathogenesis
• In 1992, Donnenberg and Kaper proposed a
three-stage model.
• In 1998 Knutton et al. proposed a four-stage
model.
Localized adherence.
• nonintimate adherence between the bacterium and the epithelial cell
• mediated by the bundle-forming pilus (BFP) and intimin along with
EspA filaments (short, surface-associated filaments).
• The adherence pattern is called ‘localized’ because organisms attach
to one or two small areas of the cell surface in microcolonies

38. Signal transduction
• The injection of the translocated intimin receptor (Tir) via a type III
secretion system directly into the host cell,
• Result in tyrosine protein kinase activation and formation of the
intimin receptor (tyrosine phosphorylated Tir) and to actin
rearrangements.
Intimate adherence.
• intimin binds to phosphorylated Tir and polymerized actin and other
cytoskeletal elements accumulate beneath the intimately attached
bacteria
Pedestal formation
• further actin polymerization and accumulation of cytoskeletal
elements at the site of bacterial attachment results in the production
of the mature A/E lesion with the characteristic pedestal structure.
• Loss of tight-junction integrity and mitochondrial function, resulting
in electrolyte loss and cell death.

40. • Have the capacity to invade interstitial epithelial cells in
vivo.
• EIEC strains are biochemically, genetically, and
pathogenetically related closely to Shigella spp.
• Like Shigella spp.,EIEC strains are generally lysine
decarboxylase negative, nonmotile, and lactose negative.
• Except
– Lower acid resistant
– Inability to produce shiga toxin
• Presentation:
– Majority - watery diarrhea.
– Occasionally - Dysentery syndrome, manifested as blood,
mucus, and leukocytes in the stool; tenesmus; and fever

41. Pathogenesis
• Acquisition of the invasive plasmid (pINV)
encodes the ability to invade host tissues.
• Comprises
– (i) epithelial cell penetration,
– (ii) lysis of the endocytic vacuole,
– (iii) intracellular multiplication,
– (iv) directional movement through the cytoplasm,
– (v) extension into adjacent epithelial cells

42. Cellular pathogenesis

43. Sereny test
• Instillation of a suspension of freshly isolated
EIEC or Shigella into the eyes of guinea pigs
• Mucopurulent conjunctivitis

44. Enteroaggregative E. coli
• Named so because they
appear aggregated in a
stacked brick formation on
Hep – 2 cells.
• second most common cause
of travelers’ diarrhea after
ETEC in both developed and
developing countries.
• commonly being recognized
as a cause of endemic and
epidemic diarrhea
worldwide.
• associated with persistent
diarrhea.

45. Pathogenesis
colonization of
intestinal
mucosa,
mucoid
biofilm
formation
elaboration of various
enterotoxins,
cytotoxins and mucosal
inflammation.
aggregative
adherence fimbriae
(AAF)
•Plamid encoded
Enteroaggregative heat
stable toxin (EAST)
•Fluid secretion
•Related to ST of ETEC

47. Diffusely Adherent E.coli
• heterogenous group that
generates a diffuse
adherence pattern on HeLa
and HEp-2 cells.
• associated with the watery
diarrhea that can become
persistent in young children
in both developing and
developed countries as well
as recurring urinary tract infections.

48. •The association of the
bacteria with the
membrane and the
formation of long finger-
like projections
emanating from the cell.
•These projections wrap
around the bacterium in a
phenotype termed
“embedding.”
Invasion is rarely seen.

49. 2 classes
Afa/Dr adhesins
(Afa/Dr DAEC)
urinary tract infections
(UTIs) (pyelonephritis,
cystitis, and asymptomatic
bacteriuria) and with
various enteric infections.
adhesin involved in diffuse
adherence (AIDA-I)
a potential cause of
infantile diarrhea.

51. • A wealth of data concerning the virulence mechanisms
of diarrheagenic E. Coli has been accumulated over the
years even though these complicated phenomena are
not yet fully understood.
• This versatile organism affects a wide range of
eukaryotic cell processes via an array of diverse genetic
elements enabling each pathotype to colonize, multiply,
and disseminate and understanding each pathogenic
step at molecular level may help in devising effective
measures for intervention in infection.

52. References
• Nataro JP, Kaper JB. Diarrheagenic Escherichia
coli. Clin Microbiol Rev 1998; 11: 142-201.
• Topley and Wilson’s Microbiology and Microbial
infections, 10th Edition
• Jafari et al , Escherichia coli: a brief review of
diarrheagenic pathotypes and their role in
diarrheal diseases in Iran, IRAN. J. MICROBIOL. 4
(3) : 102-117
• WHO Fact Sheet (April 2013) on diarrhoeal
disease