1. MIC328: Lecture 19 AIMS: To provide
Brief introduction to E. coli: a versatile pathogen
Overview of Enterotoxigenic E. coli (ETEC)

2. E. coli - a versatile pathogenic species
Single species causes a variety of different diseases
different strains possessing different sets of vir genes
Most E. coli strains - harmless intestinal commensals
Only some strains pathogenic
divided into groups based on disease produced

4. Summary of types of intestinal diseases - enteric pathogens
Secretory or watery diarrhea:
Large volume watery stools - no blood or pus
Site: Small intestine, no invasion - colonization + toxin production
Examples: V. cholerae, ETEC, EAEC
Bloody watery diarrhea:
Large volume, watery + bloody stools (sometimes + pus)
Site: Small intestine, tissue invasion
Examples: Yersinia enterocolitica, Campylobacter jejuni
Colitis or Haemorrhagic colitis:
Large volume, watery, bloody stools - no pus
Site: Large intestine - invasion absent or limited - A/E lesions
Examples: EPEC, EHEC
Dysentery:
Bloody stools + pus + mucus; scant volume
Site: Large intestine, tissue invasion
Examples: Shigella, EIEC, Entamoeba histolytica

5. Types of pathogenic E. coli
Intestinal
Disease
Enterotoxigenic E. coli (ETEC)
Enteroaggregative E. coli (EAEC)
Diffusely adhering E. coli (DAEC)
Cholera-like
watery diarrhoea
Enteropathogenic E. coli (EPEC)
Enterohaemorrhagic E. coli (EHEC)
Colitis or
haemorrhagic colitis
Enteroinvasive E. coli (EIEC)
Dysentery
Extraintestinal
Urinary tract/
pyelonephritis
Uropathogenic E. coli (UPEC)
Septicemia/
meningitis
Septic E. coli strains

6. ETEC Secretory (watery) diarrhoea: like cholera, but less severe
Like V. cholerae, ETEC
do not invade
adhere to + colonize epithelium of small intestine
produce enterotoxins that cause secretory diarrhea
Both adhesins and toxins encoded by plasmids
easily transferred among different E. coli strains
Many different ETEC strains - each usually specific for
particular host (reflects adhesion specificity)

7. Human ETEC strains Often fatal in infants and young children - a major
cause of deaths in many developing countries
Adults less susceptible – acquire natural immunity to
local ETEC strains during previous encounters
(either sub-clinical or disease)
ETEC common cause of traveller's diarrhoea in adults
- little or no immunity to ‘newly’ encountered serotypes

8. Enterotoxins produced by ETEC strains
Two different general types discovered by early ‘70s
Heat-labile enterotoxins (LT)
Heat-stable enterotoxins (ST)
Later, ‘variants’ of each type identified
mostly in: Human ETEC Animal ETEC
Type Variants
LT toxins: LT-I LT-II
ST toxins: STa (ST-I) STb (ST-II)
ETEC strains may produce LT only (30%), ST only (35%),
or both (35%)

9. E. coli Heat-labile toxin (LT)
A-subunit
ADP-ribosylates Gs
activates A1 A2
adenylate cyclase cAMP
B-pentamer B
binds to gangliosides
LT-I: GM1
LT-II: GD1a or GD1b
(binds different gangliosides)
facilitates A1 entry
LT-I: > 80% homology with CTA & CTB
LT-II: A 60% homology with CTA; B - no homology

10. Cholera usually more severe - Why ?
ETEC versus V. cholerae CT
Both colonize small intestine very effectively
LT & CTx very similar toxins - identical mechanisms
Cholera usually more severe - Why ?
Probably combination of reasons, including difference
in the way the toxins are released
Example of importance of protein secretion in bacterial
pathogenicity

11. Difference in the excretion of LT and CT toxins
E. coli
V. cholerae
Lacks GSP terminal branch
LT-A
LT-B
Sec
Sec
LT remains in periplasm
Small quantities ‘leak’ thro’ OM
CTx actively secreted by the EPS
terminal branch of GPS

12. ETEC: Heat-stable toxins (STa) - structure
STa best understood - small peptide toxin, expressed as much
larger 72 a.a. ‘prepropeptide’ precursor
N-terminal signal
sequence
‘pro-’ sequence removed outside cell. Role
unclear - possibly export thro’ OM ??
‘pre’ ‘pro’
Active toxin consists of only
the C-terminal 18 a.a. - these include 6 Cys
sec
In periplasm, protoxin folds
+ forms 3 disulphide bonds OM ?
Very stable
(resistant to gut proteases)
Type V
secretion ?

13. Heat-stable toxins (STa) - mechanism of action
‘mimics’ an intestinal hormone called guanylin
( a 15 a.a. peptide with 2 disulphide bonds)
Guanylin regulates guanylate cyclase
- ‘receptor cyclase’ located in apical membranes
Guanylin-receptor binding activates enzyme
production of cGMP in cell
STa does exactly same, but is more potent
than endogeneous hormone + larger amounts
Like excess cAMP, excess cGMP opens CFTR
increased Cl secretion net efflux of H2O

14. The second heat-stable toxin - STb
Similar size precursor (71 a.a.) as STa, with N-terminal signal
sequence secretion to periplasm by GSP, where it folds
+ forms intramolecular disulphide bonds (hence stable)
However, similarity with STa ends there.
No additional ‘processing’ after secretion to periplasm
Active toxin is a 48 a.a. peptide that does not result in
increased production of either cAMP or cGMP
Mechanism of action still not understood, though shown
to induce fluid-secretion in ligated-ileal loops.

15. ETEC Enterotoxins - Summary
LT-I & LT-II:
Very closely related to Cholera toxin - similar structure
Identical mechanism of action as CT cAMP
Like CT secreted to + assembled in periplasm
Unlike CT, not secreted thro’ OM - released by ‘leakage’
STa
Small (18 a.a.) peptide, expressed as ‘prepropeptide’
Very stable due to 3 intramolecular disulphide bonds
Mimics intestinal peptide hormone guanyline cGMP
STb
Also relatively small peptide (48 a.a.) - distinct from STa
Mechanism of action not elucidated