Foodborne Gastroenteritis Caused by Escherichia coli
649
The Enterotoxins
One of the E. coli enterotoxins is heat-labile (LT) and the other is heat-stable (STa or ST-1, and STb
or ST-II). The LT toxin is destroyed at 60
◦ C in about 30 minutes, whereas ST toxins can withstand
100
◦ C for 15 minutes.
The LT toxin is a protein with a molecular weight of about 91 kDa,
18 and it possesses enzymatic
activity similar to that of the cholera toxin (CT). Whereas CT is exported from the cytoplasm to the
outside of producing cells, LT is deposited into the periplasm of producing cells. Further, antisera to
CT neutralize LT and immunization with CT induces protection against both CT and LT challenges.
These enterotoxins are produced early in the growth phase of producing strains, with the maximum
amount of ST produced after 7 hours of growth in one study in a Casamino acids yeast extract medium
containing 0.2% glucose.
47 In a synthetic medium, ST appeared as early as 8 hours, but maximal
production required 24 hours with aeration.
8 Although LT and ST appear to be produced under all
conditions that allow cell growth, the release of LT from cells in enriched media was favored at a pH
of 7.5–8.5.
59
LT toxin is composed of two protomers: A, with a molecular weight of about 25.5 kDa, which when
nicked with trypsin becomes an enzymatically active A 1 polypeptide chain of 21 kDa linked by a
disulfide bond to an A 2 -like chain; and B, which has a molecular weight of about 59 kDa and consists
of five noncovalently linked individual polypeptide chains.
23 LTB is the binding subunit, whereas
LTA stimulates the adenylate cyclase system. LTA and LTB have immunological properties similar to
subunits A and B of the Vibrio cholerae toxin.
46 LTh and LTp designate human and porcine strains,
respectively.
STa is methanol soluble and elicits a secretory response in infant mice. It is an 18–19 amino acid
acidic peptide that contains three disulfide bonds and has a molecular weight of 1972 Da. It stimulates
particulate intestinal guanylate cyclase. STa has been chemically synthesized.
43
STb is methanol insoluble and is primarily of swine origin. It is the most prevalent toxin associated
with diarrheagenic isolates of porcine origin, and it affects the small intestine and the ligated ileum of
weaned piglets, and also the mouse intestinal loop when a protease inhibitor is added.
95 The STb gene
(estB) has been sequenced and cloned.
48 The trypsin-sensitive STb toxin is synthesized as a 71-amino
acid polypeptide that is later cleaved to yield the active 48-amino acid molecule with four cysteine
residues that pass through the inner membrane to the periplasm. Although its mode of action is yet
unclear, it has been shown to stimulate the synthesis of prostaglandin E 2 .
35
. Its receptor cell in mouse
intestinal cells is a protein with a molecular weight of 25 kDa.
34
Mode of Action of Enterotoxins. ETEC gastroenteritis is caused by the ingestion of 10
6 –10
10 viable
cells per gram that must colonize the small intestines and produce enterotoxin(s). The colonizing
factors are generally fimbriae or pili. The syndrome is characterized primarily by non-bloody diarrhea
without inflammatory exudates in stools. The diarrhea is watery and similar to that caused by V.
cholerae. Diarrhea results from enterotoxin activation of intestinal adenylate cyclase, which increases
cyclic 3,5-adenosine monophosphate (cAMP).
With regard to LT, the B protomer mediates binding of the molecule to intestinal cells. LT binds to
gangliosides, especially monosialogangliosides (GM 1 ).
23 CT also binds to GM 1 ganglioside, and CT
and LT are known to share antigenic determinants among corresponding protomers, although they do
not cross-react. Upon binding, the A polypeptide chain (of the A protomer) catalyzes ADP ribosylation
of a G protein that activates adenylate cyclase and induces increases in intracellular cAMP.
Regarding ST, STa binds irreversibly to a specific high-affinity nonganglioside receptor and initiates a transmembrane signal to activate particulate guanylate cyclase, and triggers the production of
649
The Enterotoxins
One of the E. coli enterotoxins is heat-labile (LT) and the other is heat-stable (STa or ST-1, and STb
or ST-II). The LT toxin is destroyed at 60
◦ C in about 30 minutes, whereas ST toxins can withstand
100
◦ C for 15 minutes.
The LT toxin is a protein with a molecular weight of about 91 kDa,
18 and it possesses enzymatic
activity similar to that of the cholera toxin (CT). Whereas CT is exported from the cytoplasm to the
outside of producing cells, LT is deposited into the periplasm of producing cells. Further, antisera to
CT neutralize LT and immunization with CT induces protection against both CT and LT challenges.
These enterotoxins are produced early in the growth phase of producing strains, with the maximum
amount of ST produced after 7 hours of growth in one study in a Casamino acids yeast extract medium
containing 0.2% glucose.
47 In a synthetic medium, ST appeared as early as 8 hours, but maximal
production required 24 hours with aeration.
8 Although LT and ST appear to be produced under all
conditions that allow cell growth, the release of LT from cells in enriched media was favored at a pH
of 7.5–8.5.
59
LT toxin is composed of two protomers: A, with a molecular weight of about 25.5 kDa, which when
nicked with trypsin becomes an enzymatically active A 1 polypeptide chain of 21 kDa linked by a
disulfide bond to an A 2 -like chain; and B, which has a molecular weight of about 59 kDa and consists
of five noncovalently linked individual polypeptide chains.
23 LTB is the binding subunit, whereas
LTA stimulates the adenylate cyclase system. LTA and LTB have immunological properties similar to
subunits A and B of the Vibrio cholerae toxin.
46 LTh and LTp designate human and porcine strains,
respectively.
STa is methanol soluble and elicits a secretory response in infant mice. It is an 18–19 amino acid
acidic peptide that contains three disulfide bonds and has a molecular weight of 1972 Da. It stimulates
particulate intestinal guanylate cyclase. STa has been chemically synthesized.
43
STb is methanol insoluble and is primarily of swine origin. It is the most prevalent toxin associated
with diarrheagenic isolates of porcine origin, and it affects the small intestine and the ligated ileum of
weaned piglets, and also the mouse intestinal loop when a protease inhibitor is added.
95 The STb gene
(estB) has been sequenced and cloned.
48 The trypsin-sensitive STb toxin is synthesized as a 71-amino
acid polypeptide that is later cleaved to yield the active 48-amino acid molecule with four cysteine
residues that pass through the inner membrane to the periplasm. Although its mode of action is yet
unclear, it has been shown to stimulate the synthesis of prostaglandin E 2 .
35
. Its receptor cell in mouse
intestinal cells is a protein with a molecular weight of 25 kDa.
34
Mode of Action of Enterotoxins. ETEC gastroenteritis is caused by the ingestion of 10
6 –10
10 viable
cells per gram that must colonize the small intestines and produce enterotoxin(s). The colonizing
factors are generally fimbriae or pili. The syndrome is characterized primarily by non-bloody diarrhea
without inflammatory exudates in stools. The diarrhea is watery and similar to that caused by V.
cholerae. Diarrhea results from enterotoxin activation of intestinal adenylate cyclase, which increases
cyclic 3,5-adenosine monophosphate (cAMP).
With regard to LT, the B protomer mediates binding of the molecule to intestinal cells. LT binds to
gangliosides, especially monosialogangliosides (GM 1 ).
23 CT also binds to GM 1 ganglioside, and CT
and LT are known to share antigenic determinants among corresponding protomers, although they do
not cross-react. Upon binding, the A polypeptide chain (of the A protomer) catalyzes ADP ribosylation
of a G protein that activates adenylate cyclase and induces increases in intracellular cAMP.
Regarding ST, STa binds irreversibly to a specific high-affinity nonganglioside receptor and initiates a transmembrane signal to activate particulate guanylate cyclase, and triggers the production of