Foodborne Gastroenteritis Caused by Escherichia coli
639
Enterohemorrhagic E. coli (EHEC)
These strains are both similar and dissimilar to EPEC strains. They are similar to EPEC in their
possession of the chromosomal gene eaeA (or one that is similar) and in the production of attachment–
effacement lesions (see the subsection on EPEC). In contrast to EPEC, EHEC strains affect only
the large intestine (in piglet models) and produce large quantities of Shiga-like toxins (SLT, Stx, see
below). EHECs produce a 60-MDa plasmid that encodes fimbriae that mediate attachment to culture
cells, and they do not invade HEp-2 or INT407 cell lines, although some strains have the ability to
invade some human epithelial cell lines.
64 Some EHEC strains produce curli fimbriae that facilitate
attachment of cells to surfaces.
The Toxins
Shigella dysenteriae produces a potent toxin that is referred to commonly as Shiga toxin (after K.
Shiga who first isolated and studied the organism). The toxins of EHEC strains of E. coli have been
referred to as Shiga-like toxins (verotoxin, verocytotoxin) and the two prototypes as SLT-I and SLT-II.
However, new terminology has been applied, and what was once SLT-I is now Stx1 and the former
SLT-II is Stx2.
9 The genes for Stx1 and Stx2 are encoded by temperate bacteriophages in some EHEC
strains. Stx1 differs from Stx (Shiga-toxin) by three nucleotides and one amino acid, and is neutralized
by antibodies to Stx. Stx1 and Stx2 are differentiated by a lack of cross-neutralization by homologous
polyclonal antisera, and by a lack of DNA–DNA cross-hybridization of their genes under conditions
of high stringency.
9 Both Stx2 and Stx2e (formerly SLT-IIv, VTe) are neutralized by antisera against
Stx2 but not by anti-Stx toxin. Stx2e is a variant of Stx2 that is more toxic to Vero cells than HeLa cells,
and like Stx its gene is chromosomal.
52,63 All Stxs are cytotoxic for Vero cells and lethal for mice,
and produce positive rabbit ileal loop responses. All Stxs consist of a single enzymatically active A
subunit and multiple B subunits. Stx-sensitive cells possess the toxin receptor, globotriaosylceramide
(Gb 3 ), and sodium butyrate appears to play a role in sensitizing cells to Stxs.
53 Once toxins bind to
Gb 3 , internalization follows with transport to the trans-Golgi network. Once inside host cells, the A
subunit binds to and releases an adenine residue from the 28S ribosomal RNA (rRNA) of the 60S
ribosomal subunit and this inhibits protein synthesis. The B subunits form pentamers in association
with a single A subunit and, thus, they are responsible for the binding of the toxin to the neutral
glycolipid receptors. Although serotype O157:H7 is the prototype for this group, Stxs are produced by
a number of serotypes, some of which are listed in g 27–1. For reasons that are not clear, Stx2 appears
to be more significant in the etiology of hemorrhagic colitis (HC) and hemolytic uremic syndrome
(HUS) than Stx1.
63
Growth and Stx Toxin Production
The nutritional requirements of Stx-producing strains are not unlike those for most other E. coli
strains (see Chapter 20). Reports on the effect of temperature on Stx production vary. In an early
study, temperature was found to have no effect on Stx1 synthesis, whereas iron repressed synthesis.
91
In another study, Stx production occurred at all temperatures that supported growth (Figure 27–1),
although less toxin was found when cells were grown at 21
◦ C than at 37
◦ C even though cell numbers
were similar.
1 In a ground roasted beef slurry, strain O157:H7 was found to produce Stx at either
21
◦ or 37
◦ C within 24 hours.
1 In an earlier study employing an O157:H7 strain in milk and fresh
ground beef, Stx1 was found to be produced at maximum levels at 37
◦ C in both products but at
639
Enterohemorrhagic E. coli (EHEC)
These strains are both similar and dissimilar to EPEC strains. They are similar to EPEC in their
possession of the chromosomal gene eaeA (or one that is similar) and in the production of attachment–
effacement lesions (see the subsection on EPEC). In contrast to EPEC, EHEC strains affect only
the large intestine (in piglet models) and produce large quantities of Shiga-like toxins (SLT, Stx, see
below). EHECs produce a 60-MDa plasmid that encodes fimbriae that mediate attachment to culture
cells, and they do not invade HEp-2 or INT407 cell lines, although some strains have the ability to
invade some human epithelial cell lines.
64 Some EHEC strains produce curli fimbriae that facilitate
attachment of cells to surfaces.
The Toxins
Shigella dysenteriae produces a potent toxin that is referred to commonly as Shiga toxin (after K.
Shiga who first isolated and studied the organism). The toxins of EHEC strains of E. coli have been
referred to as Shiga-like toxins (verotoxin, verocytotoxin) and the two prototypes as SLT-I and SLT-II.
However, new terminology has been applied, and what was once SLT-I is now Stx1 and the former
SLT-II is Stx2.
9 The genes for Stx1 and Stx2 are encoded by temperate bacteriophages in some EHEC
strains. Stx1 differs from Stx (Shiga-toxin) by three nucleotides and one amino acid, and is neutralized
by antibodies to Stx. Stx1 and Stx2 are differentiated by a lack of cross-neutralization by homologous
polyclonal antisera, and by a lack of DNA–DNA cross-hybridization of their genes under conditions
of high stringency.
9 Both Stx2 and Stx2e (formerly SLT-IIv, VTe) are neutralized by antisera against
Stx2 but not by anti-Stx toxin. Stx2e is a variant of Stx2 that is more toxic to Vero cells than HeLa cells,
and like Stx its gene is chromosomal.
52,63 All Stxs are cytotoxic for Vero cells and lethal for mice,
and produce positive rabbit ileal loop responses. All Stxs consist of a single enzymatically active A
subunit and multiple B subunits. Stx-sensitive cells possess the toxin receptor, globotriaosylceramide
(Gb 3 ), and sodium butyrate appears to play a role in sensitizing cells to Stxs.
53 Once toxins bind to
Gb 3 , internalization follows with transport to the trans-Golgi network. Once inside host cells, the A
subunit binds to and releases an adenine residue from the 28S ribosomal RNA (rRNA) of the 60S
ribosomal subunit and this inhibits protein synthesis. The B subunits form pentamers in association
with a single A subunit and, thus, they are responsible for the binding of the toxin to the neutral
glycolipid receptors. Although serotype O157:H7 is the prototype for this group, Stxs are produced by
a number of serotypes, some of which are listed in g 27–1. For reasons that are not clear, Stx2 appears
to be more significant in the etiology of hemorrhagic colitis (HC) and hemolytic uremic syndrome
(HUS) than Stx1.
63
Growth and Stx Toxin Production
The nutritional requirements of Stx-producing strains are not unlike those for most other E. coli
strains (see Chapter 20). Reports on the effect of temperature on Stx production vary. In an early
study, temperature was found to have no effect on Stx1 synthesis, whereas iron repressed synthesis.
91
In another study, Stx production occurred at all temperatures that supported growth (Figure 27–1),
although less toxin was found when cells were grown at 21
◦ C than at 37
◦ C even though cell numbers
were similar.
1 In a ground roasted beef slurry, strain O157:H7 was found to produce Stx at either
21
◦ or 37
◦ C within 24 hours.
1 In an earlier study employing an O157:H7 strain in milk and fresh
ground beef, Stx1 was found to be produced at maximum levels at 37
◦ C in both products but at