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In-Depth Information
STEC are characterized by enormous strain diversity. They are classified
into four groups, EHEC1, EHEC2, STEC1, and STEC2 ( Whittam, 1998 ),
based on multi-locus enzyme electrophoresis. STEC O157:H7 and its evo-
lutionary ancestor, O55:H7, constitute EHEC1. EHEC2 is the most common
group of non-O157 Stx-producing strains and is comprised of serotypes such
as O111:H8, O111:H-, O26:H11, and O111:H11. STEC1 is highly diverse;
the common serotypes in this group are O113:H21, OX3:H21, and O91:H21.
Compared with the other groups, little is known about the virulence of STEC2,
which is composed of serotypes O103:H2, O103:H6, and O45:H2 ( Whittam,
1998 ). Serotype O157:H7 and six 'non- E. coli O157' serogroups O26, O111,
O103, O121, O45, and O145 are highly significant contributors to human dis-
ease in the US ( Brooks et al., 2005 ).
History
In 1977, almost 100 years after Stx was originally identified in culture extracts
of Shigella dysenteriae , two groups independently demonstrated that certain
E. coli strains produce cytotoxins that can kill Vero cells and are neutralized
by anti-Stx serum ( Konowalchuk et al., 1977 ; O'Brien and LaVeck, 1983 ).
These cytotoxins were termed Shiga-like toxins, owing to their striking simi-
larity to Stx of Shigella . In 1982, following two hemorrhagic colitis outbreaks
in Oregon and Michigan in the US ( Centers for Disease Control, 1982 ),
STEC came to the attention of scientists and the public alike as a prominent
human pathogen. During these food-borne disease outbreaks, serotype E. coli
O157:H7 was isolated from both human patients and from frozen ground
beef patties. Shortly afterwards, in a seminal study, Karmali and colleagues
reported the presence of cytotoxins and cytotoxin-producing E. coli in the
stools of children affected with HUS ( Karmali et al., 1983 ). Subsequent stud-
ies led to the identification of Stx and E. coli O157:H7 as the causative agents
of post-diarrheal HUS. The following years were marked by several outbreaks
of STEC, predominantly E. coli O157:H7, in the US, Europe, and Japan.
Evolution
STEC evolution is characterized by the acquisition of numerous virulence fac-
tors through horizontal gene transfer effected predominantly by bacteriophage
transduction, allowing further rapid gene gain or loss through duplication or
deletion ( Kaper et al., 2004 ). E. coli O157:H7 is believed to have diverged from
a common ancestor with E. coli K-12 about 4.5 million years ago ( Reid et al.,
2000 ). Interestingly, an atypical EPEC serotype, O55:H7, is considered the
most recent precursor of E. coli O157:H7 ( Whittam, 1998 ; Reid et al., 2000 ).
The emergence of E. coli O157:H7 has been analyzed extensively by sev-
eral groups, leading to the formulation of a step-wise evolutionary model for this
pathogen. E. coli O157:H7 strains are highly clonal in nature and are distinguished
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