Escherichia coli O157   is the most common member  of pathogenic E. coli strains known diversely as enterohaemorrhagic,
verocytotoxin producing and Shiga-toxin-producing organisms.The ?rst outbreaks
induced by E. coli O157 took place in Oregon and Michigan, USA, in 1982, when it
was isolated from individuals who arise bloody diarrhea and serious abdominal
cramps after eating hamburgers in a restaurant (MMWR Morb Mortal Wkly Rep.
1982).

The Diarrheagenic Escherichia coli (DEC)
can be classified as pathotypes on the basis of gene-specific sequences.The six
known pathotypes include Diffusely adherent E.
coli (DAEC)  Enteroinvasive E. coli  (EIEC), Enterotoxigenic E. coli (ETEC),
Enteropathogenic E. coli (EPEC), Enteroaggregative E. coli
(EAEC), and Enterohemorrhagic E. coli (EHEC) (Iruka, I.N., et al., 2009).STEC
have twenty-five  serogroups identified
in this study and included four serogroups (077, 096, 0140, and 0162) not
previous described.Out of twenty-five (25) serogroups, fourteen serotypes
(02,04,07,08,09,015,(GI-0,022,026,0110,0113,0126, 0157 and 0168) have been
asoociated with human disease(CH and HUS) (Smith, H.R., Scotland, S.M.1988).It
is  recently observed that STEC as
important food-borne pathogens, especially O157, O26, O103, O111, O145, O45,
O91, O113, O121 and O128 serogroups (Momtaz, Farzan, Rahimi, Safarpoor
Dehkordi, & Souod, 2012).Within
each pathotype, E coli strains are further characterized according to
antigenic variants including O-antigen (lipopolysaccharide), H-antigen
(flagellar) and K-antigen (capsular) types (Riley LW, Remis RS, Helgerson SD,zaet al.1983).STEC
expressing somatic (O) antigen 157 and ?agellar (H) antigen and contain seven
(7) serotype most frequently isolated from human beings, and these serotype
with the potent and most bearing etiological affiliation with HUS.Strains which
express the lipopolysaccharide (LPS) O-antigen 157 (O157 strains) are usually
associated with serious clinical manifestations,including bloody diarrhea,
hemorrhagic colitis, and hemolytic uremic syndrome (Tarr, P.I. et al., 1995).A
highly conserved, non-conjugative F-like plasmid, related to pO157, which
ranges in size between 92 kb and 104 kb are mostly bears by EHEC
O157:H7.Sequence analysis shows a heterogeneous mixture conveys of genetic
elements, transposons and prophages, as well as parts of other plasmids which,
collectively, are indicative of its splotched evolution.The complete sequence
of pO157 shows 100 open reading frames; among them, 43 indicate similarities to
known proteins.However, the exact role of pO157 in disease pathogenesis is
still  not well known because published
studies have reported contradictory findings (Lim JY, Yoon J, Hovde CJ. 2010).The
dominant STEC serotype is O157:H7; it is the serotype most commonly involved in
large outbreaks (Paton, A.W., and Paton, J.C.1998).Shiga toxins (Stx1 and Stx2) or verotoxins (VT1 and VT2) are two potent phage-encoded
cytotoxins of STEC.Stx1 and Stx2 are further classified into several
subtypes; according to the new categorization suggested by Scheutz et al.
(Scheutz, F., Teel, L.D., and Beutin, L., et
al., 2012).Stx1 consists of three variants, Stx1a, Stx1c and Stx1d, whereas Stx2 is a diverse group composed of seven distinguishable variants, namely Stx2a, Stx2b, Stx2c, Stx2d, Stx2e, Stx2f and Stx2g.The
production of Stx may not be alone
responsible for pathogenesis of STEC (Farfan, M.J., and Torres, A.G.2012).

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