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 outbreaksinduced by E. coli O157 took place in Oregon and Michigan, USA, in 1982, when itwas isolated from individuals who arise bloody diarrhea and serious abdominalcramps 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 sixknown 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).STEChave twenty-five serogroups identifiedin this study and included four serogroups (077, 096, 0140, and 0162) notprevious 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 beenasoociated with human disease(CH and HUS) (Smith, H.
R., Scotland, S.M.
1988).Itis recently observed that STEC asimportant food-borne pathogens, especially O157, O26, O103, O111, O145, O45,O91, O113, O121 and O128 serogroups (Momtaz, Farzan, Rahimi, SafarpoorDehkordi, & Souod, 2012).Withineach pathotype, E coli strains are further characterized according toantigenic variants including O-antigen (lipopolysaccharide), H-antigen(flagellar) and K-antigen (capsular) types (Riley LW, Remis RS, Helgerson SD,zaet al.1983).STECexpressing somatic (O) antigen 157 and ?agellar (H) antigen and contain seven(7) serotype most frequently isolated from human beings, and these serotypewith the potent and most bearing etiological affiliation with HUS.Strains whichexpress the lipopolysaccharide (LPS) O-antigen 157 (O157 strains) are usuallyassociated with serious clinical manifestations,including bloody diarrhea,hemorrhagic colitis, and hemolytic uremic syndrome (Tarr, P.
I. et al., 1995).Ahighly conserved, non-conjugative F-like plasmid, related to pO157, whichranges in size between 92 kb and 104 kb are mostly bears by EHECO157:H7.
Sequence analysis shows a heterogeneous mixture conveys of geneticelements, transposons and prophages, as well as parts of other plasmids which,collectively, are indicative of its splotched evolution.The complete sequenceof pO157 shows 100 open reading frames; among them, 43 indicate similarities toknown proteins.However, the exact role of pO157 in disease pathogenesis isstill not well known because publishedstudies have reported contradictory findings (Lim JY, Yoon J, Hovde CJ.
2010).Thedominant STEC serotype is O157:H7; it is the serotype most commonly involved inlarge outbreaks (Paton, A.W., and Paton, J.C.1998).Shiga toxins (Stx1 and Stx2) or verotoxins (VT1 and VT2) are two potent phage-encodedcytotoxins of STEC.Stx1 and Stx2 are further classified into severalsubtypes; according to the new categorization suggested by Scheutz et al.(Scheutz, F., Teel, L.D., and Beutin, L., etal., 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.Theproduction of Stx may not be aloneresponsible for pathogenesis of STEC (Farfan, M.J., and Torres, A.G.2012).