**1. Introduction**

For the first time in the 1970s early work showed that special *E. coli* strains could produce a certain type of a toxin called verotoxin and was named after that because of its's effect on Vero cells they can be classified according to virulence factors such as toxins into different groups [1].

These toxins were also called Shiga toxins (Stx) because of their relation to the toxin produced by *Shigella dysenteriae* type 1. The group of *E. coli* strains producing these toxins is referred to as Shiga-toxin producing *E. coli* (STEC), or verocytotoxin producing *E. coli* (VTEC). The genes encoding Stx can be often carried by bacteriophages and plasmids [2].

*E. coli* O157:H7 is the most common strain of STEC, but there are many other strains of STEC as well. Anyone can get STEC infection and the whole processes begins when a person eat or drink any contaminated product, particularly raw or undercooked meat. The term enterohemorrhagic *E. coli* (EHEC) is used to designate a subset of STEC that cause severe diseases in humans, including hemorrhagic colitis (HC) (bloody diarrhea) and the hemolytic uremic syndrome (HUS).
