Verocytotoxigenic Escherichia coli causes diarrhea that can occur with bloody feces, and can be considered as the most important cause of hemorrhagic colitis in humans. The pathogenicity of VTEC strains appears to be associated with several virulence factors, including the production of several cytotoxins. These toxins are collectively called verotoxins or Shiga-like toxins, due to their great similarity to Shiga toxin. Serotype O157: H7 is the most important pathogen within the VTEC group and one of the most frequently associated with human infections worldwide. There are two kinds of verotoxin (VT) VT1 and VT2 and even some variants of VT2. These toxins are encoded by lysogenic genes, that is, genes that are located in bacteriophages that are stably integrated into the bacterial genome.