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Escherichia coli (E. coli) is a Gram-negative, rod-shaped, facultative anaerobic bacterium. This microorganism was first described by Theodor Escherich in 1885. Most E. coli strains harmlessly colonize the gastrointestinal tract of humans and animals as a normal flora. However, there are some strains that have evolved into pathogenic E. coli by acquiring virulence factors through plasmids, transposons, bacteriophages, and/or pathogenicity islands. These pathogenic E. coli can be categorized based on serogroups, pathogenicity mechanisms, clinical symptoms, or virulence factors [33, 47]. Among them, enterohemorrhagic E. coli (EHEC) is defined as pathogenic E. coli strains that produce Shiga toxins (Stxs) and cause hemorrhagic colitis (HC) and the life-threatening sequelae hemolytic uremic syndrome (HUS) in humans. Several serotypes in EHEC are frequently associated with human diseases such as O26:H11, O91:H21, O111:H8, O157:NM, and O157:H7 [44, 51]. E. coli O157:H7 is the most frequently isolated serotype of EHEC from ill persons in the United States, Japan, and the United Kingdom and it the focus of this review.

A plasmid is an extrachromosomal DNA that is capable of replicating independently of the chromosomal DNA. Plasmids are mobile elements that provide various host beneficial traits, such as resistance to antibiotics and heavy metals, production of toxins and other virulence factors, biotransformations of hydrocarbons, and symbiotic nitrogen fixation [22]. Plasmid-encoded genes are required for full pathogenesis in many enteropathogenic bacteria including Shigella, Yersinia, Salmonella, and E. colispecies.