escherichia coli (e. coli) is a normal inhabitant of the gastrointestinal tract of vertebrates, including humans. phylogenetic analysis has shown that e. coli is composed of four main phylogenetic groups (a, b1, b2 and d). group a and b1 are generally associated with commensals, whereas group b2 is associated with extra-intestinal pathotypes. most enteropathogenic isolates, however, are assigne...