 The link between phylogenetic distance ( " relatedness " ) and pathology is small , e.g. the O157 : H7 serotype strains , which form a clade ( " an exclusive group " ) — group E below — are all enterohaemorragic strains ( EHEC ) , but not all EHEC strains are closely related . In fact , four different species of Shigella are nested among E. coli strains ( vide supra ) , while E. albertii and E. fergusonii are outside of this group . Indeed , all Shigella species were placed within a single subspecies of E. coli in a phylogenomic study that included the type strain , and for this reason an according reclassification is difficult . All commonly used research strains of E. coli belong to group A and are derived mainly from Clifton 's K @-@ 12 strain ( λ ⁺ F ⁺ ; O16 ) and to a lesser degree from d 'Herelle 's Bacillus coli strain ( B strain ) ( O7 ) . 
