 Today , several hundred complete genomic sequences of Escherichia and Shigella species are available . The genome sequence of the type strain of E. coli has been added to this collection not before 2014 . Comparison of these sequences shows a remarkable amount of diversity ; only about 20 % of each genome represents sequences present in every one of the isolates , while around 80 % of each genome can vary among isolates . Each individual genome contains between 4 @,@ 000 and 5 @,@ 500 genes , but the total number of different genes among all of the sequenced E. coli strains ( the <unk> ) exceeds 16 @,@ 000 . This very large variety of component genes has been interpreted to mean that two @-@ thirds of the E. coli <unk> originated in other species and arrived through the process of horizontal gene transfer . 
