Two E.coli strains contain islands of genes specific to each strain

The circular genome maps of E. coli strains K-12 and O157:H7. The circle depicts the distribution of sequences specific to each strain. The colinear backbone common to both strains is shown in blue. The positions of O157:H7-specific sequences are shown in red. The positions of K-12-specific sequences are shown in green. The positions of O157:H7- and K-12-specific sequences at the same location are shown in tan. Hypervariable sequences are shown in purple.

[Data from N. T. Perna et al., “Genome Sequence of Enterohaemorrhagic Escherichia coli O157:H7,” Nature 409, 2001, 529-533. Courtesy of Guy Plunkett III and Frederick Blattner.]