Gene clusters from gene pair distances. a) Gene clusters with intra-genomic pair-wise distance of at most 500 base pairs. Edges indicate the number of organisms where the distance is within this cut-off (see legend). b) An overview showing the persistent genes in the S10, spc and alpha operons found in E. coli O157:H7. The cluster number and gene type ([Additional file 1: Supplemental Table S2]; red: singletons, blue: duplicates) is also indicated.