Interestingly, the ancestral IS629-deficient A2 O55:H7 strain 3256-97 is also lacking both IS629 associated regions found in the O55:H7 strains. Our analysis of common IS629 target sites demonstrated that strain 3256-97 seems to be more closely related to A4 and A5 CC strains than other A1 and A2 strains. Therefore, it is likely that IS629 has been lost in strain 3256-97 as well as in the hypothetical A3 precursor. These results may indicate that strain 3256-97 or a similar strain lacking IS629 might have given rise to IS629-deficient A4 CC strains. E. coli O157:H7 strains carry multiple IS629 copies while the non-pathogenic K-12 strain lacks

IS629 but carries other IS elements. Adriamycin molecular weight Other pathogenic E. coli strains, amongst the top six non-O157 STEC O26:H11, O111:H- and O103:H2 [25], also harbor various copies of IS629 elements in their genomes. Genome sequences for the other three most important pathogenic non-O157 STEC; O45, O145, and O121 are not available to date thus the presence

of IS629 elements is unknown. Interestingly, they also share the same reservoir with O157:H7 (e.g. cattle), shiga-toxins, haemolysin gene cluster, other virulence factors and several phages and phage-like elements [25]. Ooka et al (2009) postulated that IS-related genomic rearrangements may have significantly altered virulence and other phenotypes in O157 strains. These findings suggest that IS629 might not only have a great impact in their genomic evolution see more but might increase the pathogenicity of those strains as well. Conclusions The genomic sequence analysis showed that Guanylate cyclase 2C IS629 insertion sites exhibited a highly biased distribution. IS629 was much more frequently located on phages or prophage-like elements than in the well-conserved backbone

structure, which is Emricasan nmr consistent with the observations by Ooka et al (2009). IS629 was found to be present in the A1 and one of two A2 CC strains examined as well as in all the O157:H7 strains of A5 and A6 CC, however it was totally absent in the 6 examined SFO157 strains of A4 CC. The A4 CC strains are related to but on a divergent evolution pathway from O157:H7. These results suggest that the absence of IS629 in A4 strains probably occurred during the divergence, but it is uncertain if it contributed to the divergence. Overall, IS629 had great impact on the genomic diversification of the E. coli O157:H7 lineage and might have contributed in the emergence of the highly pathogenic O157:H7. Methods Bacterial strains The bacterial strains used in this study are listed in Table 2 and were chosen to represent typical EHEC and EPEC strains from the different clonal complexes from the evolution model for E. coli O157:H7 [11] with different serotypes (O157:H7, O157:H- and O55:H7) and different characteristics (e.g. β-glucuronidase activity (GUD), sorbitol fermentation (SOR).