Genotypic variations of Shiga toxin-converting phages from enterohaemorrhagic Escherichia coli O157 : H7 isolates

Pulsed-field gel electrophoresis (PFGE) analysis revealed that enterohaemor rhagic Escherichia coli (EHEC) O157:H7 strains had considerable variations in their genomes, This study investigated whether or not the molecular prof ile of Shiga toxin (Stx) 1- and Stx2-converting phages isolated from EHEC O 157:H7 strains, derived from various sources in the USA and Japan, correspo nded to the variations of host strains' genotypes as determined by PFGE. A total of 51 Stx-converting phages including 12 Stx1-converting phages and 3 7 Stx2-converting phages was isolated from seven USA isolates and 20 Japane se isolates. The average Dice coefficient values showed 44% similarity betw een phage DNAs in Stx2-converting phages digested with SmaI and 55% in Stx1 -converting phages digested with HindIII, indicating considerable variation among phage DNA. In particular, restriction fragment length polymorphism ( RFLP) patterns of Stx2-converting phage DNA varied according to the PFGE ty pe of their host strain, which suggests that the phage genomes have altered their genotypic characteristics with those of host genomes. However, there are several exceptions: the RFLP patterns of some Stx2-converting phages w ere quite similar irrespective of the different genotypes of the host strai ns, indicating that horizontal transfer of Stx2-converting phage may also o ccur under some circumstances.