The genome of the enterohemorrhagic Escherichia coli O157∶H7 EDL933 contains 177 “O”-islands (OIs). To study their potential contribution to the O157-specific pathogenicity, we surveyed the distribution of 22 OIs by PCR and DNA hybridization in 17 isolates of Shiga toxin producing (Stx-positive) E.coli O157∶H7, and compared with their distribution in 21 isolates of Stx-negative E.coli O157 and 21 isolates of non-O157 enteric pathogens. Fourteen of 22 OIs were present in non-O157 entericpathogens analyzed. Eight of 22 OIs were found only in the 17 Shiga toxin- (Stx) positive E.coli O157∶H7 isolates, but they were absent from the 21 Stx-negative E.coli O157∶NM and O157︰Hund isolates tested. Among the 8 OIs, only OI43 or OI48 were exclusively detected in Stx-positive E.coli O157∶H7, absent from neither of Stx-negative E.coli O157 and non-O157 enteric pathogens, such as Salmonella, Shigella, Citrobacter, Vibrio cholera, enteropathogenic E.coli (EPEC), enteroadherent E.coli (EAEC), enteroinvasive E.coli (EIEC) and enterotoxingenic E.coli (ETEC). The OI43 and OI48 are 83 kb in size and identical in DNA sequences, which encode genes for urease, tellurite resistance and adherence. By analyzing their junction genes with PCR and DNA hybridization, we found that 21 Chinese isolates have OI48 only. However, for 7 Japanese patient isolates, 4 have OI43 and 3 have OI48; for American isolates, 2 have both of OI43 and OI48, 2 have OI48 only. These data confirmed the highly plasticity of the pathogenic E.coli genome. The unique presence of OI43/OI48 in Stx-positive E.coli O157∶H7 denotes its critical role in the pathogenicity specific to this pathogen. The genome of the enterohemorrhagic Escherichia coli O157: H7 EDL933 contains 177 “O” -islands (OIs). To study their potential contribution to the O157-specific pathogenicity, we surveyed the distribution of 22 OIs by PCR and DNA hybridization in 17 isolates of Shiga toxin producing (Stx-positive) E. coli O157: H7, and compared with their distribution in 21 isolates of Stx-negative E. coli O157 and 21 isolates of non-O157 enteric pathogens. Fourteen of 22 OIs were present in non- Eight of 22 OIs were found only in the 17 Shiga toxin- (Stx) positive E. coli O157: H7 isolates, but they were absent from the 21 Stx-negative E. coli O157: NM and O157: Hund isolates tested. Among the 8 OIs, only OI43 or OI48 were exclusively detected in Stx-positive E. coli O157: H7, absent from neither of Stx-negative E. coli O157 and non-O157 enteric pathogens, such as Salmonella, Shigella, Citrobacter , Vibrio cholera, enteropathogenic E. coli (EPEC), enteroadherent E. coli The OI43 and OI48 are 83 kb in size and identical in DNA sequences, which encode genes for urease, tellurite resistance and adherence. By analyzing their in situ hybridization (i) (EAEC), enteroinvasive E. coli (EIEC) and enterotoxingenic E. coli junction genes with PCR and DNA hybridization, we found that 21 Chinese isolates have OI48 only. However, for 7 Japanese patient isolates, 4 have OI43 and 3 have OI48; for American isolates, 2 have both OI43 and OI48, 2 have OI48 only . The data confirmed the highly plasticity of the pathogenic E. coli genome. The unique presence of OI43 / OI48 in Stx-positive E. coli O157: H7 indicates its critical role in the pathogenicity specific to this pathogen.