Spontaneous reactivation of Shiga toxins in Escherichia coli O157 : H7 cells caused by transposon excision

IS1203v is an insertion sequence which has been found in inactivated Shiga toxin 2 genes (stx2) of Escherichia coli O157:H7. Using PCR amplification, we detected the wild-type stx2 genes in colonies of E. coli O157:H7 which p ossessed stx2 genes inactivated by insertion of IS1203v. This suggests that IS1203v is excised from the inactivated stx2 genes in E. coli O157: H7. We isolated the cells possessing the wild-type stx2 genes, and confirmed Stx2 productivities by reversed passive latex agglutination. We also analyzed t he frequency of the appearance of tile Stx2-producing cells using a quantit ative PCR method. As a result, the frequency was 3.00 x 10(-6) with culturi ng for 24 h at 37 degreesC, and this increased to 8.83 x 10(-5) when E. col i O157: H7 possessing the inactivated stx2 genes was transformed by an expr ession plasmid harboring the IS1203v transposase. These results showed that some Stx2-nonproducing E. coli O157:H7 strains could be spontaneously chan ged into Stx2-producing cells.