Development of verotoxin 2- and verotoxin 2 variant (VT2v)-specific oligonucleotide probes on the basis of the nucleotide sequence of the B cistron of VT2v from Escherichia coli E32511 and B2F1.

We and others have noted that there are serological differences between verotoxin 2 (VT2) (also known as Shiga-like toxin II) produced by Escherichia coli C600(933W) and the VT2 variant (VT2v) produced by strain E32511. Recent reports have described nucleotide sequence differences between the VT2v B subunit cistron of E32511 and B2F1 and that of VT2. We have confirmed the sequence differences and have used them to design oligonucleotide probes which differentiate the B subunit cistron of VT2v from that of VT2. Isolates of VT-producing E. coli obtained from human as well as food and veterinary sources were classified according to the toxin phenotype by using a toxin neutralization assay with VT2-specific monoclonal antibody and VT2v-specific polyclonal antisera. Using the oligonucleotide probes in colony hybridization, we detected 35 of 35 VT2 producers and 16 of 16 VT2v producers. One VT2 producer was falsely identified as containing the VT2v gene. The E32511 strain in our collection hybridized only with the VT2-specific probe. Southern hybridization of radiolabeled oligonucleotide probes showed that strains carried zero to one copy of the VT2 gene and zero to two copies of the VT2v gene. We conclude that colony hybridization with the VT2- and VT2-specific probes is highly predictive of the toxin phenotypes in the clinical isolates described in this study.