Evaluation of oligonucleotide probes for identification of shiga-like-toxin-producing Escherichia coli

Four synthetic oligonucleotide probes representing different regions of the Shiga-like toxin I (SLT-I) structural genes and one oligonucleotide derived from the SLT-II gene of Escherichia coli serotype O157:H7 strain 933 were examined for the identification of E. coli strains that produce cytotoxins for Vero or HeLa cells. E. coli strains that synthesize SLT-I alone or O157:H7 isolates that coexpress SLT-I and SLT-II hybridized with all four probes that were complementary to the SLT-I genes, suggesting that they have toxin genes with great homology in all the regions examined. In colony hybridization tests, these oligonucleotide probes did not react with E. coli strains that were nontoxigenic for Vero cells or that produced cytotoxins belonging to the SLT-II family. The probe derived from the slt-IIA gene distinguished E. coli strains that produced SLT-II alone from SLT-I-producing strains and hybridized to all E. coli O157:H7 strains that produced both SLT-I and SLT-II. Using two of these oligonucleotide probes that were complementary to slt-IA or slt-IIA sequences, we identified 50 of 52 cytotoxin-producing strains, whereas none of 416 nontoxigenic E. coli strains was reactive. The colony blot hybridization with the oligonucleotide probes described here can serve as a specific and sensitive test with potential diagnostic value.