A large insertion in the Shiga-like toxin 2 gene (stx2) of an Escherichia coli O157:H7 clinical isolate

Six clinical Escherichia coli O157:H7 isolates were epidemiologically linked as part of an outbreak in which the most likely source was undercooked ground beef or cross-contamination from the ground beef to other food products at a Mexican-style restaurant. These cultures were analyzed using molecular genetic, immunological and cytotoxicity procedures. All six isolates were confirmed as E. coli O157:H7 and were indistinguishable by pulsed-field gel electrophoresis using XbaI. The results of polymerase chain reaction (PCR) tests, non-isotopic gene probing, reversed passive latex agglutination (RPLA) kit results and Vero cell assays were consistent for the presence of a functional Shiga-like toxin 1 (Stx 1) protein. All six strains produced a stx2 PCR amplicon product; five strains produced a product which was consistent with the predicted amplicon size and one (SEA 6414) produced a much larger PCR product. The SEA 6414 isolate produced a protein reactive with the RPLA kit anti-Stx 2 antibody but was not cytotoxic to Vero cells. Sequencing of this region revealed that this 1310 bp insertion was very similar to a previously identified IS 1203 sequence and the insertion interrupted the carboxyl end of the coding region of the stx2 gene ‘A’ subunit.