Evaluation of a commercial enzyme immunoassay (EHEC-Tek) for detecting Escherichia coli O157 in beef and beef products

A commercial enzyme immunoassay system (EHEC-Tek) was compared with immunomagnetic separation followed by culture to cefixime-tellurite-sorbitol MacConkey agar (IMS/C) for detecting Escherichia coli O157 in artificially inoculated beef samples and naturally contaminated beefburgers. The EHEC-Tek assay used modified E. coli broth supplemented with novobiocin (mECn) as a primary enrichment medium, followed by immunomagnetic separation, secondary enrichment of magnetic beads in tryptone soya broth supplemented with acriflavine, followed by a standard monoclonal antibody-based immunoassay to detect O157 antigen. Twelve diverse strains of E. cold O157 were used to inoculate samples of minced beef in triplicate at each of three different concentrations (50, 5 and 0.5 cfu 25 g-1). When the 108 samples of minced beef were examined, there was complete agreement between EHEC-Tek and IMS/C on 87 samples. Ten samples were positive only by IMS/C and 11 samples were positive only by EHEC-Tek, such discrepant results occurring mainly at lower inoculum levels; of the latter 11, eight were confirmed culturally as containing E. coli O157. Five beefburgers naturally contaminated with E. coli O157 at c. 2 g-1 were negative by both EHEC-Tek and IMS/C after primary enrichment in mECn, but when the primary enrichment medium was buffered peptone water supplemented with vancomycin, cefixime and cefsulodin (BPW-VCC) E. coli O157 was detected in four out of five samples by EHEC-Tek and five out of five samples by IMS/C. The EHEC-Tek immunoassay is a technically simple and sensitive method for detecting E. coli O157 in beef, and requires a minimum of microbiological expertise for its performance: the assay is improved by use of BPW-VCC in place of mECn as the primary enrichment medium.