This study has evaluated the efficacy of selective enrichment and plating media used for the isolation of verocytotoxin (VT)-producing Escherichia coli (VTEC) of serogroup O157, by examining pure bacterial cultures. In addition, the performance of a variety of commercial test kits for the detection of E. coli O157 strains inoculated into minced beef was compared, using the Ampcor E. coli O157:H7 Kit, 3M Petrifilm™ Test Kit-HEC, Dynabeads anti- E coli O157, EHEC-TEK™, and the Tecra E. coli O157 visual immunoassay. The commercial Verotox F test for the determination of the VT type of VTEC isolates was compared with a polymerase chain reaction (PCR) assay for VT-coding genes. Modified E. coli broth containing novobiocin (mEC + n) and sorbitol MacConkey agar supplemented with cefixime and tellurite (CT-SMAC) were the most efficacious media for selective enrichment and isolation, respectively. After enrichment of the inoculated samples, all kits tested could detect less than one O157 VTEC cell per g of minced beef. While the results of the immunoassays need to be confirmed by isolating the organisms, the use of the immunomagnetic separation technique directly yields isolates. The results of the Verotox F test were consistent with PCR results. A sensitive and cost-effective method for the isolation of O157 VTEC from minced beef in food industry and epidemiological studies involving large numbers of samples is the following: enrichment in mEC + n at 37°C for 6 to 8 h with shaking at 100 rpm, followed by immunomagnetic separation using Dynabeads anti- E. coli O157 and spread plating of the concentrated target cells onto CT-SMAC. The Verotox F test can be used to determine whether the isolates produce VTl and/or VT2.
Keywords: O157; commercial test kits; detection media; isolation media; minced beef.