EVALUATION OF MEDIA AND TEST KITS FOR THE DETECTION AND ISOLATION OF ESCHERICHIA-COLI-O157 FROM MINCED BEEF

This study has evaluated the efficacy of selective enrichment and plat ing media used for the isolation of verocytotoxin (VT)producing Escher ichia coli (VTEC) of serogroup O157, by examining pure bacterial cultu res. 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(TM) Test Kit-HEC, Dynabeads anti-E. coli O157, EHEC-TEK(TM), and the Tecra E. coli O157 visual immunoassay. The commercial Verotox F test for th e determination of the VT type of VTEC isolates was compared with a po lymerase chain reaction (PCR) assay for VT-coding genes. Modified E. c oli broth containing novobiocin (mEC + n) and sorbitol MacConkey agar supplemented with cefixime and tellurite (CT-SMAC) were the most effic acious media for selective enrichment and isolation, respectively. Aft er enrichment of the inoculated samples, all kits tested could detect less than one O157 VTEC cell per g of minced beef. While the results o f the immunoassays need to be confirmed by isolating the organisms; th e use of the immunomagnetic separation technique directly yields isola tes. The results of the Verotox F test were consistent with PCR result s. A sensitive and cost-effective method for the isolation of O157 VTE C from minced beef in food industry and epidemiological studies involv ing large numbers of samples is the following: enrichment in mEC + n a t 37 degrees C for 6 to 8 h with shaking at 100 rpm, followed by immun omagnetic separation using Dynabeads anti-E. coli O157 and spread plat ing of the concentrated target cells onto CT-SMAC. The Verotox F test can be used to determine whether the isolates produce VT1 and/or VT2.