Low specificity of HEC O157 ELISA in screening ground beef for Escherichia coli O157:H7

During January-August, 1991, 74 retail ground beef samples were tested for the presence of Escherichia coli O157:H7 using the commercial HEC O157 ELISA kit. A total of 17 samples (23%) were presumptively positive for E. coli O157:H7. Twenty-nine isolates were taken from colonies corresponding to positive spots on the immunoreactive discs and 13 from colonies on sorbitol MacConkey agar No. 3 plates. Of these isolates, 30 were positive when retested as pure cultures with the ELISA (enzyme-linked immunoabsorbent assay). Only 15 of these 30 isolates were E. coli, 6 were identified as Hafnia alvei, and 9 were not identifiable. None of the E. coli isolates were serotype O157:H7. These results confirm that: a) the HEC O157 ELISA method for detecting E. coli O157:H7 has low specificity, and b) use of the HEC O157 ELISA necessitates thorough confirmatory testing