Comparison of template preparation methods from foods for amplification of Escherichia coli O157 shiga-like toxins type I and II DNA by multiplex polymerase chain reaction

Escherichia coli O157:H7 has been responsible for several recent food-borne outbreaks in the United States. To protect the public health, it is essential that rapid and sensitive methods be developed for detection of this pathogen in foods. Methods were compared for preparation of template DNA for the polymerase chain reaction (PCR) from enrichments of food homogenates seeded with E. coli O157:H7. Samples were enriched for 6 h at 37 degrees C in modified tryptic soy broth supplemented with vancomycin, cefsulodin, and cefixime. Aliquots of the enrichments (10 ml or 1 ml) were analyzed by either washing twice with physiological saline or incubating with antibodies to O157 coupled to immunomagnetic beads (Dynal) followed by resuspending and boiling the samples. A portion of the preparation was used in a multiplex PCR to amplify a 274-bp fragment from the sltI gene and a 364-bp fragment from the sltII gene. PCR amplification of 1-ml portions of enrichment broth was successful at inoculation levels of about 10 cells per g of food. Increasing the test sample volume to 10 ml and/or using an immunomagnetic separation step improved the PCR detection sensitivity to about 1 cell per g; the entire analysis can be completed within 12 h