Enzyme-linked immunomagnetic electrochemical detection of live Escherichia coli O157:H7 in apple juice

We describe the application of enzyme-linked immunomagnetic electrochemistry (ELIME) for the rapid detection of Escherichia coli O157:H7 in buffered apple juice. The ELIME technique entails sandwiching bacterial analyte between antibody-coated magnetic beads and an alkaline phosphatase-conjugated antibody. The beads (with or without bound bacteria) were localized onto the surface of magnetized graphite ink electrodes in a multiwell plate format. The enzyme substrate, I-naphthyl phosphate, was added, and conversion of substrate to an electroactive product was measured using electrochemical detection. With this technique, detection of whole, live E. coli O157:1-17 bacterial cells was achieved with a minimum detectable level of ca. 5 X 10(3) cells per ml in Tris-buffered saline or buffered apple juice in an assay time of ca. 80 min. With adjustment of pH, the ELIME response for the bacteria in either sampling medium was similar, indicating that apple juice components did not contribute to any discernible sample matrix effects.