Optimization of a fluorescence sandwich enzyme-linked immunosorbent assay for detection of Escherichia coli O157:H7 in apple juice.

Sandwich enzyme-linked immunosorbent assay, especially when coupled with biosensor technology, is a simple methodology that can rapidly screen juices for Escherichia coli O157:H7 contamination. However, sampling directly from apple juice and ciders has been postulated to reduce immunoassay sensitivity. In fluorescence sandwich enzyme-linked immunosorbent assays using commercially available polyclonal or monoclonal antibodies, sampling pasteurized apple juice spiked with E. coli O157:H7 compared to spiked phosphate-buffered saline shifted the range of detection. The spiked apple juice range of detection was 10(4) to 10(6) CFU/ml, whereas that for spiked phosphate-buffered saline was 10(6) to 10(8) CFU/ml, representing a hundredfold difference in sensitivity. Apple juice also increased background fluorescence intensity (P < 0.001) while reducing the net fluorescence intensity per CFU (P < 0.001). The addition of the polymer polyvinylpyrrolidone to apple juice significantly improved assay performance by increasing sensitivity and net fluorescence intensity per CFU and by reducing background fluorescence. Adjusting pH of apple juice from 3.9 to 7.4 improved assay performance but not to the degree seen with phosphate-buffered saline or polyvinylpyrrolidone-treated apple juice samples. The apple juice polyphenol, epicatechin, reduced net fluorescence intensity in a concentration-dependent manner, a change that was reversed by polyvinylpyrrolidone. Taken all together, these results suggest that polyvinylpyrrolidone can improve detection of O157:H7 in juices by reducing the effect of polyphenols on fluorescence sandwich enzyme-linked immunosorbent assay performance.