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Expression of Red-Shifted Green Fluorescent Protein by Escherichia coli O157:H7 as a Marker for the Detection of Cells on Fresh Produce

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Escherichia coli O157:H7 was transformed with a plasmid vector red-shifted green fluorescence protein (pEGFP) to express red-shifted green fluorescence protein (EGFP) from Aequorea victoria. The EGFP expression among total cells and nonviable cells was determined at the cellular level by microscopic observation of immunostained and membrane-impermeable, dye-stained cultures, respectively. E. coli O157:H7 retained pEGFP during frozen storage at-80°C. The percentage of EGFP expression was improved by repeated subculturing, reaching 83.4 ± 0.1%, although the fluorescence intensity varied among cells. A low percentage of EGFP-expressing cells was nonviable. The percentage of EGFP decreased when the culture plate was kept at 4°C, suggesting that some cells lost pEGFP during refrigeration. The storage of the culture suspension in sterile deionized water at 4°C for 24 h reduced the percentage of EGFP expression, indicating that some EGFP was denatured. The application of EGFP as a marker for E. coli O157:H7 on green leaf lettuce, cauliflower, and tomato was evaluated using confocal scanning laser microscopy. EGFP-transformed cells were readily visible under confocal scanning laser microscopy on all produce types. The numbers of E. coli O157:H7 cells detected with EGFP were equivalent to those detected with immunostaining for green leaf lettuce and cauliflower but less for tomato. E. coli O157:H7 attached preferentially to damaged tissues of green leaf lettuce and tomato over intact tissue surfaces and to flowerets of cauliflower than to stem surfaces. EGFP can serve as a marker to characterize E. coli O157:H7 attachment on green leaf lettuce and cauliflower but may not be suitable on tomato.


Document Type: Research Article

Affiliations: Center for Food Safety and Quality Enhancement, Department of Food Science and Technology, University of Georgia, Athens, Georgia 30602-2106, USA

Publication date: March 1, 2001

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