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Sample Size and Matrix Effect on the Ability of Rapid Methods to Detect E. coli O157:H7


The first objective of this work was to determine if changes in the size of a ground beef or beef trim sample, or changes in the volume of media used to enrich the sample affected rapid detection tests for E. coli O157:H7. The tests were culture isolation (established as the 'Gold Standard'), lateral flow device, and DNA amplification. The second objective looked at the effect of using these E. coli O157:H7 detection tests on different types of products (i.e. fat trim, offal, sponges, etc). The third objective of this work was to determine the best shipping conditions for samples needing to be confirmed in a third party laboratory.

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Findings: The results show that increasing size or decreasing volume will affect the accuracy of detecting E. coli O157:H7 and that test users need to revalidate the tests under their specific conditions. The results provide guidance on the use of modified enrichment medias, and the length of time needed to detect E. coli O157:H7 by different tests in different sizes of samples. All the detection methods accurately detected E. coli O157:H7 in all the different types of samples except for offal samples that contained liver. In that case, a buffered media was needed to improve detection. Finally, in mock shipping, after 24 h some samples will not test positive, especially if they have not been kept ice cold.

Wheeler, Tommy Lee; Schmidt, John; Koohmaraie, Mohammad; Kalchayanand, Norasak; Bosilevac, Joseph; Koohmaraie, Mohammad
USDA - Agricultural Research Service
IEH Laboratories & Consulting Group
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