|Koohmaraie, Mohammad - FORMER ARS EMPLOYEE|
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 9, 2008
Publication Date: May 1, 2009
Citation: Bosilevac, J.M., Guerini, M.N., Koohmaraie, M. 2009. Increased Detection of Listeria Species and Listeria Monocytogenes in Raw Beef, Using the Assurance GDS Molecular Detection System with Culture Isolation. Journal of Food Protection. 72(3):674-679. Interpretive Summary: Many species of Listeria are present throughout the environment and are harmless, but the species Listeria monocytogenes can contaminate foods and is a pathogen that can result in severe consequences (spontaneous abortion, meningitis, septicemia and death). Testing for Listeria species is challenging due to their slow growth rate. Listeria can be detected by culture isolation methods or by more rapid DNA detection methods. These studies examined the benefits of combining a Listeria DNA detection assay with a culture isolation assay. Many DNA based tests (TaqMan, iQ-Check, Qualicon BAX, Assurance GDS to name a few) can be used for detection. The Assurance GDS system was chosen for use in these studies because in addition to DNA detection it also uses an immunomagnetic concentration step that can provide an isolate of Listeria to test further if needed. Further, when the DNA detection test was used in combination with the culture method, more positive samples could be identified and the results known two days earlier, with the additional benefit of confirmed results of an isolate for further characterization. However, since the DNA detection test cannot distinguish Listeria species from L. monocytogenes, samples containing both organisms could not be identified.
Technical Abstract: Testing for Listeria is challenging due to its slow growth rate. Recently, we described a rapid Listeria culture isolation method. This method can be improved by utilizing a rapid molecular detection test such as the Assurance GDS tests for Listeria and L. monocytogenes (L. mono). These two methods (culture isolation and Assurance GDS) use different enrichment strategies that may affect the number of Listeria and L. mono detected. Therefore, after first determining Assurance GDS accurately identified common Listeria isolated from raw beef, the two methods were compared using paired ground beef samples (n=256) that had been gathered from commercial sources. The agreement of the two methods was >76% between the culture and GDS Listeria method and >77% between the culture and GDS L. mono method. Then the Assurance GDS tests were evaluated as endpoint tests in selected culture isolation enrichments. In this comparison culture isolation and Assurance GDS Listeria agreed 100% and 84.4% for Listeria positive and negative enrichments respectively. An analysis of the discrepant samples in both experiments found that ~50% of the samples identified as positive by GDS but not the culture method could be confirmed by subsequent testing, indicating that the immunomagnetic concentration step of GDS likely provides a greater level of detection than culture alone. The culture results could be known two days earlier when Assurance GDS tests were used instead of plating media. However, since the Assurance GDS Listeria test cannot distinguish Listeria species from L. mono, samples containing both organisms could not be identified.