Increased Detection of Listeria Species and Listeria monocytogenes in Raw Beef, Using the Assurance GDS Molecular Detection System with Culture Isolation†

Abstract

Testing for Listeria is challenging because of 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 Listeria monocytogenes. These two methods (culture isolation and Assurance GDS) use different enrichment strategies that may affect the number of Listeria and L. monocytogenes cells detected. Therefore, after first determining that the Assurance GDS accurately identified common Listeria strains isolated from raw beef, the two methods were compared by using paired ground beef samples (n = 256) that had been gathered from commercial sources. The agreement of the two methods was >76% for the culture and GDS Listeria method and >77% for the culture and GDS L. monocytogenes method. The molecular tests then were evaluated as endpoint tests in selected culture isolation enrichments. In this comparison, culture isolation and the molecular Listeria test agreed 100 and 84.4% of the time for Listeria-positive and -negative enrichments, respectively. An analysis of the discrepant samples in both experiments revealed that ∼50% of the samples identified as positive by the molecular method but not by the culture method could be confirmed by subsequent testing, indicating that the immunomagnetic concentration step of the GDS test likely provides a more sensitive level of detection than does culture alone. The culture results were available 2 days earlier when the molecular tests were used instead of plating media. However, because the Assurance GDS Listeria test cannot distinguish L. monocytogenes from other Listeria species such as Listeria innocua, samples containing both species could not be distinguished.