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laboratories indicated difficulty in identifying and isolating colonies from samples when using OXA plates, but not from test portions analyzed by the VIDAS LMX method. This may be due to the higher selectivity of the ALOA agar to isolate and differentiate typical Listeria colonies from competing microflora, such as Bacillus colonies. The high selectivity of the proprietary broth, the high background flora and the low selectivity of the OXA agar most likely contributed to this observation as well. For the analysis of 25 g test portions by the VIDAS LMX method, two false positives were obtained. The test results produced by the false-positive test portions (average test value of 0.22) were much lower than the test values observed with true positives (average value >2.00). By the time the coordinating laboratory received the results, the primary enrichments for these samples had been discarded so no subsequent analysis on the VIDAS LMX was possible. However, the agar plates for these test portions were shipped to the coordinating laboratory for further analysis. Up to 20 different colonies were picked for morphological and biochemical analysis using VITEK 2 GP and no Listeria colonies were identified. Additionally, the entire lawn of growth from each agar plate was swabbed, enriched in LMX broth, and incubated for 26–30 h at 37 ± 1°C. An aliquot from each tube was analyzed by the VIDAS LMX assay and negative results for L. monocytogenes were obtained. Results of this investigation led the study directors to believe that the false positives were the result of possible cross-contamination during the analysis of the samples. For the analysis of both the 25 and 125 g test portions, Laboratory 11 detected the presence of multiple types of Listeria spp. An investigation into the results indicated that colonies picked for confirmation did not meet the characteristics of Listeria spp., i.e. colonies produced Gram-negative stain reactions and were negative for motility and catalase. The results of these tests should have precluded analysis using the API strips which lead to an inaccurate identification. Due to the fact that final results reported were inconsistent with biochemical results, data produced by Laboratory 11 was removed from the statistical analysis of both the 25 and 125 g test portions. Using the POD statistical model, no significant difference in the number of positive results obtained between the two methods being compared was observed at both the low and high inoculum levels for both the 25 and 125 g test portions. No significant difference was observed between presumptive and confirmed results for the candidate method. The VIDAS Listeria monocytogenes Xpress (LMX) method with the optionalALOAagar confirmationmethod was adopted as Official First Action status for the detection of L. monocytogenes in a variety of foods, including deli ham (25 and 125 g), fermented sausage (25 g), liver pâté (25 g), processed cheese (25 g), vanilla ice cream (25 g), cooked shrimp (25 g), smoked white fish (25 g), frozen spinach (25 g), peanut butter (25 g), deli turkey (25 and 125 g), queso fresco (125 g), and ground beef (125 g). Conclusions

Acknowledgments

We would like to extend a sincere thank you to the following collaborators for their dedicated participation in this study:

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