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C rowley et al.: J ournal of AOAC International Vol. 97, No. 2, 2014 441
For the analysis of 25 g test portions by the VIDAS LPT
method, three false positives were obtained. The test results
produced by three false-positive test portions (average test value
of 0.34) 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 LPT 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 and
enriched in separate LPT broth tubes and incubated for 26–30 h
at 30 ± 1°C. An aliquot from each tube was analyzed by the
VIDAS LPT assay and negative results for Listeria spp. were
obtained. Results of this investigation lead the study directors to
believe that the false positives were the result of 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 species of
Listeria . 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, non-motile, negative catalase, or produced hemolysis
reactions not typically observed with Listeria spp.). 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 were removed from the
statistical analysis of both the 25 and 125 g test portions.
Typical growth of Listeria spp. colonies from ALOA was
easy to identify and the ALOA plates produced less background
ground from the matrix than the OXA plates for both test
portions sizes analyzed. Positive comments were received from
collaborators about the ease of use associated with the ALOA
plates.
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.
Conclusions
The VIDAS UP Listeria (LPT) method with the optional
ALOA agar confirmation method was adopted as Official First
Action status for the detection of Listeria in a variety of foods
and environmental surfaces including deli ham (25 and 125 g),
pepperoni (25 g), beef hot dogs (25 g), chicken nuggets (25 g),
chicken liver pâté (25 g), ground beef (125 g), deli turkey
(125 g), cooked shrimp (25 g), smoked salmon (25 g), whole
cantaloupe melon, bagged mixed salad (25 g), peanut butter
(25 g), black pepper (25 g), vanilla ice cream (25 g), queso
fresco (25 and 125 g), stainless steel, plastic, ceramic, and
concrete environmental surfaces.
Acknowledgments
We would like to extend a sincere thank you to the following
collaborators for their dedicated participation in this study:
John Mills and Pat Rule, bioMérieux Industry (Hazelwood,
MO)
Ben Howard, Neil Rogman, and Jacob Cannon, Certified
Laboratories (Bollingbrook, IL)
Barbara Paul, Marianna Sala-Rhatigan, and Susan Joseph,
U.S. Food and Drug Administration, Northeast Regional
Laboratory (Jamaica, NY)
Nikki Palen, Amber Stegmann, and Bryan Perry, EMS
Laboratories (St. Louis, MO)
Rachel Hiles and Tenesha Stubblefield, Silliker Laboratories
(Omaha, NE)
Nigel Nagassar, Sylvanus Owusu, and Jacqueline Zimmerman,
Silliker Laboratories (Minnetoka, MN)
Jerry Lynn Pickett, Aaron Bollenbacher, Keith Wiggins, and
Lori Cesanas, Tyson WBAAnalytical (Springdale, AR)
Bharath Brahmanda, Food Safety Net Services (San Antonio,
TX)
Andrew Capps, Grisel Rosario, Dawn Davis, Lindsey Parker,
Christine Said, and Jianfeng Li, North Carolina Department
of Agriculture and CS: Food and Drug Protection Division
(Raleigh, NC)
Keith Klemms, Bill May, Becky Hand, and Rose Burkhart,
Sherry Laboratories (Warsaw, IN)
Hesham Elgaali, Indiana State Department of Health, Food
and Dairy Microbiology Division (Indianapolis, IN)
Jennifer Jolly, Covance Laboratories (Battle Creek, MI)
Sandy Moore, Dustin Ebbing, Maggie Michels, Amanda
Kehres, and Joe Hirsch, John Morrell (Springdale, OH)
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Candidates for 2016 Method of the Year
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