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452 Crowley et al.: Journal of AOAC International Vol. 97, No. 2, 2014
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.
Conclusions
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).
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, Jacquelyn Adams, Aaron Bollenbacher,
Keith Wiggins, and Lori Cesanas-Tyson, WBA Analytical
(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 & Co. (Springdale, OH)
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