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Listeria monocytogenes
is found widespread throughout the environment, having been isolated
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from soil, vegetation, marine sediments, water as well as many different types of food products
2
[1]. While
Listeria monocytogenes
has long been known to cause illness in animals, it has only
3
more recently been identified as the cause of listeriosis in humans [1].Listeriosis, while rare, can
4
be of great concern for the elderly, pregnant women, infants and the immunocompromised, as
5
the disease can lead to septicemia, meningitis, encephalitis or death [2, 3].Outbreaks from
6
Listeria monocytogenes
have been linked to such foods as ready-to-eat deli meats, hot dogs,
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pates, dairy products, soft cheese, smoked seafood, raw sprouts and most recently cantaloupes
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[4]. The VIDAS
Listeria monocytogenes
Xpress (LMX) assay,an automated enzyme based assay
9
for the screeningof
Listeriamonocytogenes
in food, provides the ability to rapidly detect the
10
target analyte in only one to two days, depending on sample size.
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The VIDAS LMX assay utilizes two proprietary primary enrichmentsto detect
Listeria
12
monocytogenes
in food products, LMX broth with supplement for 25 g test portions and LPT
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broth for 125 g test portions. The smaller test portions require 26-30 hours of incubation, while
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larger test portions require a 24-30 hour primary enrichment incubation followed by a secondary
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enrichment in 10 mL of LPT broth for an additional 22-26 hours of incubation.For smaller test
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portion sizes, the new enrichment method produces negative and presumptive positive results the
17
next day after enrichment.
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Prior to the collaborative study, the VIDAS LMX method was validated according to AOAC
19
Guidelines [5] in a harmonized AOAC Performance Tested Method
SM
(PTM)study. The
20
objectiveof this study was to demonstrate that the VIDAS LMX method could detect
Listeria
21
monocytogenes
in a variety of foods as claimed by the manufacturer. For theVIDAS
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LMXevaluation, there were 11 matricesoriginally evaluated:processed cheese (25 g), vanilla ice
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cream (25 g), cooked shrimp (25 g), smoked white fish (25 g), frozen spinach (25 g) peanut
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butter (25 g) and five “ready to eat” (RTE) 25 g meats (hot dogs, deli turkey, deli ham,
25
fermented sausage and paté. A matrix extension was conducted to evaluate 4 additional matrices:
26
deli ham (125 g), deli turkey (125 g), queso fresco (125 g) and ground beef (125 g).
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All other PTM evaluation requirements(inclusivity, exclusivity, ruggedness, stability and
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lot to lot variability) were satisfied. The method was awarded PTM certification number 091103
29
on September 14, 2011. The matrix extension was granted approval on January 15, 2013. This
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collaborative study compared the VIDAS LMX method to the AOAC OMA 993.12
Listeria
31
monocytogenes in Milk and Dairy Products
[6]method for queso fresco at two test portion sizes,
32
25 gand 125 g.
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34
Collaborative Study
35
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Study Design
37
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For this collaborative study, one matrix, queso fresco (soft Mexican cheese), was analyzed
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using two test portion sizes: 25 gand 125 g.The queso fresco was obtained from local retailers
40
and screened for the absence of
Listeria
by the AOAC 993.12 reference method prior to analysis.
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The 25 g and 125 g test portions of queso fresco were inoculated with the same strain of
Listeria
42
monocytogenes,
ATCC 19115, at two inoculation levels: a high inoculation level of
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approximately 2-5 colony-forming units (CFU)/test portion and a low inoculation level of
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approximately 0.2-2 CFU/test portion. A set of un-inoculated control test portions were also
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included for each matrix at 0 CFU/test portion. Twelvereplicate samples from each of the three
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inoculation levels of product were analyzed. Two sets of samples (72 total) were sent to each
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laboratory for analysis by VIDAS LMXand theAOAC OMA 993.12 reference method due to
48
different sample enrichments for each method.
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2