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The current classification of the genus
Listeria
includes six well characterized species, with
1
Listeria monocytogenes,
being the species of most concern in foodborne outbreaks [1].
Listeria
2
species are short, non-sporeforming Gram positive rods that are ubiquitous in the environment
3
and can be found in soil, decaying vegetation and most environments [2]. While the number of
4
people who become ill from listeriosis, the disease caused by
Listeria,
is relatively small, the
5
high mortality rate from infection makes it one of the leading causes of death from foodborne
6
illness [2]. Of primary concern for illness from
Listeria
outbreaks are the elderly, pregnant
7
women, infants and people with compromised immune systems [3]. Outbreaks from
Listeria
8
have been linked to such foods as ready-to-eat deli meats, hot dogs, patés, dairy products, soft
9
cheeses, smoked seafood, raw sprouts and most recently cantaloupes [4]. The VIDAS UP
10
Listeria
(LPT) assay,an automated enzyme phage-ligand based assay for the screeningof
Listeria
11
in food and environmental samples, provides the ability to detect
Listeria
after only 26 hours of
12
enrichment.
13
The VIDAS LPT assay uses a primary enrichment (pre-warmed to 18-25
o
C) to detect
Listeria
14
species in 25 g test portions after 26-30 hours of enrichment. For cantaloupe melons, individual
15
melons are soaked in approximately 1 L of LPT broth and incubated following conditions
16
outlined for 25 g test portions. For larger samples sizes such as 125 g, following 24-30 primary
17
enrichment incubation, a transfer to a secondary enrichment in 10 mLsof LPT brothand an
18
additional 22-26 hours of incubation is required prior to detection. For smaller test portion sizes
19
and cantaloupe melons, the new enrichment method eliminates the need for secondary
20
enrichments and produces negative and presumptive positive results thefollowing day.
21
Prior to the collaborative study, the VIDAS LPT method was validated by expert laboratories
22
according to AOAC Guidelines [5] in a precollaborative study. The objectiveof this study was to
23
demonstrate that the VIDAS LPT method could detect
Listeria
spp. in a variety of foods and
24
environmental surfaces as claimed by the manufacturer. For theVIDAS LPTevaluation, there
25
were 19 matrices tested:deli ham (25 g & 125 g), pepperoni (25 g), beef hot dogs (25 g), chicken
26
nuggets (25 g), chicken liver pate (25 gram), ground beef (125 g), deli turkey (125 g), cooked
27
shrimp (25 g), smoked salmon (25 g), cantaloupe melon, bagged mixed salad (25 g), regular
28
peanut butter (25 g), black pepper (25 g), vanilla ice cream (25 g), queso fresco (25 g & 125 g),
29
stainless steel, plastic, ceramic and concrete environmental surfaces.
30
During the precollaborative method comparison evaluation, 525 unpaired samples were
31
analyzed by the VIDAS LPT method. One (1) false positive result and 0 false negative results
32
were observed. Using the POD statistical model, no significant difference was observed between
33
the reference method and the VIDAS LPT method for all matrices analyzed except bagged
34
mixed salad, beef hot dogs and stainless steel environmental samples. For these three matrices,
35
the VIDAS LPT detected significantly more positive samples than the reference method which
36
resulted in the statistically significant difference. The inclusivity and exclusivity evaluation
37
showed no unexpected results. The VIDAS LPT method detected all of the
Listeria
strains
38
analyzed and none of the non-
Listeria
strains analyzed. The precollaborative data and report was
39
reviewed by ERP reviewers prior to approvalof the AOAC collaborative protocol. The
40
precollaborative data is presented in Appendix 2.
41
This collaborative study compared the VIDAS LPT method to the AOAC 993.12
Listeria
42
monocytogenes in Milk and Dairy Products
[6]method for queso fresco at two test portion sizes,
43
25 gand 125 g.
44
45
Collaborative Study
46
47
Study Design
48
49
2