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propagation at temperatures lower than 1
o
C,
Listeria monocytogenes
continues to be a nuisance to
1
the food industry [2].
2
The 3M MDA
Listeria monocytogenes
method allows for the rapid and specific detection
3
of
Listeria monocytogenes
in food and environmental samplesafter 24 hours of enrichment using
4
pre-warmed (37 ± 1
o
C) Demi-Fraser (DF) broth base (without ferric ammonium citrate
5
(FAC)).After enrichment, samples are evaluated using the 3M
™
MDA
Listeria monocytogenes
6
on the 3M™ Molecular Detection System (MDS). Presumptive positive results are reported in
7
real-time while negative results are displayed after completion of the assay(75 minutes).
8
Prior to the collaborative study, the 3M MDA
Listeria monocytogenes
method was certified as
9
an AOAC
®
Performance Tested
SM
Method
(PTM) following the AOAC Guidelines for
10
Harmonized PTM Studies [3]. The goal of the PTM study was to demonstrate that the 3M MDA
11
Listeria monocytogenes
method could detect
Listeria monocytogenes
in selected foods and
12
environmental surfaces as claimed by the manufacturer. For the 3M MDA
Listeria
13
monocytogenes
evaluation, there were 8 food matrices and 2 environmental surfaces analyzed:
14
beef hot dogs (25 g & 125 g), deli turkey (25 g & 125 g), cold smoked salmon (25 g), full fat
15
cottage cheese (25 g), chocolate milk (25 g), bagged raw spinach (25 g), romaine lettuce (25 g),
16
cantaloupe (whole melon), sealed concrete (sponge in 100 mL & 225 mL) and stainless steel
17
(sponge in 225 mL) usingDF broth basewithout FAC and where applicable, a secondary
18
enrichment in Fraser broth base without FAC.All other PTM parameters (inclusivity, exclusivity,
19
ruggedness, stability and lot to lot variability) tested in the PTM studies satisfied the
20
performance requirements for PTM approval. The method was awarded PTM certification
21
number 051401 on May 23, 2014.
22
The aim of this collaborative studywas to compare the reproducibility of the 3M MDA
23
Listeria monocytogenes
method to the United States Department of Agriculture (USDA)Food
24
Safety Inspection Service (FSIS) -Microbiology Laboratory Guidebook (MLG) 8.09
Isolation
25
and Identification of Listeria monocytogenes from Red Meat, Poultry, and Egg Products and
26
Environmental Samples
[4] for deli turkey and the AOAC
®
Official Methodof Analysis
27
993.12
Listeria monocytogenes in Milk and Dairy Products
[5] reference method for full fat (4%
28
milk fat) cottage cheese.
29
30
Collaborative Study
31
32
Study Design
33
34
In this collaborative study, two matrices, deli turkey and full fat cottage cheese, were evaluated.
35
The matrices were obtained from a local retailer and screened for the absence of
Listeria
36
monocytogenes
by the appropriate reference methods prior to analysis
.
The deli turkey was
37
artificially contaminated with heat stressed cells of
Listeria monocytogenes
American Type
38
Culture Collection (ATCC)13932 and the full fat cottage cheese with non-heat stressed cells of
39
Listeria monocytogenes
ATCC 19114. There were two inoculation levels for each matrix: a high
40
inoculation level of approximately 2-5 colony-forming units (CFU)/test portion and a low
41
inoculation level of approximately 0.2-2 CFU/test portion. A set of un-inoculated control test
42
portions were also included for each matrix at 0 CFU/test portion for a total of three
43
contamination levels per method.
44
Twelve replicate samples from each of the three contamination levels were analyzed. Two
45
sets of samples (72 total) were sent to each laboratory for analysis by 3M MDA
Listeria
46
monocytogenes
and either the USDA/FSIS- MLG (deli turkey) or AOAC OMA 993.12 (full fat
47
cottage cheese) reference method due to different sample enrichment procedures between the
48
Candidates for 2016 Method of the Year
148