340 / 596
2
involving bagged leafy greens and ice cream [3].The 3M
™
Molecular Detection Assay(MDA) 2 -
1
Listeria
method,usinga combination of bioluminescence and isothermal amplification of nucleic
2
acid sequences,allows for the rapid and specific detection of
Listeria
species in a broad range of
3
food types and environmental surfaces after 24 to
2832
hoursof pre-enrichment. After
4
enrichment, samples are evaluated using the 3M MDA 2 -
Listeria
on the 3M
™
Molecular
5
Detection System (MDS).Presumptive positive results are reported in real-time while negative
6
results are displayed after completion of the assay in approximately 75 minutes.
7
Prior to the collaborative study, the 3M MDA 2 -
Listeria
method was validated according to
8
AOAC Guidelines[4] in a harmonized AOAC
®
Performance Tested Method
SM
(PTM) study.
9
The objective of the PTM study was to demonstrate that the 3M MDA 2-
Listeria
method could
10
detect
Listeria
in a broad range of food matrices and environmental surfaces as claimed by the
11
manufacturer. For the 3M MDA 2
- Listeria
PTM evaluation, 13 matrices were evaluated:hot
12
dogs (25g & 125g), salmon (25g), deli turkey (25g & 125g), cottage cheese (25g), vanilla ice
13
cream (25g), queso fresco (25g), spinach (25g), melon (whole), raw chicken leg pieces (25g),
14
raw chicken fillet (25g); concrete (sponge, 225 mL & 100 mL), stainless steel (sponge, 225 mL),
15
and plastic (Enviro
sS
wab, 10 mL) environmental samples.
16
Additional PTM parameters (inclusivity, exclusivity, ruggedness, stability and lot-to-lot
17
variability) tested in the PTM studies satisfied the performance requirements for PTM approval.
18
The method was awarded PTM certification number 111501 on November 3
rd
, 2015.
19
The purpose of this collaborative studywas to compare the reproducibility of the 3M MDA 2 -
20
Listeria
method to the United States Department of Agriculture (USDA) Food Safety Inspection
21
Service (FSIS) -Microbiology Laboratory Guidebook (MLG)Chapter 8.09
Isolation and
22
Identification of Listeria monocytogenes from Red Meat, Poultry and Egg Products, and
23
Environmental Samples
[5] for deli turkey (125 g) and raw chicken breast fillet.
24
25
Collaborative Study
26
27
Study Design
28
29
In this collaborative study, two matrices,deli turkeyand raw chicken breast fillet, wereevaluated.
30
The matrices were obtained from a local retailer and screened for the presenceof
Listeria
by the
31
USDA/FSIS MLG 8.09 reference method
.
The raw chicken breast fillet was artificially
32
contaminated with fresh unstressed cells of
Listeria monocytogenes
, American Type Culture
33
Collection (ATCC) 7644, and the deli turkeywas artificially contaminated with heat stressed
34
cells
(Table 2)
of
Listeria monocytogenes
,ATCC 19115,at two inoculation levels: a high
35
inoculation level of approximately 2-5 colony-forming units (CFU)/test portion and a low
36
inoculation level of approximately 0.2-2 CFU/test portion. A set of un-inoculated control test
37
portions (0 CFU/test portion) were also included.
38
Twelve replicate samples from each of the three inoculation levels were analyzed by each
39
method. Two sets of samples (72 total) were sent to each laboratory for analysis by 3M MDA 2 -
40
Listeria
and the USDA/FSIS MLG Chapter 8.09 reference method due to the different sample
41
enrichment procedures for each method. Additionally, collaborators were sent a 60 g test portion
42
and instructed to conduct atotal aerobic plate count (APC) using 3M
™
Petrifilm™Rapid Aerobic
43
Count Plate (AOAC Official Method 2015.13) [6] on the day samples were received for the
44
purpose of determining the total aerobic microbial load.
45
A detailed collaborative study packet outlining all necessary information related to the study
46
including media preparation, test portion preparation and documentation of results was sent to
47
each collaborating laboratory prior to the initiation of the study. A conference call was then
48