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ird
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ournal of
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nternational
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100, N
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2, 2017
were shipped on a Thursday via overnight delivery according
to the Category B Dangerous Goods shipment regulations set
forth by the International Air Transportations Association. The
two matrixes were shipped consecutively, with collaborating
laboratories performing analysis on one matrix at a time. Upon
receipt, samples were held by the collaborating laboratory at
refrigeration temperature (2–8°C) until the following Monday,
when analysis was initiated after a total equilibration time
of 96 h. All samples were packed with cold packs to target a
temperature of <7°C during shipment.
In addition to each of the test portions and a separate APC
sample, collaborators received a test portion for each matrix
labeled “temperature control.” Participants were instructed
to obtain the temperature of this portion upon receipt of
the package, document the results on the Sample Receipt
Confirmation form provided, and fax or e-mail it back to the
study director. The shipment and hold times of the inoculated
test material had been verified as a QC measure before study
initiation.
Test Portion Analysis
Collaborators were instructed to follow the appropriate
preparation and analysis as outlined in the study protocol for
each matrix, for both the 3M MDA 2 –
Listeria monocytogenes
method and the reference method. For both matrixes, each
collaborator received 72 test portions (12 high, 12 low, and
12 uninoculated controls for each method to be performed). For
the analysis of the deli turkey test portions by the 3M MDA
2 –
Listeriamonocytogenes
method, a 125 g portionwas enriched
with 1125 mL Demi-Fraser (DF) Broth with ferric ammonium
citrate (FAC) broth, homogenized for 2 min, and incubated
for 24–30 h at 37 ± 1°C. For the raw chicken breast fillet test
portions analyzed by the 3M MDA 2 –
Listeria monocytogenes
method, a 25 g portion was enriched with 475 mL DF Broth,
homogenized for 2 min, and incubated for 28–32 h at 37 ± 1°C.
After enrichment, samples were assayed by the 3M MDA
2 –
Listeria monocytogenes
method and, regardless of presumptive
result, confirmed using the USDA/FSIS MLG reference
method. Both matrixes evaluated by the 3M MDA 2 –
Listeria
monocytogenes
method were compared to samples analyzed
using the USDA/FSIS MLG reference method in an unpaired
study design. All positive test portions were biochemically
confirmed by the API
L. monocytogenes
biochemical test or
by the VITEK
®
2 Gram Positive Biochemical Identification
Method, AOAC
Official Method
2012.02
(8).
Statistical Analysis
Each collaborating laboratory recorded results for the
reference method and the 3M MDA 2 –
Listeria monocytogenes
method on the data sheets provided. At the end of each week of
testing, the data sheets were submitted to the study director for
statistical analysis. Data for each matrix were analyzed using
the probability of detection (POD) statistical model (9). POD
statistical analysis was conducted using the
AOAC Binary Data
Interlaboratory Study Workbook
, Version 2.3 (10). The PODwas
calculated as the number of positive outcomes divided by the
total number of trials. The POD was calculated for the candidate
presumptive results (POD
CP
), the candidate confirmatory results
(excluding those with presumptive negative results; POD
CC
),
the difference in collaborating laboratory POD (dLPOD) in the
candidate presumptive and confirmatory results (dLPOD
CP
),
presumptive candidate results that confirmed positive (including
those with presumptive negative results; POD
C
), the reference
method (POD
R
), and the difference between the confirmed
candidate and reference methods (dLPOD
C
). A dLPOD
C
confidence interval not containing the value 0 would indicate
a statistically significant difference between the 3M MDA
2 –
Listeria monocytogenes
and the reference method at the 5%
probability level. In addition to POD, the repeatability SD (s
r
),
the among-laboratory repeatability SD (s
L
), the reproducibility
SD (s
R
), and the homogeneity test of LPODs (P
T
) value were
calculated. The s
r
provides the variability of data within
one laboratory, the s
L
provides the difference in SD among
laboratories, and the s
R
provides the variability in data among
different laboratories. The P
T
value indicates whether adequate
sample homogeneity has occurred between laboratories.
{Applicable to the detection of
Listeria monocytogenes
in hot
dogs (25 and 125 g), salmon (25 g), deli turkey (25 and 125 g),
cottage cheese (25 g), chocolate milk (25 mL), vanilla ice cream
(25 g), queso fresco (25 g), bagged raw spinach (25 g), romaine
lettuce (25 g), melon (whole), raw chicken leg pieces (25 g), and
raw chicken breast fillet (25 g), as well as on sealed concrete
[3M Hydrated Sponge with Dey-Engley (D/E) Neutralizing
Broth; 225 and 100 mL], stainless steel (3M Hydrated Sponge
with D/E Neutralizing Broth; 225 mL), and plastic (high-density
polyethylene; 3M EnviroSwab with Letheen Broth; 10 mL)
environmental samples.}
See
Tables
2016.08A
and
2016.08B
for a summary of results
of the interlaboratory study.
See
Tables
2016.08C
and
2016.08D
for detailed results of the
interlaboratory study.
A. Principle
The 3M Molecular Detection Assay (MDA) 2 –
Listeria
monocytogenes
method is used with the 3M Molecular
Detection System (MDS) for the rapid and specific
detection of
L. monocytogenes
in enriched food and on food
process environmental samples. The 3M MDA 2 –
Listeria
monocytogenes
uses loop-mediated isothermal amplification
of unique DNA target sequences, with high specificity and
sensitivity, combined with bioluminescence to detect the
amplification. Presumptive positive results are reported in real-
time, whereas negative results are displayed after the assay
is completed. Samples are pre-enriched in DF Broth with
FAC broth.
B. Apparatus and Reagents
Items
b
–
g
are available as the 3M MDA 2 –
Listeria
monocytogenes
kit from 3M Food Safety (St. Paul, MN).
AOAC Official Method 2016.08
Listeria monocytogenes
in a Variety of Foods and Select
Environmental Surfaces
3M
™
Molecular Detection Assay (MDA) 2 –
Listeria
monocytogenes
Method
First Action 2016