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B
ird
et al
.:
J
ournal of
AOAC I
nternational
V
ol
.
100, N
o
.
2, 2017
465
Always dispose of sealed reagent tubes by soaking in a household
bleach solution (1–5%, v/v in water; 5250–6500 ppm) for 1 h
and away from the assay preparation area.
J. Results and Interpretation
An algorithm interprets the light output curve resulting from
the detection of the nucleic acid amplification. Results are
analyzed automatically by the software and are color-coded
based on the result. Apositive or negative result is determined by
analysis of a number of unique curve parameters. Presumptive
positive results are reported in real time, whereas negative and
“inspect” results will be displayed after the run is completed.
Presumptive positive samples should be confirmed as per
the laboratory’s standard operating procedures or by using
the current version of the appropriate reference method
confirmation (FDA/BAM or USDA/FSIS-MLG), beginning
with transfer from the primary enrichment to the secondary
enrichment broth (if applicable), followed by subsequent plating
and confirmation of isolates using appropriate biochemical and
serological methods.
Note
: Even a negative sample will not give a zero reading
because the system and 3M MDA 2 –
Listeria monocytogenes
amplification reagents have a “background” relative light unit
reading.
In the rare event of any unusual light output, the algorithm
labels this as inspect. 3M recommends the user to repeat the
assay for any inspect samples. If the result continues to be
inspect, proceed to confirmation testing using your preferred
method or as specified by local regulations.
Results of Collaborative Study
For this collaborative study, the 3M MDA 2 –
Listeria
monocytogenes
method was compared to the USDA/FSIS MLG
reference method for deli turkey and raw chicken breast fillet. A
total of 13 laboratories throughout the United States and Canada
participated in this study, with 11 laboratories submitting data
for the deli turkey and 12 laboratories submitting data for the
raw chicken breast fillet.
See
Table 2 for a summary of laboratory
participation for each matrix. Each laboratory analyzed 36 test
portions for each method per matrix: 12 inoculated with a high
level of
L. monocytogenes
, 12 inoculated with a low level of
L. monocytogenes
, and 12 uninoculated controls.
A background screen of the matrix using the USDA/FSIS
MLG reference method indicated an absence of indigenous
L. monocytogenes
in both matrixes. Ten replicate test portions
(randomly sampled from 50% of the total packages used in the
analysis) were screened for the presence of
L. monocytogenes
.
All test portions produced negative results for the target analyte.
Results for the heat stress analysis of the inoculum for
deli turkey are presented in Table 1. A 60.8% injury rate was
determined. The raw chicken breast fillet is not heat-treated;
therefore, it was not necessary to injure the cells. Tables
2016.08A
and
2016.08B
summarize the interlaboratory
results for all foods tested, including LPOD statistical
analysis. As per the criteria outlined in Appendix J of the
AOAC INTERNATIONAL
Methods Committee Guidelines
for Validation of Microbiological Methods for Food and
Environmental Surfaces
(4), fractional positive results were
obtained. Detailed results for each laboratory are presented
in Tables
2016.08C
and
2016.08D
. For each matrix, the level
of
L. monocytogenes
was determined by MPN testing on the
day of initiation of analysis by the coordinating laboratory.
MPN results are presented in Tables
2016.08C
and
2016.08D
.
The individual laboratory and sample results are presented
in supplemental Tables 1 and 2. The APC results for each
collaborating laboratory are presented in supplemental Table 3.
Deli turkey (125 g test portions)
.
—
Deli turkey test portions
were inoculated at a low and a high level and analyzed for the
detection of
L. monocytogenes
. Uninoculated controls were
included in each analysis. Laboratories 8 and 10 received test
portions but were unable to conduct the analysis and, therefore,
Figure 2016.08B
Table 2. Participation of each collaborating laboratory
a
Laboratory
Deli turkey
Raw chicken breast fillet
1
Y
Y
2
Y
Y
3
Y
Y
4
Y
Y
5
Y
Y
6
Y
Y
7
Y
Y
8
N
Y
9
Y
Y
10
N
Y
11
Y
N
12
Y
Y
13
Y
Y
a
Y = Collaborating laboratory analyzed the food type; N = collaborating
laboratory did not analyze the food type.