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ird
et al
.:
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ournal of
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nternational
V
ol
.
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o
.
1, 2017
method, 85 of 132 test portions confirmed positive (POD
C
of
0.64). For test portions evaluated by the USDA/FSIS MLG
8.09 reference method, 64 of 132 test portions produced
positive results. A dLPOD
C
value of 0.16 with 95% confidence
intervals of (0.04, 0.28) was obtained between the candidate
and reference methods, indicating a statistically significant
difference between the two methods, with an observed
higher proportion of positive results by the candidate method
than the reference method. A dLPOD
CP
value of 0.02 with
95% confidence intervals of (–0.10, 0.13) was obtained
between presumptive and confirmed results, indicating the
difference between presumptive and confirmed results was not
statistically significant at the 0.05 probability level.
For the high inoculum level, 131 of 132 test portions (POD
CP
of 0.99) were reported as presumptive positive by the 3M MDA
2–
Listeria
method with 132 of 132 test portions (POD
CC
of 1.00)
confirming positive. For samples that produced presumptive
positive results on the 3M MDA 2–
Listeria
method, 131 of 132
samples confirmed positive (POD
C
of 0.99). For test portions
evaluated by the USDA/FSIS MLG 8.09 reference method,
132 of 132 test portions produced positive results. A dLPOD
C
value of –0.01 with 95% confidence intervals of (–0.04, 0.02)
was obtained between the candidate and reference methods,
indicating the difference between methods was not statistically
significant at the 0.05 probability level. A dLPOD
CP
value
of –0.01 with 95% confidence intervals of (–0.04, 0.02) was
obtained between presumptive and confirmed results, indicating
the difference between presumptive and confirmed results was
not statistically significant at the 0.05 probability level.
For the uninoculated controls, 2 of 132 samples (POD
CP
of
0.02) produced a presumptive positive result by the 3M MDA
2–
Listeria
method with 1 of 132 test portions (POD
CC
of 0.01)
confirming positive. For samples that produced presumptive
positive results on the 3M MDA 2–
Listeria
method, 1 of 132
samples confirmed positive (POD
C
of 0.01). For test portions
evaluated by the USDA/FSIS MLG 8.09 reference method,
0 of 132 test portions produced positive results. A dLPOD
C
value of 0.01 with 95% confidence intervals of (–0.02, 0.04)
was obtained between the candidate and reference methods,
indicating the difference between methods was not statistically
significant at the 0.05 probability level. A dLPOD
CP
value
of 0.01 with 95% confidence intervals of (–0.03, 0.05) was
obtained between presumptive and confirmed results, indicating
the difference between presumptive and confirmed results was
not statistically significant at the 0.05 probability level.
Detailed results of the POD statistical analysis are presented
in Table
2016.07D
and Figure 2A and B.
Discussion
No negative feedback was provided by the collaborating
laboratories regarding the performance of the 3MMDA2–
Listeria
method. During the evaluation of the raw chicken breast
fillet, Laboratory 2 isolated
L. innocua
from an uninoculated
control sample. Because the organism recovered was different
from the organism that was inoculated,
L. monocytogenes,
no
just cause for removal of the data was determined, therefore,
the data were included. For the raw chicken breast fillet,
Laboratory 10 reported isolating
L. monocytogenes
from two
uninoculated control samples. The isolates were sent for further
identification and it was determined that they were the same
strain as the organism that was inoculated, indicating that cross-
contamination of the sample had occurred. Due to the fact that
cross-contamination had occurred, just cause removal of the
data was established and the data generated by laboratory 10
was, therefore, not included in the statistical analysis.
Overall, the data generated during this evaluation demonstrated
the reproducibility of this new method. For the deli turkey
analysis, the POD statistical analysis indicated the difference
between the candidate and reference methods was not statistically
significant at the 0.05 probability level, and that the difference
between presumptive and confirmed candidate methods was
not statistically significant at the 0.05 probability level. For raw
chicken breast fillet, a statistically significant difference was
observed between the reference and alternative methods. dLPOD
being significantly greater than zero showed an observed higher
proportion of positive results by the candidate method than
the reference method. One possible contribution for the higher
number of positive results observed with the 3MMDA2–
Listeria
method was the use of DF broth for the candidate method. This
enrichment media formulation may be less selective than the
modified UVMmedium used in the USDA reference method and
may have contributed to the higher level of recovery observed
during the evaluation. A second possible contribution for the
higher observed proportion of positive results with the candidate
method was the duration of the primary enrichment. Test portions
evaluated by the 3M MDA 2–
Listeria
method were incubated
for a minimum of 28 h in the primary enrichment, whereas the
USDA reference method had a maximum primary enrichment
time of 26 h. No statistically significant difference was observed
between the candidate method presumptive and confirmed results
for this matrix.
Recommendations
It is recommended that the 3M MDA 2–
Listeria
method
be adopted Official First Action status for the detection of
Listeria
in selected foods: hot dogs (25 and 125 g); salmon
(25 g); deli turkey (25 and 125 g); cottage cheese (25 g);
vanilla ice cream (25 g); queso fresco (25 g); spinach
(25 g); melon (whole); raw chicken leg pieces (25 g); raw
chicken fillet (25 g); and concrete, stainless steel, and plastic
environmental samples.
Acknowledgments
We would like to extend a sincere thank you to the following
collaborators for their dedicated participation in this study:
Robert Brooks, ATC Microbiology, LLC (North Little
Rock, AR)
Jaspreet Walia and Francisco Hernandez, Certified
Laboratories (Turlock, CA)
David Bosco and Grizelda Trevino, Food Safety Net Services
(Fresno, CA)
Alex Brandt and Chris Lopez, Food Safety Net Services (San
Antonio, TX)
Elizabeth Sjogren and Manish Shekhawat, Microbac
Laboratories, Inc. (Worcester, MA)
Li Maria Ma, Chris Timmons, and Claudia Diaz Proano,
Oklahoma State University (Stillwater, OK)
Alexandra Calle and David Campos, Texas Tech University
(Lubbock, TX)