197 / 471
1
No negative feedback was provided by the collaborating laboratories in regards to the
2
performance of the 3M MDA
Listeria monocytogenes
.Several laboratories reported difficulty in
3
isolating and identifying
Listeria
colonies on Oxford agar (OXA) from samples enriched in the
4
DF broth base (without FAC) when compared to samples enriched in the AOAC OMA 991.12
5
selective enrichment broth. This may be related to differences in formulation between the two
6
enrichments. The AOAC OMA 993.12 enrichment broth is designed to reduce the background
7
flora on OXA and is more selective than DF broth base (without FAC). In some instances, this
8
level of selectivity may cause stress on
Listeria
cells thus requiring a longer enrichment time to
9
reach a detectable level.
10
Based on the data submitted, 2 laboratories were removed from statistical consideration for
11
both the full fat cottage cheese and the deli turkey. For the cottage cheese, laboratory 6 did not
12
follow the approved incubation time andtemperature conditions for the candidate method
13
(samples were incubated for 48 hours at 30
o
C and the validated enrichment time and temperature
14
are 24 to 28 hours at 37
o
C.) and laboratory 12 confirmed growth from all plates, regardless of
15
supplementary tests that would have precluded confirmation via API
Listeria.
Due to this fact, all
16
samples confirmed via API
Listeria
produced a
Listeria
species result even if Gram reaction
17
(Gram negative), motility reaction (negative), catalase reaction (negative)andoxidase reaction
18
(positive) would indicate the orgnaisms is not of the genus
Listeria
. For the deli turkey,
19
laboratory 8 incorrectly enriched half of their candidate method samples using the reference
20
method enrichment broth (UVM) instead of DF broth. Laboratory 10 reported more confirmed
21
positive results in their uninoculated control samples (for both the candidate and reference
22
method) then for their low level contamination level indicating a substantial level of laboratory
23
cross-contamination. Based on these results, these laboratories were removed from statistical
24
analysis. No laboratories were removed from statistical analysis based on discrepancies between
25
presumptive and confirmed results.
26
During the collaborative study evaluation 7 false positive (3 for full fat cottage cheese and 4
27
for deli turkey) and 8 false negative (4 for full fat cottage cheese and 4 for deli turkey) were
28
obtained out of 792 total test portions analyzed by the 3M MDA
Listeria monocytogenes
. The
29
candidate methodcorrectly identified whether a test portion was positive or negative more than
30
98.1% of the time (false positive rate of 0.9% and false negative rate of 1.0%). Several of the
31
false positive discrepant results were obtained from uninoculated control test portions and
32
several of the false negative discrepant results were obtained from high level inoculated test
33
portions which may indicate that they are the result of laboratory error and not performance of
34
the assay. No evidence of physical cause or suspicion of cause was noted and it was determined
35
that they would be included in the statistical analysis.
36
For each matrix, the collaborative study indicated that no statistically significant difference
37
between the candidate method and the reference methods or the presumptive and confirmed
38
results of the candidate method was obtained when using the POD statistical model.
39
40
41
Recommendations
42
43
It is recommended that the 3M Molecular Detection Assay
Listeria monocytogenes
method be
44
adopted as Official First Action status for the detection of
Listeria monocytogenes
in selected
45
foods includingbeef hot dogs (25 g & 125 g), deli turkey (25 g & 125 g), cold smoked salmon
46
(25 g), full fat cottage cheese (25 g), chocolate milk (25 g), and two environmental surfaces,
47
Candidates for 2016 Method of the Year
161