148 / 471
In the rare event of any unusual light output, the algorithm labels this as “Inspect.” 3M recommends the
1
user to repeat the assay for any Inspect samples. If the result continues to be Inspect, proceed to
2
confirmation test using your preferred method or as specified by local regulations.
3
4
Results of Collaborative Study
5
6
For this collaborative study, the 3M Molecular Detection Assay (MDA)
Listeria
method was
7
comparedto the AOAC 993.12 reference method for full fat cottage cheese. A total of 15
8
laboratories throughout the United States and Canada participated in this study, with 13
9
laboratories submitting data for the full fat cottage cheese. Each laboratory analyzed 36 test
10
portions for each method: 12 inoculated with a high level of
Listeria
, 12 inoculated with a low
11
level of
Listeria
, and 12 un-inoculated controls. The 3M
™
MDA
Listeria
method produced 199
12
presumptive positive results with 196 confirming positive by traditional confirmation. There
13
were 205 confirmed positives by the reference method.
14
A background screen of the matrix indicated an absence of indigenous
Listeria
species. For each
15
matrix, the level of
Listeria
was determined by MPN determination on the day of initiation of
16
analysis by the coordinating laboratory. The individual laboratory and sample results are
17
presented in Table 2. Table 2014.1 summarizes the inter-laboratory results for all foods tested,
18
including POD statistical analysis [11]. As per criteria outlined in Appendix J of the AOAC
19
Validation Guidelines, fractional positive results were obtained.Detailed results for each
20
laboratory are presented in Tables 2014.2 and Figures 1A-1Bof the Supplementary Materials.
21
The results for each collaborating laboratory’s 3M Petrifilm Aerobic Count Plate(OMA 990.12)
22
for full fat cottage cheeseare presented in Table 2014.3 of the Supplementary Materials.
23
24
Full Fat Cottage Cheese (25 g Test Portions)
25
26
Full fat cottage cheesetest portions were inoculated at a low and high level and were analyzed
27
(Table 2) for the detection of
Listeria
spp. Un-inoculated controls were included in each
28
analysis. Laboratories 4 and 5 did not submit results to the coordinating laboratory.
29
Laboratories 6 and 13reported deviations in the protocol (laboratory 6 incorrectly incubated their
30
MDA test portions at 30
o
C instead for 48 hours instead of the required 37
o
C for 24 hours;
31
laboratory 13 confirmed all colony growth regardless of supplementary tests, (Gram stain,
32
catalase reaction) indicated that the organism would not be classified as
Listeria
(Gram negative
33
or Gram positive with spores, catalase negative))and results from these laboratories were
34
excluded from the statistical analysis. The MPN levels obtained for this test portion, with 95%
35
confidence intervals, were 0.80 CFU/test portion (0.63,1.00) for the low level and 4.83 CFU/test
36
portion (3.30, 7.70) for the high level.
37
For the high level, 132 out of 132 test portions (POD
CP
of 1.00) were reported as presumptive
38
positive by the 3M MDA
Listeria
method with all 132 test portions (POD
CC
of 1.00) confirming
39
positive.Based on the valid data submitted from each of the collaborating laboratories, 0 false
40
negative results or false positive results were obtained resulting in 132 confirmed positives
41
(POD
C
of 1.00). For the low level, 67 out of 132 test portions (POD
CP
of 0.51) were reported as
42
presumptive positive by the 3M MDA
Listeria
method with 64 test portions (POC
CC
of 0.48)
43
confirming positive.Based on the valid data submitted from each of the collaborating
44
laboratories, 3 false positive results were obtained resulting in 64 confirmed positives (POD
C
of
45
0.48). For the un-inoculated controls, 1 out of 132 samples (POD
CP
of 0.01) produced a
46
presumptive positive result by the 3M MDA
Listeria
method with all 132 test portions (POD
CC
of
47
0.00) confirming negative.. Based on the valid data submitted from each of the collaborating
48
laboratories, 0 false negative results and 0 false positive results were obtained resulting in 0
49
Candidates for 2016 Method of the Year
122