149 / 471
confirmed positives (POD
C
of 0.00). For test portions analyzed by the AOAC 993.12 Method,
1
132 out of 132 high inoculum test portions and 73 out of 132 low inoculum test portions
2
confirmed positive. For the un-inoculated controls, 0 out of 132 test portions confirmed positive.
3
For the low level inoculum, a dLPOD
C
value of -0.07 with 95% confidence intervals of
4
(-0.19, 0.06) were obtained between the 3M MDA
Listeria
method and the AOAC OMA 993.12
5
method. The confidence intervals obtained for dLPOD
C
indicated no significant difference
6
between the two methods. A dLPOD
CP
value of 0.01 with 95% confidence intervals of (-0.12,
7
0.13) were obtained between presumptive and confirmed 3M MDA
Listeria
results. The
8
confidence intervals obtained for dLPOD
CP
indicated no significant difference between the
9
presumptive and confirmed results using either confirmation process.
10
For the high level inoculum, a dLPOD
C
value of 0.00 with 95% confidence intervals of
11
(-0.03, 0.03) were obtained between the 3M MDA
Listeria
method and the AOAC OMA 993.12
12
method. The confidence intervals obtained for dLPOD
C
indicated no significant difference
13
between the two methods. A dLPOD
CP
value of 0.00 with 95% confidence intervals of (-0.03,
14
0.03) were obtained between presumptive and confirmed 3M MDA
Listeria
results. The
15
confidence intervals obtained for dLPOD
CP
indicated no significant difference between the
16
presumptive and confirmed results. Detailed results of the POD statistical analysis are presented
17
in Table 2014.2 and Figures 1A-1B of the Appendix.
18
19
20
Discussion
21
22
No negative feedback was provided by the collaborating laboratories in regards to the
23
performance of the 3M MDA
Listeria
. Several laboratories reported difficulty in isolating and
24
identifying
Listeria
colonies on Oxford agar(OXA)from samples enriched in the DF brothbase
25
(without FAC) when compared to samples enriched in the AOAC OMA 991.12 selective
26
enrichment broth. This may be related to differences in formulation between the two
27
enrichments. The AOAC OMA 993.12 enrichment brothis designed to reduce the background
28
flora on OXA and is more selective than DF broth base (without FAC). In some instances, this
29
level of selectivity may cause stress on
Listeria
cells thus requiring a longer enrichment time to
30
reach a detectable level.
31
Based on the data submitted, 2 laboratories, laboratory 6 and laboratory 13, were removed
32
from statistical consideration for the full fat cottage cheeseDuring analysis,laboratory 6 did not
33
follow the approved incubation time and temperature for the candidate method (samples were
34
incubated for 48 hours at 30
o
C and the validated enrichment time and temperature are 24 to 28
35
hours at 37
o
C.) andlaboratory 13 confirmed growth from all plates, regardless of supplementary
36
tests that would have precluded confirmation via API
Listeria.
Due to this fact, all samples
37
confirmed via API
Listeria
produced a
Listeria
species result even if Gram stain reaction (Gram
38
negative), motility reaction (negative), catalase reaction (negative) and oxidase reaction
39
(positive) would indicate the organism is not of the genus
Listeria
.
40
During the analysisof the full fat cottage cheese,3false positive results were obtainedout of 396
41
test portions analyzed with the candidate method. The 3M MDA
Listeria
correctly identified
42
whether a test portion was positive or negative more than 99.2% of the time (false positive rate
43
of 0.8%). For full fat cottage cheese, the collaborative study indicated thatno statistically
44
significant difference between the candidate method and the reference method or the
45
presumptive and confirmed results of the candidate method was obtainedwhen using the POD
46
statistical model.
47
48
Recommendations
49
Candidates for 2016 Method of the Year
123