776 / 1195
13 laboratories submitting data for the 125 g test portions as presented in Table 1. Each
1
laboratory analyzed 36 test portions for each method- 12 inoculated with a high level of
Listeria
2
monocytogenes,
12 inoculated with a low level of
Listeriamonocytogenes,
and 12 un-inoculated
3
controls. A background screen of the matrix indicated an absence of indigenous
Listeria
species.
4
As per criteria outlined in Appendix J of the AOAC guidelines, fractional positive results were
5
obtained for both the 25 g and 125 g test portions sizes. For each test portion size, the actual
6
level of
Listeria monocytogenes
was determined by MPN determination on the day of initiation
7
of analysis. The results of the inoculum heat-stress protocol are presented in Table 2. The
8
individual laboratory and sample results are presented in Tables 3-4.
Tables2013.1A-B
9
summarizes the collaborative study results for all foods tested, including POD statistical analysis
10
[10].Detailed results for each laboratory are presented in Tables 2013.2A-Dand Figures 1A-D
11
and 2A-D of the Appendix.
12
13
14
Queso Fresco (25 gram Test Portions)
15
16
Queso fresco test portions were inoculated at a low and high level and were analyzed (Table 3)
17
for the detection of
Listeriamonocytogenes
. Un-inoculated controls were included in each
18
analysis. Fourteen laboratories participated in the analysis of this matrix and the results of
19
13laboratories were included in the statistical analysis. Laboratory 11 reported eleven test
20
portions (including4 un-inoculated test portions)that produced non-
Listeria monocytogenes
21
profiles. Colonies on these plates contained one or more of the following biochemical reactions
22
not typically associated with
L. monocytogenes
: Gram-negative, Gram positive with spores,non-
23
beta-hemolytic and catalase negative.Based on the preliminary biochemical tests conducted the
24
test portions should not have been carried through for final biochemical identification on API
25
Listeria
strips which resulted in the misidentification of the test portion as
Listeria
spp. The
26
selective agar plates for these test portions were sent to the coordinating laboratory for further
27
examination. The coordinating laboratory confirmed the supplementary results (Gram stain,
28
hemolysis and catalase reaction) reported by the participating laboratory and were not able to
29
identify any
Listeria
species. Laboratory 11 also reported one un-inoculated control portion
30
positive for
L. monocytogenes.
Testing at the coordinating laboratory verified this result, which
31
indicated cross contamination with that sample.The results from this laboratory were excluded
32
from statistical analysis.The MPN obtained for this matrix, with 95% confidence intervals, were
33
0.55 CFU/test portion (0.43, 0.70) for the low inoculum level and 3.81 CFU/test portion (3.06,
34
5.48) for the high inoculum level.For VIDAS LMX test portions, no difference was observed
35
between confirmation of samples using the proprietary chromogenic ALOA agar and the Oxford
36
agar required by the reference method.
37
For the high inoculum level, 156out of 156 test portions were reported as positive by the VIDAS
38
LMX method with all test portions confirming positive. For the low inoculum level,
39
77 out of 156 test portions were reported as positive by the VIDAS LMX method with 75 test
40
portions confirming positive, indicating 2false positive results.For the un-inoculated controls, 0
41
out of 156 samples produced a presumptive positive result by the VIDAS LMX method with
42
nosamples confirmingpositive.For test portions analyzed by the AOAC 993.12 method, 153 out
43
of 156 high inoculum test portions and69 out of 156 low inoculum test portions confirmed
44
positive. For the un-inoculated controls, 0 out of 156 test portions confirmed positive.
45
For the low level inoculum, a dLPOD
C
value of 0.04 (-0.08, 0.15) was obtained between the
46
AOAC 993.12 method and the VIDAS LMX method. The confidence intervals obtained for
47
dLPOD
C
indicated no significant difference between the two methods. A dLPOD
CP
of
48
10