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laboratories indicated difficulty in identifying and isolating colonies from samples when using
1
the OXA plates, but not from test portions analyzed by the VIDAS LPT method. This may be
2
due to the higher selectivity of the ALOA agar to isolate and differentiate typical
Listeria
3
colonies from competing microflora, such as
Bacillus
colonies. The high selectivity of the
4
proprietary LPT broth, the high background flora and the low selectivity of the OXA agar most
5
likely contributed to this observation as well.
6
For the analysis of 25 gtest portions by the VIDAS LPT method, 3 false positives were obtained.
7
The test results produced by three false positive test portions (average test value of 0.34) were
8
much lower than the test values observed with true positives (average value >2.00). By the time
9
the coordinating laboratory received the results, the primary enrichments for these samples had
10
been discarded so no subsequent analysis on the VIDAS LPT was possible. However, the agar
11
plates for these test portions were shipped to the coordinating laboratory for further analysis. Up
12
to 20 different colonies were picked for morphological and biochemical analysis using VITEK 2
13
GP and no
Listeria
colonies were identified. Additionally, the entire lawn of growth from each
14
agar plate was swabbed and enriched in separate LPT broth tubes and incubated for 26-30 hours
15
at 30 ± 1
o
C. An aliquot from each tube was analyzed by the VIDAS LPT assay and negative
16
results for
Listeria
spp. were obtained. Results of this investigation lead the study directors to
17
believe that the false positives were the result of contamination during the analysis of the
18
samples.
19
For the analysis of both the 25g and 125g test portions, Laboratory 11 detected the presence of
20
multiple species
Listeria
. An investigation into the results indicated that colonies picked for
21
confirmation did not meet the characteristics of
Listeria
spp., i.e. colonies produced Gram
22
negative stain reactions, were negative for motility, catalase and produced hemolysis reactions
23
not typically observed with the
Listeria
spp.. The results of these tests should have precluded
24
analysis using the API strips which lead to an inaccurate identification. Due to the fact that final
25
results reported were inconsistent with biochemical results, data produced by Laboratory 11 was
26
removed from the statistical analysis of both the 25g and 125g test portions.
27
Typical growth of
Listeria
spp. colonies from ALOA was easy to identify and the ALOA
28
plates produced less background ground from the matrix than the OXA plates for both test
29
portions sizes analyzed. Positive comments were received from collaborators about the ease of
30
use associated with the ALOA plates.
31
Using the POD statistical model, no significant differencein the number of positive results
32
obtained between the two methods being comparedwas observed at both the low and high
33
inoculum levels for both the 25g and 125g test portions. No significant difference was observed
34
between presumptive and confirmed results for the candidate method.
35
36
37
Recommendations
38
39
It is recommended that the VIDAS
®
UP
Listeria
(LPT) method with the optional ALOAagar
40
confirmation method be adopted as Official First Action status for the detection of
Listeria
in a
41
variety of foods and environmental surfaces including deli ham (25 g & 125 g), pepperoni (25
42
g), beef hot dogs (25 g), chicken nuggets (25 g), chicken liver pate (25 g), ground beef (125 g),
43
deli turkey (125 g), cooked shrimp (25 g), smoked salmon (25 g), cantaloupe melon, bagged
44
mixed salad (25 g), peanut butter (25 g), black pepper (25 g), vanilla ice cream (25 g), queso
45
fresco (25 g & 125 g), stainless steel, plastic, ceramic and concrete environmental surfaces.
46
47
48
49
12