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February 20, 2013
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Ruggedness Testing Results
1
Results for the 3M Petrifilm SALX System ruggedness study are summarized in Tables 4-7 in the Appendix.
2
Both
Salmonella
strains were positive for all replicates and method variations. The
Citrobacter braakii
3
strain was negative for all replicates and method variations.
4
5
Shelf Life and Lot-to-Lot Consistency Methodology
6
For the shelf life study, one lot of plates and disks was evaluated. Two more lots are being produced.
7
Results from the two new lots will be submitted with the yearly PTM renewal. The target shelf-life for the
8
3M Petrifilm SALX Plates and 3M Petrifilm SALX Confirmation Disks is 18 months.
9
10
Two serovars of
Salmonella
(
Salmonella
Typhimurium ATCC 14028 and
Salmonella
Poona NCTC 4840) and
11
one non-
Salmonella
strain (
Citrobacter braakii
ATCC 13048) were analyzed. Each organism was cultured in
12
3M Salmonella Enrichment with 3M Salmonella Enrichment Supplement and then diluted in the same broth
13
to within one log limit of detection of the method.
For each organism, five replicates of were tested (10
14
positive and 5 negative) using the 3M Petrifilm Salmonella Express System Method.
15
16
Shelf Life and Lot-to-Lot Consistency Results
17
Results presented are for shelf life of the one lot of 3M Petrifilm SALX plates and disks. Results are
18
summarized in Table 8 in the Appendix. Both
Salmonella
serovars were positive for all replicates
19
tested. The
Citrobacter braakii
strain was negative for all replicates.
20
21
3M Petrifilm Salmonella Express System Method-Reference Method Comparison
The 3M Petrifilm
22
Salmonella Express System was compared to the FDA/BAM Chapter 5
Salmonella
reference method (3)
23
for shrimp, spinach, dry dog food, and stainless steel and to the USDA/FSIS MLG 4.0
5 Isolation and24
Identification of Salmonella from Meat, Poultry, Pasteurized Egg and Catfish Productsreference
25
method (4) for raw ground chicken, raw ground beef, raw ground pork, pasteurized liquid whole eggs,
26
and cooked chicken nuggets.
27
28
Sample Preparation
29
The
Salmonella
cultures were prepared to deliver a target level of 0.2 - 2 CFU / test portion.
Each
30
matrix was inoculated with a different strain of
Salmonella
species. A diluted 24 hour broth culture
31
inoculum was added to a bulk sample of each food matrix and mixed thoroughly. The cooked chicken
32
nuggets and pasteurized liquid whole eggs were inoculated with an organism that had been heat
33
stressed for 10 minutes at 50
o
C in a water bath. The degree of injury of the culture was estimated by
34
plating an aliquot of diluted culture onto Xylose Lysine Desoxycholate (XLD) and Tryptic Soy agar (TSA).
35
The agars were incubated at 37° ± 1°C for 24 ± 2 hours and the colonies were counted. The degree of
36
injury was estimated as
100 )
1(
x
n
n
nonselect
select
−
, where
n
select
= number of colonies on selective agar and
37
n
nonselect
= number of colonies on nonselective agar. The matrix was well mixed and stored at 4°C for
38
48-72 hr to allow the inoculum to equilibrate with the food environment and to cold stress the cells.
39
The inoculated bulk samples were then used to generate all inoculated test portions. Ground beef test
40
samples of 325g and 375g were made with 25g of inoculated bulk and 300 grams or 350 grams,
41
respectively, of uninoculated bulk (3M and MLG samples). Inoculated and uninoculated portions were
42
well mixed to ensure homogeneity. At the time of enrichment, the inoculated products were randomly
43
split into test groups.
44
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