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General Information
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Listeria monocytogenes
is a major cause of foodborne illnesses in the US resulting in
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94% of hospitalizations and 19% of the total food-related fatalities (1). A diverse range
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of foods such as ready-to-eat deli meats, cheeses, coleslaw etc. have been implicated
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in
Listeria
-related foodborne illnesses. The pathogen can be present in very low
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numbers on these foods and can often go undetected. Hence there is a need to
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develop a rapid, reliable and sensitive method that can detect low numbers of
Listeria
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monocytogenes
. The VIDAS
LMX method for the detection of
Listeria
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monocytogenes
was previously validated as a
Performance Tested Method
SM
(
PTM
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Certification #091103
) in a variety of foods with 25g food test portions (
Vegetables:
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spinach, peanut butter;
Seafood:
shrimp, smoked fish ;
Meat:
liver paté, raw
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fermented sausage;
Poultry:
turkey hot dogs, deli ham, deli turkey; and
Dairy:
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processed cheese, ice cream). This validation study report provides performance
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data for the proposed LMX PTM modification with 125g test portions of ground beef,
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deli ham, turkey and Mexican style soft-cheese
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Material and methods:
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Preparation of inoculums:
Listeria monocytogenes
ATCC 51776 was used for the
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study. The organism was prepared from frozen stock culture by streaking onto Tryptic
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Soy Agar (TSA, BD Diagnostic Systems, Sparks, MD) and incubating the plates at 35
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± 2°C for 24 hours. An isolated colony was inoculated into Tryptic Soy Broth (TSB,
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BD) containing 0.6% yeast extract (YE, BD) and incubated at 35 ± 2°C for at least 18
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hours. The broth cultures were centrifuged, washed, and resuspended in Butterfield’s
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Phosphate Broth (BPB). The bacterial suspension was heat stressed for 10 min at
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50°C and used only for deli ham and turkey. This heat stressed and the non-stressed
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suspension was diluted to deliver a dose of 0.2 to 2 CFU/test portion and 2-5 CFU/test
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portion.
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Food Matrices
: Products bought from a local grocery store were used during this
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study for inoculation. The products used were fully-cooked deli ham, queso fresco
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(Mexican soft-cheese), fully-cooked turkey deli and ground beef. All the samples were
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tested for absence of
Listeria
and
Listeria monocytogenes
prior to inoculation.
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Inoculation
: A 25g portion of the food matrix was inoculated with heat stressed
L.
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monocytogenes
and then the inoculated bulk lot of matrix with 100g of
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uncontaminated matrix. The samples were equilibrated at 4°C for 48 h prior to be used
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for analysis.
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Each matrix was divided into at least 3 samples.
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Control
- Uncontaminated level
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0
cfu/test portion
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5 replicates
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PTM Certification Modification Report