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Test Portion Distribution
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All samples were labeled with a randomized, blind-coded 3 digit number affixed to the
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sample container. Test portions were shipped on a Thursday via overnight delivery according to
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the Category B Dangerous Goods shipment regulations set forth by IATA. Upon receipt,
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samples were held by the collaborating laboratory at refrigeration temperature (3-5 °C) until the
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following Monday when analysis was initiated. All samples were packed with cold packs to
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target a temperature of < 7°C during shipment. In addition to each of the test portions and the
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total plate count replicate, collaborators also received a test portion for each matrix labeled as
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‘temperature control’. Participants were instructed to obtain the temperature of this portion upon
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receipt of the package, document results on the Sample Receipt Confirmation form provided and
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faxed to the study director.
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Test Portion Analysis
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Collaborators followed the appropriate preparation and analysis protocol according to the
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method for each test portion size. For both test portion sizes, each collaborator received 72 test
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portions of each food product (12 high, 12 low and 12 controls for each evaluation). For the
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analysis of the 25g test portions by the VIDAS LPTmethod, a 25 gsample replicatewasenriched
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with 225 mL of pre-warmed (18-25
o
C) LPT broth and homogenized for 2 minutes. Test portions
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were incubated for 26-30 hours at 30 ±1
o
C. For the 125 g test potions analyzed by the VIDAS
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LPTmethod, a125g sample replicate was enriched with 375mL pre-warmed (18-25
o
C) LPT broth
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andhomogenized for 2 minutes. Test portions were incubated for 24-30hours at
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30 ±1
o
C. For 125 g test potions, a 1.0 mL aliquot of the primary enrichment was transferred into
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10 mL LPT broth and incubated for an additional 22-26 hours at 30 ± 1
o
C.
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Following enrichment, samples were assayed by the VIDAS LPT method and confirmed
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following procedures outlined in the standard reference method by streaking an aliquot of the
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primary enrichment onto Oxford Agar (OXA) and a proprietary chromogenic agar, ALOA.
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Presumptive positive samples were streaked for isolation on TSA/ye and biochemically
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confirmed by morphology verification via Gram stain, hemolysis test and by VITEK 2 GP
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Biochemical Identification method (AOAC OMA 2012.02) or API Listeria biochemical test kits.
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Laboratories utilizing API Listeria kits were also required to conduct a catalase test and an
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oxidase test.
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Both test portion sizes analyzed by the VIDAS LPTmethods were compared to samples
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(25 g) analyzed using the AOAC 993.12 reference method in conjunction with VITEK 2 GP
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Biochemical Identification (OMA 2012.02) or API
Listeria
(1) for the confirmation of
Listeria
in
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an unpaired study design. Twenty-five gram testportions were enriched in pre-warmed (45
o
C)
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selective enrichment broth, homogenized for 2 minutes and incubated at 30 ± 2
o
C for 48 hours.
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Samples were streaked onto OXA and presumptive positive samples were streaked for isolation
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onto TSA/ye. Colonies from TSA/ye were confirmedby morphology verificationvia Gram stain,
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hemolysis test and by VITEK 2 GPBiochemical Identification methodor API Listeria
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biochemical test kits. Laboratories utilizing API Listeria kits were also required to conduct a
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catalase test and an oxidase test.
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Statistical Analysis
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Each collaborating laboratory recorded results for the reference method andVIDASLPT
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results. The data sheets were submitted to the study director at the end of each week of testing
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for analysis. The results of each test portion for each sample were compiled by the study director
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