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Listeria monocytogenes
in a Variety of Foods
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VIDAS
®
Listeriamonocytogenes
Xpress (LMX) Method
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First Action 2013
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(Applicable to detection of
Listeria monocytogenes
in deli ham (25g& 125 g),fermented sausage
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(25 g), liver paté (25 g), processed cheese (25 g), vanilla ice cream (25 g), cooked shrimp (25 g),
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smoked white fish (25 g), frozen spinach (25 g), peanut butter (25 g), deli turkey (25 g &125 g),
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queso fresco (125 g) and ground beef (125 g).
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See table
2013.1A-B
for a summary of results of the collaborative study. See table
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2013.2A-D
in Appendix 1 for detailed results of the collaborative study
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A.
Principle
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The VIDAS
®
Listeria monocytogenes
Xpress(LMX) method is for use on the automated
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VIDASinstrument for the detection of
Listeriamonocytogenes
antigens using the ELFA method
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(Enzyme Linked Fluorescent Assay).The Solid Phase Receptacle (SPR
®
) serves as the solid
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phase as well as the pipetting device. The interior of the SPR is coated with proteins specific for
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Listeriamonocytogenes
receptors. Reagents for the assay are ready-to-use and pre-dispensed in
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the sealed reagent strips. All of the assay steps are performed automatically by the instrument.
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The reaction medium is cycled in and out of the SPR several times. An aliquot of enrichment
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broth is dispensed into the reagent strip. The
Listeriamonocytogenes
receptors present will bind
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to the interior of the SPR. Unbound components are eliminated during the washing steps. The
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proteins conjugated to the alkaline phosphatase are cycled in and out of the SPR and will bind to
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any
Listeria monocytogenes
receptors which are themselves bound to the SPR wall. A final
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wash step removes unbound conjugate. During the final detection step, the substrate (4-Methyl-
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umbelliferyl phosphate) is cycled in and out of the SPR. The conjugate enzyme catalyzes the
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hydrolysis of the substrate into a fluorescent product (4-Methyl-umbellliferone) the fluorescence
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of which is measured at 450 nm. At the end of the assay, results are automatically analyzed by
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the instrument which calculates a test value for each sample. This value is then compared to
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internal references (thresholds) and each result is interpreted as positive or negative.
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B.
Apparatus and Reagents
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Items (a)-(h) are available as the VIDAS
Listeria monocytogenes
(LMX) assay kit from
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BioMérieux (595 Anglum Road, Hazelwood, MO 63042-2330, USA).
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B. (a)
VIDAS or miniVIDAS automated immunoassay System.
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B. (b)
LMXReagent strips-
60 polypropylene strips of 10 wells, each strip covered with a
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foil seal and label. The 10 wells contain the reagents in Table 2013.1C.
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B. (c)
Solid-phase receptacle (SPR). -
60 SPRs coated with proteins specific
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for
Listeria
receptors
.
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B. (d)
Standard-
One vial (1 x 6 mL). Ready to use. Contains purified and inactivated
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Listeriamonocytogenes
receptors + preservative + protein stabilizer.
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