Laboratory Procedure MFLP-28
June 2003
(PDF Version)
HEALTH PRODUCTS AND FOOD BRANCH
OTTAWA
MFLP-28: The Qualicon Bax® System Method for the Detection of Listeria Monocytogenes in
a Variety of Food
1. APPLICATION
This method is applicable to the detection of Listeria monocytogenes in a wide
variety of food, including raw meats, processed meats, seafood, fresh
produce/vegetables, cultured and non-cultured dairy, egg and egg products, and fruit
juices.
2. DESCRIPTION
The BAX® system is a convenient yes/no screening tool that uses Polymerase Chain
Reaction (PCR) technology for rapid amplification and fluorescent detection. Food
processors and associated laboratories can use the BAX® system as a quick method
for accurately detecting Listeria monocytogenes in a wide variety of foods.
Following 22-26 hours primary enrichment and 18-24 hours secondary enrichment, sample
preparation involves about 1 hour of user time, and the automated procedure delivers
reliable results about 4 hours later. BAX® system validation studies used the USDA-FSIS
enrichment method for meat, poultry and eggs; the AOAC method 993.12 for dairy; and the
FDA-BAM method for other food types (see Section 8: Reference Methods). The BAX®
system is designed for use by qualified lab personnel who follow standard microbiology
practices.
3. PRINCIPLE
The BAX® System uses the Polymerase Chain Reaction (PCR) to amplify a specific
fragment of bacterial DNA, which is stable and unaffected by growth environment. The
fragment is a genetic sequence that is unique to Listeria monocytogenes, thus
providing a highly reliable indicator that the organism is present. The automated BAX®
system then uses fluorescent detection to analyze PCR product for positive or negative
results.
PCR is a powerful means for quickly providing millions of copies of a specific DNA
fragment. In a typical application, sample DNA is combined with polymerase, nucleotides and
primers that are specific for a given nucleotide sequence. This mixture then undergoes a
series of timed heating and cooling cycles. Heating denatures or separates the DNA into
single strands, then as the mixture cools, primers recognize and anneal to the target DNA
sequence. DNA polymerase then uses nucleotides to extend the primers, thereby creating two
copies of the target DNA fragment. Repeated cycles of denaturing, annealing, and extending
produce exponential increases in the number of target DNA fragments within a matter of hours.
If the target sequence is not present, no detectable amplification takes place.
The BAX® system simplifies this process by combining the primers, polymerase,
nucleotides and positive control into a single sample tablet that is already packaged inside
the PCR tubes. Additionally, the automated fluorescent detection allows for closed-tube
testing, eliminating the potential for carry-over contamination with amplified DNA.
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