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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	Last Updated: 2003-09-01		Important Notices