Detection of Listeria monocytogenes in Fresh Produce Using Molecular Beacon—Real-time PCR Technology

The capability of an assay to detect Listeria monocytogenes from artificially inoculated fresh-cut produce such as cantaloupe and mixed salad was demonstrated. An oligonucleotide probe that becomes fluorescent upon hybridization to the target DNA (Molecular Beacon, MB) was used in a real-time polymerase chain reaction (PCR) assay. As few as 4 to 7 colony-forming units (CFU) of L. monocytogenes per 25 g of artificially contaminated produce could be detected. A comparison of 2 commercially available kits using MB-PCR (iQCheck, Bio-Rad Laboratories) and conventional PCR (BAX, Dupont Qualicon) was performed on artificially inoculated produce. The time required to detect L. monocytogenes (from produce to PCR) was considerably shorter for the iQ-Check protocol (approximately 26 h) compared with the BAX-PCR (approximately 52 h). The iQ-Check protocol was also used to confirm the identity of the L. monocytogenes isolates obtained during a microbiological screen of conventional and organic leaf lettuce and alfalfa sprout samples from local supermarkets. The iQ check protocol was successful in differentiating L. monocytogenes isolates from other Listeria spp. such as L. welshimeri, L. innocua, and L. ivanovii. This is the 1st report of the application of the MB probe being used for real-time detection of L. monocytogenes in whole and fresh-cut produce.