Consider a 10-hour real-time PCR technique for detecting Salmonella in ready-to-eat meats.

Developing rapid pathogen detection methodologies that have shorter pre-enrichment times and real-time data-monitoring capabilities will help the meat industry prevent the recall of contaminated meats by stopping the contaminated products from being introduced into the marketplace in the first place.

USDA-ARS scientists evaluated the efficacy of real-time polymerase chain reaction (PCR) assays for their ability to detect low levels of Salmonella in ready-to-eat (RTE) meats following eight hours of pre-enrichment. The sensitivity and accuracy of molecular beacon and TaqMan[R] probe PCR assays were compared with the conventional USDA microbiological procedure using artificially contaminated RTE meats. Molecular beacons are oligonucleotide probes that can report the presence of specific nucleic acids in homogenous solutions. They are useful when it is either not possible or desirable to isolate the probe-target hybrids from an excess of the hybridization probes, such as in the real-time monitoring of PCRs in sealed tubes or when detecting ribonucleic acid within living cells.

Fluorescent probes targeted invasion-related genes. The TaqMan probe specifically detected the invA gene. The molecular beacon probe specifically detected the iagA gene from Salmonella spp. The researchers artificially contaminated certain RTE meats--turkey, ham and bologna--with Salmonella enterica serovar typhimurium at approximately 2 cells to 4 cells per 25 g.

After 20 hours of pre-enrichment in buffered peptone water, both protocols were sensitive enough to identify all positive samples. Furthermore, all 35 inoculated RTE meat samples were detected by molecular beacon PCR assay after eight hours of pre-enrichment. However, only 23 of 35 samples were detected by the TaqMan assay following eight hours of pre-enrichment. Similarly, all 12 uninoculated controls were reported to be negative by both PCR assays and the USDA procedure.

The molecular beacon PCR assay can detect as low as 2 CFU to 4 CFU of Salmonella in 25 g of RTE meat within 10 hours.

Further information. Jitu Patel, USDA-ARS Food Safety Laboratory, Room 101, Building 201 BARC-East, Beltsville, MD, 20705; phone: [phone]; fax: [phone]; email: [e-mail].