Evaluation of a multiplex PCR system for simultaneous detection of Salmonella spp., Listeria monocytogenes, and Escherichia coli O157:H7 in foods and in food subjected to freezing.

Conventional culture methods were compared to a multiplex polymerase chain reaction (PCR) assay for simultaneous detection of Salmonella spp., Listeria monocytogenes, and Escherichia coli O157:H7 from enrichment cultures of various types of artificially inoculated and naturally contaminated foods. The multiplex PCR assay was evaluated in 44 types of spiked food samples, including meat, produce, fish, and dairy products targeting genes specific for each pathogen for simultaneous detection. The sensitivity of the assay was <or=5 CFU/25 g of inoculated sample after 20 hours of enrichment. The PCR assay was also evaluated in inoculated food samples stored at -20 degrees C for 2 weeks or 2 months. Out of 28 food samples tested, 27, 27, and 26 samples were positive for Salmonella Enteritidis, L. monocytogenes, and E. coli O157:H7, respectively, using the multiplex PCR assay, whereas only 13, 26, and 20 samples were positive, respectively, using the culture method after 2 weeks of storage at -20 degrees C. Similar results were obtained for samples stored at -20 degrees C for 2 months. The multiplex PCR assay method was capable of detecting 5 colony-forming units of each of the three pathogens per 25 g of more than 40 types of food, and the detection rate of the PCR assay was higher than that of conventional culture methods. As a result, the multiplex PCR assay is a valuable method for simultaneous rapid screening for the three pathogens in food, even after frozen storage.