Rapid and Simultaneous Quantitation of Escherichia coli O157:H7, Salmonella, and Shigella in Ground Beef by Multiplex Real-Time PCR and Immunomagnetic Separation†

Abstract

The objective of this study was to establish a multiplex real-time PCR for the simultaneous quantitation of Escherichia coli O157:H7, Salmonella, and Shigella. Genomic DNA for the real-time PCR was extracted by the boiling method. Three sets of primers and corresponding TaqMan probes were designed to target these three pathogenic bacteria. Multiplex real-time PCR was performed with TaqMan Universal PCR Master Mix in an ABI Prism 7700 Sequence Detection System. Final standard curves were calculated for each pathogen by plotting the threshold cycle value against the bacterial number (log CFU per milliliter) via linear regression. With optimized conditions, the quantitative detection range of the real-time multiplex PCR for pure cultures was 102 to 109 CFU/ml for E. coli O157:H7, 103 to 109 CFU/ml for Salmonella, and 101 to 108 CFU/ml for Shigella. When the established multiplex real-time PCR system was applied to artificially contaminated ground beef, the detection limit was 105 CFU/g for E. coli O157:H7, 103 CFU/g for Salmonella, and 104 CFU/g for Shigella. Immunomagnetic separation (IMS) was further used to separate E. coli O157:H7 and Salmonella from the beef samples. With the additional use of IMS, the detection limit was 103 CFU/g for both pathogens. Results from this study showed that TaqMan real-time PCR, combined with IMS, is potentially an effective method for the rapid and reliable quantitation of E. coli O157:H7, Salmonella, and Shigella in food.