Escherichia coli O157:H7 is a major foodborne pathogen that has posed serious problems for food safety and public health. Recent outbreaks and recalls associated with various foods contaminated by E. coli O157:H7 clearly indicate its deleterious effect on food safety. A rapid and sensitive detection assay is needed for this harmful organism to prevent foodborne illnesses and control outbreaks in a timely manner. We developed a magnetic bead-based immunoassay for detection of E. coli O157:H7 (the most well-known Shiga toxigenic E. coli strain) with a 96-well microplate as an assay platform. Immunomagnetic separation (IMS) and tyramide signal amplification were coupled to the assay to increase its sensitivity and specificity. This immunoassay was able to detect E. coli O157:H7 in pure culture with a detection limit of 50 CFU/ml in less than 3 h without an enrichment step. The detection limit was decreased 10-fold to 5 CFU/ml with addition of a 3-h enrichment step. When this assay was tested with other nontarget foodborne pathogens and common enteric bacteria, no cross-reactivity was found. When tested with artificially contaminated ground beef and milk samples, the assay sensitivity decreased two- to fivefold, with detection limits of 250 and 100 CFU/ml, respectively, probably because of the food matrix effect. The assay results also were compared with those of a sandwich-type enzyme-linked immunosorbent assay (ELISA) and an ELISA coupled with IMS; the developed assay was 25 times and 4 times more sensitive than the standard ELISA and the IMS-ELISA, respectively. Tyramide signal amplification combined with IMS can improve sensitivity and specificity for detection of E. coli O157:H7. The developed assay could be easily adapted for other foodborne pathogens and will contribute to improved food safety and public health.

Salmonella is the leading cause of bacteria-associated foodborne illnesses in the United States. Early detection of this pathogen by a rapid and sensitive assay is important to prevent salmonellosis. In this study, we describe a magnetic beadbased immunoassay for detection of Salmonella consisting of immunomagnetic separation for simple target concentration with tyramide signal amplification to increase the assay sensitivity. The developed immunoassay was able to detect Salmonella Typhimurium in culture with the detection limit of 280 CFU/mL in less than 3 h without any enrichment and further decreased to 70 CFU/mL with 3 h enrichment. When tested with ground beef and poultry samples artificially contaminated with S. Typhimurium and Enteritidis, the assay showed increased detection limits with 800 and 200 CFU/mL, respectively, due to the effect of complex food matrices. However, when 12 h enrichment was added, the detection limits in both food matrices decreased to 1 CFU. PRACTICAL APPLICATIONSThis study demonstrated that using immunomagnetic separation (IMS) and tyramide signal amplification can enhance sensitivity and specificity for the detection of Salmonella in food samples. The developed assay has great potential as a simple monitoring system for foodborne pathogens in food samples, which can improve food safety and public health. The results were also compared with sandwich type enzyme-linked immunosorbent assay, which showed the developed assay is approximately 50 times more sensitive than enzyme-linked immunosorbent assay.