2. Materials and methods2.1. Bacterial strainsThree strains each of Escherichia coli O157:H7 (ATCC 35150, ATCC 43889, and ATCC 43890), Salmonella enterica serovar Typhimurium (ATCC 19585, ATCC 43971, and DT 104), and Listeria monocytogenes (ATCC 19111, ATCC 19115, and ATCC 15313) were obtained from the Food Science and Human Nutrition culture collection at Seoul National University (Seoul, Korea). Stock cultures were kept frozen at −80°C in 0.7ml of Tryptic Soy Broth (TSB; Difco) and 0.3ml of 50% glycerol. Working cultures were streaked onto Tryptic Soy Agar (TSA; MB Cell, Seoul, Korea), incubated at 37°C for 24h and stored at 4°C.2.2. Culture and sample preparationEach strain of E. coli O157:H7, S. Typhimurium, and L. monocytogenes was cultured in 5ml TSB at 37°C for 24h and harvested by centrifugation at 4000×g for 20minat 4°C. The pelleted cells were washed three times by resuspending in sterile 0.2% Bacto Peptone (Becton-Dickinson, Sparks, MD, USA) followed by centrifugation. The final pelleted cells were resuspended in 9ml of peptone water (PW), corresponding to approximately 108–109CFU/ml. All strains of all