• Preparation of the S. typhimurium suspension
A culture of S. typhimurium (strain ATCC 13311) was obtained from Bangkok MERCEN . Stock culture on Trypticase Soy Agar (TSA; Merck Laboratories, Germany) was store at 5 °C. Prior to use, the culture was subjected to two
successive transfer by loop inocula to 9 mL was made into 100 mL Trypticase Soy Broth (TSB; pH 7.3; Merck Laboratories, Germany) incubated at 37 °C for 18 h, and used as inocula. Dilution was made and gain the final cell concentration high and low as 7.5-8 log10CFU/mL and 2.5-3 log10CFU/mL.