Microbial cultures
Escherichia coli DMST 15537, Salmonella sp. DMST 4464, Listeria monocytogenes DMST 17303, and Staphylococcus aureus DMST 6512 were obtained from the Department of Medical Sciences, Ministry of Public Health, Thailand. The microorganisms were maintained in TSA at 5 °C. Stock culture of microbial was grown in MHB (E. coli, S. aureus and Salmonella spp.) and TSB (L. monocytogenes) at 37+2 °C in a shaker incubator for 3.5 h at 110 rpm (cells in early stationary phase). The bacterial suspension was subsequently adjusted to 106 CFU/ml using MHB and TSB