The annealing temperature and concentrations of each primer pairs were optimized to ensure proper amplification efficiency for each amplicon. The annealing temperature for the multiplex PCR assay was optimized at 56 C because each specific PCR fragment could be produced at this temperature. Moreover, the concentrations of four pairs of primers were further optimized using an orthogonal experimental design (Supporting Information Table S1).The optimum concentrations of primers were: 0.32 mM for ompA primers, 0.48 mM for nuc primers, 0.48 mM for nheA primers, and 0.08 mM for 16S rRNA primers. The products of the mPCR were evaluated in a 1.5% agarose gel, and amplified fragments of 380 bp for C. sakazakii, 297 bp for S. aureus, 617 bp for B. cereus, 475 bp for IAC were obtained (Fig 1). These results showed only specific bands which indicated that the mPCR assay could detect the individual as well as the three bacteria in the same reaction.