4.9.2. Checkerboard Assay

Overnight cultures of both strains were diluted to obtain 0.5 McF in BHI and then diluted 10-fold. Bacterial suspensions were placed in 96-well plates combined with different concentrations of antibiotics: 2 MIC, MIC, 1/2 MIC, 1/4 MIC, 1/8 MIC, 1/16 MIC, 1/32 MIC, and 0 MIC. Next, the wells in columns were diluted 2-fold with PS to obtain final PS concentrations with MICs as follows: MIC, 1/2 MIC, 1/4 MIC, 1/8 MIC, 1/16 MIC, 1/32, 1/64 MIC, 1/128 MIC, 1/256 MIC, 1/512 MIC, and 0 MIC. All cells were incubated in the dark for 15 min and then exposed to irradiation at MIC doses. Next, the plates were incubated for 16–20 h at 37 °C. Bacterial growth was assessed, and the fractional inhibitory concentration index (FICI) coefficient was calculated (FICI = FIC_A + FIC_B). FIC_A/B = MIC of factor A/B in combination/MIC of factor A/B alone. Synergistic effects were observed when FICI ≤ 0.5, and antagonism was observed when FICI > 4; 4 < FICI > 0.5 means no interaction.