2.8.2. Checkerboard Assay

This method involves the MIC values established for antibiotics and the aBL treatments. Briefly, bacterial cell suspensions were prepared the same as for the MIC establishment and then transferred into a 96-well plate. Antibiotics were added to the wells vertically (to obtain 2× MIC concentration) and then 2-fold dilutions of each tested compound were performed. After 15 min of incubation, plates were exposed to different doses of blue light (2× MIC, MIC, 1/2 MIC, 1/4 MIC and 1/8 MIC). After exposure to photoinactivation, plates were protected with parafilm and incubated at 37 °C in the incubator (Thermax, Dreieich, Germany) for 16–20 h. The next day, the bacterial growth assessment indicated if the synergistic or another effect (antagonism or indifference) effect occurred. The interpretation of the checkerboard result was based on the Fractional Index (FICI). (FICI = FIC_A + FIC_B). FIC_A/B = MIC of factor A/B in combination/MIC of factor A/B alone. The synergistic effect is confirmed when FICI ≤ 0.5; antagonism was observed when FICI > 4; 4 < FICI > 0.5 means no interaction between tested factors.