4.15. Synergy Study

A checkerboard dilution test was applied to evaluate the synergistic effects of BCN-E 50-52 and thymol against all tested microorganisms [20]. The fractional inhibitory concentration (FIC) was calculated according to the following formula: FIC of each drug (FIC) = (MIC of drug in combination)/(its MIC value). The FIC index (FICI) values were calculated using the following equation: FICI = FIC_A + FIC_B [20].

The interpretation is as follows: synergy, <0.5; partial synergy, 0.5–0.75; additive effect, 0.76–1.0; indifference, >1.0–4.0; and antagonism, >4.0 [39,40]. The fractional bactericidal concentration index (FBCI) is the sum of the FBCs of each of the compounds and is calculated and interpreted the same as mentioned above for FIC [41].