A checkerboard analysis, also known as a synergy test, determines the interaction between two or more test articles when used together in vitro. A fractional inhibitory concentration index (ƩFIC) value is calculated for each pathogen strain and antimicrobial mixture in order to determine the test article combination that produces the greatest change from each individual test article’s minimum inhibitory concentration (MIC) value. If the drug combination shows increased antimicrobial activity compared to each drug alone, there is said to be a synergistic effect. Synergy is advantageous because it may reduce toxic side effects associated with large doses of a drug used alone.
TransPharm performs checkerboard assays using the broth microdilution method. Serial dilutions of test articles, alone and in combination, are added to media inoculated with a standard concentration of a susceptible pathogen. The cultures are incubated overnight. The MIC value of each test article is determined (the lowest concentration of test article that does not show bacterial or fungal growth). The FIC and ƩFIC values are calculated using the following equations:
FIC of test article A = MIC of test article A in combination with test article B ÷ MIC of test article A alone
FIC of test article B = MIC of test article B in combination with test article A ÷ MIC of test article B alone
ƩFIC = FIC of test article A + FIC of test article B