Comparing checkerboard, isobologram and CCD methods for drug combination: A case study of ciprofloxacin and plant extracts on Escherichia coli and Shigella

To investigate the interactive antimicrobial activity of conventional antibiotics, namely ciprofloxacin and Mangifera indica bark ethanol extract, Psidium guajava leaves essential oil against two Gram-negative bacteria using checkerboard, isobologram, and central composite design (CCD) methods. The checkerboard method was used for the determination of the fractional inhibitory concentration index (FICI) value needed for the interpretation of the interaction. The isobologram was constructed and the geometrical disposition of the combinations used to define the type of interaction. The CCD experimental plan was used to obtain the bactericidal probability. In general the results indicated that the methods do have few similarities in their output for the same combination. According to the results, the isobologram method can be considered as more adapted and can be confirmed using the CCD method for antimicrobials combination because of the flexibility in the testing ratio of antimicrobials to be used. This study brings into light the fact that isobologram and CCD methods should be compared when studying drug interaction. Key words: Antibiotics, central composite design, checkerboard, isobologram, plant extracts, Escherichia coli, Shigella species.