Synergy analysis with extended isobolography and its conceptual basis for the evaluation of antimicrobial effects of combined physical and chemical treatments.

To evaluate the synergistic effects of physical and chemical antimicrobial treatments, whether for cidal or static effects, I proposed the extended isobologram (EIBo) analysis method, which is a modification of the isobologram (IBo) analysis commonly used to evaluate synergy between drugs. As the method types for this analysis, the growth delay (GD) assay reported previously by the author was introduced, in addition to the conventional endpoint (EP) assay. The evaluation analysis consists of five stages, analytical procedure establishment, antimicrobial activity assay, dose-effect analysis, IBo analysis, and synergy analysis. In EIBo analysis, the fractional antimicrobial dose (FAD) is introduced for the normalization of the antimicrobial activity of each treatment. For the synergy evaluation, the synergy parameter (SP) is defined as a measure of the degree of the synergistic effect of a combined treatment. This method makes it possible to quantitatively evaluate, predict, and compare various combination treatments as a hurdle technology.