Determination of Fractional Inhibitory Concentration Index (FICI) by a Checkerboard Method

A checkerboard technique was employed to delineate the Fractional Inhibitory Concentration Index (FICI) of pleuromutilins plus antibiotic combinations (White et al., 1996). Briefly, 96 well plates containing serial dilutions of pleuromutilins + antibiotic (range, 0.125 × MIC to 4 × MIC) were inoculated with 5 × 10⁵ of S. aureus and incubated for 18 h. Control wells were free of pleuromutilins or antibiotic. After incubation, plates were screened for visual growth. The FICI were then calculated as previously described (≤0.5, synergy; 0.6–1.0, additivity; 1.1–4.0, indifference; >4.0, antagonism) (White et al., 1996; Odds, 2003).