2.7. Checkerboard Method

The microdilution checkerboard method was used to calculate the fractional inhibitory concentrations (FICs) for daptomycin combined with a cephalosporin, which was modified according to the CLSI [8,14,15]. In the three-drug combination checkerboard, daptomycin combined with a cephalosporin was also combined with 8 mg/L sulbactam in each well. The following formula was used to calculate the FIC index: the FIC of daptomycin (MIC of daptomycin in combination/MIC of daptomycin alone) + the FIC of the cephalosporin (MIC of cephalosporin in combination/MIC of cephalosporin alone). The definition of FIC₉₀ is the 90% fractional inhibitory concentration (FIC₉₀) of clinical isolates against a combination of test drugs. Synergism was defined as an FIC index of ≤0.5, a no-interaction FIC index was >0.5 but ≤4, and an antagonism FIC index >4 [25].