Minimal fractional inhibitory concentration (FIC) summations for pairwise and triple antimicrobial combinations

Drug combinationFIC sumInterpretation
Rifampin + azithromycina0.47Synergy
Minocycline + azithromycina0.43Synergy
Rifampin + minocyclinea0.41Synergy
Chloramphenicol + azithromycin0.53No interaction
Chloramphenicol + rifampina0.60No interaction
Chloramphenicol + minocycline0.75No interaction
Florfenicol + rifampina0.68No interaction
Levofloxacin + minocyclinea0.95No interaction
Levofloxacin + azithromycina1.6No interaction
Levofloxacin + rifampina1.36No interaction
Levofloxacin + chloramphenicol2.0No interaction
Ceftriaxone + azithromycina,b0.49Synergy
Ceftriaxone + levofloxacina,b0.88No interaction
Azithromycin + rifampin + minocycline0.325Synergy
Axis planesc
    Azithromycin × minocycline0.383
    Azithromycin × rifampin0.375
    Minocycline × rifampin0.375
  • a FIC data are based on finer √2-fold serial dilution rather on than standard doubling dilution matrices.

  • b FIC data for ceftriaxone combinations shown are for axenic rather than intracellular growth inhibition. Ceftriaxone did not sufficiently suppress intracellular growth at all concentrations tested, thus precluding intracellular FIC analysis.

  • c FIC values for the axis planes are calculated from within the three-dimensional data set. Two-dimensional synergy FIC values for the same antimicrobial combinations shown earlier in the table are from separate experiments.