Table 3.
Evaluation of usnic acid and polymyxin B interaction by the Checkerboard method.
| Clinical isolates | MIC (μg/mL) | MIC of the combination (μg/mL) | FICI | Interaction | |
|---|---|---|---|---|---|
| Polymyxin B | Usnic acid | Polymyxin B + usnic acid | |||
| PA05 | 4 | 500 | 0.25/15.625 | 0.094 | Synergism |
| PA07 | 4 | 500 | 0.25/15.625 | 0.094 | Synergism |
| PA18 | 8 | 500 | 0.25/31.25 | 0.094 | Synergism |
| PA19 | 4 | 500 | 0.5/15.625 | 0.156 | Synergism |
| PA46 | 32 | 250 | 0.25/7.8125 | 0.039 | Synergism |
| PA56 | 8 | 250 | 0.25/31.25 | 0.094 | Synergism |
| PA57 | 32 | 500 | 0.25/15.625 | 0.039 | Synergism |
| PA78 | 32 | 500 | 0.25/15.625 | 0.039 | Synergism |
| PA82 | 4 | 250 | 0.25/15.625 | 0.125 | Synergism |
MIC: minimum inhibitory concentration; FICI: fractional inhibitory concentration index.