TABLE 1.
S. aureus fractional inhibitory concentration index values.
| MBIC for 2CP (alone) (μg/ml) | MBIC for 2CP (combined) (μg/ml) | MBIC for antibiotic (alone) (μg/ml) | MBIC for antibiotic (combined) (μg/ml) | FICI | Interpretation | |
| S. aureus | ||||||
| Tobramycin | 1000.0 | 31 | 4.0 | 2.0 | 0.531 | Additive |
| Tetracycline | 1000.0 | 63 | 1.0 | 1.0 | 1.06 | Indifferent |
| Levofloxacin | 1000.0 | 500 | 1.0 | 0.3 | 0.8 | Additive |
| MBIC for C2DA (alone) (μg/ml) | MBIC for C2DA (combined) (μg/ml) | MBIC for antibiotic (alone) (μg/ml) | MBIC for antibiotic (combined) (μg/ml) | FICI | Interpretation | |
| Tobramycin | 1000.0 | 500 | 4.0 | 2.0 | 1.0 | Additive |
| Tetracycline | 1000.0 | 250 | 1.0 | 0.5 | 0.75 | Additive |
| Levofloxacin | 1000.0 | 250 | 1.0 | 0.5 | 0.75 | Additive |
| MBIC for T2DA (alone) (μg/ml) | MBIC for T2DA (combined) (μg/ml) | MBIC for antibiotic (alone) (μg/ml) | MBIC for antibiotic (combined) (μg/ml) | FICI | Interpretation | |
| Tobramycin | 1000.0 | 500 | 4.0 | 2.0 | 1.0 | Additive |
| Tetracycline | 1000.0 | 250 | 1.0 | 0.5 | 0.75 | Additive |
| Levofloxacin | 1000.0 | 250 | 1.0 | 0.5 | 0.75 | Additive |
Minimum biofilm inhibitory concentration (MBIC) for 2CP, C2DA, and T2DA against S. aureus when treated alone or combined with tobramycin, tetracycline, or levofloxacin. MBIC of each antibiotic against S. aureus alone is also included. These values were used in Equation 1 to determine the fractional inhibitory concentration index for each fatty acid/antibiotic combination. FICI values less than or equal to 0.5 indicate synergy, values between 0.5 and 1 indicate additivity, values between 1 and 2 indicate indifference, and values above 2 indicate antagonism.