TABLE 2.
Evaluation of synergy by checkerboard assay
| Antibiotic combination (A-B) | Concn (mg/liter) of antibiotic: |
FICa: |
Interaction result | |||
|---|---|---|---|---|---|---|
| A | B | Antibiotic A | Antibiotic B | Σ FIC | ||
| Ampicillin-azithromycin | 1 | 2 | 0.093 | 0.037 | 0.468 | Synergy |
| Ampicillin-levofloxacin | 1 | 8 | 0.281 | 0.562 | 0.843 | Indifference |
| Ampicillin-gentamicin | 1 | 0.25 | 0.039 | 0.620 | 0.659 | Indifference |
| Levofloxacin-ceftriaxone | 0.5 | 0.25 | 0.046 | 0.375 | 0.421 | Synergy |
| Levofloxacin-cefotaxime | —b | — | Rc | R | R | R |
| Levofloxacin-cefepime | 0.03 | 0.06 | 0.035 | 1.12 | 1.159 | Indifference |
| Levofloxacin-azithromycin | — | — | R | R | R | R |
| Levofloxacin-rifampin | — | — | R | R | R | R |
| Vancomycin-rifampin | 2 | 0.06 | 0.064 | 17.16 | 17.22 | Antagonism |
| Vancomycin-imipenem | 16 | 0.12 | 0.251 | 134.33 | 134.58 | Antagonism |
| Azithromycin-ciprofloxacin | 0.25 | 4 | 0.132 | 1.062 | 1.194 | Indifference |
| Ciprofloxacin-ceftriaxone | 0.06 | 0.25 | 0.312 | 0.156 | 0.468 | Synergy |
| Ciprofloxacin-cefepime | 0.06 | 0.06 | 0.124 | 0.248 | 0.372 | Synergy |
| Amoxicillin and potassium clavulanate-rifampin | 0.06 | 0.03 | 0.032 | 0.531 | 0.562 | Indifference |
| Gentamicin-ceftriaxone | 0.06 | 0.12 | 0.187 | 0.187 | 0.374 | Synergy |
a
FIC, fractional inhibitory concentration. Σ FIC (FIC A + FIC B) values: <0.5, synergy; 0.5 to 4, indifference; >4, antagonism.
b
—, no effective inhibition in growth after 24 h of incubation at 37°C compared to OD at 550 nm observed at 0 h.
c
R, resistant to the combination.