Table 3.
Fractional inhibitory concentration (FIC) and FIC indices (FICIs) of pairs of essential oil compounds (EOCs) and β-lactam antibiotics (βLAs) against MRSA strains.
| Strain | EOC–βLA | MICo | MICc | FIC | FICI | Type of Interaction |
|---|---|---|---|---|---|---|
| 1 | Methicillin–Linalyl acetate | |||||
| Methicillin (mg/L) | 1000.0 ± 0.0 | 500.0 ± 0.0 | 0.5 | 1.0 | Addition | |
| Linalyl acetate (mg/mL) | 28.2 ± 0.0 | 14.1 ± 0.0 | 0.5 | |||
| Penicillin G–Linalyl acetate | ||||||
| Penicillin G (mg/L) | 125.0 ± 0.0 | 250.0 ± 0.0 | 2.0 | 4.0 | Indifference | |
| Linalyl acetate (mg/mL) | 28.2 ± 0.0 | 56.3 ± 0.0 | 2.0 | |||
| Penicillin G–1,8-Cineole | ||||||
| Penicillin G (mg/L) | 125.0 ± 0.0 | 250.0 ± 0.0 | 2.0 | 4.0 | Indifference | |
| 1,8-Cineole (mg/mL) | 28.8 ± 0.0 | 57.6 ± 0.0 | 2.0 | |||
| 2 | Methicillin–Linalyl acetate | |||||
| Methicillin (mg/L) | 1000.0 ± 0.0 | 62.5 ± 0.0 | 0.1 | 0.2 | Synergy | |
| Linalyl acetate (mg/mL) | 112.6 ± 0.0 | 14.1 ± 0.0 | 0.1 | |||
| Penicillin G–Linalyl acetate | ||||||
| Penicillin G (mg/L) | 31.3 ± 0.0 | 93.9 ± 0.0 | 3.0 | 5.0 | Antagonism | |
| Linalyl acetate (mg/mL) | 112.6 ± 0.0 | 225.3 ± 0.0 | 2.0 | |||
| Penicillin G–1,8-Cineole | ||||||
| Penicillin G (mg/L) | 31.3 ± 0.0 | 93.9 ± 0.0 | 3.0 | 5.0 | Antagonism | |
| 1,8-Cineole (mg/mL) | 28.8 ± 0.0 | 57.6 ± 4.2 | 2.0 | |||
| 3 | Methicillin–Linalyl acetate | |||||
| Methicillin (mg/L) | 1000.0 ± 0.0 | 125.0 ± 0.0 | 0.1 | 0.4 | Synergy | |
| Linalyl acetate (mg/mL) | 112.6 ± 0.0 | 28.2 ± 0.0 | 0.3 | |||
| Penicillin G–Linalyl acetate | ||||||
| Penicillin G (mg/L) | 31.3 ± 0.0 | 62.5 ± 0.0 | 2.0 | 3.0 | Indifference | |
| Linalyl acetate (mg/mL) | 112.6 ± 0.0 | 112.6 ± 0.0 | 1.0 | |||
| Penicillin G–1,8-Cineole | ||||||
| Penicillin G (mg/L) | 31.3 ± 0.0 | 62.5 ± 0.0 | 2.0 | 5.0 | Antagonism | |
| 1,8-Cineole (mg/mL) | 28.8 ± 0.0 | 86.4 ± 0.0 | 3.0 | |||
| 4 | Methicillin–Linalyl acetate | |||||
| Methicillin (mg/L) | 31.3 ± 0.0 | 15.6 ± 0.0 | 0.5 | 1.0 | Addition | |
| Linalyl acetate (mg/mL) | 112.6 ± 0.0 | 56.3 ± 0.0 | 0.5 | |||
| Penicillin G–Linalyl acetate | ||||||
| Penicillin G (mg/L) | 3.9 ± 0.0 | 2.0 ± 0.0 | 0.5 | 0.8 | Addition | |
| Linalyl acetate (mg/mL) | 112.6 ± 0.0 | 28.2 ± 0.0 | 0.3 | |||
| Penicillin G–1,8-Cineole | ||||||
| Penicillin G (mg/L) | 3.9 ± 0.0 | 0.5 ± 0.0 | 0.1 | 0.2 | Synergy | |
| 1,8-Cineole (mg/mL) | 57.6 ± 0.0 | 7.2 ± 0.0 | 0.1 | |||
| 5 | Methicillin–Linalyl acetate | |||||
| Methicillin (mg/L) | 1000.0 ± 0.0 | 500.0 ± 0.0 | 0.5 | 1.0 | Addition | |
| Linalyl acetate (mg/mL) | 56.3 ± 0.0 | 28.2 ± 0.0 | 0.5 | |||
| Penicillin G–Linalyl acetate | ||||||
| Penicillin G (mg/L) | 125.0 ± 0.0 | 250.0 ± 0.0 | 2.0 | 5.0 | Antagonism | |
| Linalyl acetate (mg/mL) | 56.3 ± 0.0 | 168.9 ± 10.0 | 3.0 | |||
| Penicillin G–1,8-Cineole | ||||||
| Penicillin G (mg/L) | 125.0 ± 0.0 | 250.0 ± 0.0 | 2.0 | 4.0 | Indifference | |
| 1,8-Cineole (mg/mL) | 57.6 ± 0.0 | 115.2 ± 0.0 | 2.0 | |||
| 6 | Methicillin–Linalyl acetate | |||||
| Methicillin (mg/L) | 7.8 ± 0.0 | 3.9 ± 0.0 | 0.5 | 0.8 | Addition | |
| Linalyl acetate (mg/mL) | 56.3 ± 0.0 | 14.1 ± 0.0 | 0.3 | |||
| Penicillin G–Linalyl acetate | ||||||
| Penicillin G (mg/L) | 3.9 ± 0.0 | 1.0 ± 0.0 | 0.3 | 0.6 | Addition | |
| Linalyl acetate (mg/mL) | 56.3 ± 0.0 | 14.1 ± 0.0 | 0.3 | |||
| Penicillin G–1,8-Cineole | ||||||
| Penicillin G (mg/L) | 3.9 ± 0.0 | 0.5 ± 0.0 | 0.1 | 0.2 | Synergy | |
| 1,8-Cineole (mg/mL) | 57.6 ± 0.0 | 7.2 ± 0.0 | 0.1 | |||
| 7 | Methicillin–Linalyl acetate | |||||
| Methicillin (mg/L) | 1000.0 ± 0.0 | 250.0 ± 0.0 | 0.3 | 0.6 | Addition | |
| Linalyl acetate (mg/mL) | 56.3 ± 0.0 | 14.1 ± 0.0 | 0.3 | |||
| Penicillin G–Linalyl acetate | ||||||
| Penicillin G (mg/L) | 31.3 ± 0.0 | 31.3 ± 0.0 | 1.0 | 2.0 | Indifference | |
| Linalyl acetate (mg/mL) | 56.3 ± 0.0 | 56.3 ± 0.0 | 1.0 | |||
| Penicillin G–1,8-Cineole | ||||||
| Penicillin G (mg/L) | 31.3 ± 0.0 | 31.3 ± 0.0 | 1.0 | 3.0 | Indifference | |
| 1,8-Cineole (mg/mL) | 28.8 ± 0.0 | 57.6 ± 0.0 | 2.0 | |||
| 8 | Methicillin–Linalyl acetate | |||||
| Methicillin (mg/L) | 62.5 ± 0.0 | 15.6 ± 0.0 | 0.3 | 0.6 | Addition | |
| Linalyl acetate (mg/mL) | 112.6 ± 0.0 | 28.2 ± 0.0 | 0.3 | |||
| Penicillin G–Linalyl acetate | ||||||
| Penicillin G (mg/L) | 3.9 ± 0.0 | 0.5 ± 0.0 | 0.1 | 0.6 | Addition | |
| Linalyl acetate (mg/mL) | 112.6 ± 0.0 | 56.3 ± 0.0 | 0.5 | |||
| Penicillin G–1,8-Cineole | ||||||
| Penicillin G (mg/L) | 3.9 ± 0.0 | 0.2 ± 0.0 | 0.1 | 0.4 | Synergy | |
| 1,8-Cineole (mg/mL) | 57.6 ± 0.0 | 14.4 ± 0.0 | 0.3 | |||
MICo, minimal inhibitory concentration of EOC or βLA; MICc, minimal inhibitory concentration of EOC/βLA combination. FIC index = FIC of EOC + FIC of βLA. FICI < 0.5, synergy; 0.5 ≤ FICI ≤ 1.0, addition; 1.1 < FICI ≤ 4.0, indifference; FICI > 4.0, antagonism. Values are expressed as mean ± standard deviation.