TABLE 2.
Results of checkerboard synergy testing (FICIs) with non-susceptible antimicrobial combinations.
| Antimicrobial | Rangea (mg/L) | MICALONEb (mg/L) | Combination FICIMEAN ± SD (FICILOW)c | ||||
| Spectinomycin | Tilmicosin | Tylosin | Sulfamethazine | Synergy (FICIMEAN ≤0.5) | |||
| Mannheimia haemolytica ATCC 33396 × PM22 transconjugant | |||||||
| Oxytetracycline | 1–64 | 16 | 0.84 ± 0.23 (0.63) | 0.57 ± 0.24 (0.31) | 0.98 ± 0.22 (0.63) | 0.99 ± 0.17 (0.75) | No interactions |
| Spectinomycin | 64–4096 | 1024 | 0.51 ± 0.30 (0.25) | 0.76 ± 0.63 (0.31) | 0.74 ± 0.65 (0.28) | No interactions | |
| Tilmicosin | 4–256 | 32 | 0.60 ± 0.28 (0.38) | 0.54 ± 0.09 (0.38) | No interactions | ||
| Tylosin | 16–1024 | 256 | 0.63 ± 0.09 (0.53) | No interactions | |||
| Sulfamethazine | 32–2048 | 1024 | No interactions | ||||
| Pasteurella multocida CCUG 17976 × PM22 transconjugant | |||||||
| Oxytetracycline | 1–64 | 16 | 1.23 ± 0.18 (1.06) | 0.58 ± 0.05 (0.50) | 6.59 ± 5.82 (2.00) | 1.21 ± 0.18 (1.03) | No interactions |
| Spectinomycin | 64–4096 | 1024 | 0.60 ± 0.28 (0.38) | 0.59 ± 0.28 (0.38) | 1.11 ± 0.09 (1.00) | No interactions | |
| Tilmicosin | 4–256 | 32 | 0.58 ± 0.28 (0.38) | 0.93 ± 0.25 (0.56) | No interactions | ||
| Tylosin | 16–1024 | 256 | 0.84 ± 0.23 (0.56) | No interactions | |||
| Sulfamethazine | 32–2048 | 1024 | No interactions | ||||
aRange of antimicrobial concentrations tested. bMinimum inhibitory concentration (MIC) of antimicrobial tested alone. cFractional inhibitory concentration index (FICI). The FICI values are given as the mean ± standard deviation resulting from three experiments. FICI values in brackets indicate the lowest value for the given combination. Synergism: ≤0.5; No interaction, ≥0.5.