TABLE 3.
Comparison of synergy testing by checkerboard and time-kill methodologies
| Strain | MIC (μg/ml)
|
Result for drug combination by methoda
|
|||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Levofloxacin | Cefpirome | Ceftazidime | Gentamicin | Meropenem | Levofloxacin + cefpirome
|
Levofloxacin + ceftazidime
|
Levofloxacin + gentamicin
|
Levofloxacin + meropenem
|
|||||
| C | T | C | T | C | T | C | T | ||||||
| Susceptible to all compounds | |||||||||||||
| 1 | 8 | 8 | 2 | 4 | 1 | Ad | Ad | Ad | Sy (0.5/8.0) | Ad | Ad | Ad | Sy (0.5/0.5) |
| 2 | 0.5 | 8 | 2 | 2 | 1 | Ad | Sy (0.125/2) | Ad | Sy (0.125/1) | Ad | Ad | Ad | Sy (0.125/0.25) |
| 3 | 1 | 8 | 4 | 2 | 1 | Ad | Sy (0.125/4) | Ad | Ad | Ad | Ad | Ad | Sy (0.125/0.25) |
| Resistant to cefpirome (≤8.0 μg/ml) and ceftazidime | |||||||||||||
| 4 | 0.5 | 64 | 64 | 0.5 | 1 | Ad | Sy (0.125/16) | Ad | Sy (0.125/32) | Ad | Ad | Ad | Sy (0.125/0.5) |
| 5 | 2 | 64 | 128 | 2 | 1 | Ad | Sy (0.5/16) | Ad | Sy (0.5/16) | Ad | Ad | Ad | Sy (0.5/0.5) |
| 6 | 0.5 | 128 | 128 | 0.5 | 2 | Ad | Sy (0.25/64) | Ad | Sy (0.25/64) | Ad | Sy (0.25/0.25) | Ad | Sy (0.25/1) |
| Resistant to gentamicin | |||||||||||||
| 7 | 2 | 8 | 8 | 64 | 0.5 | Ad | Sy (0.5/8) | Ad | Sy (0.5/2) | Ad | Ad | Ad | Sy (0.5/0.25) |
| 8 | 1 | 8 | 4 | 8 | 4 | Ad | Sy (0.25/4) | Ad | Sy (0.25/1) | Ad | Sy (0.25/1) | Ad | Sy (0.25/0.25) |
| 9 | 0.5 | 8 | 8 | 64 | 0.5 | Ad | Sy (0.06/2) | Ad | Sy (0.06/4) | Ad | Sy (0.06/16) | Ad | Sy (0.06/0.06) |
| Resistant to meropenem | |||||||||||||
| 10 | 1 | 8 | 2 | 4 | 8 | Ad | Ad | Ad | Ad | Ad | Ad | Ad | Ad |
| 11 | 2 | 8 | 8 | 2 | 16 | Ad | Sy (0.25/8) | Sy | Sy (0.25/4) | Ad | Sy (0.25/0.25) | Ad | Sy (0.25/2) |
| 12 | 0.125 | 8 | 1 | 1 | 16 | Ad | Sy (0.25/2) | Ad | Ad | Ad | Ad | Sy | Sy (0.25/0.125) |
a
C, checkerboard titration; T, time-kill; Sy, synergistic; Ad, additive or indifferent. Values in parentheses are MICs and indicate the lowest concentration (micrograms per milliliter) of each compound that yielded sustained bactericidal activity (≥100-CFU/ml drop) at 12 h compared to that of the more active drug.