Table 1.
Structure-activity relationship of melatonin with colistin against E. coli (mcr-1)
| Analogues | Chemical structure | MICa (μg/mL) | FIC index | MICb (μg/mL) | Potentiation (fold)c |
|---|---|---|---|---|---|
| Melatonin |  |
8 | 0.063 | 0.25 | 32 |
| 1 (Serotonin) |
 |
8 | 0.188 | 1 | 8 |
| 2 (Tryptamine) |
 |
8 | 0.156 | 0.25 | 32 |
| 3 (5-Hydroxy-L-tryptophan) |
 |
8 | 1 | 4 | 2 |
| 4 (L-tryptophan) |
 |
8 | 2 | 8 | 1 |
| 5 (N-acetyl-5-hydroxytryptamine) |
 |
8 | 0.094 | 0.25 | 32 |
| 6 (5-Methoxytryptamine) |
 |
8 | 0.156 | 0.25 | 32 |
| 7 (Indole) |
 |
8 | 0.154 | 1 | 8 |
| 8 (Histamine) |
 |
8 | >4 | >32 | - |
a/b MICs of colistin in the absence or presence of sub-MIC of melatonin and its analogues;
c Degree of colistin potentiation in the presence of sub-MIC of melatonin and its analogues.