Synergistic Antibacterial Potential of Pterostilbene

The MIC assays were used to identify synergies between pterostilbene and colistin against polymyxin-resistant strains (positive for mcr-1), polymyxin-resistant strains (negative for mcr-1), and polymyxin-sensitive strains (negative for mcr-1), and were carried out using the broth microdilution method following the guidelines of the Clinical and Laboratory Standards Institute (Wiegand et al., 2008 (link); Espinel-Ingroff et al., 2016 (link)). The remaining nine antibiotics were also tested in combination with pterostilbene. The efficacies of the combinations were evaluated by calculating the fractional inhibitory concentration (FIC) index values (Ma et al., 2016 (link)). A growth curve assay was also performed to evaluate the effect of pterostilbene on the growth of the tested strains (Li et al., 2011 (link)). Briefly, K. pneumoniae ZJ02 and K. pneumoniae ZJ05 were cultured in Luria-Bertani (LB) medium at 37°C with shaking at 200 rpm to obtain an OD₆₀₀ value of 0.3. Aliquots (250 mL) of the culture were then transferred into six 50-mL Erlenmeyer flasks, and pterostilbene (or the dimethyl sulfoxide control) was added to the cultures at 0, 16, 32, 64, and 128 μg/mL, respectively. The bacteria were cultured at 37°C with shaking, and bacterial growth was estimated by measuring the OD₆₀₀ every 30 min.