Selected Polyphenols of Polish Poplar Propolis as a Key Component Shaping Its Antibacterial Properties—In Vitro and In Silico Approaches

Propolis is a natural antibacterial medicine with a varied content of phenolic compounds, which determines the activity of the ethanol extract of propolis (EEP). A new attempt was made to standardize ethanol propolis extract via its conversion into a dry concentrate (dEEP) through a two-step process. Four samples of poplar propolis from the same geographical region were used for the study. Obtained dry concentrates reconstituted in 70% ethanol (500 μg/mL) were analyzed for their antioxidant properties, total phenolic and flavonoid contents, as well as HPLC polyphenol profile. It was shown that dEEP solutions in 70% ethanol, regardless of the diversified quality of the raw material, have equalized antioxidant properties and phenolic and flavonoid contents compared to raw EEPs. However, quantitative differences in the nine individual components were still found by HPLC-DAD. The antibacterial activity of the dEEP solutions (0.03-500 µg/mL) was compared with three individual polyphenols' effect against Klebsiella pneumoniae and Streptococcus agalactiae. Based on the obtained MIC values and anti-biofilm activity of dEEPs compared to pure polyphenols, it was established that the effectiveness of the extract results from the combined action of flavonoids and phenolic acids. The antibacterial effectiveness of <i>p</i>-coumaric acid, galangin, and pinocembrin was additionally modeled using in silico analyses, suggesting promiscuous binding of all tested polyphenolic ligands to target enzymes.