Anthraquinone profile, antioxidant and antimicrobial activity of bark extracts of Rhamnus alaternus, R. fallax, R. intermedia and R. pumila

► Rhamnus fallax and R. pumila bark extracts act as strong antioxidants. ► Rhamnus spp. have demonstrated notable antimicrobial activity. ► Antioxidant activity correlates with the amount of polyphenols in bark extracts. ► Chrysophanol and physcion are the most abundant anthraquinones in Rhamnus spp. The quantity of phenols, as well as antioxidant and antimicrobial activities, were investigated in bark of Rhamnus alaternus L., R. fallax Boiss., R. intermedia Steud. et Hochst., and R. pumila Turra from natural stands in Croatia. The most abundant anthraquinones in the investigated extracts were chrysophanol in R. alaternus (3.14mg/g), emodin in R. pumila (0.339mg/g), and physcion in R. fallax (2.70mg/g) and R. intermedia (0.285mg/g). The species exhibiting the highest antioxidant activity were R. fallax and R. pumila. A positive correlation was observed between total phenolic and flavonoid levels of the extracts and antioxidant activity in some of the assays. All species showed antimicrobial activity against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans, Aspergillus niger and Microsporum gypseum with minimal inhibitory concentrations equal to or below 2.500mg/mL. The results indicate that the investigated Rhamnus species are a source of anthraquinones and other phenols, which act as multifunctional antioxidants with antimicrobial activity.