Rhus glabra L. and Pistacia vera L. Leaf Extracts as Rich Sources of Tannins with Antioxidant, Anticancer and Antimicrobial Activities

Various plant by-products have attracted increasing attention as sources of biologically active compounds, particularly polyphenols. In this study, tannin-rich polyphenolic fractions were isolated from the leaves of Rhus glabra L. and Pistacia vera L. (Anacardiaceae) using a simple and efficient extraction procedure. The chemical composition of the extracts was characterized by LC-MS analysis, and their antimicrobial, antiradical, and cytotoxic activities were evaluated. Both extracts exhibited strong antiradical activity against DPPH radicals, comparable to or exceeding that of vitamin C and Trolox. Furthermore, both extracts showed cytotoxic effects against HeLa cells while exerting virtually no effect on MRC-5 fibroblasts within the tested concentration range (1–50 µg/mL). Antibacterial activity was assessed against Gram-positive bacteria (Staphylococcus aureus ATCC 700699 (MRSA), S. aureus ATCC 8325-4 (MSSA), Staphylococcus epidermidis ATCC 51625 (MRSE), S. epidermidis ATCC 14990 (MSSE), Streptococcus mutans ATCC 25175, and Bacillus cereus ATCC 13061) and Gram-negative bacteria (Escherichia coli ATCC 35218 and Pseudomonas aeruginosa ATCC BAA-1744). The minimum inhibitory concentration (MIC) values ranged from 2 to 125 µg/mL for Gram-positive bacteria and from 125 to 500 µg/mL for Gram-negative bacteria. In all biological assays, the R. glabra extract showed greater activity than the P. vera extract, likely because of its higher flavonoid content despite similar tannin levels. Overall, the results indicate that these tannin-rich leaf extracts, particularly that of R. glabra, represent promising sources of bioactive compounds with potential practical applications. The possible contribution of synergistic interactions among extract constituents to the observed biological effects is also discussed.