Flavonoids and Phenolics Contents, Antioxidant and Antibacterial Potential of Folk Medicinal Plants Used in Northeastern Thailand

Background and objectives: Thailand has abundant traditional medicinal plant species which are efficacious for many illnesses, but most of them still lack the supportive scientific information for their healing properties. The aim of this study was to evaluate and compare the constituents and antioxidant and antibacterial activities of some of these plants. Methods: The medicinal plant extracts were assessed for their flavonoids and phenolics composition and tested for antibacterial activity using disk diffusion method. In vitro antioxidant capacity was evaluated by DPPH, ABTS, and FRAP assays. Results: Major flavonoids present in the medicinal plants were naringenin, (+)-catechin and quercetin. The highest contents of naringenin, quercetin and (+)-catechin were observed in Tinospora crispa (896.15 mg/100 g dw), Betula alnoides (521.57 mg/100 g dw) and Albizia procera (430.28 mg/100 g dw), respectively (P<0.05). Naringenin was first reported from T. crispa, quercetin and (-)-epicatechin were also found in this plant. The lowest EC50 value based on the DPPH assay was found in Capparis micracantha extracts (9.10 mg/mL). The strongest antioxidant capacities, examined by the DPPH, FRAP and ABTS assays, were found in Capparis micracantha (EC50 9.10 mg/mL), Zingiber cassumunar (334.00 mg Fe(II)/100 g dw) and Plumbago indica (61.56 mg TE/100 g dw), respectively (p<0.05). The extract of Plumbago indica root exhibited the highest antibacterial activity mainly against Bacillus subtilis (MIC = 1.56 mg/mL), Bacillus cereus (MIC = 0.39 mg/mL), Streptococcus faecalis (MIC = 0.19 mg/mL), Salmonella sp. (MIC = 0.39 mg/mL) and Salmonella typhi (MIC = 0.19 mg/mL). Conclusion: The results provided significant scientific data on phytochemical constituents and biological activities of Thai medicinal plants use in traditional medicine and the relation to their therapeutic properties.