Inhibitory Effects on Clinical Isolated Bacteria and Simultaneous HPLC Quantitative Analysis of Flavone Contents in Extracts from Oroxylum indicum

Oroxylum indicum is a medicinal plant in Thailand, which has been used as a tonic and for the treatment of various diseases. Extracts from various parts of O. indicum were reported as promoting in vitro antioxidant and antibacterial effects. Phytochemical analysis suggested that this plant contained some flavones. O. indicum fruit and seed water and ethanol extracts and their major flavonoids including baicalein, baicalin, and chrysin were tested for in vitro antibacterial activities on four clinical isolated bacteria, namely, Staphylococcus intermedius, Streptococcus suis, Pseudomonas aeruginosa, and β-Escherichia coli, using a broth micro-dilution assay. The amounts of these three major flavonoids were also quantitatively analyzed using the high-performance liquid chromatographic (HPLC) method. O. indicum fruit ethanol extract from Nakhon Pathom province (OFNE) promoted the strongest antimicrobial activity against four clinical pathogenic bacteria, including S. intermedius (IC₅₀ = 1.30 mg/mL), S. suis (13.59% inhibition at 7.81 mg/mL), P. aeruginosa (IC₅₀ = 39.20 mg/mL), and β-E. coli (IC₅₀ = 66.85 mg/mL). Baicalin showed high in vitro antibacterial effect to all tested bacteria. From the optimized and validated HPLC method, baicalin, baicalein, and chrysin contents in O. indicum extracts were 0.19 ± 0.00 − 9.45 ± 0.13, 0.14 ± 0.00 − 1.27 ± 0.02, and 0.02 ± 0.00 − 0.96 ± 0.02 g/100 g extract, respectively. Baicalin was found to be the major compound in O. indicum seed extract followed by baicalein, whereas chrysin was found in lower amounts than the amounts of the other two flavonoids in all O. indicum extracts.