Antibacterial Activity of Onopordum espinae: Identification of Hispidulin and Dehydromelitensin-8-(4'-Hydroxy-Methacrylate)

Hexane, ethyl acetate and methanol extracts of Onopordum espinae leaves were investigated for their antibacterial activities against Bacillus cereus, Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, Pseudomonas aeruginosa and Salmonella enteritidis. The in vitro antibacterial activity of Onopordum espinae extracts was qualitatively assessed by the diameter of inhibition zones (IZD) and quantitatively by the determination of the MIC and MBC values. Ethyl acetate extract was the most toxic with inhibition zone diameters to Gram-positive bacteria ranging between 15 and 30 mm and MIC values of 0.2 to 0.4 mg/mL. Two products, hispidulin 1 and dehydromelitensin-8-(4'-hydroxy-methacrylate) 2 were isolated from the ethyl acetate extract of leaves and identified by 2D-NMR for the first time from this species. Ethyl acetate extract's total flavonoid content was the highest, as 78.73 mg quercetin equivalents (QE)/g. The methanol extract showed the highest total phenolic content as 243.43 mg gallic acid equivalents (GAE)/g and gave the most important 2,2-diphenyl-1-picrylhydrazyl (DPPH)-scavenging activity (EC50 = 86 µg/mL).