Evaluation of the Combined Effects of Stilbenoid from Shorea gibbosa and Vancomycin against Methicillin-Resistant Staphylococcus aureus (MRSA)

The aim of this study is to determine the combined effects of stilbenoids from Shorea gibbosa and vancomycin against methicillin-resistant Staphylococcus aureus (MRSA). A total of nine pure compounds, five stilbenoid dimers epsilon-viniferin, ampelopsin A, balanocarpol, laevifonol and diptoindonesin G and four stilbenoid trimers alpha-viniferin, johorenol A, ampelopsin E and vaticanol G were evaluated for their antibacterial activities against ATCC 33591 and a HUKM clinical isolate. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) for each active compound were determined using the serial microdilution and plate-streak techniques. The combined effect of stilbenoids with vancomycin against MRSA was evaluated using the checkerboard assay to determine their fractional inhibitory concentration (FIC) index values. The MIC value of alpha-viniferin on both MRSA strains was 100 [mu g/mL, whereas those of johorenol A on ATCC 33591 and HUKM strain were 100 [mu g/mL and 200 [mu g/mL, respectively. The MIC values of ampelopsin E and vaticanol G were higher than 400 [mu g/mL. Out of the five stilbenoid dimers, only epsilon-viniferin was capable of inhibiting the growth of both MRSA strains at MIC 400 [mu g/mL. The MBC value of epsilon-viniferin, alpha-viniferin and johorenol A showed bacteriostatic action against MRSA. The FIC index value of c-viniferin and alpha-viniferin in combination with vancomycin showed an additive effect (0.5 < FIC <= 2.0) against both MRSA strains. Johorenol A-vancomycin combination was also additive against HUKM strain, but it showed synergistic interaction with vancomycin against ATCC 33591 (FIC < 0.5). Stilbenoid compounds from Shorea gibbosa have anti-MRSA activity and huge potential as an alternative phytotherapy in combating MRSA infections.