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 ε-viniferin, ampelopsin A, balanocarpol, laevifonol and diptoindonesin G and four stilbenoid trimers α-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 α-viniferin on both MRSA strains was 100 μg/mL, whereas those of johorenol A on ATCC 33591 and HUKM strain were 100 μg/mL and 200 μg/mL, respectively. The MIC values of ampelopsin E and vaticanol G were higher than 400 μg/mL. Out of the five stilbenoid dimers, only ε-viniferin was capable of inhibiting the growth of both MRSA strains at MIC 400 μg/mL. The MBC value of ε-viniferin, α-viniferin and johorenol A showed bacteriostatic action against MRSA. The FIC index value of ε-viniferin and α-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.