Pterostilbene Enhanced Anti-Methicillin Resistant Staphylococcus aureus (MRSA) Activity of Oxacillin

Methicillin-resistant Staphylococcus aureus (MRSA) is a deadly pathogen that initially was limited to hospital and healthcare facilities but has gradually became a growing problem in healthy children and adults. Pterostilbene belongs to the phenylpropanoid phytoalexin which is involved in plant response to various pathogen and herbivores attack. The aim of this study is to evaluate the anti-MRSA action of pterostilbene in combination with selected antibiotics; vancomycin, linezolid and oxacillin against ATCC 43300 and ATCC 33591. The minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and fractional inhibitory concentration (FIC) index values were determined. Microbroth dilution technique and microdilution checkerboard (MDC) assay were employed. The MIC and MBC of pterostilbene against ATCC 33591 was 31.25 and 62.50 µg mL^-1, respectively. While for ATCC 43300, the MBC value was also twice (62.50 µg mL^-1) its MIC value of 31.25 µg mL^-1. This indicated that pterostilbene was bacteriostatic against both MRSA strains. Our MIC/MBC study also showed that linezolid exhibited bacteriostatic action but, oxacillin and vancomycin were bactericidal. MDC study showed that pterostilbene-oxacillin combination exhibited lowest FIC value (0.56) against both MRSA strains which indicated partial synergistic interaction. On the other hand, pterostilbene was additive (FIC 1.00) in combination with vancomycin whereas pterostilbene-linezolid combination displayed indifference effect with FIC of 1.25 against both MRSA strains. Pterostilbene in combination with oxacillin partially enhanced anti-MRSA activity with twofold reduction in MIC of oxacillin by acting at different site at the bacterial cell wall from that of oxacillin but more specific to the site of action of vancomycin.