Synergistic effect of emodin in combination with ampicillin or oxacillin against methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a substantial contributor to morbidity and mortality. In search of a natural products capable of inhibiting this multidrug resistant bacteria, we have investigated the antimicrobial activity of emodin (EM) isolated from Rheum palmatum L. (Polygonaceae) against 17 different strains of the bacterium. New antimicrobial activity was found using the paper disc diffusion method, agar dilution as well as checkerboard method. Against the 17 strains, the disc diffusion test was in the range of 18-30 mm, and the minimum inhibitory concentrations (MICs) of EM were in the range of 1.5-25 μg/mL. From those results we performed the checkerboard test to determine the synergism of EM in combination with ampicillin (AM) or oxacillin (OX) against all strains. The combined activity of EM and two antimicrobial agents (AM, OX) against all strains resulted in a fractional inhibitory concentrations index (FICI) ranging from 0.37-0.5 and from 0.37-0.75, respectively. The effect of EM with AM and OX was found to be synergistic or partially synergistic. We found that EM reduced the MICs of AM and OX. EM and in combination with AM or OX could lead to the development of new combination antibiotics against MRSA infection.