The present invention is intended for the pharmaceutical field and relates to the development of a new formulation that can be used as a basic ingredient of an antibiotic against methicillin-resistant Staphylococcus aureus (MRSA) with a drug resistance index of 75%. The new formulation is based on the synergistic properties of emodin and kaempferol, two molecules isolated from the Moroccan plant Rhamnus alaternus. This invention describes the synergistic effect of emodin in increasing the inhibitory potential against MRSA at non-cytotoxic doses in the macrophage cell line. When the molecules were tested separately, emodin showed a good minimum inhibitory concentration, while kaempferol was moderately active. However, the mixture of these two molecules at very low concentrations was able to synergistically increase bioactivity.