An extract prepared from root bark of Morus australis Poir., for use as an antibacterial agent in treating a subject having a bacterial infection, wherein the extract is prepared by a process comprising the steps of (i) extracting root bark of Morus australis Poir. With a polar solvent (ii) concentrating the resulting extract solution (iii) introducing the resulting concentrate to a reverse phase chromatography column, and eluting the column with a first eluent and a second eluent in sequence, wherein the first eluent has a polarity of about 50 wt% of ethanol aqueous solution and the second eluent has a polarity of about 95 wt% of ethanol aqueous solution and (iv) collecting a second eluate from the elution of the column with the second eluent, and removing the second eluent from the second eluate by evaporation. The use of kuwanon H of formula (I) or a pharmaceutically acceptable salt thereof in treating a subject having a bacterial infection, wherein the bacteria are gram-positive bacteria such as staphylococcus aureus or streptococcus pneumonia is also provided. The extract obtained from Morus australis Poir. may comprise kuwanon H and sanggenon G of formula (II), wherein the kuwanon H and sanggenon G are subsequently separated from the extract.
