Abstract: AIM: To explore the antibacterial activity and underlying mechanism of ursolic acid combined with fusidic acid against Staphylococcus aureus (SA) ATCC29213 and Methicillin-Resistant Staphylococcus aureus (MRSA) ATCC43300 in vitro.  METHODS: The minimum inhibitory concentration (MIC) of the combined use of ursolic acid and fusidic acid on SA, MRSA was determined by the micro broth dilution method and the micro checkerboard method, and the partial inhibitory concentration index (FICI) was calculated to determine the combined effect. And the bactericidal effect of fusidic acid combined with ursolic acid was studied by the time-killing curves. The agar double dilution method was used to determine the anti-drug resistance mutation concentration (MPC) and anti-drug resistance mutation selection window (MSW) of fusidic acid alone and in combination with ursolic acid. The viable count of biofilm carrier were determined by serial dilution method and the semi-quantitative biofilm by crystal violet staining method. RESULTS: The combined use of ursolic acid and fusidic acid for SA and MRSA FICI were 0.312 5 and 0.375, respectively. The time-killing curve showed that 1MIC ursolic acid combined with 1MIC fusidic acid has a synergistic bactericidal effect on SA and MRSA. The MPC of fusidic acid to MRSA was 256 μg/mL and the MSW was 256. After fusidic acid combined with ursolic acid, the MPC decreased to 8 μg/mL. The combined group was significantly reduced compared to the fusidic acid group. The semi-quantitative and biofilm bacterial counts of combined group were markedly decreased compared to the fusidic acid group after the biofilm cultivate for 48 h and 72 h.CONCLUSION:  The combined use of UA and FA has a synergistic effect on SA and MRSA.

Key words: ursolic acid, fusidic acid, staphylococcus aureus, synergistic