Abstract: 【Objective】 This study aimed to investigate the antibacterial effect of isobavachalcone (IBC),a natural plant-derived flavonoid compound,and its combination with six commonly used antibiotics on methicillin-resistant Staphylococcus aureus (MRSA),to observe whether isobavachalcone had antibacterial synergistic effect on commonly used antibiotics,and provide reference for isobavachalcone as a potential antibacterial drug or antibacterial synergist. 【Method】 Minimal inhibitory concentrations (MIC) of six antibiotics (polymyxin B,enrofloxacin,oxytetracycline,florfenicol,ampicillin and amoxicillin) against MRSA T144 were determined by broth microdilution method.The chessboard method combined with drug sensitivity test was used to determine the effect of drug combination.The fractional inhibitory concentrations index (FICI) was used to determine their synergistic effect,and the antibacterial drugs with synergistic effect were selected.Staphylococcus aureus,Bacillus cereus,Escherichia coli and Salmonella were used as test strains to explore whether isobavachalcone combined with the screened antibacterial could have antibacterial synergistic effect on common clinical Gram-positive and Gram-negative bacteria.SYTO9-PI staining was performed to evaluate the effect of isobavachalcone on the cell membrane permeability of the tested strains.The content of antibacterials in the bacteria was determined according to the change of PI fluorescence intensity. 【Result】 The results of drug sensitivity test showed that polymyxin B,oxytetracycline,ciprofloxacin,florfenicol,ampicillin and amoxicillin all showed inhibitory effects on MRSA T144,and the MIC was 8-64 μg/mL.The results of combined drug sensitivity test showed that the combination of isobavachalcone and polymyxin B had a synergistic effect on MRSA T144 (FICI＝0.5).When isobavachalcone was combined with oxytetracycline,florfenicol,ampicillin and amoxicillin respectively,it had additive effect on MRSA T144 (FICI was 0.5625-1).There was no correlation between isobavachalcone and ciprofloxacin (FICI＝2).When isobavachalcone and polymyxin B were combined,they showed a synergistic effect on MRSA T144 (FICI＝0.5).The results of combined drug sensitivity test showed that when isobavachalcone and polymyxin B were used in combination,there was a synergistic effect on Staphylococcus aureus and Bacillus cereus (FICI was 0.258-0.375).It showed a partial synergistic effect on Escherichia coli and Salmonella (FICI was 0.531-0.563).The results of cell membrane permeability test further showed that the combined treatment group could significantly increase the number of bacteria MRSA T144,Staphylococcus aureus,Bacillus cereus,Escherichia coli and Salmonella with changed membrane permeability compared with polymyxin B and isobavachalcone alone (P＜0.05).The results of polymyxin B content in bacteria showed that compared with polymyxin B alone,the concentration of polymyxin B in Escherichia coli and MRSA T144 was significantly increased when polymyxin B was combined with isobavachalcone (P＜0.05). 【Conclusion】 Isobavachalcone combined with polymyxin B had certain antibacterial synergistic effect on MRSA T144,Staphylococcus aureus,Bacillus cereus,Escherichia coli and Salmonella,isobavachalcone could change the membrane permeability of the tested strains,thereby reducing the dose of polymyxin B.This study could provide new ideas for clinical treatment of bacterial infection,and also provide a reference for broadening the use of polymyxin B in animal husbandry in the future.

Key words: isobavachalcone; antibacterials; antibacterial synergist