Synergistic Activity of Lupinifolin in Combinations with Antibiotics Against Staphylococcus aureus

<b>Background and Objective:</b> Antibacterial resistance is one of the top global public health problems. The use of natural substances, which can enhance the antibacterial activity of currently used medications, is a promising alternative to oppose antibacterial resistance. The pharmacological activities of lupinifolin, a prenylated flavanone isolated from stems of <i>Derris reticulata</i> Craib., against growth and biofilm formation of <i>Streptococcus mutans</i> and <i>Staphylococcus aureus</i> have been previously documented. Nonetheless, interactions between lupinifolin and other antibacterial agents have not been determined. This study aimed to investigate the effects of lupinifolin in combinations with some antibacterial agents, specifically ampicillin, cloxacillin or vancomycin, against <i>S. mutans</i>, Methicillin-Sensitive <i>S. aureus</i> (MSSA) and Methicillin-Resistant <i>S. aureus</i> (MRSA). <b>Materials and Methods:</b> The checkerboard assay was performed to determine the antibacterial activity of lupinifolin plus the testing antibacterial agents. The Fractional Inhibitory Concentration Index (FICI) was calculated to indicate the interaction between lupinifolin and the antibacterial agent tested. <b>Results:</b> Lupinifolin exerted the synergistic activity when using in combination with ampicillin or cloxacillin against MSSA with the FICIs of <u><</u>0.5. The potential synergistic effect was also observed with lupinifolin plus ampicillin or cloxacillin against MRSA. However, the combination of lupinifolin plus vancomycin resulted in no interaction against MRSA. The combined effects of lupinifolin and ampicillin or cloxacillin against <i>S. mutans</i> were somewhat ambiguous with the borderline values of FICI of 0.5156 and 0.5625, respectively. <b>Conclusion:</b> Lupinifolin potentially plays a role as an antibacterial intensifier against some pathogenic gram-positive bacteria, particularly MSSA and MRSA. Nonetheless, further experiments are required to explain the precise mechanism of synergy.