Antibiofilm and antibacterial activities of lupinifolin in combination with protein synthesis inhibitors against methicillin-resistant Staphylococcus aureus

Introduction: Methicillin-resistant Staphylococcus aureus (MRSA)-derived biofilm formation is a crucial virulence factor, which essentially contributes to therapeutic challenges. This study aims to evaluate the antibiofilm and antibacterial formation activities of lupinifolin, a prenylated flavanone derived from Derris reticulata Craib. stem, in combination with protein synthesis inhibitors. Methods: The crystal violet biofilm formation assay was performed to determine the biofilm formation activity. The synergistic antibacterial activities were evaluated using the checkerboard and time-kill assays. Results: Lupinifolin and tetracycline significantly reduced MRSA biofilm formation with IC50 values of 15.32 ± 5.98 and 13.42 ± 5.90 µg/mL, respectively. On the contrary, the individual treatment of streptomycin and clindamycin tended to enhance biofilm formation. Lupinifolin at the sub-MIC of 8 µg/mL in combination with certain sub-MICs of tetracycline (8 and 16 µg/mL), streptomycin (16, 32, and 64 µg/mL), or clindamycin (4, 8, and 16 µg/mL) caused significant inhibitions against MRSA biofilm formation (P<0.05). The combination of lupinifolin and streptomycin exhibited a synergy (FIC index <0.625), confirmed in the time-kill assay. Conversely, the combination of lupinifolin and tetracycline or clindamycin resulted in no interaction (FIC indices of 1.0078 and <1.0156, respectively). Conclusion: The antibacterial synergy of lupinifolin and streptomycin possibly contributed to their antibiofilm-forming activity. However, the combinations of lupinifolin and tetracycline or clindamycin conceivably executed their antibiofilm activity directly against the MRSA biofilm formation process. These findings indicate a potential role for lupinifolin as an antibiofilm enhancer to diminish MRSA biofilm formation.