Neolignans from Piper betle Have Synergistic Activity against Antibiotic-Resistant Staphylococcus aureus

A phytochemical investigation of an extract of the leaves of Piper betle, guided by a synergistic antibacterial screen, led to the isolation and structural elucidation of 10 new neolignans, Pibeneolignan A–J (1–10), together with 11 known compounds. The structures and absolute configurations of the new compounds were elucidated on the basis of spectroscopic data, single-crystal X-ray diffraction analysis, and experimental and calculated ECD investigations. Compounds 1 and 2 are new naturally occurring neolignan skeletons, based on the cyclohept-2-ene-1,4-dione framework. We propose that these natural products are biosynthetically formed from bicyclic [3.2.1] neolignans by oxidative cleavage and ring opening at C-1′ and C-2′. Among these compounds, 9, 13, 15, and 16, in combination with norfloxacin against an effluxing S. aureus strain (SA1199B), exhibited significant synergistic activity with fractional inhibitory concentration indices (FICIs) of 0.078, 0.156, 0.125, and 0.25, respectively. Bacterial growth curves, ethidium bromide (EtBr) efflux, and qRt-PCR were further employed to verify their synergistic antibacterial mechanism. Furthermore, computational molecular modeling suggested the binding of compounds 14–17 and 19 to the active site of the modeled structure of the NorA efflux pump, which is the main efflux pump in SA1199B.