We found that an extract from Laurus nobilis L. (Lauraceae) leaves showed antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA). We purified two flavonoids as the effective compounds and identified them as kaempferol 3-O-a a-L-(2؆,4؆-di-E-p-coumaroyl)-rhamnoside (C2) and kaempferol 3-O-a a-L-(2؆-Zp-coumaroyl-4؆-E-p-coumaroyl)-rhamnoside (C3). Both compounds showed strong antibacterial activity not only against MRSA but also against vancomycin-resistant enterococci (VRE). There was low or no antibacterial activity of C2 and C3 for Streptococcus pneumoniae, Pseudomonas aeruginosa and Serratia marcescens.

In a previous study, we reported that two kaempferol glycosides isolated from Laurus nobilis L., kaempferol-3-O-a a -L-(2؆,4؆-di-E-p-coumaroyl)-rhamnoside (C2) and kaempferol-3-O-a a -L-(2؆-E-p-coumaroyl-4؆-Z-p-coumaroyl)-rhamnoside (C3), showed strong antibacterial activities against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci. Thereafter we found that these compounds greatly reduced the minimum inhibitory concentrations (MICs) of some fluoroquinolones in MRSA. In other words, C2 and C3 greatly potentiated anti-MRSA activity of fluoroquinolones. The effect of C2 and C3 with fluoroquinolones was found to be synergistic. The potentiation activity was observed with hydrophilic fluoroquinolones, such as norfloxacin and ciprofloxacin, but not with hydrophobic quinolones. We also found that norfloxacin reduced MICs of C2 and C3. The effect was synergistic. Possible mechanism of the synergistic effect was discussed.