Bacterial resistance modifying agents from Lycopus europaeus

As part of an ongoing project to identify plant natural products which modulate bacterial multidrug resistance (MDR), bioassay-guided isolation of an extract of Lycopus europaeus yielded two new isopimarane diterpenes, namely methyl-1α-acetoxy-7α 14α-dihydroxy-8,15-isopimaradien-18-oate (1) and methyl-1α,14α-diacetoxy-7α-hydroxy-8,15-isopimaradien-18-oate (2). The structures were established by spectroscopic methods. These compounds and several known diterpenes were tested for in vitro antibacterial and resistance modifying activity against strains of Staphylococcus aureus possessing the Tet(K), Msr(A), and Nor(A) multidrug resistance efflux mechanisms. At 512 μg/ml none of the compounds displayed any antibacterial activity but individually in combination with tetracycline and erythromycin, a two-fold potentiation of the activities of these antibiotics was observed against two strains of S. aureus that were highly resistant to these agents due to the presence of the multidrug efflux mechanisms Tet(K) (tetracycline resistance) and Msr(A) (macrolide resistance). Methyl-1α-acetoxy-7α,14α-dihydroxy-8,15-isopimaradien-18-oate and methyl-1α,14α-diacetoxy-7α-hydroxy-8,15-isopimaradien-18-oate were isolated from Lycopus europaeus and assessed for bacterial resistance modifying activity against multidrug resistant Staphylococcus aureus