Multi-layered antimicrobial synergism of ( E )-caryophyllene with minor compounds, tecleanatalensine B and normelicopine, from the leaves of Vepris gossweileri (I. Verd.) Mziray.

An aromatic alkaloid-rich 'absolute' extract from Vepris gossweileri inhibited Saccharomyces cerevisiae at 62.5 μg.mL ^-1 and Bacillus subtilis at 500 μg.mL ^-1 _. A loss of activity upon fractionation indicated possible synergistic effects. Three new acridones, gossweicridone A ( 3 ), B ( 4 ) and C ( 5 ) and known compounds from the extract were inactive. Combinations of compounds showed that a sub-fraction containing mixtures of minor compounds with ( Ε )-caryophyllene augmented activity by 50-folds, with MIC values of 19.6 μg.mL ^-1 for S. cerevisiae and 375.0 μg.mL ^-1 for B. subtilis , demonstrating potent ΣFIC values of 0.02 and 0.375 respectively. From the active sub-fraction, three compounds were assigned as tecleanatalensine B, 13 S -hydroxy-9 Z ,11 E ,15 E -octadecatrienoic acid and normelicopine. In combination with ( Ε )-caryophyllene they separately demonstrated MIC values of 18 μg.mL ^-1 , 34 μg.mL ^-1 and 16 μg.mL ^-1 _, respectively against S. cerevisiae . The synergistic combinations were more potent with addition of pheophytin A, suggesting that the synergistic antifungal effect of the extract is multi-layered.