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

Abstract

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