Calcium Dependence of Eugenol Tolerance and Toxicity in Saccharomyces cerevisiae

Eugenol is a plant-derived phenolic compound which has recognised therapeutical potential as an antifungal agent. However little is known of either its fungicidal activity or the mechanisms employed by fungi to tolerate eugenol toxicity. A better exploitation of eugenol as a therapeutic agent will therefore depend on addressing this knowledge gap. Eugenol initiates increases in cytosolic Ca(2+) in Saccharomyces cerevisiae which is partly dependent on the plasma membrane calcium channel, Cch1p. However, it is unclear whether a toxic cytosolic Ca(2+)elevation mediates the fungicidal activity of eugenol. In the present study, no significant difference in yeast survival was observed following transient eugenol treatment in the presence or absence of extracellular Ca(2+). Furthermore, using yeast expressing apoaequorin to report cytosolic Ca(2+) and a range of eugenol derivatives, antifungal activity did not appear to be coupled to Ca(2+) influx or cytosolic Ca(2+) elevation. Taken together, these results suggest that eugenol toxicity is not dependent on a toxic influx of Ca(2+). In contrast, careful control of extracellular Ca(2+) (using EGTA or BAPTA) revealed that tolerance of yeast to eugenol depended on Ca(2+) influx via Cch1p. These findings expose significant differences between the antifungal activity of eugenol and that of azoles, amiodarone and carvacrol. This study highlights the potential to use eugenol in combination with other antifungal agents that exhibit differing modes of action as antifungal agents to combat drug resistant infections.