AimsTo identify cellular conditions that significantly alter susceptibility of Saccharomyces cerevisiae, Candida albicans and Candida glabrata to the antimicrobial peptide, occidiofungin. Methods and ResultsGenetic and pharmacological approaches were used to determine a role for calcium signalling in occidiofungin sensitivity profiles for S. cerevisiae, C. albicans and C. glabrata strains of yeast. Minimum inhibitory concentration (MIC) and drop assays found that extracellular calcium resulted in a fourfold resistance, and this was independent of an intact calmodulin-calcineurin signalling pathway. A similar resistance was found in the presence of magnesium but not other cations. Occidiofungin was found to be ineffective against cells in a quiescent state when measured by MIC, drop assay and short-term time-kill assays. A similar resistance pattern was detected for S. cerevisiae cultures pre-exposed to cycloheximide or placed in depleted media conditions. ConclusionsExtracellular calcium results in fungal tolerance to occidiofungin bioactivity outside of the calmodulin-calcineurin pathway. In addition, the resistance of quiescent cells suggests that active cellular growth is a requirement for occidiofungin's mechanism of action. Significance and Impact of the StudyThe identification of cellular conditions that have a role in the activity of occidiofungin provided insight into potential cellular targets of this novel antifungal.