Activity of Protegrins against Yeast-Phase Candida albicans

We used a two-stage radial diffusion assay to perform a structure-activity study of the antifungal effects of protegrin-1 (PG-1) on yeast-phase Candida albicans. While doing so, we computed MICs from the radial diffusion assay data by three methods and compared the respective values with results from colony count and broth microdilution assays. This allowed us to identify several technical modifications that improved the sensitivity and accuracy of radial diffusion assays. We found that both PG-1 and enantiomeric PG-1 (composed exclusively of d-amino acids) were potently fungicidal for yeast-phase C. albicans. The protegrins PG-2, -3, and -5, but not PG-4, were as effective as PG-1. At least one intramolecular disulfide bond was required to retain optimal candidacidal activity at physiological NaCl concentrations. Truncated variants of PG-1 that lacked its first four residues showed decreased candidacidal activity, although their activity against bacteria was substantially intact. Altering the β-turn region (residues 9 to 12) of PG-1 or its variants further decreased candidacidal activity. These studies suggest that only 12 residues are needed to endow protegrin molecules with strong antibacterial activity and that at least 4 additional residues are needed to add potent antifungal properties. Thus, the 16-residue protegrin PG-2 likely represents the minimal structure needed for broad-spectrum antimicrobial activity encompassing bacteria and fungi.