ACTIVITY OF PROTEGRINS AGAINST YEAST-PHASE CANDIDA-ALBICANS

We used a two-stage radial diffusion assay to perform a structure-acti vity study of the antifungal effects of protegrin-1 (PG-1) on yeast-ph ase Candida albicans, While doing so, we computed MICs from the radial diffusion assay data by three methods and compared the respective val ues with results from colony count and broth microdilution assays. Thi s allowed us to identify several technical modifications that improved the sensitivity and accuracy of radial diffusion assays. We found tha t both PG-l and enantiomeric PG-1 (composed exclusively of D-amino aci ds) were potently fungicidal for yeast-phase C. albicans, The protegri ns 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 cand idacidal activity at physiological NaCl concentrations. Truncated vari ants of PG-l that lacked its first four residues showed decreased cand idacidal activity, although their activity against bacteria,vas substa ntially intact. Altering the beta-turn region (residues 9 to 12) of PG -l or its variants further decreased candidacidal activity. These stud ies suggest that only 12 residues are needed to endow protegrin molecu les 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 fu ngi.