In this study, antimicrobial activity of novel cyclic peptides coded as P1, P3, P5 and P8 against reference bacterial and fungal strains and clinical multidrug resistant (MDR) bacterial strains of animal origin was investigated. Good antimicrobial activity against Pseudomonas aeruginosa, Escherichia coli, Malassetia pachydermatis and Candida albicans was found, while against Gram-positive bacteria, the minimum  bactericidal concentration of peptide that killed >90% of bacteria (MBC₉₀) was >100μg/mL. Excellent activity was noticed for P3 against E. coli MDR clinical strains (MBC₉₀ 1.6-12.5μg/mL), and against P. aeruginosa MDR clinical strains (MBC₉₀ 3.2-12.5μg/mL). The peptides readily permeabilized P. aeruginosa membranes as evaluated by propidium iodide dead-stain assay. The peptides showed salt dependence with hemolytic activity close to 30% at 100μg/mL. The results so far obtained indicate good and rapid antimicrobial activity of three peptides P1, P3 and P8 that prompt for further improvement in light of a potential topical therapeutic use.

Key words: Antimicrobial peptides, Bacteria, Candida, Cyclic peptides, Malassetia, MDR veterinary isolates