Leptodactylus ocellatus (Amphibia): mechanism of defense in the skin and molecular phylogenetic relationships

Abstract

Amphibian antimicrobial peptides have been known for many decades and several of them have already been isolated. However, the number of species investigated is still small. Herein, we report on the skin secretions of Leptodactylus ocellatus, which were extracted by mild electrical stimulation and its semi-preparative reverse-phase chromatography was resolved in more than 30 fractions. Among these fractions, two novel antimicrobial peptides were isolated and their amino acid sequences determined by de novo sequencing. The ocellatins-5 and -6 (21 and 22 amino acid residues, respectively) are amidated at the C-terminus. Ocellatins inhibited the growth of reference strains of both Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus) with minimal inhibition concentration values in the range of 32-128 microg/mL. The amino acid sequence of the peptides shows structural similarity with members of the antimicrobial peptides found in the skin secretion of other leptodactylid frogs. This observation is consistent with the hypothesis that many frog skin antimicrobial peptides are related evolutionarily, having arisen from multiple duplications of an ancestral gene that existed before the radiation of the different species.