Amphibian skin is a rich source of natural compounds with diverse antimicrobial and immune defense properties. Our previous studies showed that the frog skin secretions obtained by skin micro-organs from various species of Colombian anurans have antimicrobial activities against bacteria and viruses. We purified for the first time two antimicrobial peptides from the skin micro-organs of the Orinoco lime treefrog (Sphaenorhynchus lacteus) that correspond to Buforin II (BF2) and Frenatin 2.3S (F2.3S). Here, we have synthesized the two peptides and tested them against Gram-negative and Gram-positive bacteria, observing an effective bactericidal activity at micromolar concentrations. Evaluation of BF2 and F2.3S membrane destabilization activity on bacterial cell cultures and synthetic lipid bilayers reveals a distinct membrane interaction mechanism. BF2 agglutinates E. coli cells and synthetic vesicles, whereas F2.3S shows a high depolarization and membrane destabilization activities. Interestingly, we found that F2.3S is able to internalize within bacterial cells and can bind nucleic acids, as previously reported for BF2. Moreover, bacterial exposure to both peptides alters the expression profile of genes related to stress and resistance response. Overall, these results show the multifaceted mechanism of action of both antimicrobial peptides that can provide alternative tools in the fight against bacterial resistance.

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Staphylococcus aureus infections are a common concern world-wide due to the increasing number of bacterial strains with multiresistant properties to existing antibiotics, incrementing the need for novel molecules and therapy approaches for their treatment. This study evaluated the antibacterial and immunomodulatory activity of eight new peptides (AA, KS, NS, RN, AT, GF, KV and LK) as the basis for the search of new antibacterial and therapeutic agents for topic prevention and treatment against S. aureus infections. Here, there are characterized in silico eight new antimicrobial peptides. Their antibacterial activity against S. aureus and cytotoxic activity in mammalian cell lines were evaluated in vitro with the peptides individually and combined. Three of the peptides (GF, AT and AA) immunomodulatory activity was assessed in macrophages and under three scenarios: non-stimulation, Escherichia coli LPS stimulation and S. aureus lysate stimulation. Results showed that three peptides individually showed the best antibacterial activity against the S. aureus bacteria evaluated. The peptides presented immunomodulatory activity in THP-1 macrophages by displaying different profiles, increasing or decreasing four cytokines (IL-1β, TNF-α, IL-8 and CCL2 (MCP1)). This activity depended on the peptide concentration and the stimulation in which the macrophages were exposed to. Taken together, these results demonstrate the potential of these peptides to be used in further studies as novel antimicrobial molecules for the prevention and treatment of S. aureus infections.