Introduction: Actinomycetes Streptomyces genus is recognized for their ability to produce two of the most valuable families of natural products: non-ribosomal peptides (NRPs) and poliketides (PKs) [1]. In particular, Streptomyces aculeolatus (Figure 1), belonging to the MAR4 group, is known to produce a specific group of bioactive secondary metabolites named hybrid isoprenoids (HI) [2–4]. These compounds have great bioactive potential as antibacterial, antifungal, and anticancer agents making them potentially attractive to the pharmaceutical industry [3–5]. Four S. aculeolatus stains isolated from marine sediments collected off the Madeira Archipelago (coded PTM-29, PTM-129, PTM-346 and PTM-398) revealed the production of secondary metabolites with distinct bioactivities. To date, small organic compounds are the most widely explored but now, our main goal is the screening of extracellular water soluble protein species with biological activities secreted by S. aculeolatus. Materials and methods: The antimicrobial activity of S. aculeolatus extracellular extracts (aqueous phase) was evaluated by screening the biological activity against two Methicillin Resistant strains of the human pathogen Staphyloccocus aureus, namely COL and MW2. Minimal inhibitory concentrations (MIC) were determined by growth inhibition halos and microdilution analysis. Results: Preliminary results showed antimicrobial activity against MRSA from extracellular extracts of strains PTM-29, PTM-129, PTM-346 and PTM-398, with MIC values ranging from 0.010-0.020 µg/µl. Discussion and conclusions: All the aqueous extracellular extracts exhibited inhibitory growth effect against COL and MW2 MRSA strains. In addition, the extract from PTM-346 showed the highest inhibitory activity whereas PTM-29 showed the lowest level of activity. Interestingly, PTM-129, whose organic crude extract did not show biological activity, revealed antimicrobial activity in the aqueous extracellular extract obtained in this study. In the future, the bioactive peptide species will be isolated from the extracellular extracts, purified and characterized by biochemical techniques. Ultimately, this work will deliver novel bioactive peptides that might be conducted to clinical trials, in the pursuing of lead-like agents.
