Phylogenetic and chemical diversity of MAR4 streptomycete lineage

The oceans are a highly complex microbiological environment with typical microbial abundances of 106 and 109 per ml in seawater and ocean-bottom sediments, respectively. Among them, marine actinobacteria, commonly named actinomycetes, are the greatest source of microbial derived natural products, accounting for ca. 75% of all antibiotics discovered until 2002.1 Therefore, in the last decades several efforts have been done regarding marine actinomycete natural product research, which allowed the discovery of significant novel actinomycete biodiversity. A total of 662 sediment samples were collected along Madeira Archipelago and processed for the isolation of actinomycetes. In total, 421 actinomycete strains were isolated, 80 (19%) of these, revealed an obligate requirement of seawater for growth. Among these seawater-obligate marine actinomycetes are specimens of MAR4 streptomycete lineage. The MAR4 lineage of streptomycetes (e.g. Streptomyces aculeolatus) has been reported to be largely of marine origin and recognized as a source of rare of hybrid-isoprenoid class of bacterial secondary metabolites, namely napyradiomycins (1), marinones (2), nitropyrrolins (3), and lavanducyanis (4)2, Figure 1. In addition, from a single strain, in many cases, were reported multiple bioactive derivatives within the same structural class.2 The phylogenetic tree provides clear resolution of the MAR4 streptomycete lineage, with clear separation of Streptomyces aculeolatus MAR4 strains from other Streptomyces aculeolatus strain (accession number EU741176). Our phylogenetic analysis is in accordance with previously published reports, which propose that strain (S. aculeolatus, EU741176) has not being a MAR4 strain, although it was obtained from marine sediment samples collected at the Caribbean Sea. Crude extracts were obtained from these strains; four crude-extracts showed antibacterial activity against both MRSA and VRE with MIC values of ≤ 0.04 μg/ml. Fractions of one of these strains besides antibacterial activity, showed cytotoxic activity against HCT-116 cell line, with IC50 values of 3.5 μg/ml. Additionally, this lineage has been linked to the production of hybrid isoprenoid secondary metabolites, which can display potent biological activities (e.g. the commercially important aminocoumarin antibiotics).3 Currently we are targeting their bioactive compounds for structured elucidation, which appear to be new napyradiomycin derivatives. Figure 1. Main secondary metabolites classes produced by MAR4 streptomycetes lineage. To date, phylogenetic characterization of 6 representative isolates, based on partial sequence of gene encoding 16S rRNA, confirm that these strains belong to the specie Streptomyces aculeolatus. Figure 2. Neighbour-joining phylogenetic tree created from 6 partial 16S rRNA gene sequence from Streptomyces aculeolatus strains cultured from Madeira Archipelago, based on 1000 bootstrap replicates. BLAST matches (deposited in GenBank) are included with species and strain name followed by accession number. Verrucosispora maris and Micromonospora aurantiaca were used as outgroups.