There is a real consensus that new antibiotics are urgently needed and are the best chance for combating antibiotic resistance. The phylum Actinobacteria is one of the main producers of new antibiotics, with a recent paradigm shift whereby rare actinomycetes have been increasingly targeted as a source of new secondary metabolites for the discovery of new antibiotics. However, this review shows that the genus Streptomyces is still the largest current producer of new and innovative secondary metabolites. Between January 2015 and December 2020, a significantly high number of novel Streptomyces spp. have been isolated from different environments, including extreme environments, symbionts, terrestrial soils, sediments and also from marine environments, mainly from marine invertebrates and marine sediments. This review highlights 135 new species of Streptomyces during this 6-year period with 108 new species of Streptomyces from the terrestrial environment and 27 new species from marine sources. A brief summary of the different pre-treatment methods used for the successful isolation of some of the new species of Streptomyces is also discussed, as well as the biological activities of the isolated secondary metabolites. A total of 279 new secondary metabolites have been recorded from 121 species of Streptomyces which exhibit diverse biological activity. The greatest number of new secondary metabolites originated from the terrestrial-sourced Streptomyces spp.

The islands of the South Pacific Ocean have been in the limelight for natural product biodiscovery, due to their unique and pristine tropical waters and environment. The Kingdom of Tonga is an archipelago in the central Indo-Pacific Ocean, consisting of 176 islands, 36 of which are inhabited, flourishing with a rich diversity of flora and fauna. Many unique natural products with interesting bioactivities have been reported from Indo-Pacific marine sponges and other invertebrate phyla; however, there have not been any reviews published to date specifically regarding natural products from Tongan marine organisms. This review covers both known and new/novel Marine Natural Products (MNPs) and their biological activities reported from organisms collected within Tongan territorial waters up to December 2020, and includes 109 MNPs in total, the majority from the phylum Porifera. The significant biological activity of these metabolites was dominated by cytotoxicity and, by reviewing these natural products, it is apparent that the bulk of the new and interesting biologically active compounds were from organisms collected from one particular island, emphasizing the geographic variability in the chemistry between these organisms collected at different locations.

Over the course of this study, various species of Tongan marine sponges were investigated using an NMR-based screening method and has resulted in the discovery of three new sesterterpenes and 11 known compounds. Examination of the sponge Fascaplysinopsis sp. resulted in the isolation of two novel sesterterpenes, isoluffariellolide (46) and 1-O-methylisoluffariellolide (47). Compounds 46 and 47 share the same backbone pattern as the known luffariellolide (45) and 25-Omethylluffariellolide (107) respectively, and differ only in the substitution pattern of the butenolide rings. Isoluffariellolide (46) was found to be approximately six times less cytotoxic than 1-O-methylisoluffariellolide (47). Interestingly, these results suggested that the 1-O-methyl group in compound 47 plays an important role in the cytotoxicity of the compound. Secothorectolide (49), a new ring-opened and geometric isomer of the known compound thorectolide (48), was obtained from a sponge of the order Dictyoceratida. This ring closure and opening relationship was also observed between manoalide (109) and secomanoalide (110), as well as luffariellins A (141) and B (142). Despite the different carbon skeleton, the functional groups in 141 and 142 are similar with those in 109 and 110, respectively, and not surprisingly the biological properties are almost identical. The biological activities of compounds 48 and 49 were almost the same, which would give an insight into the structure-activity relationship (SAR) between these types of compounds.