Streptomyces: Still the Biggest Producer of New Natural Secondary Metabolites, a Current Perspective

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.

Streptomyces musisoli sp. nov., an actinomycete isolated from soil

An actinobacterium, strain CH5-8^T, which formed spiral chains of spore arising from the aerial mycelium, was isolated from rhizosphere soil of Musa spp. The organism exhibited vivid greenish yellow substrate mycelium and easily produced the medium grey aerial spore mass on ISP2 medium. The typical chemotaxonomic properties of members of the genus Streptomyces were observed for strain CH5-8^T, e.g. ll-diaminopimelic acid in cell peptidoglycan, MK-9(H₈), MK-9(H₆), and MK-9(H₄) as major menaquinones and anteiso-C_15 : 0, iso-C_16 : 0, and anteiso-C_17 : 0 as major fatty acids. Diphosphatidylglycerol, phosphatidylethanolamine, hydroxyphosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol and phosphatidylinositol mannoside were detected in the cells. A combination of morphological and chemotaxonomic data supported the assignment to the genus Streptomyces. The analysis result obtained for the 16S rRNA gene sequence confirmed the taxonomic affiliation at the genus level of this strain. The novel strain CH5-8^T showed the highest 16S rRNA gene sequence values to Streptomyces echinatus NBRC 12763^T (98.9 %), followed by Streptomyces actinomycinicus RCU-197^T (98.9 %). The average nucleotide identity by blast (ANIb) and digital DNA-DNA hybridization values between CH5-8^T and its closest relatives, S. echinatus CECT 3313^T and S. actinomycinicus RCU-197^T, were ≤91.6 % and ≤47.4 %, respectively. The digital DNA G+C content of genomic DNA was 72.1 mol%. On the basis of these phenotypic and genotypic data, strain CH5-8^T represents a novel species, for which the name Streptomyces musisoli sp. nov. is proposed. The type strain is CH5-8^T (=TBRC 9950^T=NBRC 113997^T).