Streptomyces tibetensis sp. nov., an actinomycete isolated from the Tibetan Plateau

Oligomycin-producing Streptomyces sp. newly isolated from Swiss soils efficiently protect Arabidopsis thaliana against Botrytis cinerea

Botrytis cinerea is a necrotrophic phytopathogen able to attack more than 200 different plant species causing strong yield losses worldwide. Many synthetic fungicides have been developed to control this disease, resulting in the rise of fungicide-resistance B. cinerea strains. The aim of this study was to identify Streptomyces strains showing antagonistic activity against B. cinerea to contribute to plant protection in an environmentally friendly way. We isolated 15 Actinomycete strains from 9 different Swiss soils. The culture filtrates of three isolates showing antifungal activity inhibited spore germination and delayed mycelial growth of B. cinerea. Infection experiments showed that Arabidopsis thaliana plants were more resistant to this pathogen after leaf treatment with the Streptomyces filtrates. Bioassay-guided isolation of the active compounds revealed the presence of germicidins A and B as well as of oligomycins A, B, and E. While germicidins were mostly inactive, oligomycin B reduced the mycelial growth of B. cinerea significantly. Moreover, all three oligomycins inhibited this fungus' spore germination, suggesting that these molecules might contribute to the Streptomyces's ability to protect plants against infection by the broad host-pathogen Botrytis cinerea.

IMPORTANCE

This study reports the isolation of new Streptomyces strains with strong plant-protective potential mediated by their production of specialized metabolites. Using the broad host range pathogenic fungus Botrytis cinerea, we demonstrate that the cell-free filtrate of selected Streptomyces isolates efficiently inhibits different developmental stages of the fungus, including mycelial growth and the epidemiologically relevant spore germination. Beyond in vitro experiments, the strains and their metabolites also efficiently protected plants against the disease caused by this pathogen. This work further identifies oligomycins as active compounds involved in the observed antifungal activity of the strains. This work shows that we can harness the natural ability of soil-borne microbes and of their metabolites to efficiently fight other microbes responsible for significant crop losses. This opens the way to the development of environmentally friendly health protection measures for crops of agronomical relevance, based on these newly isolated strains or their metabolic extracts containing oligomycins.

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 boncukensis sp. nov., isolated from saltern soil

A novel Gram-stain positive, aerobic, non-motile Actinobacterium, designated strain SB3404^T, was isolated from saltern soil collected from Boncuk Saltern, Sungurlu-Çorum, Turkey, and subjected to a polyphasic taxonomic approach. The organism has shown to have phylogenetic, chemotaxonomic, cultural and morphological properties consistent with its classification in the genus Streptomyces. 16S rRNA gene sequence analysis of strain SB3404^T showed that it is closely related to Streptomyces albus NBRC 13014^T (97.2% sequence similarity), Streptomyces xishensis YIM M 10378^T (96.7%) and Streptomyces abyssalis YIM M 10400^T (96.5%). The cell wall of the strain contained LL-diaminopimelic acid and the cell-wall sugars were glucose, mannose and ribose. The predominant menaquinones were identified as MK-9(H₈) and MK-9(H₆). The major cellular fatty acids were found to be iso-C_16:0, anteiso-C_17:0 and anteiso-C_15:0. Consequently, strain SB3404^T is considered to represent a novel species in the genus Streptomyces, for which the name Streptomyces boncukensis sp. nov. is proposed. The type strain is SB3404^T (= KCTC 49371^T = JCM 34018^T).