Nonactin biosynthesis: setting limits on what can be achieved with precursor-directed biosynthesis

Streptomyces Endophytes in Edible Plants: New Insights into their Chemistry and Health Benefits

Streptomyces is the largest source of microbial antibiotics with about 50 % of marketed antimicrobial drugs originating from this genus. Endophytic streptomyces are the link between medicinal plants and the microbial world. Endophytic Streptomyces in edible plants were not targeted before despite their uniqueness and importance. In this review, we analyzed the chemical diversity of more than 150 compounds belonging to endophytic Streptomyces chemical classes such as alkaloids, polyketides, peptides, macrolides and terpenes and their biological activities. This analysis showed a dominant antimicrobial effect for most of the isolated compounds and highlighted an underestimated diversity to be studied or repurposed for other biological activities. Return to edible plants use and conducting toxicity studies to rationalize their nutraceutical potential based on their beneficial endophytes is urged. Although there are many studies for non-vertebrates, the nutraceutical potential of these plants is expected to improve the gut microbiota since they are enriched with bioactive compounds from streptomyces species. This is the first review to discuss edible plants associated streptomyces, and we prospect that many studies will follow to unravel the mysterious health benefits of streptomyces in the human microbiome and encourage the revival of a correct lifestyle for the sake of a healthier microbiome.

Strain Streptomyces sp. P-56 Produces Nonactin and Possesses Insecticidal, Acaricidal, Antimicrobial and Plant Growth-Promoting Traits

Streptomycetes produce a huge variety of bioactive metabolites, including antibiotics, enzyme inhibitors, pesticides and herbicides, which offer promise for applications in agriculture as plant protection and plant growth-promoting products. The aim of this report was to characterize the biological activities of strain Streptomyces sp. P-56, previously isolated from soil as an insecticidal bacterium. The metabolic complex was obtained from liquid culture of Streptomyces sp. P-56 as dried ethanol extract (DEE) and possessed insecticidal activity against vetch aphid (Medoura viciae Buckt.), cotton aphid (Aphis gossypii Glov.), green peach aphid (Myzus persicae Sulz.), pea aphid (Acyrthosiphon pisum Harr.) and crescent-marked lily aphid (Neomyzus circumflexus Buckt.), as well as two-spotted spider mite (Tetranychus urticae). Insecticidal activity was associated with production of nonactin, which was purified and identified using HPLC-MS and crystallographic techniques. Strain Streptomyces sp. P-56 also showed antibacterial and antifungal activity against various phytopathogenic bacteria and fungi (mostly for Clavibacfer michiganense, Alternaria solani and Sclerotinia libertiana), and possessed a set of plant growth-promoting traits, such as auxin production, ACC deaminase and phosphate solubilization. The possibilities for using this strain as a biopesticide producer and/or biocontrol and a plant growth-promoting microorganism are discussed.

Natural products targeting cancer cell dependency

Precision cancer medicine is a tailored treatment approach for individual cancer patients with different genomic characteristics. Mutated or hyperactive oncogenes have served as main drug targets in current precision cancer medicine, while defective or inactivated tumor suppressors in general have not been considered as druggable targets. Synthetic lethality is one of very few approaches that enable to target defective tumor suppressors with pharmacological agents. Synthetic lethality exploits cancer cell dependency on a protein or pathway, which arises when the function of a tumor suppressor is defective. This approach has been proven to be effective in clinical settings since the successful clinical introduction of BRCA-PARP synthetic lethality for the treatment of breast and ovarian cancer with defective BRCA. Subsequently, large-scale screenings with RNAi, CRISPR/Cas9-sgRNAs, and chemical libraries have been applied to identify synthetic lethal partners of tumor suppressors. Natural products are an important source for the discovery of pharmacologically active small molecules. However, little effort has been made in the discovery of synthetic lethal small molecules from natural products. This review introduces recent advances in the discovery of natural products targeting cancer cell dependency and discusses potentials of natural products in the precision cancer medicine.

The Structural Diversity of Marine Microbial Secondary Metabolites Based on Co-Culture Strategy: 2009-2019

Marine microorganisms have drawn great attention as novel bioactive natural product sources, particularly in the drug discovery area. Using different strategies, marine microbes have the ability to produce a wide variety of molecules. One of these strategies is the co-culturing of marine microbes; if two or more microorganisms are aseptically cultured together in a solid or liquid medium in a certain environment, their competition or synergetic relationship can activate the silent biosynthetic genes to produce cryptic natural products which do not exist in monocultures of the partner microbes. In recent years, the co-cultivation strategy of marine microbes has made more novel natural products with various biological activities. This review focuses on the significant and excellent examples covering sources, types, structures and bioactivities of secondary metabolites based on co-cultures of marine-derived microorganisms from 2009 to 2019. A detailed discussion on future prospects and current challenges in the field of co-culture is also provided on behalf of the authors’ own views of development tendencies.

A Novel Cellular Spheroid-Based Autophagy Screen Applying Live Fluorescence Microscopy Identifies Nonactin as a Strong Inducer of Autophagosomal Turnover

Dysregulation of the basal autophagic flux has been linked to several pathological conditions, including neurodegenerative diseases and cancer. In addition, autophagy has profound effects on the response of tumor cells to therapy. Hence, the search for pharmacological modulators of autophagy is of great clinical relevance. We established a drug screening assay in which the autophagic flux is measured by recording the fluorescence emission of the tandem fusion protein mRFP-GFP-LC3 by dynamic live-cell imaging. We optimized the assay for the identification of autophagy modulators in three dimensions with U343 glioma cell spheroids, which represent a more realistic cancer model than conventional 2D cell cultures. We validated the assay by screening a library of known autophagy modulators. As the first application, a small library of 94 natural compounds was screened for its impact on autophagy. We discovered the cyclic ionophore nonactin as a new and potent autophagy inducer. This novel autophagy screening assay based on 3D tumor spheroids is robust, reproducible, and scalable. It provides a valuable tool for both basic research and drug screening campaigns.

Dereplication of Streptomyces sp. AMC 23 polyether ionophore antibiotics by accurate-mass electrospray tandem mass spectrometry

Actinomycetes, especially those belonging to the genus Streptomyces, are economically important from a biotechnological standpoint: they produce antibiotics, anticancer compounds and a variety of bioactive substances that are potentially applicable in the agrochemical and pharmaceutical industries. This paper combined accurate-mass electrospray tandem mass spectrometry in the full scan and product ion scan modes with compounds library data to identify the major compounds in the crude extract produced by Streptomyces sp. AMC 23; it also investigated how sodiated nonactin ([M + Na](+)) fragmented. Most product ions resulted from elimination of 184 mass units due to consecutive McLafferty-type rearrangements. The data allowed identification of four macrotetrolides homologous to nonactin (monactin, isodinactin, isotrinactin/trinactin and tetranactin) as well as three related linear dimer compounds (nonactyl nonactoate, nonactyl homononactoate and homononactyl homononactoate). The major product ions of the sodiated molecules of these compounds also originated from elimination of 184 and 198 mass units. UPLC-MS/MS in the neutral loss scan mode helped to identify these compounds on the basis of the elimination of 184 and 198 mass units. This method aided monitoring of the relative production of these compounds for 32 days and revealed that the biosynthetic process began with increased production of linear dimers as compared with macrotetrolides. These data could facilitate dereplication and identification of these compounds in other microbial crude extracts.

Follow-up of natural products isolation

Follow-up of natural products isolation refers to re-isolation of compound(s) of interest in larger amounts for further pharmacological testing, conclusive structure elucidation, structure modifications to synthesize analogs for structure-activity relationships (SAR) studies, preformulation and formulation studies or clinical trials. In addition to conventional synthetic chemistry approaches, several other methodologies can be applied for following-up natural products isolation. This chapter outlines, with specific examples, various strategies and methods involved in follow-up of natural products isolation.

Natural product derivatives with bactericidal activity against Gram-positive pathogens including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecalis

We have shown that the intentional engineering of a natural product biosynthesis pathway is a useful way to generate stereochemically complex scaffolds for use in the generation of combinatorial libraries that capture the structural features of both natural products and synthetic compounds. Analysis of a prototype library based upon nonactic acid lead to the discovery of triazole-containing nonactic acid analogs, a new structural class of antibiotic that exhibits bactericidal activity against drug resistant, Gram-positive pathogens including Staphylococcus aureus and Enterococcus faecalis.