Longicatenamides A-D, Two Diastereomeric Pairs of Cyclic Hexapeptides Produced by Combined-culture of Streptomyces sp. KUSC_F05 and Tsukamurella pulmonis TP-B0596

Unearthing naturally-occurring cyclic antibacterial peptides and their structural optimization strategies

Natural products with antibacterial activity are highly desired globally to combat against multidrug-resistant (MDR) bacteria. Antibacterial peptide (ABP), especially cyclic ABP (CABP), is one of the abundant classes. Most of them were isolated from microbes, demonstrating excellent bactericidal effects. With the improved proteolytic stability, CABPs are normally considered to have better druggability than linear peptides. However, most clinically-used CABP-based antibiotics, such as colistin, also face the challenges of drug resistance soon after they reached the market, urgently requiring the development of next-generation succedaneums. We present here a detail review on the novel naturally-occurring CABPs discovered in the past decade and some of them are under clinical trials, exhibiting anticipated application potential. According to their chemical structures, they were broadly classified into five groups, including (i) lactam/lactone-based CABPs, (ii) cyclic lipopeptides, (iii) glycopeptides, (iv) cyclic sulfur-rich peptides and (v) multiple-modified CABPs. Their chemical structures, antibacterial spectrums and proposed mechanisms are discussed. Moreover, engineered analogs of these novel CABPs are also summarized to preliminarily analyze their structure-activity relationship. This review aims to provide a global perspective on research and development of novel CABPs to highlight the effectiveness of derivatives design in identifying promising antibacterial agents. Further research efforts in this area are believed to play important roles in fighting against the multidrug-resistance crisis.

Oxytetracycline hyper-production through targeted genome reduction of Streptomyces rimosus

Most biosynthetic gene clusters (BGC) encoding the synthesis of important microbial secondary metabolites, such as antibiotics, are either silent or poorly expressed; therefore, to ensure a strong pipeline of novel antibiotics, there is a need to develop rapid and efficient strain development approaches. This study uses comparative genome analysis to instruct rational strain improvement, using Streptomyces rimosus, the producer of the important antibiotic oxytetracycline (OTC) as a model system. Sequencing of the genomes of two industrial strains M4018 and R6-500, developed independently from a common ancestor, identified large DNA rearrangements located at the chromosome end. We evaluated the effect of these genome deletions on the parental S. rimosus Type Strain (ATCC 10970) genome where introduction of a 145 kb deletion close to the OTC BGC in the Type Strain resulted in massive OTC overproduction, achieving titers that were equivalent to M4018 and R6-500. Transcriptome data supported the hypothesis that the reason for such an increase in OTC biosynthesis was due to enhanced transcription of the OTC BGC and not due to enhanced substrate supply. We also observed changes in the expression of other cryptic BGCs; some metabolites, undetectable in ATCC 10970, were now produced at high titers. This study demonstrated for the first time that the main force behind BGC overexpression is genome rearrangement. This new approach demonstrates great potential to activate cryptic gene clusters of yet unexplored natural products of medical and industrial value.IMPORTANCEThere is a critical need to develop novel antibiotics to combat antimicrobial resistance. Streptomyces species are very rich source of antibiotics, typically encoding 20-60 biosynthetic gene clusters (BGCs). However, under laboratory conditions, most are either silent or poorly expressed so that their products are only detectable at nanogram quantities, which hampers drug development efforts. To address this subject, we used comparative genome analysis of industrial Streptomyces rimosus strains producing high titers of a broad spectrum antibiotic oxytetracycline (OTC), developed during decades of industrial strain improvement. Interestingly, large-scale chromosomal deletions were observed. Based on this information, we carried out targeted genome deletions in the native strain S. rimosus ATCC 10970, and we show that a targeted deletion in the vicinity of the OTC BGC significantly induced expression of the OTC BGC, as well as some other silent BGCs, thus suggesting that this approach may be a useful way to identify new natural products.

OSMAC Strategy: A promising way to explore microbial cyclic peptides

Microbial secondary metabolites are pivotal for the development of novel drugs. However, conventional culture techniques, have left a vast array of unexpressed biosynthetic gene clusters (BGCs) in microorganisms, hindering the discovery of metabolites with distinct structural features and diverse biological functions. To address this limitation, several innovative strategies have been emerged. The "One Strain Many Compounds" (OSMAC) strategy, which involves altering microbial culture conditions, has proven to be particularly effective in mining numerous novel secondary metabolites for the past few years. Among these, microbial cyclic peptides stand out. These peptides often comprise rare amino acids, unique chemical structures, and remarkable biological function. With the advancement of the OSMAC strategy, a plethora of new cyclic peptides have been identified from diverse microbial genera. This work reviews the progress in mining novel compounds using the OSMAC strategy and the applications of this strategy in discovering 284 microbial cyclic peptides from 63 endophytic strains, aiming to offer insights for the further explorations into novel active cyclic peptides.

Amoxetamide A, a new anoikis inducer, produced by combined-culture of Amycolatopsis sp. and Tsukamurella pulmonis

Cancer cells including colorectal cancer cells are resistant to anoikis, an anchorage-independent programmed death, which enables metastasis and subsequent survival in a new tumor microenvironment. In this study, we identified a new anoikis inducer, amoxetamide A (1) with a β-lactone moiety, that was produced by combined-culture of Amycolatopsis sp. 26-4 and mycolic acid-containing bacteria (MACB) Tsukamurella pulmonis TP-B0596. The structure of 1 including the stereochemistry of C8 was determined by MS and NMR spectroscopy and modified Mosher's method, and the absolute configurations of C11 and C12 were suggested as 11R and 12S, respectively, by GIAO NMR calculations. Amoxetamide A (1) exhibited anoikis-inducing activity in human colorectal cancer HT-29 cells in anchorage-independent culture conditions.

Genomic Insights into Paucibacter aquatile DH15, a Cyanobactericidal Bacterium, and Comparative Genomics of the Genus Paucibacter

Microcystis blooms threaten ecosystem function and cause substantial economic losses. Microorganism-based methods, mainly using cyanobactericidal bacteria, are considered one of the most ecologically sound methods to control Microcystis blooms. This study focused on gaining genomic insights into Paucibacter aquatile DH15 that exhibited excellent cyanobactericidal effects against Microcystis. Additionally, a pan-genome analysis of the genus Paucibacter was conducted to enhance our understanding of the ecophysiological significance of this genus. Based on phylogenomic analyses, strain DH15 was classified as a member of the species Paucibacter aquatile. The genome analysis supported that strain DH15 can effectively destroy Microcystis, possibly due to the specific genes involved in the flagellar synthesis, cell wall degradation, and the production of cyanobactericidal compounds. The pan-genome analysis revealed the diversity and adaptability of the genus Paucibacter, highlighting its potential to absorb external genetic elements. Paucibacter species were anticipated to play a vital role in the ecosystem by potentially providing essential nutrients, such as vitamins B7, B12, and heme, to auxotrophic microbial groups. Overall, our findings contribute to understanding the molecular mechanisms underlying the action of cyanobactericidal bacteria against Microcystis and shed light on the ecological significance of the genus Paucibacter.

Total syntheses of surugamides and thioamycolamides toward understanding their biosynthesis

Peptidic natural products have received much attention as potential drug leads, and biosynthetic studies of peptidic natural products have contributed to the field of natural product chemistry over the past several decades. However, the key biosynthetic intermediates are generally not isolated from natural sources, and this can hamper a detailed analysis of biosynthesis. Furthermore, reported unusual structures, which are targets for biosynthetic studies, are sometimes the results of structural misassignments. Chemical synthesis techniques are imperative in solving these problems. This review focuses on the chemical syntheses of surugamides and thioamycolamides toward understanding their biosynthesis. These studies can provide the key biosynthetic intermediates that can reveal the biosynthetic pathways and/or true structures of these natural products.

Solid-phase total synthesis and structural confirmation of antimicrobial longicatenamide A

Longicatenamides A–D are cyclic hexapeptides isolated from the combined culture of Streptomyces sp. KUSC_F05 and Tsukamurella pulmonis TP-B0596. Because these peptides are not detected in the monoculture broth of the actinomycete, they are key tools for understanding chemical communication in the microbial world. Herein, we report the solid-phase total synthesis and structural confirmation of longicatenamide A. First, commercially unavailable building blocks were chemically synthesized with stereocontrol. Second, the peptide chain was elongated via Fmoc-based solid-phase peptide synthesis. Third, the peptide chain was cyclized in the solution phase, followed by simultaneous cleavage of all protecting groups to afford longicatenamide A. Chromatographic analysis corroborated the chemical structure of longicatenamide A. Furthermore, the antimicrobial activity of synthesized longicatenamide A was confirmed. The developed solid-phase synthesis is expected to facilitate the rapid synthesis of diverse synthetic analogues.

Secondary Metabolites Produced by Coculture of Pleurotus ostreatus SY10 and Pleurotus eryngii SY302

A new isovaleryl-monoterpene, pleurotusin A (1), and a new cyclopentenone derivative, pleurotusin B (7), together with five related known terpenoids (2-6), were isolated from the coculture broth of two edible fungi, Pleurotus ostreatus SY10 and Pleurotus eryngii SY302. The absolute configurations of 1 and 7 were elucidated by comprehensively using the density functional theory calculation of NMR and ECD data, DP4+ probability analysis, Mo2(OAc)4-based CD experiment and optical rotation. The antimicrobial activities of these compounds except for the unstable compound 7, were evaluated against two types of human-pathogenic fungi, Candida albicans and Cryptococcus neoformans, and three types of human-pathogenic bacteria, Staphylococcus aureus, Escherichia coli,  and Shigella sp. Compound 1 displayed moderate activity against S. aureus with an MIC50 value of 90.3 μM. In addition, the antioxidant activities of high-yielding 2-6 were tested using DPPH, and compound 4 showed moderate activity with an EC50 value of 573 μM.

NMR characterization of streptogramin B and L-156,587, a non-synergistic pair of the streptogramin family antibiotic complexes produced inductively by a combined culture of Streptomyces albogriseolus and Tsukamurella pulmonis

The complete ¹ H and ¹³ C NMR characterization of streptogramin B (1), the major component of a clinically important synergistic antibiotic complex, was presented for the first time, along with those of L-156,587 (2), a dehydrated congener of streptogramin A (3). Compounds 1 and 2 were not synergistic and produced by Streptomyces albogriseolus in co-culture with Tsukamurella pulmonis, which poses a question on the adaptive significance of the induced production of this antibiotic pair.

Activation of Secondary Metabolism in Red Soil-Derived Streptomycetes via Co-Culture with Mycolic Acid-Containing Bacteria

Our previous research has demonstrated a promising capacity of streptomycetes isolated from red soils to produce novel secondary metabolites, most of which, however, remain to be explored. Co-culturing with mycolic acid-containing bacteria (MACB) has been used successfully in activating the secondary metabolism in Streptomyces. Here, we co-cultured 44 strains of red soil-derived streptomycetes with four MACB of different species in a pairwise manner and analyzed the secondary metabolites. The results revealed that each of the MACB strains induced changes in the metabolite profiles of 35–40 streptomycetes tested, of which 12–14 streptomycetes produced “new” metabolites that were not detected in the pure cultures. Moreover, some of the co-cultures showed additional or enhanced antimicrobial activity compared to the pure cultures, indicating that co-culture may activate the production of bioactive compounds. From the co-culture-induced metabolites, we identified 49 putative new compounds. Taking the co-culture of Streptomyces sp. FXJ1.264 and Mycobacterium sp. HX09-1 as a case, we further explored the underlying mechanism of co-culture activation and found that it most likely relied on direct physical contact between the two living bacteria. Overall, our results verify co-culture with MACB as an effective approach to discover novel natural products from red soil-derived streptomycetes.

Identification and Total Synthesis of an Unstable Anticancer Macrolide Presaccharothriolide Z Produced by Saccharothrix sp. A1506

Saccharothriolides A-F are 10-membered microbial macrolides proposed to be generated from their precursors presaccharothriolides X-Z. Previously, we isolated presaccharothriolide X, and its unique natural prodrug-like properties have intrigued us. However, the other congeners were not detected. Herein, we detected presaccharothriolide Z using our highly sensitive labeling reagent. Moreover, chemical synthesis of presaccharothriolide Z, the first total synthesis of saccharothriolide-class macrolides, was achieved, and the structure and biological activity of presaccharothriolide Z were determined.

A bioprocess perspective on the production of secondary metabolites by Streptomyces in submerged co-cultures

Filamentous microorganisms are potent sources of bioactive secondary metabolites, the molecules formed in response to complex environmental signals. The chemical diversity encoded in microbial genomes is only partially revealed by following the standard microbiological approaches. Mimicking the natural stimuli through laboratory co-cultivation is one of the most effective methods of awakening the formation of high-value metabolic products. Whereas the biosynthetic outcomes of co-cultures are reviewed extensively, the bioprocess aspects of such efforts are often overlooked. The aim of the present review is to discuss the submerged co-cultivation strategies used for triggering and enhancing secondary metabolites production in Streptomyces, a heavily investigated bacterial genus exhibiting an impressive repertoire of secondary metabolites, including a vast array of antibiotics. The previously published studies on influencing the biosynthetic capabilities of Streptomyces through co-cultivation are comparatively analyzed in the bioprocess perspective, mainly with the focus on the approaches of co-culture initiation, the experimental setup, the design of experimental controls and the ways of influencing the outcomes of co-cultivation processes. These topics are discussed in the general context of secondary metabolites production in submerged microbial co-cultures by referring to the Streptomyces-related studies as illustrative examples.