Polycyclic tetrahydroxanthones from Streptomyces chrestomyceticus BCC 24770

Antioxidant and Antibacterial Properties of 1,3-Dihydroxy-,2', 2'-Dimethylpyrano-(5,6)-Xanthone from Streptomyces sp. SU84

<b>Background and Objective:</b> The SU84 was isolated from the rhizosphere of <i>Curcuma longa</i> and identified to be <i>Streptomyces</i> sp. via analysis of its 16S rDNA sequence, chemotaxonomy and morphology. This study aimed to isolate major compounds from the extract culture of strain SU84 and evaluate their antibacterial activity. <b>Materials and Methods:</b> The TLC and silica gel column chromatography were used to purify major compounds, elucidate 1,3-dihydroxy-,2',2'-dimethylpyrano-(5,6)-xanthone (compound <b>1</b>) and lupeol (compound <b>2</b>) using mass spectrometry and nuclear magnetic resonance. One new chemical, compound <b>1</b>, was first isolated from microbial sources. Antibacterial, antioxidant and cytotoxic properties of these compounds were carried out. <b>Results:</b> Various bioassays showed that compound <b>1</b> displayed antibacterial property against Gram-positive bacteria, with a minimum inhibitory concentration of 8-32 μg/mL and minimum bactericidal concentration of 32-128 μg/mL. In addition, the purified compounds were tested against normal cell lines using tetrazolium assay. The results did not show cytotoxic property against L929 and Vero cells, with IC₅₀ values of >512.00 μg/mL. Compounds <b>1</b> and <b>2</b> have also antioxidant properties, with IC₅₀ values of 16.67±7.48 and 38.86±8.45 μg/mL, respectively. <b>Conclusion:</b> The findings suggested that compounds of <i>Streptomyces</i> sp. SU84 displayed antibacterial and antioxidant properties without cytotoxic activity. Extensive studies of compound <b>1</b> may be useful for the advancement of improved methods for avoidance, control and management of bacterial infections and metabolic-related free radical contribution.

Metabolic and genomic analysis deciphering biocontrol potential of endophytic Streptomyces albus RC2 against crop pathogenic fungi

Plant diseases caused by phytopathogenic fungi are one of the leading factors affecting crop loss. In the present study, sixty-one Streptomyces strains were screened for their antifungal activity against relevant wide range fungal pathogens prominent in Vietnam, namely Lasiodiplodia theobromae, Fusarium fujikuroi, and Scopulariopsis gossypii. Endophytic strain RC2 was the most effective strain in the mycelial inhibition of the tested fungi. Based on phenotypic characteristics, 16S rDNA gene analysis, and genomic analysis, strain RC2 belonged to Streptomyces albus. An ethyl acetate extract of S. albus RC2 led to the strong growth inhibition of S. gossypii Co1 and F. fujikuroi L3, but not L. theobromae N13. The crude extract also suppressed the spore germination of S. gossypii Co1 and F. fujikuroi L3 to 92.4 ± 3.2% and 87.4% ± 1.9%, respectively. In addition, the RC2 extract displayed potent and broad-spectrum antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, and the phytopathogenic bacteria Ralstonia solanacearum and Xanthomonas oryzae. The genome of strain RC2 was sequenced and revealed the presence of 15 biosynthetic gene clusters (BGCs) with similarities ≥ 45% to reference BGCs available in the antiSMASH database. The UPLC-HRMS analysis led to the identification of 8 other secondary metabolites, which have not been reported in S. albus. The present study indicated that RC2 could be a potent biocontrol agent against phytopathogenic fungi. Further attention should be paid to antifungal metabolites without functional annotation, development of product prototypes, and greenhouse experiments to demonstrate effective control of the plant diseases.

Mode of action of antimicrobial agents albofungins in eradicating penicillin- and cephalosporin-resistant Vibrio parahaemolyticus biofilm

Albofungin is a promising broad-spectrum antimicrobial compound against multidrug-resistant bacteria. In the present study, we further investigated albofungin's biofilm eradication activity and its potential mode of action against drug-resistant Vibrio parahaemolyticus. Among all derivatives, albofungin exhibited the best antibiofilm and antibacterial activity with rapid killing effects at 0.12 µg mL-1. Confocal microscopy observation exhibited that albofungin disrupted V. parahaemolyticus biofilms by killing or dispersing biofilm cells. Meanwhile, scanning electron microscope and fluorescent staining experiments demonstrated that albofungin rapidly destroyed the integrity and permeability of the bacterial cell membrane. Moreover, this study revealed an antibiofilm mechanism of albofungin involving inhibition of peptidoglycan biosynthesis, flagella assembly pathways, and secretion system proteins in V. parahaemolyticus by quantitative proteomics and validation experiments. Our results highlighted albofungin's mechanism of action in planktonic cells and biofilms and suggested further development and potential applications of albofungin for treating infections caused by penicillins-and-cephalosporins-resistant V. parahaemolyticus. IMPORTANCE Infections caused by multidrug-resistant bacteria, as well as a scarcity of new antibiotics, have become a major health threat worldwide. To tackle the demand for new and effective treatments, we investigated the mechanism of action of albofungin, a natural product derived from Streptomyces, which exhibits potent antimicrobial activity against multidrug-resistant bacteria. Albofungin showed potent biofilm eradication activity against penicillins-and-cephalosporins-resistant Vibrio parahaemolyticus, which expresses a novel metallo-β-lactamase and, thus, reduces their sensitivity to various antibiotics. We observed membrane disruption and permeation mechanisms in planktonic cells and biofilms after albofungin treatment, while albofungin had a weak interaction with bacterial DNA. Moreover, the antibiofilm mechanism of albofungin included inhibition of peptidoglycan biosynthesis, flagellar assembly pathways, and secretion system proteins. Our finding suggested potential applications of albofungin as an antibacterial and antibiofilm therapeutic agent.

Antifungal Metabolites of Streptomyces chrestomyceticus STR-2 Inhibits Magnaporthe oryzae, the Incitant of Rice Blast

Rice, a staple food crop worldwide, suffers devastating yield losses as a result of blast disease caused by Magnaporthe oryzae Cav. The adverse effects of chemicals on the environment are rising concerns for sustainable and eco-friendly approaches. The use of antagonistic microbes for the management of rice blast appears to be a sustainable solution to this challenge. Herein, we isolated 20 Streptomyces strains from rice rhizosphere, among which the isolate STR-2 exhibited maximum inhibition of mycelial growth of M. oryzae accounting for 50% reduction over control. The isolate STR-2 was identified as S. chrestomyceticus through 16S rRNA gene sequencing. In vitro tests demonstrated its ability to produce antifungal and bioactive compounds and also synthesize siderophore, IAA, and phosphate-solubilizing agents, thereby promoting plant growth upon inoculation on rice seeds. GC-MS analysis showed the presence of volatiles, antifungal, antimicrobial, and antioxidant compounds with different retention times. The crude antibiotic extract of 0.5% of S. chrestomyceticus STR-2 reduced the mycelial growth of M. oryzae over the control. Application of talc-based formulation of Streptomyces chrestomyceticus STR-2 resulted in the least disease incidence (15.89%) with the highest disease reduction of 65.26% over untreated control under field condition. These findings indicate the potential of S. chrestomyceticus as a potential bio-inoculant against rice blast disease.

Chemistry and biosynthesis of bacterial polycyclic xanthone natural products

Covering: up to the end of 2021Bacterial polycyclic xanthone natural products (BPXNPs) are a growing family of natural xanthones featuring a pentangular architecture with various modifications to the tricyclic xanthone chromophore. Their structural diversities and various activities have fueled biosynthetic and chemical synthetic studies. Moreover, their more potent activities than the clinically used drugs make them potential candidates for the treatment of diseases. Future unraveling of structure activity relationships (SARs) will provide new options for the (bio)-synthesis of drug analogues with higher activities. This review summarizes the isolation, structural elucidation and biological activities and more importantly, the recent strategies for the microbial biosynthesis and chemical synthesis of BPXNPs. Regarding their biosynthesis, we discuss the recent progress in enzymes that synthesize tricyclic xanthone, the protein candidates for structural moieties (methylene dioxygen bridge and nitrogen heterocycle), tailoring enzymes for methylation and halogenation. The chemical synthesis part summarizes the recent methodology for the division synthesis and coupling construction of achiral molecular skeletons. Ultimately, perspectives on the biosynthetic study of BPXNPs are discussed.

Genetic and Biochemical Characterization of Halogenation and Drug Transportation Genes Encoded in the Albofungin Biosynthetic Gene Cluster

Albofungin, a hexacyclic aromatic natural product, exhibits broad-spectrum antimicrobial activity. Its biosynthesis, regulation, and resistance remain elusive. Here, we report the albofungin (abf) biosynthetic gene cluster (BGC) from its producing strain Streptomyces tumemacerans JCM5050. The nascent abf BGC encodes 70 putative genes, including regulators, transporters, type II polyketide synthases (PKSs), oxidoreductase, and tailoring enzymes. To validate the intactness and functionality of the BGC, we developed an Escherichia coli-Streptomyces shuttle bacterial artificial chromosome system, whereby the abf BGC was integrated into the genome of a nonproducing host via heterologous conjugation, wherefrom albofungin can be produced, confirming that the BGC is in effect. We then delimited the boundaries of the BGC by means of in vitro CRISPR-Cas9 DNA editing, concluding a minimal but essential 60-kb abf BGC ranging from orfL to abf58. The orfA gene encoding a reduced flavin adenine dinucleotide (FADH₂)-dependent halogenase was examined and is capable of transforming albofungin to halogen-substituted congeners in vivo and in vitro. The orfL gene encoding a transporter was examined in vivo. The presence/absence of orfA or orfL demonstrated that the MIC of albofungin is subject to alteration when an extracellular polysaccharide intercellular adhesin was formed. Despite that halogenation of albofungin somewhat increases binding affinity to transglycosylase (TGase), albofungin with/without a halogen substituent manifests similar in vitro antimicrobial activity. Halogenation, however, limits overall dissemination and effectiveness given a high secretion rate, weak membrane permeability, and high hydrophobicity of the resulting products, whereby the functions of orfA and orfL are correlated with drug detoxification/resistance for the first time.

IMPORTANCE Albofungin, a natural product produced from Streptomycetes, exhibits bioactivities against bacteria, fungi, and tumor cells. The biosynthetic logic, regulations, and resistance of albofungin remain yet to be addressed. Herein, the minimal albofungin (abf) biosynthetic gene cluster (BGC) from the producing strain Streptomyces tumemacerans JCM5050 was precisely delimited using the Escherichia coli-Streptomyces shuttle bacterial artificial chromosome system, of which the gene essentiality was established in vivo and in vitro. Next, we characterized two genes orfA and orfL encoded in the abf BGC, which act as a reduced flavin adenine dinucleotide (FADH₂)-dependent halogenase and an albofungin-congeners transporter, respectively. While each testing microorganism exhibited different sensitivities to albofungins, the MIC values of albofungins against testing strains with/without orfA and/or orfL were subject to considerable changes. Halogen-substituted albofungins mediated by OrfA manifested overall compromised dissemination and effectiveness, revealing for the first time that two functionally distinct proteins OrfA and OrfL are associated together, exerting a novel “belt and braces” mechanism in antimicrobial detoxification/resistance.

Discovery, Yield Improvement, and Application in Marine Coatings of Potent Antifouling Compounds Albofungins Targeting Multiple Fouling Organisms

Marine biofouling caused huge economic losses of maritime industries. We aim to develop high-efficient, less-toxic, and cost-effective antifoulants to solve the problems of biofouling. In this study, we described the antifouling compounds albofungin and its derivatives (albofungin A, chrestoxanthone A, and chloroalbofungin) isolated from the metabolites of bacterium Streptomyces chrestomyceticus BCC 24770, the construction of high-yield strains for albofungin production, and application of albofungin-based antifouling coatings. Results showed that these albofungins have potent antibiofilm activities against Gram-positive and Gram-negative bacteria and anti-macrofouling activities against larval settlement of major fouling organisms with low cytotoxicity. With the best antifouling activity and highest yield in bacterial culture, albofungin was subsequently incorporated with hydrolyzable and degradable copolymer to form antifouling coatings, which altered biofilm structures and prevented the settlement of macrofouling organisms in marine environments. Our results suggested that albofungins were promising antifouling compounds with potential application in marine environments.

Isolation, Biosynthesis, and Biological Activity of Polycyclic Xanthones From Actinomycetes

Natural products from actinomycetes serve as a crucial source of clinical pharmaceuticals, especially antibiotics and anticancer agents. Among them, polycyclic xanthones belong to a growing group of highly oxygenated aromatic polyketides with a xanthone-containing angular hexacyclic framework. These biosynthetically unique small molecules are of great interest due to their wide spectrum of biological activities, especially the remarkable antibacterial activity against gram-positive bacteria and the significant antineoplastic effects toward various cancer cells at nanomolar concentrations. Their complex structures and significant bioactivities have aroused considerable attention in the chemical and biological communities in recent decades. This review covers the isolation, the biosynthesis, and the biological studies toward these structurally complex and biologically active molecules.

Sattahipmycin, a Hexacyclic Xanthone Produced by a Marine-Derived Streptomyces

Sattahipmycin was isolated from the mycelium of marine-derived Streptomyces sp. GKU 257-1 by following the antibiofilm activity against E. coli NBRC 3972 throughout the purification steps. The structure of sattahipmycin was determined to be a new polycyclic xanthone related to xantholipin but lacking a dioxymethylene and a chlorinated carbon. This compound showed activity toward Gram-positive bacteria and Plasmodium falciparum, antibiofilm formation of Escherichia coli, and cytotoxicity to human cancer cell lines. Using genome sequence data, a biosynthetic pathway leading to sattahipmycin has been proposed involving an uncharacterized type II polyketide synthase biosynthetic gene cluster.

Albofungin and chloroalbofungin: antibiotic crystals with 2D but not 3D isostructurality

The potent antibiotics albofungin [systematic name: (1S,4R,8aR)-13-amino-1,15,16-trihydroxy-4-methoxy-12-methyl-3,4,8a,13-tetrahydro-1H-xantheno[4',3',2':4,5][1,3]benzodioxino[7,6-g]isoquinoline-14,17(2H,9H)-dione, C₂₇H₂₄N₂O₉, 1] and its chlorinated analogue chloroalbofungin (the 11-chloro analogue, C₂₇H₂₃ClN₂O₉, 2) have been crystallized following their isolation from the bacterial strain Streptomyces chrestomyceticus and their structures determined by single-crystal X-ray diffraction. The novel N-aminoquinolone molecular arrangement shows N-N bond lengths of 1.4202 (16) and 1.424 (2) Å in 1 and 2, respectively. The regiochemistry of chloro substitution in the A-ring is para to the quinolone O atom, with a C-Cl bond length of 1.741 (2) Å. The absolute stereochemistry at three chiral centres of the xanthone rings (i.e. 10S, 13R and 19R) is confirmed. Both compounds crystallize in chiral Sohncke space groups consistent with enantiopurity, but are not fully isostructural. A preserved supramolecular construct (SC) confers two-dimensional (2D) isostructurality, but the SC self-associates via either a twofold screw operation in 1, giving a monoclinic P2₁ structure, or a twofold rotation in 2, affording a monoclinic C2 structure with a doubled unit-cell axis.

A marine microbiome antifungal targets urgent-threat drug-resistant fungi

New antifungal drugs are urgently needed to address the emergence and transcontinental spread of fungal infectious diseases, such as pandrug-resistant Candida auris. Leveraging the microbiomes of marine animals and cutting-edge metabolomics and genomic tools, we identified encouraging lead antifungal molecules with in vivo efficacy. The most promising lead, turbinmicin, displays potent in vitro and mouse-model efficacy toward multiple-drug-resistant fungal pathogens, exhibits a wide safety index, and functions through a fungal-specific mode of action, targeting Sec14 of the vesicular trafficking pathway. The efficacy, safety, and mode of action distinct from other antifungal drugs make turbinmicin a highly promising antifungal drug lead to help address devastating global fungal pathogens such as C. auris.

Control of the rubber anthracnose fungus Colletotrichum gloeosporioides using culture filtrate extract from Streptomyces deccanensis QY-3

Colletotrichum gloeosporioides is a main cause of rubber anthracnose, which results in a huge loss for the natural rubber industry. In this study, an actinomycete strain QY-3 was isolated and had good antagonistic activity against C. gloeosporioides with an inhibition rate of 86.6%. Strain QY-3 was identified as Streptomyces deccanensis preliminarily. Millet medium was selected as the optimal fermentation broth for antifungal metabolites production by S. deccanensis QY-3. The culture filtrate extract (CFE) from the millet broth of S. deccanensis QY-3 exhibits broad-spectrum antifungal activity against plant pathogenic fungi, and its EC₅₀ inhibiting the mycelial growth of C. gloeosporioides is 6.3 μg/mL. The CFE has good thermal and pH stabilities, and it can break the hyphae and inhibit the conidial germination of C. gloeosporioides. 100 μg/mL of CFE had an obvious control effect on rubber anthracnose, and the control efficacy was 63.7% on 5 days after inoculation. Two compounds with inhibitory effects on C. gloeosporioides, anthranilic acid and sangivamycin, were isolated from the CFE. The MICs of both compounds against C. gloeosporioides were 29.3 and 23.0 μg/mL respectively. In conclusion, the CFE from S. deccanensis QY-3 has great potential to be a promising fungicide for rubber anthracnose.

Dinactin from a new producer, Streptomyces badius gz-8, and its antifungal activity against the rubber anthracnose fungus Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is a main cause of rubber anthracnose, which results in very large losses for the natural rubber industry. In this study, an actinomycete strain gz-8 was isolated and had strong antagonistic activity against C. gloeosporioides, with an inhibition rate of 72.5 %. Strain gz-8 was identified as Streptomyces badius. Three active compounds were separated from S. badius gz-8 and identified as feigrisolide B, feigrisolide C and dinactin according to the mass spectrometry and NMR-spectra results. In the three compounds, dinactin exhibited the best antifungal activity against C. gloeosporioides, with an EC₅₀ value of 2.55 μg/mL, and its minimum inhibitory concentration was 44 μg/mL. Dinactin had broad inhibitory activities against nine other pathogenic fungi, and it also had an obvious control effect on rubber anthracnose comparable to that of chlorothalonil. Dinactin could inhibit the conidiogenesis and spore germination of C. gloeosporioides. This report will contribute to understanding the antifungal activity of dinactin against C. gloeosporioides.

A novel chresdihydrochalcone from Streptomyces chrestomyceticus exhibiting activity against Gram-positive bacteria

Microbial-derived natural products provide unique bioactivities and serve as a unique source of drug leads. In the present study, we isolated one new chresdihydrochalcone (1), one new chresphenylacetone (2), and one known streptimidone (3) from Streptomyces chrestomyceticus BCC 24770 using antibacterial activity-guided isolation and purification procedures. We determined their molecular weights using MS and HRMS and elucidated their chemical structures from their 1D and 2D NMR and electronic circular dichroism (ECD) spectra. Compound 1 showed moderate inhibitory activities against the Gram-positive bacteria Methicillin-resistant Staphylococcus aureus, Bacillus subtilis, and Micrococcus luteus. Cytotoxicity and hemolytic activity were not observed at a concentration of up to 100 μg ml^-1. The specific antimicrobial activity and low toxicity of compound 1 indicate this compound to be a potential antibiotic candidate, especially as antibiotic resistance has become a significant public health threat.

Cytotoxic and Antibacterial Cervinomycins B1-4 from a Streptomyces Species

AntiSMASH analysis of genome DNA of Streptomyces CPCC 204980, a soil isolate with potent antibacterial activity, revealed a gene cluster for polycyclic xanthones. A subsequent chemical study confirmed that the microorganism produced polycyclic xanthone cervinomycin A₂ (1) and the new congeners cervinomycins B_1-4 (2-5). The structures of 1-5 were determined by comprehensive analyses of MS and NMR data, which indicated that 2-5 featured a common dihydro-D ring in the polycyclic xanthone core moiety of their molecules. 2-5 are toxic to human cancer cells and active against Gram-positive bacteria.

MDN-0185, an Antiplasmodial Polycyclic Xanthone Isolated from Micromonospora sp. CA-256353

A potent antiplasmodial polycyclic xanthone, MDN-0185 (1), was isolated from an unidentified species of the genus Micromonospora. The planar structure of 1 was established as a seven-ring polycyclic xanthone with partial structures very similar to two known natural products, namely, xantholipin and Sch 54445. Using ROESY correlations, the relative stereochemistry of the two independent stereoclusters of compound 1 could be determined. Mosher analysis and comparison of the specific rotation of compound 1 with that of xantholipin allowed the determination of its absolute configuration. Compound 1 exhibited an IC₅₀ of 9 nM against Plasmodium falciparum 3D7 parasites.