Cyclic tripeptides from the halotolerant fungus Aspergillus sclerotiorum PT06-1

Hidden Treasure: Halophilic Fungi as a Repository of Bioactive Lead Compounds

The pressing demand for novel compounds to address contemporary health challenges has prompted researchers to venture into uncharted territory, including extreme ecosystems, in search of new natural pharmaceuticals. Fungi capable of tolerating extreme conditions, known as extremophilic fungi, have garnered attention for their ability to produce unique secondary metabolites crucial for defense and communication, some of which exhibit promising clinical significance. Among these, halophilic fungi thriving in high-salinity environments have particularly piqued interest for their production of bioactive molecules. This review highlights the recent discoveries regarding novel compounds from halotolerant fungal strains isolated from various saline habitats. From diverse fungal species including Aspergillus, Penicillium, Alternaria, Myrothecium, and Cladosporium, a plethora of intriguing molecules have been elucidated, showcasing diverse chemical structures and bioactivity. These compounds exhibit cytotoxicity against cancer cell lines such as A549, HL60, and K-562, antimicrobial activity against pathogens like Escherichia coli, Bacillus subtilis, and Candida albicans, as well as radical-scavenging properties. Notable examples include variecolorins, sclerotides, alternarosides, and chrysogesides, among others. Additionally, several compounds display unique structural motifs, such as spiro-anthronopyranoid diketopiperazines and pentacyclic triterpenoids. The results emphasize the significant promise of halotolerant fungi in providing bioactive compounds for pharmaceutical, agricultural, and biotechnological uses. However, despite their potential, halophilic fungi are still largely unexplored as sources of valuable compounds.

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.

Antimicrobial and Anti-inflammatory Cyclic Tetrapeptides from the Co-cultures of Two Marine-Derived Fungi

Violaceotides B-E (1-4), four new cyclic tetrapeptides, along with seven known compounds, were identified from the sponge-associated Aspergillus insulicola IMB18-072 co-cultivated with the marine-derived Alternaria angustiovoidea IMB20-805. Their structures were elucidated by extensive analysis of spectroscopic data, including HRESIMS, 1D and 2D NMR, and MS/MS data. The absolute configurations were determined by the advanced Marfey's method. Compounds 2, 3, and violaceotide A (5) displayed selective antimicrobial activities against the aquatic pathogenic bacteria Edwardsiella tarda and E. ictaluri. In addition, compounds 1-5 showed inhibitory activities against the LPS-induced expression of the inflammatory mediator IL-6 in RAW264.7 cells at a concentration of 10 μM.

Recent Advances in Polypeptide Antibiotics Derived from Marine Microorganisms

In the post-antibiotic era, the rapid development of antibiotic resistance and the shortage of available antibiotics are triggering a new health-care crisis. The discovery of novel and potent antibiotics to extend the antibiotic pipeline is urgent. Small-molecule antimicrobial peptides have a wide variety of antimicrobial spectra and multiple innovative antimicrobial mechanisms due to their rich structural diversity. Consequently, they have become a new research hotspot and are considered to be promising candidates for next-generation antibiotics. Therefore, we have compiled a collection of small-molecule antimicrobial peptides derived from marine microorganisms from the last fifteen years to show the recent advances in this field. We categorize these compounds into three classes-cyclic oligopeptides, cyclic depsipeptides, and cyclic lipopeptides-according to their structural features, and present their sources, structures, and antimicrobial spectrums, with a discussion of the structure activity relationships and mechanisms of action of some compounds.

Peptides from Marine-Derived Fungi: Chemistry and Biological Activities

Marine natural products are well-recognized as potential resources to fill the pipeline of drug leads to enter the pharmaceutical industry. In this circumstance, marine-derived fungi are one of the unique sources of bioactive secondary metabolites due to their capacity to produce diverse polyketides and peptides with unique structures and diverse biological activities. The present review covers the peptides from marine-derived fungi reported from the literature published from January 1991 to June 2023, and various scientific databases, including Elsevier, ACS publications, Taylor and Francis, Wiley Online Library, MDPI, Springer, Thieme, Bentham, ProQuest, and the Marine Pharmacology website, are used for a literature search. This review focuses on chemical characteristics, sources, and biological and pharmacological activities of 366 marine fungal peptides belonging to various classes, such as linear, cyclic, and depsipeptides. Among 30 marine-derived fungal genera, isolated from marine macro-organisms such as marine algae, sponges, coral, and mangrove plants, as well as deep sea sediments, species of Aspergillus were found to produce the highest number of peptides (174 peptides), followed by Penicillium (23 peptides), Acremonium (22 peptides), Eurotium (18 peptides), Trichoderma (18 peptides), Simplicillium (17 peptides), and Beauveria (12 peptides). The cytotoxic activity against a broad spectrum of human cancer cell lines was the predominant biological activity of the reported marine peptides (32%), whereas antibacterial, antifungal, antiviral, anti-inflammatory, and various enzyme inhibition activities ranged from 7% to 20%. In the first part of this review, the chemistry of marine peptides is discussed and followed by their biological activity.

Marine Cyclic Peptides: Antimicrobial Activity and Synthetic Strategies

Oceans are a rich source of structurally unique bioactive compounds from the perspective of potential therapeutic agents. Marine peptides are a particularly interesting group of secondary metabolites because of their chemistry and wide range of biological activities. Among them, cyclic peptides exhibit a broad spectrum of antimicrobial activities, including against bacteria, protozoa, fungi, and viruses. Moreover, there are several examples of marine cyclic peptides revealing interesting antimicrobial activities against numerous drug-resistant bacteria and fungi, making these compounds a very promising resource in the search for novel antimicrobial agents to revert multidrug-resistance. This review summarizes 174 marine cyclic peptides with antibacterial, antifungal, antiparasitic, or antiviral properties. These natural products were categorized according to their sources—sponges, mollusks, crustaceans, crabs, marine bacteria, and fungi—and chemical structure—cyclic peptides and depsipeptides. The antimicrobial activities, including against drug-resistant microorganisms, unusual structural characteristics, and hits more advanced in (pre)clinical studies, are highlighted. Nocathiacins I–III (91–93), unnarmicins A (114) and C (115), sclerotides A (160) and B (161), and plitidepsin (174) can be highlighted considering not only their high antimicrobial potency in vitro, but also for their promising in vivo results. Marine cyclic peptides are also interesting models for molecular modifications and/or total synthesis to obtain more potent compounds, with improved properties and in higher quantity. Solid-phase Fmoc- and Boc-protection chemistry is the major synthetic strategy to obtain marine cyclic peptides with antimicrobial properties, and key examples are presented guiding microbiologist and medicinal chemists to the discovery of new antimicrobial drug candidates from marine sources.

Cryptic Metabolites from Marine-Derived Microorganisms Using OSMAC and Epigenetic Approaches

Marine microorganisms have proven to be a source of new natural products with a wide spectrum of biological activities relevant in different industrial sectors. The ever-increasing number of sequenced microbial genomes has highlighted a discrepancy between the number of gene clusters potentially encoding the production of natural products and the actual number of chemically characterized metabolites for a given microorganism. Homologous and heterologous expression of these biosynthetic genes, which are often silent under experimental laboratory culture conditions, may lead to the discovery of new cryptic natural products of medical and biotechnological interest. Several new genetic and cultivation-based strategies have been developed to meet this challenge. The OSMAC approach (one strain-many compounds), based on modification of growth conditions, has proven to be a powerful strategy for the discovery of new cryptic natural products. As a direct extension of this approach, the addition of chemical elicitors or epigenetic modifiers have also been used to activate silent genes. This review looks at the structures and biological activities of new cryptic metabolites from marine-derived microorganisms obtained using the OSMAC approach, the addition of chemical elicitors, and enzymatic inhibitors and epigenetic modifiers. It covers works published up to June 2021.

Cyclic Peptides from the Soft Coral-Derived Fungus Aspergillus sclerotiorum SCSIO 41031

Three novel cyclic hexapeptides, sclerotides C-E (1-3), and a new lipodepsipeptide, scopularide I (4), together with a known cyclic hexapeptide sclerotide A (5), were isolated from fermented rice cultures of a soft coral-derived fungus: Aspergillus sclerotiorum SCSIO 41031. The structures of the new peptides were determined by 1D and 2D NMR spectroscopic analysis, Marfey's method, ESIMS/MS analysis, and single crystal X-ray diffraction analysis. Scopularide I (4) exhibited acetylcholinesterase inhibitory activity with an IC50 value of 15.6 μM, and weak cytotoxicity against the human nasopharyngeal carcinoma cell line HONE-EBV with IC50 value of 10.1 μM.

Harnessing the natural pool of polyketide and non-ribosomal peptide family: A route map towards novel drug development

Emergence of communicable and non-communicable diseases possess health challenge to millions of people worldwide and is a major threat to the economic and social development in the coming century. The occurrence of recent pandemic, SARS-CoV-2 caused by lethal severe acute respiratory syndrome coronavirus 2 is one such example. Rapid research and development of drugs for the treatment and management of these diseases has been an incredibly challenging task for the pharmaceutical industry. Although, substantial focus has been made in the discovery of therapeutic compounds from natural sources having significant medicinal potential, their synthesis has shown a slow progress. Hence, the discovery of new targets by the application of the latest biotechnological and synthetic biology approaches is very much the need of the hour. Polyketides (PKs) and non-ribosomal peptides (NRPs) found in bacteria, fungi and plants are a large diverse family of natural products synthesized by two classes of enzymes: polyketide synthases (PKS) and non-ribosomal peptide synthetases (NRPS). These enzymes possess immense biomedical potential due to their simple architecture, catalytic capacity, as well as diversity. With the advent of latest in-silico and in-vitro strategies, these enzymes and their related metabolic pathways, if targeted, can contribute highly towards the biosynthesis of an array of potentially natural drug leads that have antagonist effects on biopolymers associated with various human diseases. In the face of the rising threat from the multidrug-resistant pathogens, this will further open new avenues for the discovery of novel and improved drugs by combining the natural and the synthetic approaches. This review discusses the relevance of polyketides and non-ribosomal peptides and the improvement strategies for the development of their derivatives and scaffolds, and how they will be beneficial to the future bioprospecting and drug discovery.

Bioactive Metabolites From Acid-Tolerant Fungi in a Thai Mangrove Sediment

Despite being potentially useful extremophile resources, there have been few reports on acid-tolerant fungi and their bioactive metabolites. Acidophilic/aciduric fungi (n = 237) were isolated from Thai mangrove sediments in an acidic medium. Using fungal identification technology (including morphologic observation, chemical screening, and sequence comparisons) all the isolates were identified and 41 representative isolates were selected for analysis of the phylogenetic relationships (ITS rDNA, β-tubulin, calmodulin, and actin gene sequences). There were seven genera identified – Penicillium; Aspergillus; Talaromyces; Cladosporium; Allophoma; Alternaria; and Trichoderma – in four taxonomic orders of the phylum Ascomycota, and Penicillium, Aspergillus, and Talaromyces were the dominant genera. Acidity tolerance was evaluated and 95% of the isolates could grow under extremely acidic conditions (pH 2). Six strains were classed as acidophilic fungi that cannot survive under pH 7, all of which had an extraordinarily close genetic relationship and belonged to the genus Talaromyces. This is the first report on the acidophilic characteristics of this genus. The antimicrobial, anti-tumor, and antiviral activities of the fermentation extracts were evaluated. Nearly three-quarters of the extracts showed cytotoxic activity, while less than a quarter showed antimicrobial or anti-H1N1 activity. The typical aciduric fungus Penicillium oxalicum OUCMDZ-5207 showed similar growth but completely different chemical diversity at pH 3 and 7. The metabolites of OUCMDZ-5207 that were obtained only at pH 3 were identified as tetrahydroauroglaucin (1), flavoglaucin (2), and auroglaucin (3), among which auroglaucin showed strong selective inhibition of A549 cells with an IC₅₀ value of 5.67 μM. These results suggest that acid stress can activate silent gene clusters to expand the diversity of secondary metabolites, and the bioprospecting of aciduric/acidophilic microorganism resources in Thai mangrove sediments may lead to the discovery of compounds with potential medicinal applications.

Structure elucidation of a novel cyclic tripeptide from the marine-derived fungus Aspergillus ochraceopetaliformis DSW-2

A novel cyclic tripeptide, sclerotiotide M (1), was isolated from the culture of a marine derived fungus Aspergillus ochraceopetaliformis DSW-2, together with four known compounds (2-5). The planar structure of 1 was established by 1 D and 2 D NMR data, supported by mass spectrometry, and the relative configuration was established by calculated NMR chemical shifts coupled with a statistical method (DP4+). All the compounds (1-5) displayed weak cytotoxic activities against cancer cell lines HPAC and BXPC3, with IC₅₀ values over 20 μM.

Antibacterial Cyclic Tripeptides from Antarctica-Sponge-Derived Fungus Aspergillus insulicola HDN151418

Three new aspochracin-type cyclic tripeptides, sclerotiotides M–O (1–3), together with three known analogues, sclerotiotide L (4), sclerotiotide F (5), and sclerotiotide B (6), were obtained from the ethyl acetate extract of the fungus Aspergillus insulicola HDN151418, which was isolated from an unidentified Antarctica sponge. Spectroscopic and chemical approaches were used to elucidate their structures. The absolute configuration of the side chain in compound 4 was elucidated for the first time. Compounds 1 and 2 showed broad antimicrobial activity against a panel of pathogenic strains, including Bacillus cereus, Proteus species, Mycobacterium phlei, Bacillus subtilis, Vibrio parahemolyticus, Edwardsiella tarda, MRCNS, and MRSA, with MIC values ranging from 1.56 to 25.0 µM.

Molecules derived from the extremes of life: a decade later

Covering: Early 2008 until the end of 2019Microorganisms which survive (extreme-tolerant) or even prefer (extremophilic) living at the limits of pH, temperature, salinity and pressure found on earth have proven to be a rich source of novel structures. In this update we summarise the wide variety of new molecules which have been isolated from extremophilic and extreme-tolerant microorganisms since our original 2009 review, highlighting the range of bioactivities these molecules have been reported to possess.

Antimicrobial lead compounds from marine plants

Marine environment is a home to a very wide diversity of flora and fauna, which includes an array of genetically diverse coastline and under seawater plant species, animal species, microbial species, their habitats, ecosystems, and supporting ecological processes. The Earth is home to an estimated 10 million species, of which a large chunk belongs to marine environment. Marine plants are a store house of a variety of antimicrobial compounds like classes of marine flavonoids—flavones and flavonols, terpenoids, alkaloids, peptides, carbohydrates, fatty acids, polyketides, polysaccharides, phenolic compounds, and steroids. Lot of research today is directed toward marine species, which have proved to be a potent source of structurally widely diverse and yet highly bioactive secondary metabolites. Varied species of phylum Porifera, algae including diatoms, Chlorophyta, Euglenophyta, Dinoflagellata, Chrysophyta, cyanobacteria, Rhodophyta, and Phaeophyta, bacteria, fungi, and weeds have been exploited by mankind for their inherent indigenous biological antimicrobial compounds, produced under the extreme stressful underwater conditions of temperature, atmospheric pressure, light, and nutrition. The present study aims at presenting a brief review of bioactive marine compounds possessing antimicrobial potency.

From genomics to metabolomics, moving toward an integrated strategy for the discovery of fungal secondary metabolites

Fungal secondary metabolites are defined by bioactive properties that ensure adaptation of the fungus to its environment. Although some of these natural products are promising sources of new lead compounds especially for the pharmaceutical industry, others pose risks to human and animal health. The identification of secondary metabolites is critical to assessing both the utility and risks of these compounds. Since fungi present biological specificities different from other microorganisms, this review covers the different strategies specifically used in fungal studies to perform this critical identification. Strategies focused on the direct detection of the secondary metabolites are firstly reported. Particularly, advances in high-throughput untargeted metabolomics have led to the generation of large datasets whose exploitation and interpretation generally require bioinformatics tools. Then, the genome-based methods used to study the entire fungal metabolic potential are reported. Transcriptomic and proteomic tools used in the discovery of fungal secondary metabolites are presented as links between genomic methods and metabolomic experiments. Finally, the influence of the culture environment on the synthesis of secondary metabolites by fungi is highlighted as a major factor to consider in research on fungal secondary metabolites. Through this review, we seek to emphasize that the discovery of natural products should integrate all of these valuable tools. Attention is also drawn to emerging technologies that will certainly revolutionize fungal research and to the use of computational tools that are necessary but whose results should be interpreted carefully.

New Butenolides and Cyclopentenones from Saline Soil-Derived Fungus Aspergillus Sclerotiorum

Three new γ-hydroxyl butenolides (1–3), a pair of new enantiomeric spiro-butenolides (4a and 4b), a pair of enantiomeric cyclopentenones (5a new and 5b new natural), and six known compounds (6–11), were isolated from Aspergillus sclerotiorum. Their structures were established by spectroscopic data and electronic circular dichroism (ECD) spectra. Two pairs of enantiomers [(+)/(–)-6c and (+)/(–)-6d] obtained from the reaction of 6 with acetyl chloride (AcCl) confirmed that 6 was a mixture of two pairs of enantiomers. In addition, the X-ray data confirmed that 7 was also a racemate. The new metabolites (1−5) were evaluated for their inhibitory activity against cancer and non-cancer cell lines. As a result, compound 1 exhibited moderate cytotoxicity to HL60 and A549 with IC₅₀ values of 6.5 and 8.9 µM, respectively, and weak potency to HL-7702 with IC₅₀ values of 17.6 µM. Furthermore, compounds 1−9 were screened for their antimicrobial activity using the micro-broth dilution method. MIC values of 200 μg/mL were obtained for compounds 2 and 3 towards Staphylococcus aureus and Escherichia coli, while compound 8 exhibited a MIC of 50 μ/mL towards Candida albicans.

Screening and identification of Aspergillus activity against Xanthomonas oryzae pv. oryzae and analysis of antimicrobial components

To screen for Aspergillus activity against Xanthomonas oryzae pv. oryzae and analyse the antimicrobial components involved, 60 Aspergillus spp. were isolated and purified from fruits, soil and other habitats. As-75, an Aspergillus strain that can antagonize Xanthomonas oryzae pv. oryzae, was identified based on the zone of inhibition formed during co-culture. According to morphological, ITS rDNA gene sequencing and phylogenetic tree results, the strain showed close homology to Aspergillus sclerotiorum. The biochemical characterization tests showed that the fermentation broth of strain As-75 exhibited a high capacity for environmental adaptation. The results of the antimicrobial spectrum experiments demonstrated that As-75 exhibited fairly strong antagonistic activity against five plant pathogenic fungi and six plant pathogenic bacteria in vitro. The fermentation broth of strain As-75 displayed maximum stability under fluorescent illumination at temperatures below 60°C at pH 6.5. A substance with antagonistic activity was obtained from strain As-75 via fractional extraction, silica gel column chromatography and thin-layer chromatography. Through mass spectrometry, nuclear magnetic resonance and electrospray ionization mass spectrometry (ESI-MS) analyses, the target compound was identified as (2Z)-2-butenedioic acid-2-(1-methylethenyl)-4-methyl ester; its molecular weight of 170.06 daltons and formula of C₈H₁₀O₄ identify it as a novel compound. Trials of the preventative and curative effects demonstrated that compound S1 exhibited a better control efficiency than the control against rice bacterial blight. Additionally, the M1 processing method was better, and the efficiency of compound S1 in preventing rice bacterial blight in six rice varieties, TN1, IR24, ZF802, Zhonghua 11, Wuyunjing 21, and Nipponbare, was 78.3%, 77.5%, 74.2%, 75.3%, 70.9%, and 72.1%, respectively.

Fungi-derived lipopeptide antibiotics developed since 2000

Lipopeptide antibiotics have linear or cyclic structures with one or more hydrocarbon tails linked to the N-terminus of a short oligopeptide that may be chemically modified and/or contain unusual amino acid residues in their structures. They possess huge potential as pharmaceutical drugs and biocontrol agents, and ˜30 representative genera of fungi are known to produce them. Some chemically synthesised derivatives have already been developed into commercial products or subjected to clinical trials, including cilofungin, caspofungin, micafungin, anidulafungin, rezafungin, emodepside, fusafungine and destruxins. This review summarizes 200 fungi-derived compounds reported since 2000, including 95 cyclic depsipeptides, 67 peptaibiotics (including 35 peptaibols, eight lipoaminopeptides, and five lipopeptaibols), and 38 non-depsipeptide and non-peptaibiotic lipopeptides. Their sources, structural sequences, antibiotic activities (e.g. antibacterial, antifungal, antiviral, antimycobacterial, antimycoplasmal, antimalarial, antileishmanial, insecticidal, antitrypanosomal and nematicidal), structure-activity relationships, mechanisms of action, and specific relevance are discussed. These compounds have attracted considerable interest within the pharmaceutical and agrochemical industries.

Recent Status and Advancements in the Development of Antifungal Agents: Highlights on Plant and Marine Based Antifungals

The developing resistance in fungi has become a key challenge, which is being faced nowadays with the available antifungal agents in the market. Further search for novel compounds from different sources has been explored to meet this problem. The current review describes and highlights recent advancement in the antifungal drug aspects from plant and marine based sources. The current available antifungal agents act on specific targets on the fungal cell wall, like ergosterol synthesis, chitin biosynthesis, sphingolipid synthesis, glucan synthesis etc. We discuss some of the important anti-fungal agents like azole, polyene and allylamine classes that inhibit the ergosterol biosynthesis. Echinocandins inhibit β-1, 3 glucan synthesis in the fungal cell wall. The antifungals poloxins and nikkomycins inhibit fungal cell wall component chitin. Apart from these classes of drugs, several combinatorial therapies have been carried out to treat diseases due to fungal resistance. Recently, many antifungal agents derived from plant and marine sources showed potent activity. The renewed interest in plant and marine derived compounds for the fungal diseases created a new way to treat these resistant strains which are evident from the numerous literature publications in the recent years. Moreover, the compounds derived from both plant and marine sources showed promising results against fungal diseases. Altogether, this review article discusses the current antifungal agents and highlights the plant and marine based compounds as a potential promising antifungal agents.

Natalenamides A⁻C, Cyclic Tripeptides from the Termite-Associated Actinomadura sp. RB99

In recent years, investigations into the biochemistry of insect-associated bacteria have increased. When combined with analytical dereplication processes, these studies provide a powerful strategy to identify structurally and/or biologically novel compounds. Non-ribosomally synthesized cyclic peptides have a broad bioactivity spectrum with high medicinal potential. Here, we report the discovery of three new cyclic tripeptides: natalenamides A–C (compounds 1–3). These compounds were identified from the culture broth of the fungus-growing termite-associated Actinomadura sp. RB99 using a liquid chromatography (LC)/ultraviolet (UV)/mass spectrometry (MS)-based dereplication method. Chemical structures of the new compounds (1–3) were established by analysis of comprehensive spectroscopic methods, including one-dimensional (¹H and ¹³C) and two-dimensional (¹H-¹H-COSY, HSQC, HMBC) nuclear magnetic resonance spectroscopy (NMR), together with high-resolution electrospray ionization mass spectrometry (HR-ESIMS) data. The absolute configurations of the new compounds were elucidated using Marfey’s analysis. Through several bioactivity tests for the tripeptides, we found that compound 3 exhibited significant inhibitory effects on 3-isobutyl-1-methylxanthine (IBMX)-induced melanin production. The effect of compound 3 was similar to that of kojic acid, a compound extensively used as a cosmetic material with a skin-whitening effect.

α-Glucosidase Inhibitors: Diphenyl Ethers and Phenolic Bisabolane Sesquiterpenoids from the Mangrove Endophytic Fungus Aspergillus flavus QQSG-3

Two new diphenyl ethers (1 and 2) and four new phenolic bisabolane sesquiterpenoids (3–6), together with five known related derivatives, were isolated from the culture of the endophytic fungus Aspergillus flavus QQSG-3 obtained from a fresh branch of Kandelia obobata, which was collected from Huizhou city in the province of Guangdong, China. The structures of compounds 1–6 were determined by analyzing NMR and HRESIMS data. The absolute configurations of 5 and 6 were assigned by comparing their experimental ECD spectra with those reported for similar compounds in the literature. All isolates were evaluated for their α-glucosidase inhibitory activity, of which compounds 3, 5, 10, and 11 showed strong inhibitory effects with IC₅₀ values in the range of 1.5–4.5 μM.

Polyketides From the Endophytic Fungus Cladosporium sp. Isolated From the Mangrove Plant Excoecaria agallocha

Five new polyketides (2–6) and ten known compounds (1 and 7–15) were obtained from the fermentation products of the endophytic fungus Cladosporium sp. OUCMDZ-302 with the mangrove plant, Excoecaria agallocha (Euphorbiaceae). The new structures of 2–6 were established on the basis of ECD, specific rotation and spectroscopic data as well as the chemical calculation. Compound 7 showed cytotoxicity against H1975 cell line with an IC₅₀ value of 10.0 μM. Compounds 4 and 8–10 showed radical scavenging activity against DPPH with the IC₅₀ values of 2.65, 0.24, 5.66, and 6.67 μM, respectively. In addition, the absolute configuration of compound 1 was solidly determined by X-ray and sugar analysis of the acidic hydrolysates for the first time as well as those of compounds 8–10 in this paper.

New Anti-inflammatory Cyclopeptides From a Sponge-Derived Fungus Aspergillus violaceofuscus

Three new cyclic peptides including a cyclic tetrapeptide (1), an aspochracin-type cyclic tripeptide sclerotiotide L (2) and a diketopiperazine dimer (3), have been isolated from the ethyl acetate extract of a marine sponge-derived fungus Aspergillus violaceofuscus. The structures of all compounds were unambiguously elucidated on the basis of HRESIMS, 1D and 2D NMR spectroscopic data, MS/MS experiments and chemical methods. Compounds 1 and 3 showed anti-inflammatory activity against IL-10 expression of the LPS-induced THP-1 cells with inhibitory rates of 84.3 and 78.1% respectively at concentration of 10 μM.

Novel Natural Products from Extremophilic Fungi

Extremophilic fungi have been found to develop unique defences to survive extremes of pressure, temperature, salinity, desiccation, and pH, leading to the biosynthesis of novel natural products with diverse biological activities. The present review focuses on new extremophilic fungal natural products published from 2005 to 2017, highlighting the chemical structures and their biological potential.

A critical review of producers of small lactone mycotoxins: patulin, penicillic acid and moniliformin

A very large number of filamentous fungi has been reported to produce the small lactone mycotoxins patulin, penicillic acid and moniliformin. Among the 167 reported fungal producers of patulin, only production by 29 species could be confirmed. Patulin is produced by 3 Aspergillus species, 3 Paecilomyces species, 22 Penicillium species from 7 sections of Penicillium, and one Xylaria species. Among 101 reported producers of penicillic acid, 48 species could produce this mycotoxin. Penicillic acid is produced by 23 species in section Aspergillus subgenus Circumdati section Circumdati, by Malbranchea aurantiaca and by 24 Penicillium species from 9 sections in Penicillium and one species that does not actually belong to Penicillium (P. megasporum). Among 40 reported producers of moniliformin, five species have been regarded as doubtful producers of this mycotoxin or are now regarded as taxonomic synonyms. Moniliformin is produced by 34 Fusarium species and one Penicillium species. All the accepted producers of patulin, penicillic acid and moniliformin were revised according to the new one fungus – one name nomenclatural system, and the most recently accepted taxonomy of the species.

Fatty acid derivatives from the halotolerant fungus Cladosporium cladosporioides

Halotolerant fungus Cladosporium cladosporioides OUCMDZ-187 was isolated from the mangrove plant Rhizophora stylosa collected in Shankou, Guangxi Province of China. Three new fatty acid esters cladosporesters A-C (1-3) and 5 new fatty acids cladosporacids A-E (4-8) were isolated from the ethyl acetate extract of the fermentation broth of OUCMDZ-187 in a hypersaline (10% salt) medium. Their structures were elucidated by UV, IR, MS, specific rotation, and 1D and 2D NMR data.

Structural Diversity and Biological Activities of Fungal Cyclic Peptides, Excluding Cyclodipeptides

Cyclic peptides are cyclic compounds formed mainly by the amide bonds between either proteinogenic or non-proteinogenic amino acids. This review highlights the occurrence, structures and biological activities of fungal cyclic peptides (excluding cyclodipeptides, and peptides containing ester bonds in the core ring) reported until August 2017. About 293 cyclic peptides belonging to the groups of cyclic tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, undeca-, dodeca-, tetradeca-, and octadecapeptides as well as cyclic peptides containing ether bonds in the core ring have been isolated from fungi. They were mainly isolated from the genera Aspergillus, Penicillium, Fusarium, Acremonium and Amanita. Some of them were screened to have antimicrobial, antiviral, cytotoxic, phytotoxic, insecticidal, nematicidal, immunosuppressive and enzyme-inhibitory activities to show their potential applications. Some fungal cyclic peptides such as the echinocandins, pneumocandins and cyclosporin A have been developed as pharmaceuticals.

Nonribosomal Peptides from Marine Microbes and Their Antimicrobial and Anticancer Potential

Marine environments are largely unexplored and can be a source of new molecules for the treatment of many diseases such as malaria, cancer, tuberculosis, HIV etc. The Marine environment is one of the untapped bioresource of getting pharmacologically active nonribosomal peptides (NRPs). Bioprospecting of marine microbes have achieved many remarkable milestones in pharmaceutics. Till date, more than 50% of drugs which are in clinical use belong to the nonribosomal peptide or mixed polyketide-nonribosomal peptide families of natural products isolated from marine bacteria, cyanobacteria and fungi. In recent years large numbers of nonribosomal have been discovered from marine microbes using multi-disciplinary approaches. The present review covers the NRPs discovered from marine microbes and their pharmacological potential along with role of genomics, proteomics and bioinformatics in discovery and development of nonribosomal peptides drugs.

Photo-induced synthesis and in vitro antitumor activity of Fenestin A analogs

Two bioactive Fenestin A analogs were synthesized by photo-induced cyclization.

α-Glucosidase Inhibitors from the Marine-Derived Fungus Aspergillus flavipes HN4-13

Three new butenolide derivatives, flavipesolides A-C (1-3), along with 13 known compounds (4-13, aspulvinone Q, monochlorosulochrin, and dihydrogeodin), were isolated from the marine-derived Aspergillus flavipes HN4-13 from a Lianyungang coastal sediment sample. The structures were elucidated by spectroscopic evidence. Compounds 4-6 and 9 were noncompetitive α-glucosidase inhibitors with K_i/IC₅₀ values of 0.43/34, 2.1/37, 0.79/19, and 2.8/90 μM, respectively. Compounds 1-3, 8, 10, and 13 are mixed α-glucosidase inhibitors with K_i/IC₅₀ values of (2.5, 19)/44, (3.4, 14)/57, (9.2, 4.7)/95, (6.3, 5.5)/55, (1.4, 0.60)/9.9, and (2.5, 7.2)/33 μM, respectively (IC₅₀ 101 μM for acarbose and 79 μM for 1-deoxynojirimycin).

Antifungal potential of marine natural products

Fungal diseases represent an increasing threat to human health worldwide which in some cases might be associated with substantial morbidity and mortality. However, only few antifungal drugs are currently available for the treatment of life-threatening fungal infections. Furthermore, plant diseases caused by fungal pathogens represent a worldwide economic problem for the agriculture industry. The marine environment continues to provide structurally diverse and biologically active secondary metabolites, several of which have inspired the development of new classes of therapeutic agents. Among these secondary metabolites, several compounds with noteworthy antifungal activities have been isolated from marine microorganisms, invertebrates, and algae. During the last fifteen years, around 65% of marine natural products possessing antifungal activities have been isolated from sponges and bacteria. This review gives an overview of natural products from diverse marine organisms that have shown in vitro and/or in vivo potential as antifungal agents, with their mechanism of action whenever applicable. The natural products literature is covered from January 2000 until June 2015, and we are reporting the chemical structures together with their biological activities, as well as the isolation source.

Anti-Candida albicans natural products, sources of new antifungal drugs: A review

INTRODUCTION

Candida albicans is the most prevalent fungal pathogen in humans. Due to the development of drug resistance, there is today a need for new antifungal agents for the efficient management of C. albicans infections. Therefore, we reviewed antifungal activity, mechanisms of action, possible synergism with antifungal drugs of all natural substances experimented to be efficient against C. albicans for future.

METHODS

An extensive and systematic review of the literature was undertaken and all relevant abstracts and full-text articles analyzed and included in the review.

REVIEW

A total of 111 documents were published and highlighted 142 anti-C. albicans natural products. These products are mostly are reported in Asia (44.37%) and America (28.17%). According to in vitro model criteria, from the 142 natural substances, antifungal activity can be considered as important for 40 (28.20%) and moderate for 24 (16.90%). Sixteen products have their antifungal activity confirmed by in vivo gold standard experimentation. Microbial natural products, source of antifungals, have their antifungal mechanism well described in the literature: interaction with ergosterol (polyenes), inhibition 1,3-β-d-glucan synthase (Echinocandins), inhibition of the synthesis of cell wall components (chitin and mannoproteins), inhibition of sphingolipid synthesis (serine palmitoyltransferase, ceramide synthase, inositol phosphoceramide synthase) and inhibition of protein synthesis (sordarins). Natural products from plants mostly exert their antifungal effects by membrane-active mechanism. Some substances from arthropods are also explored to act on the fungal membrane. Interestingly, synergistic effects were found between different classes of natural products as well as between natural products and azoles.

CONCLUSION

Search for anti-C. albicans new drugs is promising since the list of natural substances, which disclose activity against this yeast is today long. Investigations must be pursued not only to found more new anti-Candida compounds from plants and organisms but also to carried out details on molecules from already known anti-Candida compounds and to more elucidate mechanisms of action.

Anthranilic acid-containing cyclic tetrapeptides: at the crossroads of conformational rigidity and synthetic accessibility

Each amino acid in a peptide contributes three atom units to main-chains, hence natural cyclic peptides can be 9, 12, 15, …. i.e. 3n membered-rings, where n is the number of amino acids. Cyclic peptides that are 9 or 12-membered ring compounds tend to be hard to prepare because of strain, while their one amino acid homologs (15-membered cyclic pentapeptides) are not conformationally homogeneous unless constrained by strategically placed proline or d-amino acid residues. We hypothesized that replacing one genetically encoded amino acid in a cyclic tetrapeptide with a rigid β-amino acid would render peptidomimetic designs that rest at a useful crossroads between synthetic accessibility and conformational rigidity. Thus this research explored non-proline containing 13-membered ring peptides 1 featuring one anthranilic acid (Anth) residue. Twelve cyclic peptides of this type were prepared, and in doing so the viability of both solution- and solid-phase methods was demonstrated. The library produced contained a complete set of four diastereoisomers of the sequence 1aaf (i.e. cyclo-AlaAlaPheAnth). Without exception, these four diastereoisomers each adopted one predominant conformation in solution; basically these conformations feature amide N-H vectors puckering above and below the equatorial plane, and approximately oriented their N-H[combining low line] atoms towards the polar axis. Moreover, the shapes of these conformers varied in a logical and predictable way (NOE, temperature coefficient, D/H exchange, circular dichroism). Comparisons were made of the side-chain orientations presented by compounds 1aaa in solution with ideal secondary structures and protein-protein interaction interfaces. Various 1aaa stereoisomers in solution present side-chains in similar orientations to regular and inverse γ-turns, and to the most common β-turns (types I and II). Consistent with this, compounds 1aaa have a tendency to mimic various turns and bends at protein-protein interfaces. Finally, proteolytic- and hydrolytic stabilities of the compounds at different pHs indicate they are robust relative to related linear peptides, and rates of permeability through an artificial membrane indicate their structures are conducive to cell permeability.

Gymnopeptides A and B, Cyclic Octadecapeptides from the Mushroom Gymnopus fusipes

Mycochemical study of the mushroom Gymnopus fusipes led to the discovery of two new cyclopeptides. The two compounds, named as gymnopeptides A and B, are unprecedented highly N-methylated cyclic octadecapeptides. Detailed spectroscopic studies, Marfey's analysis, and a preliminary molecular modeling study suggested that both are natural cyclic β hairpins. The isolated compounds exhibited striking antiproliferative activity on several human cancer cell lines, with nanomolar IC50 values.

New α-glucosidase inhibitors from a marine sponge-derived fungus, Aspergillus sp. OUCMDZ-1583

18 new compounds named aspergones A–Q and 6-O-demethylmonocerin, along with five known analogues, were isolated from the fermentation broth of Aspergillus sp. OUCMDZ-1583 associated with an unidentified marine sponge from the Xisha Islands of China.

New insights into the echinocandins and other fungal non-ribosomal peptides and peptaibiotics

Non-ribosomal peptide synthetases (NRPSs) are a primary modality for fungal peptidic natural product assembly and are responsible for some of the best known, most useful, and most destructive fungal metabolites. Through genome sequencing and computer-assisted recognition of modular motifs of catalytic domains, one can now confidently identify most NRPS biosynthetic genes of a fungal strain. The biosynthetic gene clusters responsible for two of the most important classes of NRP fungal derived drugs, cyclosporine and the echinocandins, have been recently characterized by genomic sequencing and annotation. Complete biosynthetic gene clusters for the pneumocandins and echinocandins have been mapped at the genetic level and functionally characterized to some extent. Genomic sequencing of representative strains of most of the variants in the echinocandin family, including the wild-type of the three fungal strains employed for industrial-scale production of caspofungin, micafungin and anidulofungin, has enabled characterization of the basic architecture of the echinocandin NRPS pathways. A comparative analysis of how pathway genes cause variations in lipoinitiation, biosynthesis of the non-proteinogenic amino acids, amino acid substitutions, and hydroxylations and sulfonations of the core peptide and contribute to the molecular diversity of the family is presented. We also review new information on the natural functions of NRPs, the differences between fungal and bacterial NRPSs, and functional characterization of selected NRPS gene clusters. Continuing discovery of the new fungal nonribosomal peptides has contributed new structural diversity and potential insights into their biological functions among other natural peptides and peptaibiotics. We therefore provide an update on new peptides, depsipeptides and peptaibols discovered in the Fungi since 2009.

Thiodiketopiperazines from the marine-derived fungus Phoma sp. OUCMDZ-1847

Three new thiodiketopiperazines, named phomazines A-C (1-3), along with 10 known analogues (4-13), were isolated from the fermentation broth of an endophytic fungus, Phoma sp. OUCMDZ-1847, associated with the mangrove plant Kandelia candel. The structures including the absolute configurations of the new compounds were unambiguously elucidated by spectroscopic, X-ray crystallographic, and Mosher's methods along with quantum ECD and (13)C NMR calculations. Compounds 2, 4, 5, 11, and 12 showed cytotoxicities against the HL-60, HCT-116, K562, MGC-803, and A549 cell lines with IC50 values in the range 0.05 to 8.5 μM.

Pestalafuranones F-J, five new furanone analogues from the endophytic fungus Nigrospora sp. BM-2

Five new 2(5H)-furanone-type derivatives, pestalafuranones F-J (compounds 3-7), together with two known compounds, pestalafuranones A (1) and B (2), were isolated from the ethyl acetate extract from the fermentation broth of the endophytic fungus Nigrospora sp. BM-2 in a hypersaline medium. The structures of these metabolites were elucidated by EIMS, HREIMS and NMR spectroscopic data. Compounds 1-7 exhibited no cytotoxic activities against the MDA-MB-231 and Caski cancer cell lines.

Anabaenolysins, novel cytolytic lipopeptides from benthic Anabaena cyanobacteria

Two novel cyclic lipopeptides, anabaenolysin A and anabaenolysin B, were isolated from two benthic cyanobacterial strains of the genus Anabaena. This novel class of cyanobacterial lipopeptides has a general structure of a small peptide ring consisting of four amino acids from which two are proteinogenic and two unusual; glycine¹, glycine², 2-(3-amino-5-oxytetrahydrofuran-2-yl)-2-hydroxyacetic acid³ and a long unsaturated C₁₈ β-amino acid⁴ with a conjugated triene structure. They are distinguished by the presence of a conjugated dienic structure in the C18 β-amino acid present in anabaenolysin A but not in anabaenolysin B. Conjugated triene structure generates a typical UV spectrum for anabaenolysins for easy recognition. Anabaenolysin A constituted up to 400 ppm of the cyanobacterial dry weight. We found evidence of thirteen variants of anabaenolysins in one cyanobacterial strain. This suggests that the anabaenolysins are an important class of secondary metabolites in benthic Anabaena cyanobacteria. Both anabaenolysin A and B had cytolytic activity on a number of mammalian cell lines.

Unraveling the Numerous Biosynthetic Products of the Marine Sediment-Derived Fungus, Aspergillus insulicola

A new tripeptide, pre-sclerotiotide F (3), was isolated from a marine sediment-derived fungus, Aspergillus insulicola, along with five known compounds, one of which was new at the time of isolation, scerotiotide F (4). The absolute configuration elucidation of the new compound was determined using a combination of NMR, HR-ESI-MS, and optical rotation analyses. Cytotoxicities were measured in vitro against selected cancer cells. The effects of pre-sclerotiotide F (3) and sclerotiotide F (4) on LPS-induced NF-κB and iNOS expression were also measured.

p-Terphenyl and diterpenoid metabolites from endophytic Aspergillus sp. YXf3

Six new p-terphenyl derivatives, named 4″-deoxy-3-hydroxyterphenyllin (1), 4″-deoxy-5'-desmethyl-terphenyllin (2), 5'-desmethylterphenyllin (3), 4″-deoxycandidusin A (4), 4,5-dimethoxycandidusin A (5), and terphenolide (6), four new diterpenoids with norcleistanthane (aspergiloid A (12) and aspergiloid B (13)), cleistanthane (aspergiloid C (14)), and isopimarane (aspergiloid D (15)) type skeletons, and five known p-terphenyl compounds (7-11) were isolated from the fermentation broth of the plant endophytic fungus Aspergillus sp. Their structures were elucidated on the basis of detailed spectroscopic analysis and by comparison of their NMR data with those reported in the literature. Compounds 4, 6, 7, and 9 displayed moderate neuraminidase inhibitory activity with IC(50) values ranging from 4.34 to 9.17 μM.

Bioactive secondary metabolites from marine microbes for drug discovery

The isolation and extraction of novel bioactive secondary metabolites from marine microorganisms have a biomedical potential for future drug discovery as the oceans cover 70% of the planet's surface and life on earth originates from sea. Wide range of novel bioactive secondary metabolites exhibiting pharmacodynamic properties has been isolated from marine microorganisms and many to be discovered. The compounds isolated from marine organisms (macro and micro) are important in their natural form and also as templates for synthetic modifications for the treatments for variety of deadly to minor diseases. Many technical issues are yet to overcome before wide-scale bioprospecting of marine microorganisms becomes a reality. This chapter focuses on some novel secondary metabolites having antitumor, antivirus, enzyme inhibitor, and other bioactive properties identified and isolated from marine microorganisms including bacteria, actinomycetes, fungi, and cyanobacteria, which could serve as potentials for drug discovery after their clinical trials.