Nocardiopsistins A-C: New angucyclines with anti-MRSA activity isolated from a marine sponge-derived Nocardiopsis sp. HB-J378

An Overview on Nocardiopsis Species Originating From North African Biotopes as a Promising Source of Bioactive Compounds and In Silico Genome Mining Analysis of Three Sequenced Genomes

Actinobacteria are renowned for their prolific production of diverse bioactive secondary metabolites. In recent years, there has been an increasing focus on exploring "rare" genera within this phylum for biodiscovery purposes, notably the Nocardiopsis genus, which will be the subject of the present study. Recognizing the absence of articles describing the research process of finding bioactive molecules from the genus Nocardiopsis in North African environments. We, therefore, present a historical overview of the discoveries of bioactive molecules of the genus Nocardiopsis originating from the region, highlighting their biological activities and associated reported molecules, providing a snapshot of the current state of the field, and offering insights into future opportunities and challenges for drug discovery. Additionally, we present a genome mining analysis of three genomes deposited in public databases that have been reported to be bioactive. A total of 36 biosynthetic gene clusters (BGCs) were identified, including those known to encode bioactive molecules. Notably, a substantial portion of the BGCs showed little to no similarity to those previously described, suggesting the possibility that the analyzed strains could be potential producers of new compounds. Further research on these genomes is essential to fully uncovering their biotechnological potential. Moving forward, we discuss the experimental designs adopted in the reported studies, as well as new avenues to guide the exploration of the Nocardiopsis genus in North Africa.

Genomic and Metabolomic Analyses of Nocardiopsis maritima YSL2 as the Mycorrhizosphere Bacterium of Suaeda maritima (L.) Dumort

Nine bacteria were isolated from the episphere of Suaeda maritima (L.) Dumort. Among them, the bacterial strain YSL2 displayed the highest antimicrobial activity on agar plates and exhibited significant novelty compared with other bacteria based on 16S rRNA analysis. Consequently, Nocardiopsis maritima YSL2T was subjected to phenotypic characterization and whole-genome sequencing. Phylogenetic analysis revealed its close association with Nocardiopsis aegyptia SNG49T. Furthermore, genomic analysis of strain YSL2T revealed the presence of various gene clusters, indicating its potential for producing antimicrobial secondary metabolites. Upon cultivation on a large scale, maritiamides A and B (1 and 2) were isolated and characterized as cyclic hexapeptides based on nuclear magnetic resonance, ultraviolet, infrared, and mass spectrometric data. The absolute configurations of the amino acid residues in the maritiamides were determined through chiral derivatization, utilizing FDAA and GITC. Maritiamides 1 and 2 exhibited promising antibacterial activities against Staphylococcus epidermidis and weakly inhibited the growth of Escherichia coli and Pseudomonas fluorescens.

New Insights on Biological Activities, Chemical Compositions, and Classifications of Marine Actinomycetes Antifouling Agents

Biofouling is the assemblage of undesirable biological materials and macro-organisms (barnacles, mussels, etc.) on submerged surfaces, which has unfavorable impacts on the economy and maritime environments. Recently, research efforts have focused on isolating natural, eco-friendly antifouling agents to counteract the toxicities of synthetic antifouling agents. Marine actinomycetes produce a multitude of active metabolites, some of which acquire antifouling properties. These antifouling compounds have chemical structures that fall under the terpenoids, polyketides, furanones, and alkaloids chemical groups. These compounds demonstrate eminent antimicrobial vigor associated with antiquorum sensing and antibiofilm potentialities against both Gram-positive and -negative bacteria. They have also constrained larval settlements and the acetylcholinesterase enzyme, suggesting a strong anti-macrofouling activity. Despite their promising in vitro and in vivo biological activities, scaled-up production of natural antifouling agents retrieved from marine actinomycetes remains inapplicable and challenging. This might be attributed to their relatively low yield, the unreliability of in vitro tests, and the need for optimization before scaled-up manufacturing. This review will focus on some of the most recent marine actinomycete-derived antifouling agents, featuring their biological activities and chemical varieties after providing a quick overview of the disadvantages of fouling and commercially available synthetic antifouling agents. It will also offer different prospects of optimizations and analysis to scale up their industrial manufacturing for potential usage as antifouling coatings and antimicrobial and therapeutic agents.

Advances in Natural Products from the Marine-Sponge-Associated Microorganisms with Antimicrobial Activity in the Last Decade

Microorganisms are the dominating source of food and nutrition for sponges and play an important role in sponge structure, chemical defense, excretion and evolution. In recent years, plentiful secondary metabolites with novel structures and specific activities have been identified from sponge-associated microorganisms. Additionally, as the phenomenon of the drug resistance of pathogenic bacteria is becoming more and more common, it is urgent to discover new antimicrobial agents. In this paper, we reviewed 270 secondary metabolites with potential antimicrobial activity against a variety of pathogenic strains reported in the literature from 2012 to 2022. Among them, 68.5% were derived from fungi, 23.3% originated from actinomycetes, 3.7% were obtained from other bacteria and 4.4% were discovered using the co-culture method. The structures of these compounds include terpenoids (13%), polyketides (51.9%), alkaloids (17.4%), peptides (11.5%), glucosides (3.3%), etc. Significantly, there are 124 new compounds and 146 known compounds, 55 of which have antifungal activity in addition to antipathogenic bacteria. This review will provide a theoretical basis for the further development of antimicrobial drugs.

Investigation on Metabolites in Structure and Biosynthesis from the Deep-Sea Sediment-Derived Actinomycete Janibacter sp. SCSIO 52865

For exploring structurally diverse metabolites and uniquely metabolic mechanisms, we systematically investigated the chemical constituents and putative biosynthesis of Janibacter sp. SCSIO 52865 derived from the deep-sea sediment based on the OSMAC strategy, molecular networking tool, in combination with bioinformatic analysis. As a result, one new diketopiperazine (1), along with seven known cyclodipeptides (2-8), trans-cinnamic acid (9), N-phenethylacetamide (10) and five fatty acids (11-15), was isolated from the ethyl acetate extract of SCSIO 52865. Their structures were elucidated by a combination of comprehensive spectroscopic analyses, Marfey's method and GC-MS analysis. Furthermore, the analysis of molecular networking revealed the presence of cyclodipeptides, and compound 1 was produced only under mBHI fermentation condition. Moreover, bioinformatic analysis suggested that compound 1 was closely related to four genes, namely jatA-D, encoding core non-ribosomal peptide synthetase and acetyltransferase.

Secondary Metabolites from Marine-Derived Bacteria with Antibiotic and Antibiofilm Activities against Drug-Resistant Pathogens

The search for new antibiotics against drug-resistant microbes has been expanded to marine bacteria. Marine bacteria have been proven to be a prolific source of a myriad of novel compounds with potential biological activities. Therefore, this review highlights novel and bioactive compounds from marine bacteria reported during the period of January 2016 to December 2021. Published articles containing novel marine bacterial secondary metabolites that are active against drug-resistant pathogens were collected. Previously described compounds (prior to January 2016) are not included in this review. Unreported compounds during this period that exhibited activity against pathogenic microbes were discussed and compared in order to find the cue of the structure-bioactivity relationship. The results showed that Streptomyces are the most studied bacteria with undescribed bioactive compounds, followed by other genera in the Actinobacteria. We have categorized the structures of the compounds in the present review into four groups, based on their biosynthetic origins, as polyketide derivatives, amino acid derivatives, terpenoids, as well as compounds with mixed origin. These compounds were active against one or more drug-resistant pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus epidermidis (MRSE), vancomycin-resistant Enterococci (VRE), multidrug-resistant Mycobacterium tuberculosis (MDR-TB), and amphotericin B-resistant Candida albicans. In addition, some of the compounds also showed activity against biofilm formation of the test bacteria. Some previously undescribed compounds, isolated from marine-derived bacteria during this period, could have a good potential as lead compounds for the development of drug candidates to overcome multidrug-resistant pathogens.

The Diversity of Deep-Sea Actinobacteria and Their Natural Products: An Epitome of Curiosity and Drug Discovery

Bioprospecting of novel antibiotics has been the conventional norm of research fostered by researchers worldwide to combat drug resistance. With the exhaustion of incessant leads, the search for new chemical entities moves into uncharted territories such as the deep sea. The deep sea is a furthermost ecosystem with much untapped biodiversity thriving under extreme conditions. Accordingly, it also encompasses a vast pool of ancient natural products. Actinobacteria are frequently regarded as the bacteria of research interest due to their inherent antibiotic-producing capabilities. These interesting groups of bacteria occupy diverse ecological habitats including a multitude of different deep-sea habitats. In this review, we provide a recent update on the novel species and compounds of actinomycetes from the deep-sea environments within a period of 2016–2022. Within this period, a total of 24 new species of actinomycetes were discovered and characterized as well as 101 new compounds of various biological activities. The microbial communities of various deep-sea ecosystems are the emerging frontiers of bioprospecting.

Brominated and Sulfur-Containing Angucyclines Derived from a Single Pathway: Identification of Nocardiopsistins D-F

One novel brominated nocardiopsistin D (1) and two new sulfur-containing nocardiopsistins E-F (2-3) were identified from Nocardiopsis sp. HB-J378. The biosynthetic gene cluster ncd featuring a brominase was identified. Compounds 1-3 exhibited significant anti-methicillin-resistant Staphylococcus aureus (anti-MRSA) activities with minimum inhibitory concentrations (MICs) of 0.098, 3.125, and 0.195 μg/mL, respectively. The single bromination in 1 drastically enhanced the anti-MRSA activity by 128-fold without altering cell toxicity and acquired new activities against the bacterial pathogens vancomycin-resistant S. aureus (VRSA), Enterococcus faecium, and Bacillus cereus.

Marine Actinobacteria a New Source of Antibacterial Metabolites to Treat Acne Vulgaris Disease—A Systematic Literature Review

Acne vulgaris is a multifactorial disease that remains under-explored; up to date it is known that the bacterium Cutibacterium acnes is involved in the disease occurrence, also associated with a microbial dysbiosis. Antibiotics have become a mainstay treatment generating the emergence of antibiotic-resistant bacteria. In addition, there are some reported side effects of alternative treatments, which indicate the need to investigate a different therapeutic approach. Natural products continue to be an excellent option, especially those extracted from actinobacteria, which represent a prominent source of metabolites with a wide range of biological activities, particularly the marine actinobacteria, which have been less studied than their terrestrial counterparts. Therefore, this systematic review aimed to identify and evaluate the potential anti-infective activity of metabolites isolated from marine actinobacteria strains against bacteria related to the development of acne vulgaris disease. It was found that there is a variety of compounds with anti-infective activity against Staphylococcus aureus and Staphylococcus epidermidis, bacteria closely related to acne vulgaris development; nevertheless, there is no report of a compound with antibacterial activity or quorum-sensing inhibition toward C. acnes, which is a surprising result. Since two of the most widely used antibiotics for the treatment of acne targeting C. acnes were obtained from actinobacteria of the genus Streptomyces, this demonstrates a great opportunity to pursue further studies in this field, considering the potential of marine actinobacteria to produce new anti-infective compounds.

Genus Nocardiopsis: A Prolific Producer of Natural Products

Actinomycetes are currently one of the major sources of bioactive secondary metabolites used for medicine development. Accumulating evidence has shown that Nocardiopsis, a key class of actinomycetes, has the ability to produce novel bioactive natural products. This review covers the sources, distribution, bioactivities, biosynthesis, and structural characteristics of compounds isolated from Nocardiopsis in the period between March 2018 and 2021. Our results reveal that 67% of Nocardiopsis-derived natural products are reported for the first time, and 73% of them are isolated from marine Nocardiopsis. The chemical structures of the Nocardiopsis-derived compounds have diverse skeletons, concentrating on the categories of polyketides, peptides, terphenyls, and alkaloids. Almost 50% of the natural products isolated from Nocardiopsis have been discovered to display various bioactivities. These results fully demonstrate the great potential of the genus Nocardiopsis to produce novel bioactive secondary metabolites that may serve as a structural foundation for the development of novel drugs.

Selective isolation, antimicrobial screening and phylogenetic diversity of marine actinomycetes derived from the Coast of Bejaia City (Algeria), a polluted and microbiologically unexplored environment

AIMS

The current study aimed to evaluate the occurrence of actinomycetes in the Coast of Bejaia City using selective isolation, as well as their bioactivity and phylogenitic diversity.

METHODS AND RESULTS

Different selective media and methods were used, leading to the isolation of 103 actinomycete strains. The number of strains was influenced by isolation procedures and their interactions based on a three-way ANOVA and a post hoc Tukey test, which revealed that using M2 medium, dilution of samples followed by moderate heat treatment, and sampling at 10-20 m yielded the highest numbers of actinomycetes. The isolates were screened for their antimicrobial activity against human pathogenic microorganisms using agar and well diffusion methods. Of all the isolates, ten displayed activity against at least one Gram-positive bacterium, of which P21 showed the highest activity against Staphylococcus aureus, Methicillin-resistant S. aureus and Bacillus subtilis, with a diameter of 32, 28 and 25 mm respectively. Subsequently, active isolates were assigned to Streptomyces spp. and Nocardiopsis spp. based on 16S rRNA gene sequencing, including a putative new Streptomyces species (S3). The phenotypic characteristics of the P21 strain were determined, and interesting enzymatic capacities were shown.

CONCLUSION

The recovery of actinomycetes along the Coast of Bejaia City was influenced by the isolation procedure. Ten strains displayed interesting antibacterial activity against Gram-positive bacteria, of which the P21 strain was selected as the most active strain.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work provides a new insight into the occurrence of actinobacteria in the Coast of Bejaia. It suggests also that polluted environments such as Bejaia Bay could provide access to interesting actinomycetes as sources of antibiotic leads.

Genome-Guided Discovery of Highly Oxygenated Aromatic Polyketides, Saccharothrixins D-M, from the Rare Marine Actinomycete Saccharothrix sp. D09

Angucyclines and angucyclinones are aromatic polyketides with intriguing structures and therapeutic value. Genome mining of the rare marine actinomycete Saccharothrix sp. D09 led to the identification of a type II polyketide synthase biosynthetic gene cluster, sxn, which encodes several distinct subclasses of oxidoreductases, implying that this strain has the potential to produce novel polycyclic aromatic polyketides with unusual redox modifications. The "one strain-many compounds" (OSMAC) strategy and comparative metabolite analysis facilitated the discovery of 20 angucycline derivatives from the D09 strain, including six new highly oxygenated saccharothrixins D-I (1-6), four new glycosylated saccharothrixins J-M (7-10), and 10 known analogues (11-20). Their structures were elucidated based on detailed HRESIMS, NMR spectroscopic, and X-ray crystallographic analysis. With the help of gene disruption and heterologous expression, we proposed their plausible biosynthetic pathways. In addition, compounds 3, 4, and 8 showed antibacterial activity against Helicobacter pylori with MIC values ranging from 16 to 32 μg/mL. Compound 3 also revealed anti-inflammatory activity by inhibiting the production of NO with an IC₅₀ value of 28 μM.

Two strains of airborne Nocardiopsis alba producing different volatile organic compounds (VOCs) as biofungicide for Ganoderma boninense

Nocardiopsis are actinobacteria which produce active compounds, such as antifungals and volatile compounds. Ganoderma boninense is a pathogenic and aggressive fungus that decreases palm oil yield during production. In this study, we isolated two strains of Nocardia (GME01 and GME22) from airborne contaminants on the actinobacteria culture collection in the laboratory. The aim of this study is to identify two strains of Nocardiopsis and to obtain the antifungal potency of volatile organic compounds (VOCs) against G. boninese. We characterized the morphology using Scanning Electrone Microscope (SEM), molecular properties and whole-cell protein spectra using Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS), antifungal assay on G. boninense and VOCs analysis of Nocardia using solid phase micro extraction/gas chromatography (SPME/GC). The two Nocardiopsis strains had the similar characteristic such as white aerial mycelium and spores, aerobic, grow well on ISP-2, TSA and NA medium without diffusible pigment and had the highest similarity with Nocardiopsis alba DSM 43377 (99.63% and 99.55% similarity for GME01 and GME22, respectively), Different morphological feature was found in aerial mycelium and spores. GME22 has a clearly fragmented mycelium whereas GME01 has none. Other features also showed different on the whole-cell protein spectra, antifungal activity and VOCs profiles. Antifungal activity assay on G. boninense showed that N. alba GME22 has higher antifungal activity than GME01 related with the VOCs abundance in two strains. Almost 38.3% (18 VOCs) of N. alba GME22 and 25.5% (12 VOCs) of N. alba GME01 were found specifically in each strain, and 36.2% (the 17 same VOCs) produced by both. The known volatile antifungal compounds S-methyl ethanethioate, 1,2-dimethyldisulfane, acetic acid, 2-methyl propanoic acid, 3-methyl-butanoic acid, nonan-2-one, undecan-2-one and 2-isopropyl-5-methylcyclohexan-1-ol only produced by N. alba GME22 and 1,3-dimethyltrisulfane only produced by N. alba GME01. A total of two known antifungal compounds 1,2-dimethyldisulfane and 6-methylheptan-2-one were produced by both N. alba. The abundance of antifungal VOCs produced by these bacteria is potentially to be used as biocontrol agent for pathogenic fungi in plants.

Natural Products from Actinomycetes Associated with Marine Organisms

The actinomycetes have proven to be a rich source of bioactive secondary metabolites and play a critical role in the development of pharmaceutical researches. With interactions of host organisms and having special ecological status, the actinomycetes associated with marine animals, marine plants, macroalgae, cyanobacteria, and lichens have more potential to produce active metabolites acting as chemical defenses to protect the host from predators as well as microbial infection. This review focuses on 536 secondary metabolites (SMs) from actinomycetes associated with these marine organisms covering the literature to mid-2021, which will highlight the taxonomic diversity of actinomycetes and the structural classes, biological activities of SMs. Among all the actinomycetes listed, members of Streptomyces (68%), Micromonospora (6%), and Nocardiopsis (3%) are dominant producers of secondary metabolites. Additionally, alkaloids (37%), polyketides (33%), and peptides (15%) comprise the largest proportion of natural products with mostly antimicrobial activity and cytotoxicity. Furthermore, the data analysis and clinical information of SMs have been summarized in this article, suggesting that some of these actinomycetes with multiple host organisms deserve more attention to their special ecological status and genetic factors.

Himalaquinones A-G, Angucyclinone-Derived Metabolites Produced by the Himalayan Isolate Streptomyces sp. PU-MM59

Himalaquinones A-G, seven new anthraquinone-derived metabolites, were obtained from the Himalayan-based Streptomyces sp. PU-MM59. The chemical structures of the new compounds were identified based on cumulative analyses of HRESIMS and NMR spectra. Himalaquinones A-F were determined to be unique anthraquinones that contained unusual C-4a 3-methylbut-3-enoic acid aromatic substitutions, while himalaquinone G was identified as a new 5,6-dihydrodiol-bearing angucyclinone. Comparative bioactivity assessment (antimicrobial, cancer cell line cytotoxicity, impact on 4E-BP1 phosphorylation, and effect on axolotl embryo tail regeneration) revealed cytotoxic landomycin and saquayamycin analogues to inhibit 4E-BP1p and inhibit regeneration. In contrast, himalaquinone G, while also cytotoxic and a regeneration inhibitor, did not affect 4E-BP1p status at the doses tested. As such, this work implicates a unique mechanism for himalaquinone G and possibly other 5,6-dihydrodiol-bearing angucyclinones.

Marine Actinomycetes, New Sources of Biotechnological Products

The Actinomycetales order is one of great genetic and functional diversity, including diversity in the production of secondary metabolites which have uses in medical, environmental rehabilitation, and industrial applications. Secondary metabolites produced by actinomycete species are an abundant source of antibiotics, antitumor agents, anthelmintics, and antifungals. These actinomycete-derived medicines are in circulation as current treatments, but actinomycetes are also being explored as potential sources of new compounds to combat multidrug resistance in pathogenic bacteria. Actinomycetes as a potential to solve environmental concerns is another area of recent investigation, particularly their utility in the bioremediation of pesticides, toxic metals, radioactive wastes, and biofouling. Other applications include biofuels, detergents, and food preservatives/additives. Exploring other unique properties of actinomycetes will allow for a deeper understanding of this interesting taxonomic group. Combined with genetic engineering, microbial experimental evolution, and other enhancement techniques, it is reasonable to assume that the use of marine actinomycetes will continue to increase. Novel products will begin to be developed for diverse applied research purposes, including zymology and enology. This paper outlines the current knowledge of actinomycete usage in applied research, focusing on marine isolates and providing direction for future research.

Marine natural products

This review covers the literature published between January and December in 2018 for marine natural products (MNPs), with 717 citations (706 for the period January to December 2018) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1554 in 469 papers for 2018), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. The proportion of MNPs assigned absolute configuration over the last decade is also surveyed.

Progress in Research on Bioactive Secondary Metabolites from Deep-Sea Derived Microorganisms

Deep sea has an extreme environment which leads to biodiversity of microorganisms and their unique physical and biochemical mechanisms. Deep-sea derived microorganisms are more likely to produce novel bioactive substances with special mechanism of action for drug discovery. This article reviews secondary metabolites with biological activities such as anti-tumor, anti-bacterial, anti-viral, and anti-inflammatory isolated from deep-sea fungi and bacteria during 2018-2020. Effective methods for screening and obtaining natural active compounds from deep-sea microorganisms are also summarized, including optimizing the culture conditions, using genome mining technology, biosynthesis and so on. The comprehensive application of these methods makes broader prospects for the development and application of deep sea microbial bioactive substances.

Novel Bioactive Polyketides Isolated from Marine Actinomycetes: An Update Review from 2013 to 2019

Marine organism-associated actinobacteria represent a valuable resource for marine drugs due to their abundant secondary metabolites. The special environments in the ocean, for instance, high salt, high pressure, low temperature and oligotrophy, not only adapt to survival of actinomycetes but also enhance molecular diversity of actinomycete secondary metabolites production, thus making marine actinomycetes important sources of marine-based bioactive compounds, especially polyketides. Herein, we summarized the structures and pharmacological activities of polyketides from actinobacteria associated with marine organisms from 2013 to 2019; moreover, the main source species of actinomycetes were discussed as well. We expected that this review would be helpful for future in-depth research and development of marine-based bioactive polyketides.

Antimicrobial compounds from marine actinomycetes

Marine actinomycetes were the main origin of marine natural products in the past 40 years. This review was to present the sources, structures and antimicrobial activities of 313 new natural products from marine actinomycetes reported from 1976 to 2019.

Applying microbial ecology to antimicrobial discovery

Introduction of antibiotics into clinical use has contributed to some of the greatest improvements to public health in the 20th century. Most antibiotics are based on antimicrobials that were isolated from environmental microorganisms over 50 years ago, but emerging resistance requires discovery of new molecules and development of these molecules into therapeutics. Bioinformatic analyses of microbial genomes indicate that many more microbial bioactive molecules remain undiscovered. Understanding when, where, and why these molecules are produced informs efforts to tap into the hidden unexplored chemical diversity. Expanding the search to undersampled ecological niches and improving culturing techniques will ensure discovery of new antibiotics.

Antibiotics Development and the Potentials of Marine-Derived Compounds to Stem the Tide of Multidrug-Resistant Pathogenic Bacteria, Fungi, and Protozoa

As the search for new antibiotics continues, the resistance to known antimicrobial compounds continues to increase. Many researchers around the world, in response to antibiotics resistance, have continued to search for new antimicrobial compounds in different ecological niches such as the marine environment. Marine habitats are one of the known and promising sources for bioactive compounds with antimicrobial potentials against currently drug-resistant strains of pathogenic microorganisms. For more than a decade, numerous antimicrobial compounds have been discovered from marine environments, with many more antimicrobials still being discovered every year. So far, only very few compounds are in preclinical and clinical trials. Research in marine natural products has resulted in the isolation and identification of numerous diverse and novel chemical compounds with potency against even drug-resistant pathogens. Some of these compounds, which mainly came from marine bacteria and fungi, have been classified into alkaloids, lactones, phenols, quinones, tannins, terpenes, glycosides, halogenated, polyketides, xanthones, macrocycles, peptides, and fatty acids. All these are geared towards discovering and isolating unique compounds with therapeutic potential, especially against multidrug-resistant pathogenic microorganisms. In this review, we tried to summarize published articles from 2015 to 2019 on antimicrobial compounds isolated from marine sources, including some of their chemical structures and tests performed against drug-resistant pathogens.

Marine Actinomycetes-derived Natural Products

Actinomycetes is an abundant resource for discovering a large number of lead compounds, which play an important role in microbial drug discovery. Compared to terrestrial microorganisms, marine actinomycetes have unique metabolic pathways because of their special living environment, which has the potential to produce a variety of bioactive substances. In this paper, secondary metabolites isolated from marine actinomycetes are reviewed (2013-2018), most of which exhibited cytotoxic, antibacterial, and antiviral biological activities.

Advances in microbial culturing conditions to activate silent biosynthetic gene clusters for novel metabolite production

Natural products (NPs) produced by bacteria and fungi are often used as therapeutic agents due to their complex structures and wide range of bioactivities. Enzymes that build NPs are encoded by co-localized biosynthetic gene clusters (BGCs), and genome sequencing has recently revealed that many BGCs are “silent” under standard laboratory conditions. There are numerous methods used to activate “silent” BGCs that rely either upon altering culture conditions or genetic modification. In this review, we discuss several recent microbial cultivation methods that have been used to expand the scope of NPs accessible in the laboratory. These approaches are divided into three categories: addition of a physical scaffold, addition of small molecule elicitors, and co-cultivation with another microbe.

Kenalactams A-E, Polyene Macrolactams Isolated from Nocardiopsis CG3

In our screening program for new biologically active secondary metabolites, a new strain, Nocardiopsis CG3 (DSM 106572), isolated from the saltpan of Kenadsa, was found to produce five new polyene macrolactams, the kenalactams A-E (1-5). Their structures were elucidated by spectral methods (NMR and HRESIMS), and the absolute configuration was derived by chemical derivatization (Mosher's method). Through a feeding experiment, alanine was proven to be the nitrogen-bearing starter unit involved in biosynthesis of the polyketide kenalactam A (1). Kenalactam E (5) was cytotoxic against human prostate cancer PC-3 cells with an IC₅₀ value of 2.1 μM.