Molecular beacons to identify gifted microbes for genome mining

Recent advances in discovery and biosynthesis of natural products from myxobacteria: an overview from 2017 to 2023

Covering: 2017.01 to 2023.11Natural products biosynthesized by myxobacteria are appealing due to their sophisticated chemical skeletons, remarkable biological activities, and intriguing biosynthetic enzymology. This review aims to systematically summarize the advances in the discovery methods, new structures, and bioactivities of myxobacterial NPs reported in the period of 2017-2023. In addition, the peculiar biosynthetic pathways of several structural families are also highlighted.

A metagenomic library cloning strategy that promotes high-level expression of captured genes to enable efficient functional screening

Functional screening of environmental DNA (eDNA) libraries is a potentially powerful approach to discover enzymatic "unknown unknowns", but is usually heavily biased toward the tiny subset of genes preferentially transcribed and translated by the screening strain. We have overcome this by preparing an eDNA library via partial digest with restriction enzyme FatI (cuts CATG), causing a substantial proportion of ATG start codons to be precisely aligned with strong plasmid-encoded promoter and ribosome-binding sequences. Whereas we were unable to select nitroreductases from standard metagenome libraries, our FatI strategy yielded 21 nitroreductases spanning eight different enzyme families, each conferring resistance to the nitro-antibiotic niclosamide and sensitivity to the nitro-prodrug metronidazole. We showed expression could be improved by co-expressing rare tRNAs and encoded proteins purified directly using an embedded His6-tag. In a transgenic zebrafish model of metronidazole-mediated targeted cell ablation, our lead MhqN-family nitroreductase proved ∼5-fold more effective than the canonical nitroreductase NfsB.

A metagenomic library cloning strategy that promotes high-level expression of captured genes to enable efficient functional screening

Functional screening of environmental DNA (eDNA) libraries is a potentially powerful approach to discover enzymatic "unknown unknowns", but is usually heavily biased toward the tiny subset of genes preferentially transcribed and translated by the screening strain. We have overcome this by preparing an eDNA library via partial digest with restriction enzyme FatI (cuts CATG), causing a substantial proportion of ATG start codons to be precisely aligned with strong plasmid-encoded promoter and ribosome-binding sequences. Whereas we were unable to select nitroreductases from standard metagenome libraries, our FatI strategy yielded 21 nitroreductases spanning eight different enzyme families, each conferring resistance to the nitro-antibiotic niclosamide and sensitivity to the nitro-prodrug metronidazole. We showed expression could be improved by co-expressing rare tRNAs and encoded proteins purified directly using an embedded His6-tag. In a transgenic zebrafish model of metronidazole-mediated targeted cell ablation, our lead MhqN-family nitroreductase proved ~5-fold more effective than the canonical nitroreductase NfsB.

Constructing a Myxobacterial Natural Product Database to Facilitate NMR-Based Metabolomics Bioprospecting of Myxobacteria

Myxobacteria are fascinating prokaryotes featuring a potent capacity for producing a wealth of bioactive molecules with intricate chemical topology as well as intriguing enzymology, and thus it is critical to developing an efficient pipeline for bioprospecting. Herein, we construct the database MyxoDB, the first public compendium solely dedicated to myxobacteria, which enabled us to provide an overview of the structural diversity and taxonomic distribution of known myxobacterial natural products. Moreover, we demonstrated that the cutting-edge NMR-based metabolomics was effective to differentiate the biosynthetic priority of myxobacteria, whereby MyxoDB could greatly streamline the dereplication of multifarious known compounds and accordingly speed up the discovery of new compounds. This led to the rapid identification of a class of linear di-lipopeptides (archangimins) and a rare rearranged sterol (corasterol) that were endowed with unique chemical architectures and/or biosynthetic enzymology. We also showcased that NMR-based metabolomics, MyxoDB, and genomics can also work concertedly to accelerate the targeted discovery of a polyketidic compound pyxipyrrolone C. All in all, this study sets the stage for the discovery of many more novel natural products from underexplored myxobacterial resources.

Concepts and conjectures concerning predatory performance of myxobacteria

Myxobacteria are excellent model organisms for investigation of predator-prey interactions and predatory shaping of microbial communities. This review covers interdisciplinary topics related to myxobacterial predation and provides current concepts and challenges for determining predatory performance. Discussed topics include the role of specialized metabolites during predation, genetic determinants for predatory performance, challenges associated with methodological differences, discrepancies between sequenced and environmental myxobacteria, and factors that influence predation.

Biological Dark Matter Exploration using Data Mining for the Discovery of Antimicrobial Natural Products

The discovery of novel antimicrobials has significantly slowed down over the last three decades. At the same time, humans rely increasingly on antimicrobials because of the progressive antimicrobial resistance in medical practices, human communities, and the environment. Data mining is currently considered a promising option in the discovery of new antibiotics. Some of the advantages of data mining are the ability to predict chemical structures from sequence data, anticipation of the presence of novel metabolites, the understanding of gene evolution, and the corroboration of data from multiple omics technologies. This review analyzes the state-of-the-art for data mining in the fields of bacteria, fungi, and plant genomic data, as well as metabologenomics. It also summarizes some of the most recent research accomplishments in the field, all pinpointing to innovation through uncovering and implementing the next generation of antimicrobials.

Genome mining for drug discovery: progress at the front end

Microbial genome mining for drug discovery and development has been accelerating in recent years, driven by technical advancements in genome sequencing, bioinformatics, metabolomics/metabologenomics, and synthetic biology. Microbial genome mining is a multistep process that starts with the sequencing of microbes that encode multiple secondary metabolites and identifying new and novel secondary metabolite biosynthetic gene clusters (BGCs) to pursue. The initial steps in the process are critical for the overall success, and they encompass the most innovative new technologies to revitalize natural product discovery. As microbial genome mining has matured in recent years, unvalidated conjectures about what microbes to pursue, how to identify legitimate secondary metabolite BGCs, and how to sequence DNA to satisfactory levels of completion have been identified. The solutions to correct the misconceptions around these topics are beginning to be implemented.

Novel Alkaloids from Marine Actinobacteria: Discovery and Characterization

The marine environment is an excellent resource for natural products with therapeutic potential. Its microbial inhabitants, often associated with other marine organisms, are specialized in the synthesis of bioactive secondary metabolites. Similar to their terrestrial counterparts, marine Actinobacteria are a prevalent source of these natural products. Here, we discuss 77 newly discovered alkaloids produced by such marine Actinobacteria between 2017 and mid-2021, as well as the strategies employed in their elucidation. While 12 different classes of alkaloids were unraveled, indoles, diketopiperazines, glutarimides, indolizidines, and pyrroles were most dominant. Discoveries were mainly based on experimental approaches where microbial extracts were analyzed in relation to novel compounds. Although such experimental procedures have proven useful in the past, the methodologies need adaptations to limit the chance of compound rediscovery. On the other hand, genome mining provides a different angle for natural product discovery. While the technology is still relatively young compared to experimental screening, significant improvement has been made in recent years. Together with synthetic biology tools, both genome mining and extract screening provide excellent opportunities for continued drug discovery from marine Actinobacteria.

Assessment of Evolutionary Relationships for Prioritization of Myxobacteria for Natural Product Discovery

Discoveries of novel myxobacteria have started to unveil the potentially vast phylogenetic diversity within the family Myxococcaceae and have brought about an updated approach to myxobacterial classification. While traditional approaches focused on morphology, 16S gene sequences, and biochemistry, modern methods including comparative genomics have provided a more thorough assessment of myxobacterial taxonomy. Herein, we utilize long-read genome sequencing for two myxobacteria previously classified as Archangium primigenium and Chondrococcus macrosporus, as well as four environmental myxobacteria newly isolated for this study. Average nucleotide identity and digital DNA-DNA hybridization scores from comparative genomics suggest previously classified as A. primigenium to instead be a novel member of the genus Melittangium, C. macrosporus to be a potentially novel member of the genus Corallococcus with high similarity to Corallococcus exercitus, and the four isolated myxobacteria to include another novel Corallococcus species, a novel Pyxidicoccus species, a strain of Corallococcus exiguus, and a potentially novel Myxococcus species with high similarity to Myxococcus stipitatus. We assess the biosynthetic potential of each sequenced myxobacterium and suggest that genus-level conservation of biosynthetic pathways support our preliminary taxonomic assignment. Altogether, we suggest that long-read genome sequencing benefits the classification of myxobacteria and improves determination of biosynthetic potential for prioritization of natural product discovery.

Antibacterial Performance of a Streptomyces spectabilis Strain Producing Metacycloprodigiosin

After separation of bacterial colonies on solid plates, purification, and screening through the agar cup-plate method, an antibiotic-resistant bacterial isolate was obtained, and named strain L20190601, the 16S rRNA gene sequence data of strain L20190601 to GenBank, NCBI have provided GenBank accession number MW931615. 16S rRNA gene sequencing revealed that this isolate was highly similar to a number of Streptomyces species. Among them, the homology with S. spectabilis was the highest, reaching 99.9, together with curved hyphal morphology and biochemical tests, allowed us to identify strain L20190601 as S. spectabilis. The red pigment produced by S. spectabilis strain L20190601 was structurally identified. An acid-base color reaction assay showed that when this pigment was dissolved in a solution at pH 3.0 and 9.0, the color of the solution was red and yellow, respectively. In addition, the analysis of absorption spectra revealed that at pH 8.0 and 3.0, the maximum absorption peaks were at 466 and 531 nm, respectively. These results are consistent with the spectral absorption characteristics of metacycloprodigiosin reported in the literature. Moreover, the retention time of purified pigments was identical to those of standard metacycloprodigiosin solutions. Mass spectrometry analysis revealed that the molecular weight of the red compound was 392.2 [M + H]⁺. Finally, metacycloprodigiosin was found to be effective against eight clinically common pathogens: Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Streptococcus pyogenes, Pseudomonas aeruginosa, Bacillus typhi, Candida albicans, and Trichophyton rubrum. In summary, metacycloprodigiosin exhibited strong antibacterial activity and a broad antibacterial spectrum, and thus is a promising compound for the development of a new type of antibacterial drug.

Genome mining for drug discovery: cyclic lipopeptides related to daptomycin

The cyclic lipopeptide antibiotics structurally related to daptomycin were first reported in the 1950s. Several have common lipopeptide initiation, elongation, and termination mechanisms. Initiation requires the use of a fatty acyl-AMP ligase (FAAL), a free-standing acyl carrier protein (ACP), and a specialized condensation (C^III) domain on the first NRPS elongation module to couple the long chain fatty acid to the first amino acid. Termination is carried out by a dimodular NRPS that contains a terminal thioesterase (Te) domain (CAT-CATTe). Lipopeptide BGCs also encode ABC transporters, apparently for export and resistance. The use of this mechanism of initiation, elongation, and termination, coupled with molecular target-agnostic resistance, has provided a unique basis for robust natural and experimental combinatorial biosynthesis to generate a large variety of structurally related compounds, some with altered or different antibacterial mechanisms of action. The FAAL, ACP, and dimodular NRPS genes were used as molecular beacons to identify phylogenetically related BGCs by BLASTp analysis of finished and draft genome sequences. These and other molecular beacons have identified: (i) known, but previously unsequenced lipopeptide BGCs in draft genomes; (ii) a new daptomycin family BGC in a draft genome of Streptomyces sedi; and (iii) novel lipopeptide BGCs in the finished genome of Streptomyces ambofaciens and the draft genome of Streptomyces zhaozhouensis.

Comparative Metabologenomics Analysis of Polar Actinomycetes

Biosynthetic and chemical datasets are the two major pillars for microbial drug discovery in the omics era. Despite the advancement of analysis tools and platforms for multi-strain metabolomics and genomics, linking these information sources remains a considerable bottleneck in strain prioritisation and natural product discovery. In this study, molecular networking of the 100 metabolite extracts derived from applying the OSMAC approach to 25 Polar bacterial strains, showed growth media specificity and potential chemical novelty was suggested. Moreover, the metabolite extracts were screened for antibacterial activity and promising selective bioactivity against drug-persistent pathogens such as Klebsiella pneumoniae and Acinetobacter baumannii was observed. Genome sequencing data were combined with metabolomics experiments in the recently developed computational approach, NPLinker, which was used to link BGC and molecular features to prioritise strains for further investigation based on biosynthetic and chemical information. Herein, we putatively identified the known metabolites ectoine and chrloramphenicol which, through NPLinker, were linked to their associated BGCs. The metabologenomics approach followed in this study can potentially be applied to any large microbial datasets for accelerating the discovery of new (bioactive) specialised metabolites.

Diversity of nonribosomal peptide synthetase and polyketide synthase gene clusters in the genus Acrocarpospora

Acrocarpospora is a rare, recently established actinomycete genus of the family Streptosporangiaceae. In the present study, we sequenced whole genomes of the type strains of Acrocarpospora corrugate, Acrocarpospora macrocephala, and Acrocarpospora pleiomorpha to assess their potency as secondary metabolite producers; we then surveyed their nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) gene clusters. The genome sizes of A. corrugate NBRC 13972^T, A. macrocephala NBRC 16266^T, and A. pleiomorpha NBRC 16267^T were 9.3 Mb, 12.1 Mb, and 11.8 Mb, respectively. Each genome contained 12-17 modular NRPS and PKS gene clusters. Among the 23 kinds of NRPS and PKS gene clusters identified from the three strains, eight clusters were conserved in all the strains, six were shared between A. macrocephala and A. pleiomorpha, and the remaining nine were strain-specific. We predicted the chemical structures of the products synthesized by these gene clusters based on bioinformatic analyses. Since the chemical structures are diverse, Acrocarpospora strains are considered an attractive source of diverse nonribosomal peptide and polyketide compounds.

Deep Hypersaline Anoxic Basins as Untapped Reservoir of Polyextremophilic Prokaryotes of Biotechnological Interest

Deep-sea hypersaline anoxic basins (DHABs) are considered to be among the most extreme ecosystems on our planet, allowing only the life of polyextremophilic organisms. DHABs’ prokaryotes exhibit extraordinary metabolic capabilities, representing a hot topic for microbiologists and biotechnologists. These are a source of enzymes and new secondary metabolites with valuable applications in different biotechnological fields. Here, we review the current knowledge on prokaryotic diversity in DHABs, highlighting the biotechnological applications of identified taxa and isolated species. The discovery of new species and molecules from these ecosystems is expanding our understanding of life limits and is expected to have a strong impact on biotechnological applications.

Metagenome Driven Discovery of Nonribosomal Peptides

Declining rates of novel natural product discovery and exponential rates of rediscovery heralded the end of the 1940s to 1960s "golden era" of antibiotic discovery. Fifty years later, the implementation of molecular screening methodologies revealed that standard culture-based screening approaches had failed to capture the vast majority of environmental bacteria and that even for the cultivable isolates only a small fraction of the biosynthetic potential had been tapped. A diversity of metagenomic screening and synthetic biology approaches have been developed to address these issues. The nonribosomal peptides have received particular focus, owing to their high levels of bioactivity and the predictability of the biosynthetic logic of the genetically encoded assembly lines that produce them. By uniting advances in next-generation sequencing and bioinformatic analysis with a diversity of traditional disciplines, several pioneering teams have proven that this previously inaccessible resource is no longer out of reach.

The complete genomic sequence of Streptomyces spectabilis NRRL-2792 and identification of secondary metabolite biosynthetic gene clusters

This is the first report of a fully annotated genomic sequence of Streptomyces spectabilis NRRL-2792, isolated and identified by The Upjohn Company in 1961. The genome was assembled into a single scaffold for annotation and analysis. The chromosome is linear, 9.5 Mb in size which is one of the largest Streptomyces genomes yet described, has a G+C content of 72%, and encodes for approximately 7943 genes. Antibiotic Secondary Metabolite Analysis Shell (antiSMASH) and Basic Local Alignment Search Tool (BLAST) bioinformatics analyses identified six complete secondary metabolite biosynthetic gene clusters for ectoine, melanin, albaflavenone, spectinomycin, 2-methylisoborneol and coelichelin. Additionally, biosynthetic clusters were identified that shared ≥ 90% gene content with complestatin, hopene, neoaureothin, or undecylprodigiosin. Thirty-one other likely secondary metabolite gene clusters were identified by antiSMASH. BLAST identified two subsets of undecylprodigiosin biosynthetic genes at polar opposites of the chromosome; their duplication was subsequently confirmed by primer walking.

Natural product drug discovery in the genomic era: realities, conjectures, misconceptions, and opportunities

Natural product discovery from microorganisms provided important sources for antibiotics, anti-cancer agents, immune-modulators, anthelminthic agents, and insecticides during a span of 50 years starting in the 1940s, then became less productive because of rediscovery issues, low throughput, and lack of relevant new technologies to unveil less abundant or not easily detected drug-like natural products. In the early 2000s, it was observed from genome sequencing that Streptomyces species encode about ten times as many secondary metabolites as predicted from known secondary metabolomes. This gave rise to a new discovery approach—microbial genome mining. As the cost of genome sequencing dropped, the numbers of sequenced bacteria, fungi and archaea expanded dramatically, and bioinformatic methods were developed to rapidly scan whole genomes for the numbers, types, and novelty of secondary metabolite biosynthetic gene clusters. This methodology enabled the identification of microbial taxa gifted for the biosynthesis of drug-like secondary metabolites. As genome sequencing technology progressed, the realities relevant to drug discovery have emerged, the conjectures and misconceptions have been clarified, and opportunities to reinvigorate microbial drug discovery have crystallized. This perspective addresses these critical issues for drug discovery.

Toward a global picture of bacterial secondary metabolism

Bacterial metabolism is comprised of primary metabolites, the intracellular molecules of life that enable growth and proliferation, and secondary metabolites, predominantly extracellular molecules that facilitate a microbe's interaction with its environment. While our knowledge of primary metabolism and its web of interconnected intermediates is quantitative and holistic, significant knowledge gaps remain in our understanding of the secondary metabolomes of bacteria. In this Perspective, I discuss the main challenges involved in obtaining a global, comprehensive picture of bacterial secondary metabolomes, specifically in biosynthetically "gifted" microbes. Recent methodological advances that can meet these challenges will be reviewed. Applications of these methods combined with ongoing innovations will enable a detailed picture of global secondary metabolomes, which will in turn shed light onto the biology, chemistry, and enzymology underlying natural products and simultaneously aid drug discovery.

Natural products from anaerobes

Natural product discovery in the microbial world has historically been biased toward aerobes. Recent in silico analysis demonstrates that genomes of anaerobes encode unexpected biosynthetic potential for natural products, however, chemical data on natural products from the anaerobic world are extremely limited. Here, we review the current body of work on natural products isolated from strictly anaerobic microbes, including recent genome mining efforts to discover polyketides and non-ribosomal peptides from anaerobes. These known natural products of anaerobes have demonstrated interesting molecular scaffolds, biosynthetic logic, and/or biological activities, making anaerobes a promising reservoir for future natural product discovery.

Synthetic biology advances and applications in the biotechnology industry: a perspective

Synthetic biology is a logical extension of what has been called recombinant DNA (rDNA) technology or genetic engineering since the 1970s. As rDNA technology has been the driver for the development of a thriving biotechnology industry today, starting with the commercialization of biosynthetic human insulin in the early 1980s, synthetic biology has the potential to take the industry to new heights in the coming years. Synthetic biology advances have been driven by dramatic cost reductions in DNA sequencing and DNA synthesis; by the development of sophisticated tools for genome editing, such as CRISPR/Cas9; and by advances in informatics, computational tools, and infrastructure to facilitate and scale analysis and design. Synthetic biology approaches have already been applied to the metabolic engineering of microorganisms for the production of industrially important chemicals and for the engineering of human cells to treat medical disorders. It also shows great promise to accelerate the discovery and development of novel secondary metabolites from microorganisms through traditional, engineered, and combinatorial biosynthesis. We anticipate that synthetic biology will continue to have broadening impacts on the biotechnology industry to address ongoing issues of human health, world food supply, renewable energy, and industrial chemicals and enzymes.

Comparison of Strategies to Overcome Drug Resistance: Learning from Various Kingdoms

Drug resistance, especially antibiotic resistance, is a growing threat to human health. To overcome this problem, it is significant to know precisely the mechanisms of drug resistance and/or self-resistance in various kingdoms, from bacteria through plants to animals, once more. This review compares the molecular mechanisms of the resistance against phycotoxins, toxins from marine and terrestrial animals, plants and fungi, and antibiotics. The results reveal that each kingdom possesses the characteristic features. The main mechanisms in each kingdom are transporters/efflux pumps in phycotoxins, mutation and modification of targets and sequestration in marine and terrestrial animal toxins, ABC transporters and sequestration in plant toxins, transporters in fungal toxins, and various or mixed mechanisms in antibiotics. Antibiotic producers in particular make tremendous efforts for avoiding suicide, and are more flexible and adaptable to the changes of environments. With these features in mind, potential alternative strategies to overcome these resistance problems are discussed. This paper will provide clues for solving the issues of drug resistance.

Synthetic biology, genome mining, and combinatorial biosynthesis of NRPS-derived antibiotics: a perspective

Combinatorial biosynthesis of novel secondary metabolites derived from nonribosomal peptide synthetases (NRPSs) has been in slow development for about a quarter of a century. Progress has been hampered by the complexity of the giant multimodular multienzymes. More recently, advances have been made on understanding the chemical and structural biology of these complex megaenzymes, and on learning the design rules for engineering functional hybrid enzymes. In this perspective, I address what has been learned about successful engineering of complex lipopeptides related to daptomycin, and discuss how synthetic biology and microbial genome mining can converge to broaden the scope and enhance the speed and robustness of combinatorial biosynthesis of NRPS-derived natural products for drug discovery.