Regulation of specialised metabolites in Actinobacteria - expanding the paradigms

Culturomics of the pig tonsil microbiome identifies new species and an untapped source of novel antimicrobials

BACKGROUND

In humans and pigs, altered composition of the microbiota associated with the epithelium of the palatine tonsils has been associated with bacterial or viral infection and lymphoid tissue inflammation. Tonsil lymphoid tissue is important for immunity and considered an important portal of entry for pathogens such as Streptococcus suis. Little is known about correlations between tonsil-associated microbiota, tonsillar infections, and the species that might confer colonization resistance against pathogens. Here, we describe a large collection of representative bacterial species from the tonsil surface biofilm and used genome mining and in vitro assays to assess their potential as probiotics to reduce infections by S. suis and other pathogens.

RESULTS

Data on tonsil microbiota composition from over 100 piglets from 11 farms and 3 countries revealed a core microbiota comprising Actinobacillus, Streptococcus, and Moraxella and 11 other less abundant but prevalent genera. To establish a collection of culturable core species, we plated 5 tonsil swabs taken from healthy piglets on different farms and countries on 8 different media and isolated 518 pure cultures belonging to 23 genera. To identify candidate probiotic strains, we tested for antagonistic activity against a panel of pathogens and in silico genome mining to find biosynthetic gene clusters (BGCs) in isolates that might produce antimicrobial compounds. We identified two novel species with potential probiotic activities: a Brevibacterium species and Corynebacterium species producing a heat and proteolytically stable lanthipeptide variant of flavucin, inhibiting in vitro growth of the opportunistic pathogens S. suis and Staphylococcus aureus.

CONCLUSIONS

We defined the core tonsil microbiota of piglets and cultured representative single bacterial isolates for research on microbiota-host interactions in the oral cavity. Several isolates inhibiting the growth of bacterial pathogens that might be exploited as probiotics to promote colonization resistance were deposited in publicly available strain repositories. Our mining of genomes from cultured isolates suggests that the tonsil microbiota is an untapped source of novel antimicrobials Video Abstract.

Microbiome properties in the root nodules of Prosopis cineraria, a leguminous desert tree

We conducted a comprehensive analysis of the total microbiome and transcriptionally active microbiome communities in the roots and root nodules of Prosopis cineraria, an important leguminous tree in arid regions of many Asian countries. Mature P. cineraria trees growing in the desert did not exhibit any detected root nodules. However, we observed root nodules on the roots of P. cineraria growing on a desert farm and on young plants growing in a growth chamber, when inoculated with rhizosphere soil, including with rhizosphere soil from near desert tree roots that had no nodules. Compared to nearby soil, non-nodulated roots were enriched with Actinobacteria (e.g., Actinophytocola sp.), whereas root nodules sampled from the desert farm and growth chamber had abundant Alphaproteobacteria (e.g., Ensifer sp.). These nodules yielded many microbes in addition to such nitrogen-fixing bacteria as Ensifer and Sinorhizobium species. Significant differences exist in the composition and abundance of microbial isolates between the nodule surface and the nodule endosphere. Shotgun metagenome analysis of nodule endospheres revealed that the root nodules comprised over 90% bacterial DNA, whereas metatranscriptome analysis showed that the plant produces vastly more transcripts than the microbes in these nodules. Control inoculations demonstrated that four out of six Rhizobium, Agrobacterium, or Ensifer isolates purified from P. cineraria nodules produced nodules in the roots of P. cineraria seedlings under greenhouse conditions. The best nodulation was achieved when seedlings were inoculated with a mixture of those bacterial strains. Though root nodulation could be achieved under water stress conditions, nodule number and nodule biomass increased with copious water availability. .IMPORTANCEMicrobial communities were investigated in roots and root nodules of Prosopis cineraria, a leguminous tree species in arid Asian regions that is responsible for exceptionally important contributions to soil fertility in these dramatically dry locations. Soil removed from regions near nodule-free roots on these mature plants contained an abundance of bacteria with the genetic ability to generate nodules and fix nitrogen but did not normally nodulate in their native rhizosphere environment, suggesting a very different co-evolved relationship than that observed for herbaceous legumes. The relative over-expression of the low-gene-density plant DNA compared to the bacterial DNA in the nodules was also unexpected, indicating a very powerful induction of host genetic contributions within the nodule. Finally, the water dependence of nodulation in inoculated seedlings suggested a possible link between early seedling growth (before a deep root system can be developed) and the early development of nitrogen-fixing capability.

Natural product discovery in soil actinomycetes: unlocking their potential within an ecological context

Natural products (NPs) produced by bacteria, particularly soil actinomycetes, often possess diverse bioactivities and play a crucial role in human health, agriculture, and biotechnology. Soil actinomycete genomes contain a vast number of predicted biosynthetic gene clusters (BGCs) yet to be exploited. Understanding the factors governing NP production in an ecological context and activating cryptic and silent BGCs in soil actinomycetes will provide researchers with a wealth of molecules with potential novel applications. Here, we highlight recent advances in NP discovery strategies employing ecology-inspired approaches and discuss the importance of understanding the environmental signals responsible for activation of NP production, particularly in a soil microbial community context, as well as the challenges that remain.

Harnessing regulatory networks in Actinobacteria for natural product discovery

Microbes typically live in complex habitats where they need to rapidly adapt to continuously changing growth conditions. To do so, they produce an astonishing array of natural products with diverse structures and functions. Actinobacteria stand out for their prolific production of bioactive molecules, including antibiotics, anticancer agents, antifungals, and immunosuppressants. Attention has been directed especially towards the identification of the compounds they produce and the mining of the large diversity of biosynthetic gene clusters (BGCs) in their genomes. However, the current return on investment in random screening for bioactive compounds is low, while it is hard to predict which of the millions of BGCs should be prioritized. Moreover, many of the BGCs for yet undiscovered natural products are silent or cryptic under laboratory growth conditions. To identify ways to prioritize and activate these BGCs, knowledge regarding the way their expression is controlled is crucial. Intricate regulatory networks control global gene expression in Actinobacteria, governed by a staggering number of up to 1000 transcription factors per strain. This review highlights recent advances in experimental and computational methods for characterizing and predicting transcription factor binding sites and their applications to guide natural product discovery. We propose that regulation-guided genome mining approaches will open new avenues toward eliciting the expression of BGCs, as well as prioritizing subsets of BGCs for expression using synthetic biology approaches.

ONE-SENTENCE SUMMARY

This review provides insights into advances in experimental and computational methods aimed at predicting transcription factor binding sites and their applications to guide natural product discovery.

Manipulation and epigenetic control of silent biosynthetic pathways in actinobacteria

Most biosynthetic gene clusters (BGCs) of Actinobacteria are either silent or expressed less than the detectable level. The non-genetic approaches including biological interactions, chemical agents, and physical stresses that can be used to awaken silenced pathways are compared in this paper. These non-genetic induction strategies often need screening approaches, including one strain many compounds (OSMAC), reporter-guided mutant selection, and high throughput elicitor screening (HiTES) have been developed. Different types of genetic manipulations applied in the induction of cryptic BGCs of Actinobacteria can be categorized as genome-wide pleiotropic and targeted approaches like manipulation of global regulatory systems, modulation of regulatory genes, ribosome and engineering of RNA polymerase or phosphopantheteine transferases. Targeted approaches including genome editing by CRISPR, mutation in transcription factors and modification of BGCs promoters, inactivation of the highly expressed biosynthetic pathways, deleting the suppressors or awakening the activators, heterologous expression, or refactoring of gene clusters can be applied for activation of pathways which are predicted to synthesize new bioactive structures in genome mining studies of Acinobacteria. In this review, the challenges and advantages of employing these approaches in induction of Actinobacteria BGCs are discussed. Further, novel natural products needed as drug for pharmaceutical industry or as biofertilizers in agricultural industry can be discovered even from known species of Actinobactera by the innovative approaches of metabolite biosynthesis elicitation.

Trends in the two-component system's role in the synthesis of antibiotics by Streptomyces

Despite the advances in understanding the regulatory networks for secondary metabolite production in Streptomyces, the participation of the two-component systems (TCS) in this process still requires better characterization. These sensing systems and their responses to environmental stimuli have been described by evaluating mutant strains with techniques that allow in-depth regulatory responses. However, defining the stimulus that triggers their activation is still a task. The transmembrane nature of the sensor kinases and the high content of GC in the streptomycetes represent significant challenges in their study. In some examples, adding elements to the assay medium has determined the respective ligand. However, a complete TCS description and characterization requires specific amounts of the involved proteins that are most difficult to obtain. The availability of enough sensor histidine kinase concentrations could facilitate the identification of the ligand-protein interaction, and besides would allow the establishment of its phosphorylation mechanisms and determine their tridimensional structure. Similarly, the advances in the development of bioinformatics tools and novel experimental techniques also promise to accelerate the TCSs description and provide knowledge on their participation in the regulation processes of secondary metabolite formation. This review aims to summarize the recent advances in the study of TCSs involved in antibiotic biosynthesis and to discuss alternatives to continue their characterization. KEY POINTS: • TCSs are the environmental signal transducers more abundant in nature. • The Streptomyces have some of the highest number of TCSs found in bacteria. • The study of signal transduction between SHKs and RRs domains is a big challenge.

Streptomyces and their specialised metabolites for phytopathogen control - comparative in vitro and in planta metabolic approaches

In the search for new crop protection microbial biocontrol agents, isolates from the genus Streptomyces are commonly found with promising attributes. Streptomyces are natural soil dwellers and have evolved as plant symbionts producing specialised metabolites with antibiotic and antifungal activities. Streptomyces biocontrol strains can effectively suppress plant pathogens via direct antimicrobial activity, but also induce plant resistance through indirect biosynthetic pathways. The investigation of factors stimulating the production and release of Streptomyces bioactive compounds is commonly conducted in vitro, between Streptomyces sp. and a plant pathogen. However, recent research is starting to shed light on the behaviour of these biocontrol agents in planta, where the biotic and abiotic conditions share little similarity to those of controlled laboratory conditions. With a focus on specialised metabolites, this review details (i) the various methods by which Streptomyces biocontrol agents employ specialised metabolites as an additional line of defence against plant pathogens, (ii) the signals shared in the tripartite system of plant, pathogen and biocontrol agent, and (iii) an outlook on new approaches to expedite the identification and ecological understanding of these metabolites under a crop protection lens.

On the evolution of natural product biosynthesis

Natural products are the raw material for drug discovery programmes. Bioactive natural products are used extensively in medicine and agriculture and have found utility as antibiotics, immunosuppressives, anti-cancer drugs and anthelminthics. Remarkably, the natural role and what mechanisms drive evolution of these molecules is relatively poorly understood. The exponential increase in genome and chemical data in recent years, coupled with technical advances in bioinformatics and genetics have enabled progress to be made in understanding the evolution of biosynthetic gene clusters and the products of their enzymatic machinery. Here we discuss the diversity of natural products, incorporating the mechanisms that govern evolution of metabolic pathways and how this can be applied to biosynthetic gene clusters. We build on the nomenclature of natural products in terms of primary, integrated, secondary and specialised metabolism and place this within an ecology-evolutionary-developmental biology framework. This eco-evo-devo framework we believe will help to clarify the nature and use of the term specialised metabolites in the future.

Megalochelin, a Tridecapeptide Siderophore from a Talented Streptomycete

Streptomycetes are bacteria known for their extraordinary biosynthetic capabilities. Herein, we describe the genome and metabolome of a particularly talented strain, Streptomyces ID71268. Its 8.4-Mbp genome harbors 32 bioinformatically predicted biosynthetic gene clusters (BGCs), out of which 10 are expressed under a single experimental condition. In addition to five families of known metabolites with previously assigned BGCs (nigericin, azalomycin F, ectoine, SF2766, and piericidin), we were able to predict BGCs for three additional metabolites: streptochlorin, serpetene, and marinomycin. The strain also produced two families of presumably novel metabolites, one of which was associated with growth inhibitory activity against the human opportunistic pathogen Acinetobacter baumannii in an iron-dependent manner. Bioassay-guided fractionation, followed by extensive liquid chromatography-mass spectrometry (LC-MS) and NMR analyses, established that the molecule responsible for the observed antibacterial activity is an unusual tridecapeptide siderophore with a ring-and-tail structure: the heptapeptide ring is formed through a C-C bond between a 2,3-dihydroxybenzoate (DHB) cap on Gly1 and the imidazole moiety of His7, while the hexapeptide tail is sufficient for binding iron. This molecule, named megalochelin, is the largest known siderophore. The megalochelin BGC encodes a 13-module nonribosomal peptide synthetase for the synthesis of the tridecapeptide, and a copper-dependent oxidase, likely responsible for the DHB-imidazole cross-link, whereas the genes for synthesis of the DHB starter unit are apparently specified in trans by a different BGC. Our results suggest that prolific producers of specialized metabolites may conceal hidden treasures within a background of known compounds.

Enhanced Rishirilide Biosynthesis by a Rare In-Cluster Phosphopantetheinyl Transferase in Streptomyces xanthophaeus

Phosphopantetheinyl transferases (PPTases) play important roles in activating apo-acyl carrier proteins (apo-ACPs) and apo-peptidyl carrier proteins (apo-PCPs) in both primary and secondary metabolism. PPTases catalyze the posttranslational modifications of those carrier proteins by covalent attachment of the 4'-phosphopantetheine group to a conserved serine residue. The protein-protein interactions between a PPTase and a cognate acyl or peptidyl carrier protein have important regulatory functions in microbial biosynthesis, but the molecular mechanism underlying their specific recognition remains elusive. In this study, we identified a new rishirilide biosynthetic gene cluster with a rare in-cluster PPTase from Streptomyces xanthophaeus no2. The function of this Sfp-type PPTase, SxrX, in rishirilide production was confirmed using genetic mutagenesis and biochemical characterization. We applied molecular modeling and site-directed mutagenesis to identify key residues mediating the protein-protein interaction between SxrX and its cognate ACP. In addition, six natural products were isolated from wild-type S. xanthophaeus no2 and the ΔsxrX mutant, including rishirilide A and lupinacidin A, that exhibited antimicrobial and anticancer activities, respectively. SxrX is the first Sfp-type PPTase identified from an aromatic polyketide biosynthetic gene cluster and shown to be responsible for high-level production of rishirilide derivatives. IMPORTANCE Genome mining has been a vital means for natural product drug discovery in the postgenomic era. The rishirilide-type polyketides have attracted attention due to their potent bioactivity, but the poor robustness of production hosts has limited further research and development. This study not only identifies a hyperproducer of rishirilides but also reveals a rare, in-cluster PPTase SxrX that plays an important role in boosting rishirilide biosynthesis. Experimental and computational investigations revealed new insights on the protein-protein interaction between SxrX and its cognate ACP with wide implications for understanding polyketide biosynthesis.

ActinoBase: tools and protocols for researchers working on Streptomyces and other filamentous actinobacteria

Actinobacteria is an ancient phylum of Gram-positive bacteria with a characteristic high GC content to their DNA. The ActinoBase Wiki is focused on the filamentous actinobacteria, such as Streptomyces species, and the techniques and growth conditions used to study them. These organisms are studied because of their complex developmental life cycles and diverse specialised metabolism which produces many of the antibiotics currently used in the clinic. ActinoBase is a community effort that provides valuable and freely accessible resources, including protocols and practical information about filamentous actinobacteria. It is aimed at enabling knowledge exchange between members of the international research community working with these fascinating bacteria. ActinoBase is an anchor platform that underpins worldwide efforts to understand the ecology, biology and metabolic potential of these organisms. There are two key differences that set ActinoBase apart from other Wiki-based platforms: [1] ActinoBase is specifically aimed at researchers working on filamentous actinobacteria and is tailored to help users overcome challenges working with these bacteria and [2] it provides a freely accessible resource with global networking opportunities for researchers with a broad range of experience in this field.

N-Acetyl-Cysteinylated Streptophenazines from Streptomyces

Here, we describe two N-acetyl-cysteinylated streptophenazines (1 and 2) produced by the soil-derived Streptomyces sp. ID63040 and identified through a metabolomic approach. These metabolites attracted our interest due to their low occurrence frequency in a large library of fermentation broth extracts and their consistent presence in biological replicates of the producer strain. The compounds were found to possess broad-spectrum antibacterial activity while exhibiting low cytotoxicity. The biosynthetic gene cluster from Streptomyces sp. ID63040 was found to be highly similar to the streptophenazine reference cluster in the MIBiG database, which originates from the marine Streptomyces sp. CNB-091. Compounds 1 and 2 were the main streptophenazine products from Streptomyces sp. ID63040 at all cultivation times but were not detected in Streptomyces sp. CNB-091. The lack of obvious candidates for cysteinylation in the Streptomyces sp. ID63040 biosynthetic gene cluster suggests that the N-acetyl-cysteine moiety derives from cellular functions, most likely from mycothiol. Overall, our data represent an interesting example of how to leverage metabolomics for the discovery of new natural products and point out the often-neglected contribution of house-keeping cellular functions to natural product diversification.

Type S Non‐Ribosomal Peptide Synthetases for the Rapid Generation of Tailormade Peptide Libraries**

Bacterial natural products in general, and non‐ribosomally synthesized peptides in particular, are structurally diverse and provide us with a broad range of pharmaceutically relevant bioactivities. Yet, traditional natural product research suffers from rediscovering the same scaffolds and has been stigmatized as inefficient, time‐, labour‐ and cost‐intensive. Combinatorial chemistry, on the other hand, can produce new molecules in greater numbers, cheaper and in less time than traditional natural product discovery, but also fails to meet current medical needs due to the limited biologically relevant chemical space that can be addressed. Consequently, methods for the high throughput generation of new natural products would offer a new approach to identifying novel bioactive chemical entities for the hit to lead phase of drug discovery programs. As a follow‐up to our previously published proof‐of‐principle study on generating bipartite type S non‐ribosomal peptide synthetases (NRPSs), we now envisaged the de novo generation of non‐ribosomal peptides (NRPs) on an unreached scale. Using synthetic zippers, we split NRPSs in up to three subunits and rapidly generated different bi‐ and tripartite NRPS libraries to produce 49 peptides, peptide derivatives, and de novo peptides at good titres up to 145 mg L⁻¹. A further advantage of type S NRPSs not only is the possibility to easily expand the created libraries by re‐using previously created type S NRPS, but that functions of individual domains as well as domain‐domain interactions can be studied and assigned rapidly.

Effects of dual deletion of glnR and mtrA on expression of nitrogen metabolism genes in Streptomyces venezuelae

GlnR activates nitrogen metabolism genes under nitrogen‐limited conditions, whereas MtrA represses these genes under nutrient‐rich conditions in Streptomyces. In this study, we compared the transcription patterns of nitrogen metabolism genes in a double deletion mutant (ΔmtrA‐glnR) lacking both mtrA and glnR and in mutants lacking either mtrA (ΔmtrA) or glnR (ΔglnR). The nitrogen metabolism genes were expressed similarly in ΔmtrA‐glnR and ΔglnR under both nitrogen‐limited and nutrient‐rich conditions, with patterns distinctly different from that of ΔmtrA, suggesting a decisive role for GlnR in the control of nitrogen metabolism genes and further suggesting that regulation of these genes by MtrA is GlnR‐dependent. MtrA and GlnR utilize the same binding sites upstream of nitrogen metabolism genes, and we showed stronger in vivo binding of MtrA to these sites under nutrient‐rich conditions and of GlnR under nitrogen‐limited conditions, consistent with the higher levels of MtrA or GlnR under those respective conditions. In addition, we showed that both mtrA and glnR are self‐regulated. Our study provides new insights into the regulation of nitrogen metabolism genes in Streptomyces.

The ubiquitous catechol moiety elicits siderophore and angucycline production in Streptomyces

Actinobacteria are a rich source of bioactive molecules, and genome sequencing has shown that the vast majority of their biosynthetic potential has yet to be explored. However, many of their biosynthetic gene clusters (BGCs) are poorly expressed in the laboratory, which prevents discovery of their cognate natural products. To exploit their full biosynthetic potential, better understanding of the signals that promote the expression of BGCs is needed. Here, we show that the human stress hormone epinephrine (adrenaline) elicits siderophore production by Actinobacteria. Catechol was established as the likely eliciting moiety, since similar responses were seen for catechol and for the catechol-containing molecules dopamine and catechin but not for related molecules. Exploration of the catechol-responsive strain Streptomyces sp. MBT84 using mass spectral networking revealed elicitation of a BGC that produces the angucycline glycosides aquayamycin, urdamycinone B and galtamycin C. Heterologous expression of the catechol-cleaving enzymes catechol 1,2-dioxygenase or catechol 2,3-dioxygenase counteracted the eliciting effect of catechol. Thus, our work identifies the ubiquitous catechol moiety as a novel elicitor of the expression of BGCs for specialized metabolites.

Rare actinobacteria isolated from the hypersaline Ojo de Liebre Lagoon as a source of novel bioactive compounds with biotechnological potential

The Ojo de Liebre Lagoon is a Marine Protected Area that lies within a UNESCO World Heritage Site and is a critical habitat for important migratory species such as the grey whale and bird species. Unique hypersaline environments, such as the Ojo de Liebre Lagoon, are underexplored in terms of their bacterial and chemical diversity, representing a potential source for new bioactive compounds with pharmacological properties. Actinobacteria are one of the most diverse and prolific taxonomic bacterial groups in terms of marine bioactive compounds. This study aimed to identify the culturable actinobacterial community inhabiting the Lagoon, as well as to test their potential as new sources of anticancer compounds with pharmacological potential. A selective isolation approach focused on spore-forming bacteria from 40 sediment samples generated a culture collection of 64 strains. The 16S rRNA gene analyses identified three phyla in this study, the Actinobacteria, Firmicutes and Proteobacteria, where the phylum Actinobacteria dominated (57%) the microbial community profiles. Within the Actinobacteria, nine different genera were isolated including the Actinomadura, Micromonospora, Nocardiopsis, Plantactinospora and Streptomyces sp. We observed seasonal differences on actinobacteria recovery. For instance, Micromonospora strains were recovered during the four sampling seasons, while Arthrobacter and Pseudokineococcus were only isolated in February 2018, and Blastococcus, Rhodococcus and Streptomyces were uniquely isolated in June 2018. Ethyl acetate crude extracts derived from actinobacterial cultures were generated and screened for cytotoxic activity against six cancer cell lines. Strains showed promising low percentages of viability on lung (H1299), cervical (SiHa), colon (Caco-2) and liver (HepG2) cancer lines. Molecular networking results suggest many of the metabolites produced by these strains are unknown and they might harbour novel chemistry. Our results showed the Ojo de Liebre Lagoon is a novel source for isolating diverse marine actinobacteria which produce promising bioactive compounds for potential biotechnological use as anticancer agents.

Into the Unknown: Microbial Communities in Caves, Their Role, and Potential Use

Caves have been an item of amateur and professional exploration for many years. Research on the karst caves has revealed great diversity of bacteria, algae, and fungi living on stone walls and speleothems, in mud puddles or sediments. They have become the source of interest for various research groups including geologists, chemists, ecologists, or microbiologists. The adaptations of cave-dwelling organisms applied to their survival are complex and some of their properties show potential to be used in various areas of human life. Secondary metabolites produced by cave’s bacteria show strong antimicrobial, anti-inflammatory, or anticancer properties. Furthermore, bacteria that can induce mineral precipitation could be used in the construction industry and for neutralization of radioisotopes. In this review we focus on bacteria and algae present in cave ecosystems, their role in shaping such specific environment, and their biotechnological and medical potential.

Genome analysis suggests the bacterial family Acetobacteraceae is a source of undiscovered specialized metabolites

Acetobacteraceae is an economically important family of bacteria that is used for industrial fermentation in the food/feed sector and for the preparation of sorbose and bacterial cellulose. It comprises two major groups: acetous species (acetic acid bacteria) associated with flowers, fruits and insects, and acidophilic species, a phylogenetically basal and physiologically heterogeneous group inhabiting acid or hot springs, sludge, sewage and freshwater environments. Despite the biotechnological importance of the family Acetobacteraceae, the literature does not provide any information about its ability to produce specialized metabolites. We therefore constructed a phylogenomic tree based on concatenated protein sequences from 141 type strains of the family and predicted the presence of small-molecule biosynthetic gene clusters (BGCs) using the antiSMASH tool. This dual approach allowed us to associate certain biosynthetic pathways with particular taxonomic groups. We found that acidophilic and acetous species contain on average ~ 6.3 and ~ 3.4 BGCs per genome, respectively. All the Acetobacteraceae strains encoded proteins involved in hopanoid biosynthesis, with many also featuring genes encoding type-1 and type-3 polyketide and non-ribosomal peptide synthases, and enzymes for aryl polyene, lactone and ribosomal peptide biosynthesis. Our in silico analysis indicated that the family Acetobacteraceae is a potential source of many undiscovered bacterial metabolites and deserves more detailed experimental exploration.

Transcriptional regulation of congocidine (netropsin) biosynthesis and resistance

The production of specialized metabolites by Streptomyces bacteria is usually temporally regulated. This regulation is complex and frequently involves both global and pathway-specific mechanisms. Streptomyces ambofaciens ATCC23877 produces several specialized metabolites, including spiramycins, stambomycins, kinamycins and congocidine. The production of the first three molecules has been shown to be controlled by one or several cluster-situated transcriptional regulators. However, nothing is known regarding the regulation of congocidine biosynthesis. Congocidine (netropsin) belongs to the family of pyrrolamide metabolites, which also includes distamycin and anthelvencins. Most pyrrolamides bind into the minor groove of DNA, specifically in A/T-rich regions, which gives them numerous biological activities, such as antimicrobial and antitumoral activities. We previously reported the characterization of the pyrrolamide biosynthetic gene clusters of congocidine (cgc) in S. ambofaciens ATCC23877, distamycin (dst) in Streptomyces netropsis DSM40846 and anthelvencins (ant) in Streptomyces venezuelae ATCC14583. The three gene clusters contain a gene encoding a putative transcriptional regulator, cgc1, dst1 and ant1 respectively. Cgc1, Dst1 and Ant1 present a high percentage of amino acid sequence similarity. We demonstrate here that Cgc1, an atypical orphan response regulator, activates the transcription of all cgc genes in the stationary phase of S. ambofaciens growth. We also show that the cgc cluster is constituted of eight main transcriptional units. Finally, we show that congocidine induces the expression of the transcriptional regulator Cgc1 and of the operon containing the resistance genes (cgc20 and cgc21, coding for an ABC transporter), and propose a model for the transcriptional regulation of the cgc gene cluster. Importance Understanding the mechanisms of regulation of specialized metabolite production can have important implications both at the level of specialized metabolism study (expression of silent gene clusters) and the biotechnological level (increase of the production of a metabolite of interest). We report here a study on the regulation of the biosynthesis of a metabolite from the pyrrolamide family, congocidine. We show that congocidine biosynthesis and resistance is controlled by Cgc1, a cluster-situated regulator. As the gene clusters directing the biosynthesis of the pyrrolamides distamycin and anthelvencin encode a homolog of Cgc1, our findings may be relevant for the biosynthesis of other pyrrolamides. In addition, our results reveal a new type of feed-forward induction mechanism, in which congocidine induces its own biosynthesis through the induction of the transcription of cgc1.

Biodiversity of Secondary Metabolites Compounds Isolated from Phylum Actinobacteria and Its Therapeutic Applications

The current review aims to summarise the biodiversity and biosynthesis of novel secondary metabolites compounds, of the phylum Actinobacteria and the diverse range of secondary metabolites produced that vary depending on its ecological environments they inhabit. Actinobacteria creates a wide range of bioactive substances that can be of great value to public health and the pharmaceutical industry. The literature analysis process for this review was conducted using the VOSviewer software tool to visualise the bibliometric networks of the most relevant databases from the Scopus database in the period between 2010 and 22 March 2021. Screening and exploring the available literature relating to the extreme environments and ecosystems that Actinobacteria inhabit aims to identify new strains of this major microorganism class, producing unique novel bioactive compounds. The knowledge gained from these studies is intended to encourage scientists in the natural product discovery field to identify and characterise novel strains containing various bioactive gene clusters with potential clinical applications. It is evident that Actinobacteria adapted to survive in extreme environments represent an important source of a wide range of bioactive compounds. Actinobacteria have a large number of secondary metabolite biosynthetic gene clusters. They can synthesise thousands of subordinate metabolites with different biological actions such as anti-bacterial, anti-parasitic, anti-fungal, anti-virus, anti-cancer and growth-promoting compounds. These are highly significant economically due to their potential applications in the food, nutrition and health industries and thus support our communities' well-being.

Biotransformation of Daidzein, Genistein, and Naringenin by Streptomyces Species Isolated from High-Altitude Soil of Nepal

Flavonoids have achieved widespread importance in pharmaceutical, food, and cosmetics industries. Furthermore, modification of these naturally occurring flavonoids to structurally diverse compounds through whole cell biotransformation with enhanced biological activities has numerous biotechnological applications. The present study investigated the biotransformation potential of Streptomyces species isolated from a high-altitude-soil sample towards selected flavonoid molecules. The biotransformed metabolites were confirmed by comparing the HPLC chromatogram with authentic compounds and LC-MS/MS analysis. Of these isolates, Streptomyces species G-18 (Accession number: MW663767.1) catalyzed isoflavone molecules daidzein and genistein to produce hydroxylated products at 24 h of reaction condition in a whole cell system. The hydroxylation of daidzein (4′,7-dihydroxyisoflavone) was confirmed at 3′-position of the B ring to produce 3′,4′,7-trihydroxyisoflavone. In addition, Streptomyces species G-14 (Accession number: MW663770.1) and Streptomyces species S4L (Accession number: MW663769.1) also revealed the transformation of daidzein (4′,7-dihydroxyisoflavone) to hydroxy daidzein at a distinct position than that of G-18 isolates, whereas thee Streptomyces species S4L reaction mixture with naringenin as a substrate also revealed the hydroxylated product. Our results demonstrated that microorganisms isolated from different ecological niches have broad application.

Diversity and plant growth-promoting potential of (un)culturable bacteria in the Hedera helix phylloplane

Background

A diverse community of microbes naturally exists on the phylloplane, the surface of leaves. It is one of the most prevalent microbial habitats on earth and bacteria are the most abundant members, living in communities that are highly dynamic. Today, one of the key challenges for microbiologists is to develop strategies to culture the vast diversity of microorganisms that have been detected in metagenomic surveys.

Results

We isolated bacteria from the phylloplane of Hedera helix (common ivy), a widespread evergreen, using five growth media: Luria–Bertani (LB), LB01, yeast extract–mannitol (YMA), yeast extract–flour (YFlour), and YEx. We also included a comparison with the uncultured phylloplane, which we showed to be dominated by Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes. Inter-sample (beta) diversity shifted from LB and LB01 containing the highest amount of resources to YEx, YMA, and YFlour which are more selective. All growth media equally favoured Actinobacteria and Gammaproteobacteria, whereas Bacteroidetes could only be found on LB01, YEx, and YMA. LB and LB01 favoured Firmicutes and YFlour was most selective for Betaproteobacteria. At the genus level, LB favoured the growth of Bacillus and Stenotrophomonas, while YFlour was most selective for Burkholderia and Curtobacterium. The in vitro plant growth promotion (PGP) profile of 200 isolates obtained in this study indicates that previously uncultured bacteria from the phylloplane may have potential applications in phytoremediation and other plant-based biotechnologies.

Conclusions

This study gives first insights into the total bacterial community of the H. helix phylloplane, including an evaluation of its culturability using five different growth media. We further provide a collection of 200 bacterial isolates underrepresented in current databases, including the characterization of PGP profiles. Here we highlight the potential of simple strategies to obtain higher microbial diversity from environmental samples and the use of high-throughput sequencing to guide isolate selection from a variety of growth media.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02119-z.

Complete Genome Sequence of Microbacterium sp. Strain Nx66, Isolated from Waters Contaminated with Petrochemicals in El Saf-Saf Valley, Algeria

Microbacterium sp. strain Nx66 was isolated from waters contaminated by petrochemical effluents collected in Algeria. Its genome was sequenced using Illumina MiSeq (2 × 150-bp read pairs) and Oxford Nanopore (long reads) technologies and was assembled using Unicycler. It is composed of one chromosome of 3.42 Mb and one plasmid of 34.22 kb.

Microbacterium sp. strain Nx66 was isolated from waters contaminated by petrochemical effluents collected in Algeria. Its genome was sequenced using Illumina MiSeq (2 × 150-bp read pairs) and Oxford Nanopore (long reads) technologies and was assembled using Unicycler. It is composed of one chromosome of 3.42 Mb and one plasmid of 34.22 kb.

Combinatorial Control of Plant Specialized Metabolism: Mechanisms, Functions, and Consequences

Plants constantly perceive internal and external cues, many of which they need to address to safeguard their proper development and survival. They respond to these cues by selective activation of specific metabolic pathways involving a plethora of molecular players that act and interact in complex networks. In this review, we illustrate and discuss the complexity in the combinatorial control of plant specialized metabolism. We hereby go beyond the intuitive concept of combinatorial control as exerted by modular-acting complexes of transcription factors that govern expression of specialized metabolism genes. To extend this discussion, we also consider all known hierarchical levels of regulation of plant specialized metabolism and their interfaces by referring to reported regulatory concepts from the plant field. Finally, we speculate on possible yet-to-be-discovered regulatory principles of plant specialized metabolism that are inspired by knowledge from other kingdoms of life and areas of biological research.

Evolving Antibiotics against Resistance: a Potential Platform for Natural Product Development?

To avoid an antibiotic resistance crisis, we need to develop antibiotics at a pace that matches the rate of evolution of resistance. However, the complex functions performed by antibiotics-combining, e.g., penetration of membranes, counteraction of resistance mechanisms, and interaction with molecular targets-have proven hard to achieve with current methods for drug development, including target-based screening and rational design. Here, we argue that we can meet the evolution of resistance in the clinic with evolution of antibiotics in the laboratory. On the basis of the results of experimental evolution studies of microbes in general and antibiotic production in Actinobacteria in particular, we propose methodology for evolving antibiotics to circumvent mechanisms of resistance. This exploits the ability of evolution to find solutions to complex problems without a need for design. We review evolutionary theory critical to this approach and argue that it is feasible and has important advantages over current methods for antibiotic discovery.

Evolutionary dynamics of natural product biosynthesis in bacteria

Covering: 2008 up to 2019The forces of biochemical adaptive evolution operate at the level of genes, manifesting in complex phenotypes and the global biodiversity of proteins and metabolites. While evolutionary histories have been deciphered for some other complex traits, the origins of natural product biosynthesis largely remain a mystery. This fundamental knowledge gap is surprising given the many decades of research probing the genetic, chemical, and biophysical mechanisms of bacterial natural product biosynthesis. Recently, evolutionary thinking has begun to permeate this otherwise mechanistically dominated field. Natural products are now sometimes referred to as 'specialized' rather than 'secondary' metabolites, reinforcing the importance of their biological and ecological functions. Here, we review known evolutionary mechanisms underlying the overwhelming chemical diversity of bacterial secondary metabolism, focusing on enzyme promiscuity and the evolution of enzymatic domains that enable metabolic traits. We discuss the mechanisms that drive the assembly of natural product biosynthetic gene clusters and propose formal definitions for 'specialized' and 'secondary' metabolism. We further explore how biosynthetic gene clusters evolve to synthesize related molecular species, and in turn how the biological and ecological roles that emerge from metabolic diversity are acted on by selection. Finally, we reconcile chemical, functional, and genetic data into an evolutionary model, the dynamic chemical matrix evolutionary hypothesis, in which the relationships between chemical distance, biomolecular activity, and relative fitness shape adaptive landscapes.

Nanoparticles from Actinobacteria: A Potential Target to Antimicrobial Therapy

Nanoparticles have gained significant importance in the past two decades, due to their multifaceted applications in the field of nanomedicine. As our ecosystems and habitats are changing due to global warming, many new diseases are emerging continuously. Treating these costs a lot of money and mostly ends up in failure. In addition, frequent use of antibiotics to control the emerging diseases has led the pathogens to develop resistance to antibiotics. Hence, the nanoparticles are targeted to treat such diseases instead of the costly antibiotics. In particular, the biosynthesized nanoparticles have received considerable attention due to their simple, eco-friendly and promising activity. To highlight, microbial mediated nanoparticles have been found to possess higher activity and thus have a promising role in antimicrobial therapy to fight against the emerging drug-resistant pathogens. In this context, this review article is aimed at highlight the role of nanoparticles in the field of nanomedicine and importance of actinobacteria in the nanoparticle synthesis and their need in antimicrobial therapy. This is a comprehensive review, focusing on the potential of actinobacteria-mediated nanoparticles in the field of nanomedicine.

Unlocking the Spatial Control of Secondary Metabolism Uncovers Hidden Natural Product Diversity in Nostoc punctiforme

Filamentous cyanobacteria belong to the most prolific producers of structurally unique and biologically active natural products, yet the majority of biosynthetic gene clusters predicted for these multicellular collectives are currently orphan. Here, we present a systems analysis of secondary metabolite gene expression in the model strain Nostoc punctiforme PCC73102 using RNA-seq and fluorescence reporter analysis. Our data demonstrate that the majority of the cryptic gene clusters are not silent but are expressed with regular or sporadic pattern. Cultivation of N. punctiforme using high-density fermentation overrules the spatial control and leads to a pronounced upregulation of more than 50% of biosynthetic gene clusters. Our data suggest that a combination of autocrine factors, a high CO₂ level, and high light account for the upregulation of individual pathways. Our overarching study not only sheds light on the strategies of filamentous cyanobacteria to share the enormous metabolic burden connected with the production of specialized molecules but provides an avenue for the genome-based discovery of natural products in multicellular cyanobacteria as exemplified by the discovery of highly unusual variants of the tricyclic peptide microviridin.

Archaea dominate the microbial community in an ecosystem with low-to-moderate temperature and extreme acidity

BACKGROUND

The current view suggests that in low-temperature acidic environments, archaea are significantly less abundant than bacteria. Thus, this study of the microbiome of Parys Mountain (Anglesey, UK) sheds light on the generality of this current assumption. Parys Mountain is a historically important copper mine and its acid mine drainage (AMD) water streams are characterised by constant moderate temperatures (8-18 °C), extremely low pH (1.7) and high concentrations of soluble iron and other metal cations.

RESULTS

Metagenomic and SSU rRNA amplicon sequencing of DNA from Parys Mountain revealed a significant proportion of archaea affiliated with Euryarchaeota, which accounted for ca. 67% of the community. Within this phylum, potentially new clades of Thermoplasmata were overrepresented (58%), with the most predominant group being "E-plasma", alongside low-abundant Cuniculiplasmataceae, 'Ca. Micrarchaeota' and 'Terrestrial Miscellaneous Euryarchaeal Group' (TMEG) archaea, which were phylogenetically close to Methanomassilicoccales and clustered with counterparts from acidic/moderately acidic settings. In the sediment, archaea and Thermoplasmata contributed the highest numbers in V3-V4 amplicon reads, in contrast with the water body community, where Proteobacteria, Nitrospirae, Acidobacteria and Actinobacteria outnumbered archaea. Cultivation efforts revealed the abundance of archaeal sequences closely related to Cuniculiplasma divulgatum in an enrichment culture established from the filterable fraction of the water sample. Enrichment cultures with unfiltered samples showed the presence of Ferrimicrobium acidiphilum, C. divulgatum, 'Ca. Mancarchaeum acidiphilum Mia14', 'Ca. Micrarchaeota'-related and diverse minor (< 2%) bacterial metagenomic reads.

CONCLUSION

Contrary to expectation, our study showed a high abundance of archaea in this extremely acidic mine-impacted environment. Further, archaeal populations were dominated by one particular group, suggesting that they are functionally important. The prevalence of archaea over bacteria in these microbiomes and their spatial distribution patterns represents a novel and important advance in our understanding of acidophile ecology. We also demonstrated a procedure for the specific enrichment of cell wall-deficient members of the archaeal component of this community, although the large fraction of archaeal taxa remained unculturable. Lastly, we identified a separate clustering of globally occurring acidophilic members of TMEG that collectively belong to a distinct order within Thermoplasmata with yet unclear functional roles in the ecosystem.

Streptomyces coelicolor

Streptomyces coelicolor A3(2) is amongst the best studied representatives of the genus Streptomyces, which is the largest genus within the Actinobacteria. Streptomycetes have a remarkably complex developmental life cycle and the capacity to produce a plethora of natural products. Whilst referred to as S. coelicolor A3(2), this strain is more closely related to Streptomyces violaceoruber ISP5049 than to the type strain for the species, S. coelicolor Müller. However, the name was maintained as it had become the workhorse for genetics and a model for development and antibiotic production. Streptomycetes are multicellular mycelial bacteria that grow as vegetative hyphae, which are compartmentalized via cross-walls. Reproduction occurs when specialized aerial hyphae differentiate into chains of spores. Streptomycetes produce around half of the clinically used antibiotics and other pharmaceutically useful natural products such as anthelmintics, anticancer agents, and immunosuppressives.

Mining Actinomycetes for Novel Antibiotics in the Omics Era: Are We Ready to Exploit This New Paradigm?

The current spread of multi-drug resistance in a number of key pathogens and the lack of therapeutic solutions in development to address most of the emerging infections in the clinic that are difficult to treat have become major concerns. Microbial natural products represent one of the most important sources for the discovery of potential new antibiotics and actinomycetes have been one of the most relevant groups that are prolific producers of these bioactive compounds. Advances in genome sequencing and bioinformatic tools have collected a wealth of knowledge on the biosynthesis of these molecules. This has revealed the broad untapped biosynthetic diversity of actinomycetes, with large genomes and the capacity to produce more molecules than previously estimated, opening new opportunities to identify the novel classes of compounds that are awaiting to be discovered. Comparative genomics, metabolomics and proteomics and the development of new analysis and genetic engineering tools provide access to the integration of new knowledge and better understanding of the physiology of actinomycetes and their tight regulation of the production of natural products antibiotics. This new paradigm is fostering the development of new genomic-driven and culture-based strategies, which aims to deliver new chemical classes of antibiotics to be developed to the clinic and replenish the exhausted pipeline of drugs for fighting the progression of infection diseases in the near future.