Activity of the thiopeptide antibiotic nosiheptide against contemporary strains of methicillin-resistant Staphylococcus aureus

Thiostrepton: multifaceted biological activities and its applications in treatment of inflammatory diseases

Thiostrepton (TST) is a naturally occurring oligopeptide antibiotic with a demonstrated capacity to antagonize a broad spectrum of Gram-positive bacteria. It has been utilized as a safe antimicrobial agent in veterinary medicine. Despite its therapeutic potential, the clinical application of TST has been constrained by its poor solubility and bioavailability. However, an increasing number of studies indicate that TST possesses diverse pharmacological activities, including its significant role in microbe resistance and cancer countering. Notably, recent studies have pointed out that TST also possesses anti-inflammatory potential. It has exhibited promising therapeutic efficacy across various in vivo and in vitro disease models, including non-alcoholic fatty liver disease, inflammatory bowel disease, sepsis, psoriasis-like inflammation, and periodontitis. In this review, we describe the various pharmacological activities of TST, particularly its anti-inflammatory activity demonstrated in a variety of inflammatory diseases and the underlying mechanisms. These effects highlight the potential of TST as an anti-inflammatory agent for the treatment of inflammation diseases and for enhancing cellular therapies.

Discovery and Biosynthesis of Persiathiacins: Unusual Polyglycosylated Thiopeptides Active Against Multidrug Resistant Tuberculosis

Thiopeptides are ribosomally biosynthesized and post-translationally modified peptides (RiPPs) that potently inhibit the growth of Gram-positive bacteria by targeting multiple steps in protein biosynthesis. The poor pharmacological properties of thiopeptides, particularly their low aqueous solubility, has hindered their development into clinically useful antibiotics. Antimicrobial activity screens of a library of Actinomycetota extracts led to discovery of the novel polyglycosylated thiopeptides persiathiacins A and B from Actinokineospora sp. UTMC 2448. Persiathiacin A is active against methicillin-resistant Staphylococcus aureus and several Mycobacterium tuberculosis strains, including drug-resistant and multidrug-resistant clinical isolates, and does not significantly affect the growth of ovarian cancer cells at concentrations up to 400 μM. Polyglycosylated thiopeptides are extremely rare and nothing is known about their biosynthesis. Sequencing and analysis of the Actinokineospora sp. UTMC 2448 genome enabled identification of the putative persiathiacin biosynthetic gene cluster (BGC). A cytochrome P450 encoded by this gene cluster catalyzes the hydroxylation of nosiheptide in vitro and in vivo, consistent with the proposal that the cluster directs persiathiacin biosynthesis. Several genes in the cluster encode homologues of enzymes known to catalyze the assembly and attachment of deoxysugars during the biosynthesis of other classes of glycosylated natural products. One of these encodes a glycosyl transferase that was shown to catalyze attachment of a D-glucose residue to nosiheptide in vitro. The discovery of the persiathiacins and their BGC thus provides the basis for the development of biosynthetic engineering approaches to the creation of novel (poly)glycosylated thiopeptide derivatives with enhanced pharmacological properties.

Cell-Free Systems: Ideal Platforms for Accelerating the Discovery and Production of Peptide-Based Antibiotics

Peptide-based antibiotics (PBAs), including antimicrobial peptides (AMPs) and their synthetic mimics, have received significant interest due to their diverse and unique bioactivities. The integration of high-throughput sequencing and bioinformatics tools has dramatically enhanced the discovery of enzymes, allowing researchers to identify specific genes and metabolic pathways responsible for producing novel PBAs more precisely. Cell-free systems (CFSs) that allow precise control over transcription and translation in vitro are being adapted, which accelerate the identification, characterization, selection, and production of novel PBAs. Furthermore, these platforms offer an ideal solution for overcoming the limitations of small-molecule antibiotics, which often lack efficacy against a broad spectrum of pathogens and contribute to the development of antibiotic resistance. In this review, we highlight recent examples of how CFSs streamline these processes while expanding our ability to access new antimicrobial agents that are effective against antibiotic-resistant infections.

Opportunities and challenges of RiPP-based therapeutics

Covering: up to 2024Ribosomally synthesised and post-translationally modified peptides (RiPPs) comprise a substantial group of peptide natural products exhibiting noteworthy bioactivities ranging from antiinfective to anticancer and analgesic effects. Furthermore, RiPP biosynthetic pathways represent promising production routes for complex peptide drugs, and the RiPP technology is well-suited for peptide engineering to produce derivatives with specific functions. Thus, RiPP natural products possess features that render them potentially ideal candidates for drug discovery and development. Nonetheless, only a small number of RiPP-derived compounds have successfully reached the market thus far. This review initially outlines the therapeutic opportunities that RiPP-based compounds can offer, whilst subsequently discussing the limitations that require resolution in order to fully exploit the potential of RiPPs towards the development of innovative drugs.

Rule-based omics mining reveals antimicrobial macrocyclic peptides against drug-resistant clinical isolates

Antimicrobial resistance remains a significant global threat, driving up mortality rates worldwide. Ribosomally synthesized and post-translationally modified peptides have emerged as a promising source of novel peptide antibiotics due to their diverse chemical structures. Here, we report the discovery of new aminovinyl-(methyl)cysteine (Avi(Me)Cys)-containing peptide antibiotics through a synergistic approach combining biosynthetic rule-based omics mining and heterologous expression. We first bioinformatically identify 1172 RiPP biosynthetic gene clusters (BGCs) responsible for Avi(Me)Cys-containing peptides formation from a vast pool of over 50,000 bacterial genomes. Subsequently, we successfully establish the connection between three identified BGCs and the biosynthesis of five peptide antibiotics via biosynthetic rule-guided metabolic analysis. Notably, we discover a class V lanthipeptide, massatide A, which displays excellent activity against gram-positive pathogens, including drug-resistant clinical isolates like linezolid-resistant S. aureus and methicillin-resistant S. aureus, with a minimum inhibitory concentration of 0.25 μg/mL. The remarkable performance of massatide A in an animal infection model, coupled with a relatively low risk of resistance and favorable safety profile, positions it as a promising candidate for antibiotic development. Our study highlights the potential of Avi(Me)Cys-containing peptides in expanding the arsenal of antibiotics against multi-drug-resistant bacteria, offering promising drug leads in the ongoing battle against infectious diseases.

Antagonistic actions of Paucibacter aquatile B51 and its lasso peptide paucinodin toward cyanobacterial bloom-forming Microcystis aeruginosa PCC7806

Superior antagonistic activity against axenic Microcystis aeruginosa PCC7806 was observed with Paucibacter sp. B51 isolated from cyanobacterial bloom samples among 43 tested freshwater bacterial species. Complete genome sequencing, analyzing average nucleotide identity and digital DNA-DNA hybridization, designated the B51 strain as Paucibacter aquatile. Electron and fluorescence microscopic image analyses revealed the presence of the B51 strain in the vicinity of M. aeruginosa cells, which might provoke direct inhibition of the photosynthetic activity of the PCC7806 cells, leading to perturbation of cellular metabolisms and consequent cell death. Our speculation was supported by the findings that growth failure of the PCC7806 cells led to low pH conditions with fewer chlorophylls and down-regulation of photosystem genes (e.g., psbD and psaB) during their 48-h co-culture condition. Interestingly, the concentrated ethyl acetate extracts obtained from B51-grown supernatant exhibited a growth-inhibitory effect on PCC7806. The physical separation of both strains by a filter system led to no inhibitory activity of the B51 cells, suggesting that contact-mediated anti-cyanobacterial compounds might also be responsible for hampering the growth of the PCC7806 cells. Bioinformatic tools identified 12 gene clusters that possibly produce secondary metabolites, including a class II lasso peptide in the B51 genome. Further chemical analysis demonstrated anti-cyanobacterial activity from fractionated samples having a rubrivinodin-like lasso peptide, named paucinodin. Taken together, both contact-mediated inhibition of photosynthesis and the lasso peptide secretion of the B51 strain are responsible for the anti-cyanobacterial activity of P. aquatile B51.

Selective biosynthesis of a rhamnosyl nosiheptide by a novel bacterial rhamnosyltransferase

Nosiheptide (NOS) is a thiopeptide antibiotic produced by the bacterium Streptomyces actuosus. The hydroxyl group of 3-hydroxypyridine in NOS has been identified as a promising site for modification, which we therefore aimed to rhamnosylate. After screening, Streptomyces sp. 147326 was found to regioselectively attach a rhamnosyl unit to the 3-hydroxypyridine site in NOS, resulting in the formation of a derivative named NOS-R at a productivity of 24.6%. In comparison with NOS, NOS-R exhibited a 17.6-fold increase in aqueous solubility and a new protective effect against MRSA infection in mice, while maintaining a similar in vitro activity. Subsequently, SrGT822 was identified as the rhamnosyltransferase in Streptomyces sp. 147326 responsible for the biosynthesis of NOS-R using dTDP-L-rhamnose. SrGT822 demonstrated an optimal reaction pH of 10.0 and temperature of 55°C, which resulted in a NOS-R yield of 74.9%. Based on the catalytic properties and evolutionary analysis, SrGT822 is anticipated to be a potential rhamnosyltransferase for use in the modification of various complex scaffolds.

Design of a chimeric glycosyltransferase OleD for the site-specific O-monoglycosylation of 3-hydroxypyridine in nosiheptide

To identify the potential role of the 3-hydroxyl group of the pyridine ring in nosiheptide (NOS) for its antibacterial activity against Gram-positive pathogens, enzymatic glycosylation was utilized to regio-selectively create a monoglycosyl NOS derivative, NOS-G. For this purpose, we selected OleD, a UDP glycosyltransferase from Streptomyces antibioticus that has a low productivity for NOS-G. Activity of the enzyme was increased by swapping domains derived from OleI, both single and in combination. Activity enhancement was best in mutant OleD-10 that contained four OleI domains. This chimer was engineered by site-directed mutagenesis (single and in combination) to increase its activity further, whereby variants were screened using a newly-established colorimetric assay. OleD-10 with I117F and T118G substitutions (FG) had an increased NOS-G productivity of 56%, approximately 70 times higher than that of wild-type OleD. The reason for improved activity of FG towards NOS was structurally attributed to a closer distance (<3 Å) between NOS/sugar donor and the catalytic amino acid H25. The engineered enzyme allowed sufficient activity to demonstrate that the produced NOS-G had enhanced stability and aqueous solubility compared to NOS. Using a murine MRSA infection model, it was established that NOS-G resulted in partial protection within 20 h of administration and delayed the death of infected mice. We conclude that 3-hydroxypyridine is a promising site for structural modification of NOS, which may pave the way for producing nosiheptide derivatives as a potential antibiotic for application in clinical treatment.

Nosiheptide Harbors Potent In Vitro and Intracellular Inhbitory Activities against Mycobacterium tuberculosis

Multidrug-resistant tuberculosis (MDR-TB) is often associated with poor clinical outcomes. In this study, we evaluated the potential of nosiheptide (NOS) as a new drug candidate for treating Mycobacterium tuberculosis infections, including MDR-TB. The antimicrobial susceptibility testing was performed to determine the MICs of NOS against 18 reference strains of slowly growing mycobacteria (SGM) and 128 clinical isolates of M. tuberculosis. The postantibiotic effects (PAE) and interaction with other antituberculosis drugs of NOS were also evaluated using M. tuberculosis H37Rv. Fifteen out of the 18 tested reference strains of SGM had MICs far below 1 μg/mL. From the 128 M. tuberculosis clinical isolates, the MIC₅₀ and MIC₉₀ were 0.25 μg/mL and 1 μg/mL, respectively; the tentative epidemiological cutoff (ECOFF) was defined at 1 μg/mL. Furthermore, a Lys89Thr mutation was found in one M. tuberculosis isolate with a MIC of NOS >8 μg/mL. After 24 h of incubation, NOS at 1 μg/mL inhibited 25.79 ± 1.22% of intracellular bacterial growth, which was comparable with the inhibitory rate of 25.71 ± 3.67% achieved by rifampin at 2 μg/mL. Compared to rifampicin and isoniazid (INH), NOS had a much longer PAE, i.e., a value of about 16 days. In addition, a partial synergy between NOS and INH was observed. NOS has potent inhibitory activities against M. tuberculosis in vitro as well as in macrophages. Furthermore, the long PAE and partial synergistic effect with INH, in addition to the added safety of long-term use as a feed additive in husbandry, provide support for NOS being a promising drug candidate for tuberculosis treatment. IMPORTANCE This study is aimed at chemotherapy for MDR-TB, mainly to explore the anti-TB activity of the existing chemotherapeutic reagent. We found that NOS has potent inhibitory activities against M. tuberculosis in vitro regardless of the drug-resistant profile. Furthermore, NOS also showed the long PAE and partial synergistic effect with INH and is nontoxic, providing support for its promise as a drug candidate for drug-resistant tuberculosis treatment.

Mechanism of Action of Ribosomally Synthesized and Post-Translationally Modified Peptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a natural product class that has undergone significant expansion due to the rapid growth in genome sequencing data and recognition that they are made by biosynthetic pathways that share many characteristic features. Their mode of actions cover a wide range of biological processes and include binding to membranes, receptors, enzymes, lipids, RNA, and metals as well as use as cofactors and signaling molecules. This review covers the currently known modes of action (MOA) of RiPPs. In turn, the mechanisms by which these molecules interact with their natural targets provide a rich set of molecular paradigms that can be used for the design or evolution of new or improved activities given the relative ease of engineering RiPPs. In this review, coverage is limited to RiPPs originating from bacteria.

Micrococcin P2 Targets Clostridioides difficile

Clostridioides difficile infection is a global public health threat. Extensive in vitro assays using clinical isolates have identified micrococcin P2 (MP2, 1) as a particularly effective anti-C. difficile agent. MP2 possesses a mode of action that differs from other antibiotics and pharmacokinetic properties that render it especially promising. Its time-kill studies have been investigated using hypervirulent C. difficile ribotype 027. DSS (dextran sulfate sodium)-induced in vivo mouse studies with that strain indicate that 1 is better than vancomycin and fidaxomicin. Thus, micrococcin P2 is a valuable platform to be exploited for the development of new anti-C. difficile antibiotics.

Marine Based Natural Products: Exploring the Recent Developments in the Identification of Antimicrobial Agents

The marine ecosystem is the less explored, biologically diverse, and vastest resource to discover novel antimicrobial agents. In recent decades' antimicrobial drugs are losing their effectiveness due to the growing resistance among pathogens, which causes diseases to have considerable death rates across the globe. Therefore, there is a need for the discovery of new antibacterials that can reach the market. There is a gradual growth of compounds from marine sources which are entering the clinical trials. Thus, the prominence of marine natural products in the field of drug design and discovery across the academia and pharmaceutical industry is gaining attention. Herein,  the present review covers nearly 200 marine based antimicrobial agents of 11 structural classes discovered from the year 2010 to 2022. All the discussed compounds have exhibited medium to high antimicrobial activity in inhibiting various microorganisms.

In vitro and intracellular inhibitory activities of nosiheptide against Mycobacterium abscessus

The high level of inherent drug resistance of Mycobacterium abscessus makes the infection caused by it very difficult to be treated. The objective of this study was to evaluate the potential of nosiheptide (NOS) as a new drug candidate for treating M. abscessus infections. The microplate AlamarBlue assay was performed to determine the minimum inhibitory concentrations (MICs) of NOS for 28 reference strains of rapidly growing mycobacteria (RGM) and 77 clinical isolates of M. abscessus. Time-kill kinetic and post-antibiotic effect (PAE) of NOS against M. abscessus was evaluated. Its bactericidal activity against M. abscessus in macrophages was determined by an intracellular colony numerating assay. NOS manifested good activity against the reference strains of RGM and M. abscessus clinical isolates in vitro. The MICs of NOS against M. abscessus clinical isolates ranged from 0.0078 to 1 μg/ml, and the MIC₅₀ and MIC₉₀ were 0.125 μg/ml and 0.25 μg/ml, respectively. The pattern of growth and kill by NOS against M. abscessus was moderate with apparent concentration-dependent characteristics, and the PAE value of NOS was found to be ~6 h. Furthermore, NOS had low cell toxicity against the THP-1 cell line after 48 h of exposure (IC₅₀ = 106.9 μM). At 4 μg/ml, NOS exhibited high intracellular bactericidal activity against M. abscessus reference strains with an inhibitory rate of 66.52% ± 1.51%, comparable with that of clarithromycin at 2 μg/ml. NOS showed suitable inhibitory activities against M. abscessus in vitro and in macrophages and could be a potential drug candidate to treat M. abscessus infection.

Diversity-oriented routes to thiopeptide antibiotics: total synthesis and biological evaluation of micrococcin P2

We report the first total synthesis of micrococcin P2 (MP2, 1) by a diversity-oriented route that incorporates a number of refinements relative to earlier syntheses. Biological data regarding the activity of 1 against a range of human pathogens are also provided. Furthermore, we disclose a chemical property of MP2 that greatly facilitates medicinal chemistry work in the micrococcin area and describe a method to obtain MP2 by fermentation in B. subtilis.

Inner residues of macrothiolactone in autoinducer peptides-I/IV circumvents S-to-O acyl transfer to the upstream serine residue

Autoinducing peptides I and IV (AIP-I/IV) are naturally occurring cyclic thiodepsipeptides (CTPs) bearing a Ser–Thr–Cys–Asp/Tyr (STC[D/Y]) tetrapeptide motif, where the Cys thiol (^HSC) in the side-chain is linked to the Met C-terminal carboxylic acid (M_COOH) to form 5-residue macrothiolactones,^−SC(D/Y)FIM_CO−. We have recently reported that CTPs containing SX₁CX₂ motifs spontaneously undergo macrolactonization to yield cyclic depsipeptides (CDPs) by an unprecedented rapid S-to-O acyl transfer to the upstream Ser hydroxyl group. Interestingly, even though the STC[D/Y] motif in AIP-I/IV is a member of the SX₁CX₂ motif family, it maintains the CTP form. This suggests that AIP-I/IV have a structural or chemical motive for avoiding such an S-to-O acyl transfer, thus retaining the CTP form intact. Here we have used genetic code reprogramming to ribosomally synthesize various AIP-I analogs and studied what the determinant is to control the formation of CTP vs. CDP products. The study revealed that a Gly substitution of the inner Asp/Tyr or Met residues in the thiolactone drastically alters the resistance to the promotion of the S-to-O acyl transfer, giving the corresponding CDP product. This suggests that the steric hindrances originating from the α-substituted sidechain in these two amino acids in the AIP-I/IV thiolactone likely play a critical role in controlling the resistance against macrolactone rearrangement to the upstream Ser residue.

In AIP-I/IV, single Gly mutation at the thiolactone induces S-to-O acyl shift to yield a corresponding ring-expanded lactone form.

Applications of ultrasound in total synthesis of bioactive natural products: A promising green tool

Total synthesisis frequently compared to climbing as it provides a suitable route to reach a high point from the floor, the complex natural product from simple and commercially available materials. The total synthesis has a privileged position of trust in confirming the hypothetical complex structures of natural products despite sophisticated analytical and spectroscopic instrumentation and techniques that are available presently. Moreover, total synthesis is also useful to prepare rare bioactive natural products in the laboratory as several bioactive secondary metabolites are obtained in small quantities from natural sources. The artistic aspect of the total synthesis of bioactive natural products continues to be praised today as it may provide environmental protection through the concept of green or clean chemistry. The use of ultrasound waves as a non-polluting source of energy is of great interest in the field of sustainable and pharmaceutical chemistry as it differs from conventional energy sources in terms of reaction rates, yields, selectivities, and purity of the products. The present review highlights the application of ultrasound as a green tool in the total synthesis of bioactive natural products as well as this article is also aimed to offer an overview of natural sources, structures, and biological activities of the promising natural products for the first time from 2005 to 2020 elegantly.

Thiocillin and micrococcin exploit the ferrioxamine receptor of Pseudomonas aeruginosa for uptake

BACKGROUND

Thiopeptides are a class of antibiotics that are active against Gram-positive bacteria and inhibit translation. They were considered inactive against Gram-negative bacteria due to their inability to cross the outer membrane. However, we discovered previously that a member of this class, thiostrepton (TS), has activity against Pseudomonas aeruginosa and Acinetobacter baumannii under iron-limiting conditions. TS hijacks the pyoverdine siderophore receptors of P. aeruginosa to cross the outer membrane and synergizes with iron chelators.

OBJECTIVES

To test other thiopeptides for antimicrobial activity against P. aeruginosa and determine their mechanism of uptake, action and spectrum of activity.

METHODS

Eight thiopeptides were screened in chequerboard assays against a mutant of P. aeruginosa PA14 lacking both pyoverdine receptors. Thiopeptides that retain activity against a pyoverdine receptor-null mutant may use alternative siderophore receptors for entry. Susceptibility testing against siderophore receptor mutants was used to determine thiopeptide mechanism of uptake.

RESULTS

The thiopeptides thiocillin (TC) and micrococcin (MC) use the ferrioxamine siderophore receptor (FoxA) for uptake and inhibit the growth of P. aeruginosa at low micromolar concentrations. The activity of TC required the TonB-ExbBD system used to energize siderophore uptake. TC acted through its canonical mechanism of action of translation inhibition.

CONCLUSIONS

Multiple thiopeptides have antimicrobial activity against P. aeruginosa, countering the historical assumption that they cannot cross the outer membrane. These results demonstrate the potential for thiopeptides to act as antipseudomonal antibiotics.

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.

Synthesis of the 26-Membered Core of Thiopeptide Natural Products by Scalable Thiazole-Forming Reactions of Cysteine Derivatives and Nitriles

The increased resistance of bacteria to clinical antibiotics is one of the major dilemmas facing human health and without solutions the problem will grow exponentially worse. Thiopeptide natural products have shown promising antibiotic activities and provide an opportunity for the development of a new class of antibiotics. Attempts to directly translate these compounds into human medicine have been limited due to poor physiochemical properties. The synthesis of the core structure of the 26-membered class of thiopeptide natural products is reported using chemistry that enables the synthesis of large quantities of synthetic intermediates and the common core structure. The use of cysteine/nitrile condensation reactions followed by oxidation to generate thiazoles has been key in enabling large academic scale reactions that in many instances avoided chromatography further aiding in accessing large amounts of key synthetic intermediates.

Enzymatic macrocyclization of ribosomally synthesized and posttranslational modified peptides via C-S and C-C bond formation

Covering: 2000 to 2020 Ribosomally synthesized and posttranslational modified peptides (RiPPs) are a rapidly growing class of bioactive natural products. Many members of RiPPs contain macrocyclic structural units constructed by modification enzymes through macrocyclization of linear precursor peptides. In this study, we summarize recent progress in the macrocyclization of RiPPs by C-S and C-C bond formation with a focus on the current understanding of the enzymatic mechanisms.

Cell wall distraction and biofilm inhibition of marine Streptomyces derived angucycline in methicillin resistant Staphylococcus aureus

The emergence of life threatening antibiotic resistant pathogens and its associated mortality and morbidity necessitates many new antibiotics from diverse ecological habitats. Marine sponge associated microbes are promising to provide such antimicrobial compounds. In the present study, we report antibacterial and anti-biofilm potential of the angucycline antibiotic 8-O-metyltetrangomycin from Streptomyces sp. SBRK2 isolated from a marine sponge of Gulf of Mannar, Rameswaram, India. Our screening program to tackle methicillin-resistant Staphylococcus aureus (MRSA) drug resistance from marine sponge associated actinobacteria yielded the bioactive strain SBRK2. Based on 16S rRNA gene phylogenetic analysis the isolate was found to closely related with Streptomyces longispororuber NBRC 13488^T. In vitro production by agar plate fermentation, solvent based extraction, TLC, HPLC purification and LC-MS based de-replication revealed the bioactive compound as 8-O-metyltetrangomycin. The antibacterial minimum inhibitory concentrations against MRSA was identified as 2 μg/mL. Sub-inhibitory concentration of the compound 8-O-metyltetrangomycin reduced the biofilm formation of S. aureus ATCC25923 and increased the cell surface hydrophobicity index. Scanning electron microscopic observation of the sub-inhibitory concentration exposure revealed a wrinkled membrane surface and slight cellular damage shows the cell wall distracting property of the compound. Zebrafish embryo based toxicity assays exhibited 100 μg/mL of compound as maximal non-lethal concentration which had demonstrated the positive relationship in safety index. The angucycline compound 8-O-metyltetrangomycin could be a potential candidate for the development of anti-biofilm agents against drug resistant pathogens.

Evaluation of Anti-Mycobacterial Compounds in a Silkworm Infection Model with Mycobacteroides abscessus

Among four mycobacteria, Mycobacterium avium, M. intracellulare, M. bovis BCG and Mycobacteroides (My.) abscessus, we established a silkworm infection assay with My. abscessus. When silkworms (fifth-instar larvae, n = 5) were infected through the hemolymph with My. abscessus (7.5 × 10⁷ CFU/larva) and bred at 37 °C, they all died around 40 h after injection. Under the conditions, clarithromycin and amikacin, clinically used antimicrobial agents, exhibited therapeutic effects in a dose-dependent manner. Furthermore, five kinds of microbial compounds, lariatin A, nosiheptide, ohmyungsamycins A and B, quinomycin and steffimycin, screened in an in vitro assay to observe anti-My. abscessus activity from 400 microbial products were evaluated in this silkworm infection assay. Lariatin A and nosiheptide exhibited therapeutic efficacy. The silkworm infection model with My. abscessus is useful to screen for therapeutically effective anti-My. abscessus antibiotics.

Introduction to Thiopeptides: Biological Activity, Biosynthesis, and Strategies for Functional Reprogramming

Thiopeptides (also known as thiazolyl peptides) are structurally complex natural products with rich biological activities. Known for over 70 years for potent killing of Gram-positive bacteria, thiopeptides are experiencing a resurgence of interest in the last decade, primarily brought about by the genomic revolution of the 21st century. Every area of thiopeptide research-from elucidating their biological function and biosynthesis to expanding their structural diversity through genome mining-has made great strides in recent years. These advances lay the foundation for and inspire novel strategies for thiopeptide engineering. Accordingly, a number of diverse approaches are being actively pursued in the hope of developing the next generation of natural-product-inspired therapeutics. Here, we review the contemporary understanding of thiopeptide biological activities, biosynthetic pathways, and approaches to structural and functional reprogramming, with a special focus on the latter.

Phenomic and genomic approaches to studying the inhibition of multiresistant Salmonella enterica by microcin J25

In livestock production, antibiotics are used to promote animal growth, control infections and thereby increase profitability. This practice has led to the emergence of multiresistant bacteria such as Salmonella, of which some serovars are disseminated in the environment. The objective of this study is to evaluate microcin J25 as an inhibitor of Salmonella enterica serovars of various origins including human, livestock and food. Among the 116 isolates tested, 37 (31.8%) were found resistant to at least one antibiotic, and 28 were multiresistant with 19 expressing the penta-resistant phenotype ACSSuT. Microcin J25 inhibited all isolates, with minimal inhibitory concentration values ranging from 0.06 μg/ml (28.4 nM) to 400 μg/ml (189 μM). Interestingly, no cross-resistance was found between microcin J25 and antibiotics. Multiple sequence alignments of genes encoding for the different proteins involved in the recognition and transport of microcin J25 showed that only ferric-hydroxamate uptake is an essential determinant for susceptibility of S. enterica to microcin J25. Examination of Salmonella strains exposed to microcin J25 by transmission electronic microscopy showed for the first-time involvement of a pore formation mechanism. Microcin J25 was a strong inhibitor of several multiresistant isolates of Salmonella and may have a great potential as an alternative to antibiotics.

Selected Metabolites Profiling of Staphylococcus aureus Following Exposure to Low Temperature and Elevated Sodium Chloride

Staphylococcus aureus is one of the main foodborne pathogens that can cause food poisoning. Due to this reason, one of the essential aspects of food safety focuses on bacterial adaptation and proliferation under preservative conditions. This study was aimed to determine the metabolic changes that can occur following the exposure of S. aureus to either low temperature conditions or elevated concentrations of sodium chloride (NaCl). The results revealed that most of the metabolites measured were reduced in cold-stressed cells, when compared to reference controls. The major reduction was observed in nucleotides and organic acids, whereas mannitol was significantly increased in response to low temperature. However, when S. aureus was exposed to elevated NaCl, a significant increase was observed in the metabolite levels, particularly purine and pyrimidine bases along with organic acids. The majority of carbohydrates remained constant in the cells grown under ideal conditions and those exposed to elevated NaCl concentrations. Partial least square discriminate analysis (PLS-DA) of the metabolomic data indicated that both, prolonged cold stress and osmotic stress conditions, generated cells with different metabolic profiles, in comparison to the reference controls. These results provide evidence that, when bacterial cells exposed to low temperatures or high concentrations of NaCl, experience in situ homeostatic alterations to adapt to new environmental conditions. These data supported the hypothesis that changes in metabolic homeostasis were critical to the adaptive processes required for survival under alterations in the environmental conditions.

Phylogenetic Analysis and Screening of Antimicrobial and Antiproliferative Activities of Culturable Bacteria Associated with the Ascidian Styela clava from the Yellow Sea, China

Over 1,000 compounds, including ecteinascidin-743 and didemnin B, have been isolated from ascidians, with most having bioactive properties such as antimicrobial, antitumor, and enzyme-inhibiting activities. In recent years, direct and indirect evidence has shown that some bioactive compounds isolated from ascidians are not produced by ascidians themselves but by their symbiotic microorganisms. Isolated culturable bacteria associated with ascidians and investigating their potential bioactivity are an important approach for discovering novel compounds. In this study, a total of 269 bacteria were isolated from the ascidian Styela clava collected from the coast of Weihai in the north of the Yellow Sea, China. Phylogenetic relationships among 183 isolates were determined using their 16S rRNA gene sequences. Isolates were tested for antimicrobial activity against seven indicator strains, and an antiproliferative activity assay was performed to test for inhibition of human hepatocellular carcinoma Bel 7402 and human cervical carcinoma HeLa cell proliferation. Our results showed that the isolates belonged to 26 genera from 18 families in four phyla (Firmicutes, Actinobacteria, Proteobacteria, and Bacteroidetes). Bacillus and Streptomyces were the most dominant genera; 146 strains had potent antimicrobial activities and inhibited at least one of the indicator strains. Crude extracts from 29 strains showed antiproliferative activity against Bel 7402 cells with IC₅₀ values below 500 μg·mL^-1, and 53 strains showed antiproliferative activity against HeLa cells, with IC₅₀ values less than 500 μg·mL^-1. Our results suggest that culturable bacteria associated with the ascidian Styela clava may be a promising source of novel bioactive compounds.

Potential of marine natural products against drug-resistant bacterial infections

Natural products have been a rich source of compounds with structural and chemical diversity for drug discovery. However, antibiotic resistance in bacteria has been reported for nearly every antibiotic once it is used in clinical practice. In the past decade, pharmaceutical companies have reduced their natural product discovery projects because of challenges, such as high costs, low return rates, and high rediscovery rates. The largely unexplored marine environment harbours substantial diversity and is a large resource to discover novel compounds with novel modes of action, which is essential for the treatment of drug-resistant bacterial infections. In this Review, we report compounds derived from marine sources that have shown in-vivo and in-vitro efficacy against drug-resistant bacteria. Analysis of the physicochemical properties of these marine natural products with activity against drug-resistant bacteria showed that 60% of the compounds have oral bioavailability potential. Their overall distribution pattern of drug characteristics agrees with the observation that marketed antibacterial drugs have a polar distribution, with a lower median calculated logP. The aim of this Review is to summarise the diversity of these marine natural products, with a special focus on analysis of drug bioavailability. Such biologically active compounds, with high degrees of bioavailability, have the potential to be developed as effective drugs against infectious diseases.

Discovery of Nosiheptide, Griseoviridin, and Etamycin as Potent Anti-Mycobacterial Agents against Mycobacterium avium Complex

Mycobacterium avium complex (MAC) is a serious disease mainly caused by M. avium and M. intracellulare. Although the incidence of MAC infection is increasing worldwide, only a few agents are clinically used, and their therapeutic effects are limited. Therefore, new anti-MAC agents are needed. Approximately 6600 microbial samples were screened for new anti-mycobacterial agents that inhibit the growth of both M. avium and M. intracellulare, and two culture broths derived from marine actinomycete strains OPMA1245 and OPMA1730 had strong activity. Nosiheptide (1) was isolated from the culture broth of OPMA1245, and griseoviridin (2) and etamycin (viridogrisein) (3) were isolated from the culture broth of OPMA1730. They had potent anti-mycobacterial activity against M. avium and M. intracellulare with minimum inhibitory concentrations (MICs) between 0.024 and 1.56 μg/mL. In addition, a combination of 2 and 3 markedly enhanced the anti-mycobacterial activity against both M. avium and M. intracellulare. Furthermore, a combination 2 and 3 had a therapeutic effect comparable to that of ethambutol in a silkworm infection assay with M. smegmatis.

Capturing Intermediates in the Reaction Catalyzed by NosN, a Class C Radical S-Adenosylmethionine Methylase Involved in the Biosynthesis of the Nosiheptide Side-Ring System

Nosiheptide is a ribosomally synthesized and post-translationally modified thiopeptide natural product that possesses antibacterial, anticancer, and immunosuppressive properties. It contains a bicyclic structure composed of a large macrocycle and a unique side-ring system containing a 3,4-dimethylindolic acid bridge connected to the side chains of Glu6 and Cys8 of the core peptide via ester and thioester linkages, respectively. In addition to the structural peptide, encoded by the nosM gene, the biosynthesis of the side-ring structure requires the actions of NosI, -J, -K, -L, and -N. NosN is annotated as a class C radical S-adenosylmethionine (SAM) methylase, but its true function is to transfer a C1 unit from SAM to C4 of 3-methyl-2-indolic acid (MIA) with concomitant formation of a bond between the carboxylate of Glu6 of the core peptide and the nascent C1 unit. However, exactly when NosN performs its function during the biosynthesis of nosiheptide is unknown. Herein, we report the syntheses and use of three peptide mimics as potential substrates designed to address the timing of NosN's function. Our results show that NosN clearly closes the side ring before NosO forms the pyridine ring and most likely before NosD/E catalyzes formation of the dehydrated amino acids, although the possibility of a more random process (i.e., NosN acting after NosD/E) cannot be ruled out. Using a substrate mimic containing a rigid structure, we also identify and characterize two reaction-based adducts containing SAM fused to C4 of MIA. The two SAM adducts are derived from a consensus radical-containing species proposed to be the key intermediate-or a derivative of the key intermediate-in our proposed catalytic mechanism of NosN.

I2/TBHP-Mediated tandem cyclization and oxidation reaction: Facile access to 2-substituted thiazoles and benzothiazoles

The efficient synthesis of 2-substituted thiazoles and benzothiazoles has been accomplished employing readily available cysteine esters and 2-aminobenzenethiols as N and S sources.

Radical S-Adenosyl-l-methionine Tryptophan Lyase (NosL): How the Protein Controls the Carboxyl Radical •CO2- Migration

The radical S-adenosyl-l-methionine tryptophan lyase uses radical-based chemistry to convert l-tryptophan into 3-methyl-2-indolic acid, a fragment in the biosynthesis of the thiopeptide antibiotic nosiheptide. This complex reaction involves several successive steps corresponding to (i) the activation by a specific hydrogen-atom abstraction, (ii) an unprecedented •CO₂^- radical migration, (iii) a cyanide fragment release, and (iv) the termination of the radical-based reaction. In vitro study of this reaction is made more difficult because the enzyme produces a significant amount of a shunt product instead of the natural product. Here, using a combination of X-ray crystallography, electron paramagnetic resonance spectroscopy, and quantum and hybrid quantum mechanical/molecular mechanical calculations, we have deciphered the fine mechanism of the key •CO₂^- radical migration, highlighting how the preorganized active site of the protein tightly controls this reaction.

Streptomyces as a Prominent Resource of Future Anti-MRSA Drugs

Methicillin-resistant Staphylococcus aureus (MRSA) pose a significant health threat as they tend to cause severe infections in vulnerable populations and are difficult to treat due to a limited range of effective antibiotics and also their ability to form biofilm. These organisms were once limited to hospital acquired infections but are now widely present in the community and even in animals. Furthermore, these organisms are constantly evolving to develop resistance to more antibiotics. This results in a need for new clinically useful antibiotics and one potential source are the Streptomyces which have already been the source of several anti-MRSA drugs including vancomycin. There remain large numbers of Streptomyces potentially undiscovered in underexplored regions such as mangrove, deserts, marine, and freshwater environments as well as endophytes. Organisms from these regions also face significant challenges to survival which often result in the production of novel bioactive compounds, several of which have already shown promise in drug development. We review the various mechanisms of antibiotic resistance in MRSA and all the known compounds isolated from Streptomyces with anti-MRSA activity with a focus on those from underexplored regions. The isolation of the full array of compounds Streptomyces are potentially capable of producing in the laboratory has proven a challenge, we also review techniques that have been used to overcome this obstacle including genetic cluster analysis. Additionally, we review the in vivo work done thus far with promising compounds of Streptomyces origin as well as the animal models that could be used for this work.

Differential anti-microbial secondary metabolites in different ESKAPE pathogens explain their adaptation in the hospital setup

Nosocomial infections are caused by ESKAPE (E. faecium, S. aureus, K. pneumoniae, A. baumannii, P. aeruginosa, and E. cloacae) pathogens, and their co-existence is associated with their ability to survive in the hospital setup. They may produce molecules, which helps in the better survival of one ESKAPE pathogens over other. We have identified all secondary metabolite gene clusters in six ESKAPE pathogens and predicted antimicrobial and anti-biofilm properties of their product secondary metabolites. To validate our model, we have taken the secondary metabolites of ESKAPE pathogens and studied their interaction with diguanylate cyclase (involved in quorum sensing) and biofilm-associated protein (involved in biofilm formation) of Acinetobacter baumannii. Results suggest the presence of differential secondary metabolites in all ESKAPE pathogens with only three common non-antimicrobial secondary metabolites. Out of twenty-three antimicrobial secondary metabolites, TP-1161, nosiheptide and meilingmycin, showed the best antimicrobial activity and nineteen showed high anti-biofilm activity. Interaction study showed that secondary metabolites produced by other ESKAPE pathogens (non-Acinetobacter) have very good interaction with diguanylate cyclase and biofilm-associated protein of A. baumannii. This concludes that better survival of these ESKAPE pathogens in hospital setup can be correlated with differential production of antimicrobial secondary metabolites. The present study also investigates the molecular mechanism of the competition of different pathogens living in similar hospital setup (similar habitat). Therefore, the present study will initiate research that might lead to the discovery of antibiotics from one ESKAPE pathogen that controls the infection of other ESKAPE pathogens or other pathogens.

NosN, a Radical S-Adenosylmethionine Methylase, Catalyzes Both C1 Transfer and Formation of the Ester Linkage of the Side-Ring System during the Biosynthesis of Nosiheptide

Nosiheptide, a member of the e series of macrocyclic thiopeptide natural products, contains a side-ring system composed of a 3,4-dimethylindolic acid (DMIA) moiety connected to Glu6 and Cys8 of the thiopeptide backbone via ester and thioester linkages, respectively. Herein, we show that NosN, a predicted class C radical S-adenosylmethionine (SAM) methylase, catalyzes both the transfer of a C1 unit from SAM to 3-methylindolic acid linked to Cys8 of a synthetic substrate surrogate as well as the formation of the ester linkage between Glu6 and the nascent C4 methylene moiety of DMIA. In contrast to previous studies that indicated that 5'-methylthioadenosine is the immediate methyl donor in the reaction, in our studies, SAM itself plays this role, giving rise to S-adenosylhomocysteine as a coproduct of the reaction.

Biosynthesis of the nosiheptide indole side ring centers on a cryptic carrier protein NosJ

Nosiheptide is a prototypal thiopeptide antibiotic, containing an indole side ring in addition to its thiopeptide-characteristic macrocylic scaffold. This indole ring is derived from 3-methyl-2-indolic acid (MIA), a product of the radical S-adenosylmethionine enzyme NosL, but how MIA is incorporated into nosiheptide biosynthesis remains to be investigated. Here we report functional dissection of a series of enzymes involved in nosiheptide biosynthesis. We show NosI activates MIA and transfers it to the phosphopantetheinyl arm of a carrier protein NosJ. NosN then acts on the NosJ-bound MIA and installs a methyl group on the indole C4, and the resulting dimethylindolyl moiety is released from NosJ by a hydrolase-like enzyme NosK. Surface plasmon resonance analysis show that the molecular complex of NosJ with NosN is much more stable than those with other enzymes, revealing an elegant biosynthetic strategy in which the reaction flux is controlled by protein-protein interactions with different binding affinities.Thiopeptides such as nosiheptide are clinically-interesting antimicrobial natural products. Here the authors show the functional dissection of a series of enzymes involved in nosiheptide biosynthesis, revealing a unique biosynthetic pathway that centers on a previously-unknown carrier protein.

The diversity and antibiotic properties of actinobacteria associated with endemic deepwater amphipods of Lake Baikal

The emergence of pathogenic bacteria resistant to antibiotics increases the need for discovery of new effective antimicrobials. Unique habitats such as marine deposits, wetlands and caves or unexplored biological communities are promising sources for the isolation of actinobacteria, which are among the major antibiotic producers. The present study aimed at examining cultivated actinobacteria strains associated with endemic Lake Baikal deepwater amphipods and estimating their antibiotic activity. We isolated 42 actinobacterial strains from crustaceans belonging to Ommatogammarus albinus and Ommatogammarus flavus. To our knowledge, this is the first report describing the isolation and initial characterization of representatives of Micromonospora and Pseudonocardia genera from Baikal deepwater invertebrates. Also, as expected, representatives of the genus Streptomyces were the dominant group among the isolated species. Some correlations could be observed between the number of actinobacterial isolates, the depth of sampling and the source of the strains. Nevertheless, >70% of isolated strains demonstrated antifungal activity. The dereplication analysis of extract of one of the isolated strains resulted in annotation of several known compounds that can help to explain the observed biological activities. The characteristics of ecological niche and lifestyle of deepwater amphipods suggests that the observed associations between crustaceans and isolated actinobacteria are not random and might represent long-term symbiotic interactions.

Muscarine, imidazole, oxazole and thiazole alkaloids

Covering: July 2012 to June 2015. Previous review: Nat. Prod. Rep., 2013, 30, 869-915The structurally diverse imidazole-, oxazole-, and thiazole-containing secondary metabolites are widely distributed in terrestrial and marine environments, and exhibit extensive pharmacological activities. In this review the latest progress involving the isolation, biological activities, and chemical and biogenetic synthesis studies on these natural products has been summarized.

Antibacterial Compounds from Marine Bacteria, 2010-2015

This review summarizes the reports on antibacterial compounds that have been obtained from marine-derived bacteria during the period 2010-2015. Over 50 active compounds were isolated during this period, most of which (69%) were obtained from Actinobacteria. Several compounds were already known, such as etamycin A (11) and nosiheptide (65), and new experiments with them showed some previously undetected antibacterial activities, highlighting the fact that known natural products may be an important source of new antibacterial leads. New broad-spectrum antibacterial compounds were reported with activity against antibiotic resistant Gram-positive and Gram-negative bacteria. Anthracimycin (33), kocurin (66), gageotetrins A-C (72-74), and gageomacrolactins 1-3 (86-88) are examples of compounds that display promising properties and could be leads to new antibiotics. A number of microbes produced mixtures of metabolites sharing similar chemical scaffolds, and structure-activity relationships are discussed.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

Strategies to Develop Inhibitors of Motif-Mediated Protein-Protein Interactions as Drug Leads

Protein-protein interactions are fundamental for virtually all functions of the cell. A large fraction of these interactions involve short peptide motifs, and there has been increased interest in targeting them using peptide-based therapeutics. Peptides benefit from being specific, relatively safe, and easy to produce. They are also easy to modify using chemical synthesis and molecular biology techniques. However, significant challenges remain regarding the use of peptides as therapeutic agents. Identification of peptide motifs is difficult, and peptides typically display low cell permeability and sensitivity to enzymatic degradation. In this review, we outline the principal high-throughput methodologies for motif discovery and describe current methods for overcoming pharmacokinetic and bioavailability limitations.

Antibacterial activity and mechanism of action of auranofin against multi-drug resistant bacterial pathogens

Traditional methods employed to discover new antibiotics are both a time-consuming and financially-taxing venture. This has led researchers to mine existing libraries of clinical molecules in order to repurpose old drugs for new applications (as antimicrobials). Such an effort led to the discovery of auranofin, a drug initially approved as an anti-rheumatic agent, which also possesses potent antibacterial activity in a clinically achievable range. The present study demonstrates auranofin’s antibacterial activity is a complex process that involves inhibition of multiple biosynthetic pathways including cell wall, DNA, and bacterial protein synthesis. We also confirmed that the lack of activity of auranofin observed against Gram-negative bacteria is due to the permeability barrier conferred by the outer membrane. Auranofin’s ability to suppress bacterial protein synthesis leads to significant reduction in the production of key methicillin-resistant Staphylococcus aureus (MRSA) toxins. Additionally, auranofin is capable of eradicating intracellular MRSA present inside infected macrophage cells. Furthermore, auranofin is efficacious in a mouse model of MRSA systemic infection and significantly reduces the bacterial load in murine organs including the spleen and liver. Collectively, this study provides valuable evidence that auranofin has significant promise to be repurposed as a novel antibacterial for treatment of invasive bacterial infections.

Identification and Functional Analysis of the Nocardithiocin Gene Cluster in Nocardia pseudobrasiliensis

Nocardithiocin is a thiopeptide compound isolated from the opportunistic pathogen Nocardia pseudobrasiliensis. It shows a strong activity against acid-fast bacteria and is also active against rifampicin-resistant Mycobacterium tuberculosis. Here, we report the identification of the nocardithiocin gene cluster in N. pseudobrasiliensis IFM 0761 based on conserved thiopeptide biosynthesis gene sequence and the whole genome sequence. The predicted gene cluster was confirmed by gene disruption and complementation. As expected, strains containing the disrupted gene did not produce nocardithiocin while gene complementation restored nocardithiocin production in these strains. The predicted cluster was further analyzed using RNA-seq which showed that the nocardithiocin gene cluster contains 12 genes within a 15.2-kb region. This finding will promote the improvement of nocardithiocin productivity and its derivatives production.