Pyrrolocin C and equisetin inhibit bacterial acetyl-CoA carboxylase

Insights into the methodology of acetyl-CoA carboxylase inhibition

Acetyl-CoA carboxylase catalyzes the first committed and regulated step in fatty acid synthesis in all animals, plants and bacteria. In most Gram-positive and Gram-negative bacteria, the enzyme is composed of three proteins: biotin carboxylase, biotin carboxyl carrier protein and carboxyltransferase. The reaction consists of two half-reactions. The first half reaction is catalyzed by biotin carboxylase and involves the carboxylation of the vitamin biotin which is covalently attached to the biotin carboxyl carrier protein. The second half reaction catalyzed by carboxyltransferase involves the transfer of the carboxyl group from biotin to acetyl-CoA to form malonyl-CoA. This chapter will describe the inhibitors of both the biotin carboxylase and carboxyltransferase components of bacterial acetyl-CoA carboxylase. Inhibitors that were used in the elucidation of the structure and mechanism of the enzyme will be discussed first. The second half will focus on inhibitors that also possess antibacterial activity.

New Fusarochromanone Derivatives from the Marine Fungus Fusarium equiseti UBOCC-A-117302

Two new fusarochromanone derivatives, deacetylfusarochromene (1) and deacetamidofusarochrom-2',3-diene (2), along with the previously reported metabolites fusarochromanone TDP-2 (3), fusarochromene (4), 2,2-dimethyl-5-amino-6-(2'E-ene-4'-hydroxylbutyryl)-4-chromone (5), fusarochromanone (6), (-)-chrysogine (7), and equisetin (8), were isolated from the marine fungus Fusarium equiseti UBOCC-A-117302. The structures of the compounds were determined by extensive spectrometric (HRMS) and spectroscopic (1D and 2D NMR) analyses, as well as specific rotation. Among them, 2 and 5 showed inhibition of three protein kinases with IC50 values ranging from 1.42 to 25.48 μM. Cytotoxicity and antimicrobial activity of all isolated compounds were also evaluated. Six fusarochromanone derivatives (1-6) exhibited diverse activities against three cell lines, RPE-1, HCT-116, and U2OS (IC50 values ranging from 0.058 to 84.380 μM). Equisetin (8) showed bactericidal activities against Bacillus cereus and Listeria monocytogenes (MBC values of 7.8 and 31.25 µM, respectively), and bacteriostatic activity against Enterococcus faecalis (MIC value of 31.25 µM). Compounds 2 and 4 showed bacteriostatic activities against Listeria monocytogenes (MIC of 125 µM).

Equisetin protects from atherosclerosis in vivo by binding to STAT3 and inhibiting its activity

Atherosclerosis is a chronic inflammatory vascular disease characterized by lipid metabolism disorder and lipid accumulation. Equisetin (EQST) is a hemiterpene compound isolated from fungus of marine sponge origin, which has antibacterial, anti-inflammatory, lipid-lowering, and weight loss effects. Whether EQST has anti-atherosclerotic activity has not been reported. In this study, we revealed that EQST displayed anti- atherosclerosis effects through inhibiting macrophage inflammatory response, lipid uptake and foam cell formation in vitro, and finally ameliorated high-fat diet (HFD)-induced atherosclerosis in AopE-/- mice in vivo. Mechanistically, EQST directly bound to STAT3 with high-affinity by forming hydrophobic bonds at GLN247 and GLN326 residues, as well as hydrogen bonds at ARG325 and THR346 residues. EQST interacted with STAT3 physically, and functionally inhibited the transcription activity of STAT3, thereby regulating atherosclerosis. Therefore, these results supports EQST as a candidate for developing anti-atherosclerosis therapeutic agent.

Equisetin inhibits adiposity through AMPK-dependent regulation of brown adipocyte differentiation

Obesity has a significant impact on endocrine function, which leads to metabolic diseases including diabetes, insulin resistance, and other complications associated with obesity. Development of effective and safe anti-obesity drugs is imperative and necessary. Equisetin (EQST), a tetramate-containing marine fungal product, was reported to inhibit bacterial fatty acid synthesis and affect mitochondrial metabolism. It is tempting to speculate that EQST might have anti-obesity effects. This study was designed to explore anti-obesity effects and underlying mechanism of EQST on 3T3-L1 adipocytes differentiated from 3T3-L1 cells. Oil Red O staining showed that EQST reduced lipid accumulation in 3T3-L1 adipocytes. Quantitative real-time polymerase chain reaction and Western blot analysis revealed that EQST significantly inhibited expression of adipogenesis/lipogenesis-related genes C/ebp-α, Ppar-γ, Srebp1c, Fas, and reduced protein levels. There was also increased expression of key genes and protein levels involved in lipolysis (Perilipin, Atgl, Hsl), brown adipocyte differentiation (Prdm16, Ucp1), mitochondrial biogenesis (Pgc1α, Tfam) and β-oxidation Acsl1, Cpt1. Moreover, mitochondrial content, their membrane potential ΔΨM, and respiratory chain genes Mt-Co1, Cox7a1, Cox8b, and Cox4 (and protein) exhibited marked increase in expression upon EQST treatment, along with increased protein levels. Importantly, EQST induced expression and activation of AMPK, which was compromised by the AMPK inhibitor dorsomorphin, leading to rescue of EQST-downregulated Fas expression and a reduction of the EQST-increased expression of Pgc1α, Ucp1, and Cox4. Together, EQST robustly promotes fat clearance through the AMPK pathway, these results supporting EQST as a strong candidate for the development into an anti-obesity therapeutic agent.

Lipid-Centric Approaches in Combating Infectious Diseases: Antibacterials, Antifungals and Antivirals with Lipid-Associated Mechanisms of Action

One of the global challenges of the 21st century is the increase in mortality from infectious diseases against the backdrop of the spread of antibiotic-resistant pathogenic microorganisms. In this regard, it is worth targeting antibacterials towards the membranes of pathogens that are quite conservative and not amenable to elimination. This review is an attempt to critically analyze the possibilities of targeting antimicrobial agents towards enzymes involved in pathogen lipid biosynthesis or towards bacterial, fungal, and viral lipid membranes, to increase the permeability via pore formation and to modulate the membranes' properties in a manner that makes them incompatible with the pathogen's life cycle. This review discusses the advantages and disadvantages of each approach in the search for highly effective but nontoxic antimicrobial agents. Examples of compounds with a proven molecular mechanism of action are presented, and the types of the most promising pharmacophores for further research and the improvement of the characteristics of antibiotics are discussed. The strategies that pathogens use for survival in terms of modulating the lipid composition and physical properties of the membrane, achieving a balance between resistance to antibiotics and the ability to facilitate all necessary transport and signaling processes, are also considered.

Insights into the genomic architecture of a newly discovered endophytic Fusarium species belonging to the Fusarium concolor complex from India

In this study, a new species Fusarium indicum belonging to the Fusarium concolor species complex is established to accommodate an endophytic fungus isolated from Bambusa sp. and collected from Himachal Pradesh. The identity of this isolate was confirmed based on the asexual morphs, its cultural characteristics, and phylogenetic analyses. This isolate revealed out to be distinct by showing less similarity with described species in the genus Fusarium based on molecular sequence data, approximately 93.9% similarity based on translation elongation factor 1-alpha, and 94.2% similarity based on RNA polymerase II subunit. Furthermore, to increase knowledge about this novel species, whole-genome sequencing was carried out. The results displayed that Fusarium indicum NFCCI 5145 possesses a 40.2 Mb genome and 48.39% of GC content. Approximately 12,963 functional protein-coding genes were carefully predicted and annotated using different BLAST databases, such as Uniprot, Kyoto Encyclopedia of Genes and Genomes (KEGG), Gene Ontology (GO), Pathogen Host Interactions (PHI), Clusters of Orthologous Groups (COG), and Carbohydrate-Active enzymes (CAZy). The orthologous proteins were identified using OrthoFinder and used for the phylogenetic analysis. ANIb confirmed that the isolate is closely related to the F. concolor species complex. It is known that Fusarium strains can produce a wide range of bioactive secondary metabolites. Therefore, in-depth mining for biosynthetic gene clusters for secondary metabolite biosynthesis of Fusarium indicum NFCCI 5145 was investigated using Antibiotics and Secondary Metabolites Analysis Shell (AntiSMASH) annotation. AntiSMASH results displayed that this isolate possesses 45 secondary metabolites of biosynthetic gene clusters (BGCs). These findings significantly improved our understanding of the strain Fusarium indicum NFCCI 5145 and its possible applications in different sectors including industry for the secondary metabolites and enzymes it can produce.

Itaconic acid and dimethyl itaconate exert antibacterial activity in carbon-enriched environments through the TCA cycle

Itaconic acid (IA), a metabolite generated by the tricarboxylic acid (TCA) cycle in eukaryotic immune cells, and its derivative dimethyl itaconate (DI) exert antibacterial functions in intracellular environments. Previous studies suggested that IA and DI only inhibit bacterial growth in carbon-limited environments; however, whether IA and DI maintain antibacterial activity in carbon-enriched environments remains unknown. Here, IA and DI inhibited the bacteria with minimum inhibitory concentrations of 24.02 mM and 39.52 mM, respectively, in a carbon-enriched environment. The reduced bacterial pathogenicity was reflected in cell membrane integrity, motility, biofilm formation, AI-2/luxS, and virulence. Mechanistically, succinate dehydrogenase (SDH) activity and fumaric acid levels decreased in the IA and DI treatments, while isocitrate lyase (ICL) activity was upregulated. Inhibited TCA circulation was also observed through untargeted metabolomics. In addition, energy-related aspartate metabolism and lysine degradation were suppressed. In summary, these results indicated that IA and DI reduced bacterial pathogenicity while exerting antibacterial functions by inhibiting TCA circulation. This study enriches knowledge on the inhibition of bacteria by IA and DI in a carbon-mixed environment, suggesting an alternative method for treating bacterial infections by immune metabolites.

Elucidation of the stereocontrol mechanisms of the chemical and biosynthetic intramolecular Diels-Alder cycloaddition for the formation of bioactive decalins

The intramolecular Diels-Alder reaction (IMDA) is a powerful method for regioselective and stereoselective construction of functionalised decalin skeletons, and the recent discovery of enzymes that catalyse IMDA cycloaddition in biosynthesis has generated considerable interest. This study focused on the role of the absolute configuration of the C-6 carbon of the substrate polyene in the stereocontrol of the IMDA reaction catalysed by Fsa2 and Phm7, which construct different enantiomeric decalin skeletons. Their enantiomeric precursor polyenes were synthesised and subjected to enzymatic or thermal IMDA reactions to isolate various diastereomeric decalines and determine their absolute configuration. Furthermore, density functional theory calculations were performed to elucidate the stereocontrol mechanism underlying the formation of decalin. The results showed that Fsa2 exhibits the same equisetin-type stereoselectivity for enantiomeric substrates regardless of the 6-methyl group configuration of the substrate, while Phm7 shows two types of stereoselectivity depending on the configuration of the 6-methyl group. We also found a unique stereochemistry-activity relationship in antibacterial activity for decalin diastereomers, including new derivatives. This study provides new insights into the stereoselectivity of DAase, which is important in the synthesis of natural product skeletons.

cis-Decalin-containing tetramic acids as inhibitors of insect steroidogenic glutathione S-transferase Noppera-bo

Decalin-containing tetramic acid is a bioactive scaffold primarily produced by filamentous fungi. The structural diversity of this group of compounds is generated by characteristic enzymes of fungal biosynthetic pathways, including polyketide synthase/nonribosomal peptide synthetase hybrid enzymes and decalin synthase, which are responsible for the construction of a linear polyenoyl tetramic acid structure and stereoselective decalin formation via the intramolecular Diels-Alder reaction, respectively. Compounds that differed only in the decalin configuration were collected from genetically engineered mutants derived from decalin-containing tetramic acid-producing fungi and used for a structure-activity relationship study. Our evaluation of biological activities, such as cytotoxicity against several cancer cell lines and antibacterial, antifungal, antimalarial, and mitochondrial inhibitory activities, demonstrated that the activity for each assay varies depending on the decalin configurations. In addition to these known biological activities, we revealed that the compounds showed inhibitory activity against the insect steroidogenic glutathione S-transferase Noppera-bo. Engineering the decalin configurations would be useful not only to find derivatives with better biological activities but also to discover overlooked biological activities.

Genus Equisetum L: Taxonomy, toxicology, phytochemistry and pharmacology

INTRODUCTION

The genus Equisetum (Equisetaceae) is cosmopolitan in distribution, with 41 recognized species. Several species of Equisetum are widely used in treating genitourinary and related diseases, inflammatory and rheumatic problems, hypertension, and wound healing in traditional medicine practices worldwide. This review intends to present information on the traditional uses, phytochemical components, pharmacological activities, and toxicity of Equisetum spp. and to analyze the new insights for further study.

METHODS

Relevant literature has been scanned and collected via various electronic repositories, including PubMed, Science Direct, Google Scholar, Springer Connect, and Science Online, from 1960 to 2022.

RESULTS

Sixteen Equisetum spp. were documented as widely used in traditional medicine practices by different ethnic groups throughout the world. A total of 229 chemical compounds were identified from Equisetum spp. with the major group of constituents being flavonol glycosides and flavonoids. The crude extracts and phytochemicals of Equisetum spp. exhibited significant antioxidant, antimicrobial, anti-inflammatory, antiulcerogenic, antidiabetic, hepatoprotective, and diuretic properties. A wide range of studies have also demonstrated the safety of Equisetum spp.

CONCLUSION

The reported pharmacological properties of Equisetum spp. support its use in traditional medicine, though there are gaps in understanding the traditional usage of these plants for clinical experiments. The documented information revealed that the genus is not only a great herbal remedy but also has several bioactives with the potential to be discovered as novel drugs. Detailed scientific investigation is still needed to fully understand the efficacy of this genus; hence, very few Equisetum spp. were studied in detail for phytochemical and pharmacological investigation. Moreover, its bioactives, structure-activity connection, in vivo activity, and associated mechanism of action ought to be explored further.

β-Lactams from the Ocean

The title of this essay is as much a question as it is a statement. The discovery of the β-lactam antibiotics-including penicillins, cephalosporins, and carbapenems-as largely (if not exclusively) secondary metabolites of terrestrial fungi and bacteria, transformed modern medicine. The antibiotic β-lactams inactivate essential enzymes of bacterial cell-wall biosynthesis. Moreover, the ability of the β-lactams to function as enzyme inhibitors is of such great medical value, that inhibitors of the enzymes which degrade hydrolytically the β-lactams, the β-lactamases, have equal value. Given this privileged status for the β-lactam ring, it is therefore a disappointment that the exemplification of this ring in marine secondary metabolites is sparse. It may be that biologically active marine β-lactams are there, and simply have yet to be encountered. In this report, we posit a second explanation: that the value of the β-lactam to secure an ecological advantage in the marine environment might be compromised by its close structural similarity to the β-lactones of quorum sensing. The steric and reactivity similarities between the β-lactams and the β-lactones represent an outside-of-the-box opportunity for correlating new structures and new enzyme targets for the discovery of compelling biological activities.

Equisetin Targets Intracellular Staphylococcus aureus through a Host Acting Strategy

Mammalian cells act as reservoirs of internalized bacteria to circumvent extracellular antibacterial compounds, resulting in relapse and reinfection diseases. The intracellular persistence of Staphylococcus aureus renders most traditional antibiotics useless, due to their inadequate subcellular accumulation. To replenish our antibiotic arsenal, we found that a marine-derived compound, equisetin, efficiently eliminates intracellular S. aureus by potentiating the host autophagy and inducing mitochondrial-mediated ROS generation to clear the invading S. aureus. The remarkable anti-infection activity of equisetin was validated in a peritonitis-infected mouse model. The marine product equisetin utilizes a unique dual mechanism to modulate the host-pathogen interaction in the clearance of intracellular bacteria. Thus, equisetin is an inspiring host-acting candidate for overcoming intracellular pathogens.

Mining Fatty Acid Biosynthesis for New Antimicrobials

Antibiotic resistance is a serious public health concern, and new drugs are needed to ensure effective treatment of many bacterial infections. Bacterial type II fatty acid synthesis (FASII) is a vital aspect of bacterial physiology, not only for the formation of membranes but also to produce intermediates used in vitamin production. Nature has evolved a repertoire of antibiotics inhibiting different aspects of FASII, validating these enzymes as potential targets for new antibiotic discovery and development. However, significant obstacles have been encountered in the development of FASII antibiotics, and few FASII drugs have advanced beyond the discovery stage. Most bacteria are capable of assimilating exogenous fatty acids. In some cases they can dispense with FASII if fatty acids are present in the environment, making the prospects for identifying broad-spectrum drugs against FASII targets unlikely. Single-target, pathogen-specific FASII drugs appear the best option, but a major drawback to this approach is the rapid acquisition of resistance via target missense mutations. This complication can be mitigated during drug development by optimizing the compound design to reduce the potential impact of on-target missense mutations at an early stage in antibiotic discovery. The lessons learned from the difficulties in FASII drug discovery that have come to light over the last decade suggest that a refocused approach to designing FASII inhibitors has the potential to add to our arsenal of weapons to combat resistance to existing antibiotics.

Isolation, Screening, and Active Metabolites Identification of Anti-Vibrio Fungal Strains Derived From the Beibu Gulf Coral

The Beibu Gulf harbors abundant underexplored marine microbial resources, which are rich in diversified secondary metabolites. The genera Vibrio is a well-known pathogenic bacterium of aquatic animals. In this study, 22 fungal strains were isolated and identified from the Beibu Gulf coral via the serial dilution method and internal transcribed spacer (ITS) sequence analysis, which were further divided into three branches by phylogenetic tree analysis. The crude extracts of them via small-scale fermentation were selected for the screening of inhibitory activity against Vibrio alginalyticus, Vibrio coralliilyticus, Vibrio harveyi, Vibrio parahaemolyticus, Vibrio owensii, and Vibrio shilonii. The results showed that eight fungal extracts displayed anti-Vibrio activity via the filter paper disk assay. Several of them showed strong inhibitory effects. Then, two tetramic acid alkaloids, equisetin (1) and 5'-epiequisetin (2), were identified from Fusarium equiseti BBG10 by bioassay-guided isolation, both of which inhibited the growth of Vibrio spp. with the MIC values of 86-132 μg/ml. The scanning electron microscope results showed that cell membranes of Vibrio became corrugated, distorted or ruptured after treatment with 1 and 2. Taken together, this study provided eight fungal isolates with anti-Vibrio potentials, and two alkaloid-type antibiotics were found with anti-Vibrio effects from the bioactive strain F. equiseti BBG10. Our findings highlight the importance of exploring promising microbes from the Beibu Gulf for the identification of anti-Vibrio for future antibiotic development.

Equisetin is an anti-obesity candidate through targeting 11β-HSD1

Obesity is increasingly prevalent globally, searching for therapeutic agents acting on adipose tissue is of great importance. Equisetin (EQST), a meroterpenoid isolated from a marine sponge-derived fungus, has been reported to display antibacterial and antiviral activities. Here, we revealed that EQST displayed anti-obesity effects acting on adipose tissue through inhibiting adipogenesis in vitro and attenuating HFD-induced obesity in mice, doing so without affecting food intake, blood pressure or heart rate. We demonstrated that EQST inhibited the enzyme activity of 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), a therapeutic target of obesity in adipose tissue. Anti-obesity properties of EQST were all offset by applying excessive 11β-HSD1's substrates and 11β-HSD1 inhibition through knockdown in vitro or 11β-HSD1 knockout in vivo. In the 11β-HSD1 bypass model constructed by adding excess 11β-HSD1 products, EQST's anti-obesity effects disappeared. Furthermore, EQST directly bond to 11β-HSD1 protein and presented remarkable better intensity on 11β-HSD1 inhibition and better efficacy on anti-obesity than known 11β-HSD1 inhibitor. Therefore, EQST can be developed into anti-obesity candidate compound, and this study may provide more clues for developing higher effective 11β-HSD1 inhibitors.

Metal Binding and Its Amelioration in Tetramates

Metal chelation in tetramates may be ameliorated by changing the ligating group and by steric blocking, which in turn leads to a change in their antibacterial properties; the former was achieved by replacement of an amide with a C-9 C═N bond and the latter by the synthesis of cysteine-derived tetramates with functionalization at the C-6 or C-9 enolic groups. In both cases, the metal-chelating ability was weak, and a loss of antibacterial activity was observed. Tetramate alkylations with an extended tricarbonyl-conjugated system could be achieved under Mitsunobu conditions which led to regioisomers, distinguishable by careful heteronuclear multiple bond coherence correlation and carbonyl carbon chemical shift analysis. C-9 and C-6 O-alkylation were observed but not C-8 O-alkylation for tetramate carboxamides; interestingly, C-7 alkylation with allyl and prenyl derivatives was also observed, and this arose by the rearrangement of initially formed O-alkyl products. Only the C-7 alkylated tetramate derivatives 13a and 13d with no metal-chelating ability demonstrated promising antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), with the most active analogue exhibiting a minimum inhibitory concentration of ≤ 1.95 μg/mL against MRSA, suggesting a mechanism of action independent of metal chelation. Otherwise, modifications at C-6/C-9 of tetramates led to a complete loss of metal-chelating ability, which correlated with the loss of antibacterial activity. This work further confirms that the metal-chelating capability is of fundamental importance in the biological activity of tetramates.

γ-Lactam-Based Antifungal Compounds against the Wheat Pathogen Zymoseptoria tritici

As new environmentally friendly and effective antifungal agents are deeply needed, efficient ecofriendly strategies were designed to access two series of compounds inspired from natural γ-lactams. Designed compounds were fully characterized and evaluated as antifungal candidates against Zymoseptoria tritici, the main pathogen on wheat crops. The targeted derivatives were prepared from natural resources using green solvents, simple procedures, and limited purification steps. These bio-inspired compounds revealed as good candidates for further development of efficient crop protection products. Indeed, the HIT compounds exhibited IC₅₀ around 1 μg/mL and were more active than the references tebuconazole and bixafen towards some multidrug-resistant strains. Two dozen of derivatives have been obtained for each series and allowed to establish early structure-activity relationships useful for the development of next generation of γ-lactam derivatives with improved efficacy.

Biosynthetic versatility of marine-derived fungi on the delivery of novel antibacterial agents against priority pathogens

Despite the increasing number of novel marine natural products being reported from fungi in the last three decades, to date only the broad-spectrum cephalosporin C can be tracked back as marine fungal-derived drug. Cephalosporins were isolated in the early 1940s from a strain of Acremonium chrysogenum obtained in a sample collected in sewage water in the Sardinian coast, preliminary findings allowing the discovery of cephalosporin C. Since then, bioprospection of marine fungi has been enabling the identification of several metabolites with antibacterial effects, many of which proving to be active against multi-drug resistant strains, available data suggesting also that some might fuel the pharmaceutical firepower towards some of the bacterial pathogens classified as a priority by the World Health Organization. Considering the success of their terrestrial counterparts on the discovery and development of several antibiotics that are nowadays used in the clinical setting, marine fungi obviously come into mind as producers of new prototypes to counteract antibiotic-resistant bacteria that are no longer responding to available treatments. We mainly aim to provide a snapshot on those metabolites that are likely to proceed to advanced preclinical development, not only based on their antibacterial potency, but also considering their targets and modes of action, and activity against priority pathogens.

Five Tetramic Acid Derivatives Isolated from the Iranian Fungus Colpoma quercinum CCTU A372

Submerged mycelial cultures of the ascomycete Colpoma quercinum CCTU A372 were found to produce five previously undescribed tetramic acids, for which we propose the trivial names colposetins A-C (1-3) and colpomenoic acids A and B (4 and 5), along with the known compounds penicillide (6) and monodictyphenone (7). The planar structures of 1-5 were determined by high-resolution electrospray ionization mass spectrometry (HR-ESIMS) and extensive 1D and 2D nuclear magnetic resonance (NMR) spectroscopy. Their absolute configurations were determined by the combination of electronic circular dischroism (ECD) analysis, J-based configurational analysis, and a rotating-frame Overhauser effect spectroscopy (ROESY) experiment. Colposetin B displayed weak antimicrobial activity against Bacillus subtilis and Mucor hiemalis (MIC 67 µg/mL).

Biosynthetic Cyclization Catalysts for the Assembly of Peptide and Polyketide Natural Products

Many biologically active natural products are synthesized by nonribosomal peptide synthetases (NRPSs), polyketide synthases (PKSs) and their hybrids. These megasynthetases contain modules possessing distinct catalytic domains that allow for substrate initiation, chain extension, processing and termination. At the end of a module, a terminal domain, usually a thioesterase (TE), is responsible for catalyzing the release of the NRPS or PKS as a linear or cyclized product. In this review, we address the general cyclization mechanism of the TE domain, including oligomerization and the fungal C-C bond forming Claisen-like cyclases (CLCs). Additionally, we include examples of cyclization catalysts acting within or at the end of a module. Furthermore, condensation-like (CT) domains, terminal reductase (R) domains, reductase-like domains that catalyze Dieckmann condensation (RD), thioesterase-like Dieckmann cyclases, trans-acting TEs from the penicillin binding protein (PBP) enzyme family, product template (PT) domains and others will also be reviewed. The studies summarized here highlight the remarkable diversity of NRPS and PKS cyclization catalysts for the production of biologically relevant, complex cyclic natural products and related compounds.

A Marine Antibiotic Kills Multidrug-Resistant Bacteria without Detectable High-Level Resistance

Antibiotic resistance nowadays is spreading much faster than the introduction of new antibiotics into clinical practice. There is an urgent need for potential compounds to combat multidrug-resistant (MDR) bacteria. Marine fungi provide a promising source for chemical diversity with antibiotic-like molecules. To identify structurally distinct compounds that effectively eradicate MDR pathogens and to control the development of antibiotic resistance, we have reinvestigated equisetin, a previously reported meroterpenoid isolated from a marine sponge-derived fungus. Equisetin exerted efficient antibacterial activities against either MRSA or VRE without detectable high-level resistance. Meanwhile, equisetin, as an antibiotic adjuvant, restores colistin susceptibility to colistin-resistant bacteria toward diverse Gram-negative pathogens. Intriguingly, the low-level equisetin-resistant Staphylococcus aureus displayed collateral sensitivity to multiple classes of existing antibiotics with decreased capacity to produce biofilm. Lastly, equisetin showed efficacy with MRSA in three infected animal models. This work suggests that equisetin derived from marine natural products is a promising lead to overcome antibiotic resistance, providing new insight in future antibiotic discovery and development.