Natural products to drugs: daptomycin and related lipopeptide antibiotics

Metabolic engineering of "last-line antibiotic" colistin in Paenibacillus polymyxa

Colistin, also known as polymyxin E, is a lipopeptide antibiotic used to treat infections caused by multidrug-resistant gram-negative bacteria. It is considered a "last-line antibiotic", but its clinical development is hindered by low titer and impurities resulting from the presence of diverse homologs in microbial fermentation. To ensure consistent pharmaceutical activity and kinetics, it is crucial to have high-purity colistin active pharmaceutical ingredient (API) in the pharmaceutical industry. This study focused on the metabolic engineering of a natural colistin producer strain to produce colistin with a high titer and purity. Guided by genome mining, we identified Paenibacillus polymyxa ATCC 842 as a natural colistin producer capable of generating a high proportion of colistin A. By systematically inactivating seven non-essential biosynthetic gene clusters (BGCs) of peptide metabolites that might compete precursors with colistin or inhibit colistin production, we created an engineered strain, P14, which exhibited an 82% increase in colistin titer and effectively eliminated metabolite impurities such as tridecaptin, paenibacillin, and paenilan. Additionally, we engineered the L-2,4-diaminobutyric acid (L-2,4-DABA) pathway to further enhance colistin production, resulting in the engineered strain P19, which boosted a remarkable colistin titer of 649.3 mg/L - a 269% improvement compared to the original strain. By concurrently feeding L-isoleucine and L-leucine, we successfully produced high-purity colistin A, constituting 88% of the total colistin products. This study highlights the potential of metabolic engineering in improving the titer and purity of lipopeptide antibiotics in the non-model strain, making them more suitable for clinical use. These findings indicate that efficiently producing colistin API in high purity directly from fermentation can now be achieved in a straightforward manner.

Chemoenzymatic synthesis of macrocyclic peptides and polyketides via thioesterase-catalyzed macrocyclization

Chemoenzymatic strategies that combine synthetic and enzymatic transformations offer efficient approaches to yield target molecules, which have been increasingly employed in the synthesis of bioactive natural products. In the biosynthesis of macrocyclic nonribosomal peptides, polyketides, and their hybrids, thioesterase (TE) domains play a significant role in late-stage macrocyclization. These domains can accept mimics of native substrates in vitro and exhibit potential for use in total synthesis. This review summarizes the recent advances of TE domains in the chemoenzymatic synthesis for these natural products that aim to address the common issues in classical synthetic approaches and increase synthetic efficiencies, which have the potential to facilitate further pharmaceutical research.

Antiviral Effect of Antimicrobial Peptoid TM9 and Murine Model of Respiratory Coronavirus Infection

New antiviral agents are essential to improving treatment and control of SARS-CoV-2 infections that can lead to the disease COVID-19. Antimicrobial peptoids are sequence-specific oligo-N-substituted glycine peptidomimetics that emulate the structure and function of natural antimicrobial peptides but are resistant to proteases. We demonstrate antiviral activity of a new peptoid (TM9) against the coronavirus, murine hepatitis virus (MHV), as a closely related model for the structure and antiviral susceptibility profile of SARS-CoV-2. This peptoid mimics the human cathelicidin LL-37, which has also been shown to have antimicrobial and antiviral activity. In this study, TM9 was effective against three murine coronavirus strains, demonstrating that the therapeutic window is large enough to allow the use of TM9 for treatment. All three isolates of MHV generated infection in mice after 15 min of exposure by aerosol using the Madison aerosol chamber, and all three viral strains could be isolated from the lungs throughout the 5-day observation period post-infection, with the peak titers on day 2. MHV-A59 and MHV-A59-GFP were also isolated from the liver, heart, spleen, olfactory bulbs, and brain. These data demonstrate that MHV serves as a valuable natural murine model of coronavirus pathogenesis in multiple organs, including the brain.

In Vitro Module Editing Of NRPS Enables Production Of Highly Potent Gq -Signaling Inhibitor FR900359 Derived From Unculturable Plant Symbiont

Heterotrimeric G proteins are key mediators in the signaling of G protein-coupled receptors (GPCR) that are involved in a plethora of important physiological processes and thus major targets of pharmaceutical drugs. The cyclic depsipeptides YM-254890 and FR900359 are strong and selective inhibitors of the Gq subfamily of G proteins. FR900359 was first reported to be produced by unculturable plant symbiont, however, a culturable FR900359 producer was discovered recently by the standard strategy, screening of the producing strain from the environment. As another strategy, we introduce herein the different way to supply natural compounds of unculturable microorganism origin. We therefore embarked on constructing an artificial biosynthetic gene cluster (BGC) for FR900359 with YM-254890 BGC as a template using "in vitro module editing" technology, first developed for the modification of type-I PKS BGCs, to edit YM-254890 BGC. The resulting artificial BGCs coding FR900359 were heterologously expressed in the Pseudomonas putida KT2440 host strain.

Dilarmycins A-C, Calcium-Dependent Lipopeptide Antibiotics with a Non-canonical Ca2+-Binding Motif

Genome analysis of strain Streptomyces sp. CA-278952 revealed a biosynthetic gene cluster encoding a putative lipopeptide with a sequence containing an Asp-Gly-Glu-Ala motif. We envisioned that this motif could mimic the canonical Asp-X-Asp-Gly sequence found in previously reported calcium-dependent lipopeptide antibiotics. Chemical investigation of the producing strain led to the discovery of three novel lipodepsipeptides, dilarmycins A-C. The calcium-dependent antibacterial activity of the new compounds was confirmed against the Gram-positive pathogens methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus.

Gausemycin Antibiotic Family Acts via Ca2+-Dependent Membrane Targeting

We report the molecular mechanism of action of gausemycins and the isolation of new members of the family, gausemycins C (1c), D (1d), E (1e), and F (1f), the minor components of the mixture. To elucidate the mechanism of action of gausemycins, we investigated the antimicrobial activity of the most active compounds, gausemycins A and B, in the presence of Ca2+, other metal ions, and phosphate. Gausemycins require a significantly higher Ca2+ concentration for maximum activity than daptomycin but lower than that required for malacidine and cadasides. Species-specific antimicrobial activity was found upon testing against a wide panel of Gram-positive bacteria. Membranoactivity of gausemycins was demonstrated upon their interactions with model lipid bilayers and micelles. The pore-forming ability was found to be dramatically dependent on the Ca2+ concentration and the membrane lipid composition. An NMR study of gausemycin B in zwitterionic and anionic micelles suggested the putative structure of the gausemycin/membrane complex and revealed the binding of Ca2+ by the macrocyclic domain of the antibiotic.

Synthesis and antimicrobial studies of cadasides analogues via on-resin esterification

A series of cadasides analogues have been prepared via a combination of solid-phase peptide synthesis and solution-phase cyclization. Primary structure-activity relationship studies of cadasides have also been established and revealed the critical roles of unnatural amino acid residues, which will facilitate the further development of cadasides analogues with improved antimicrobial activities.

Utilizing a cell-free protein synthesis platform for the biosynthesis of a natural product, caffeine

Natural products are a valuable source of pharmaceuticals, providing a majority of the small-molecule drugs in use today. However, their production through organic synthesis or in heterologous hosts can be difficult and time-consuming. Therefore, to allow for easier screening and production of natural products, we demonstrated the use of a cell-free protein synthesis system to partially assemble natural products in vitro using S-Adenosyl Methionine (SAM)-dependent methyltransferase enzyme reactions. The tea caffeine synthase, TCS1, was utilized to synthesize caffeine within a cell-free protein synthesis system. Cell-free systems also provide the benefit of allowing the use of substrates that would normally be toxic in a cellular environment to synthesize novel products. However, TCS1 is unable to utilize a compound like S-adenosyl ethionine as a cofactor to create ethylated caffeine analogs. The automation and reduced metabolic engineering requirements of cell-free protein synthesis systems, in combination with other synthesis methods, may enable the more efficient generation of new compounds. Graphical Abstract.

Antibiotics and Antibiotic Resistance-Mur Ligases as an Antibacterial Target

The emergence of Multidrug Resistance (MDR) strains of bacteria has accelerated the search for new antibacterials. The specific bacterial peptidoglycan biosynthetic pathway represents opportunities for the development of novel antibacterial agents. Among the enzymes involved, Mur ligases, described herein, and especially the amide ligases MurC-F are key targets for the discovery of multi-inhibitors, as they share common active sites and structural features.

Occurrence of D-amino acids in natural products

Since the identified standard genetic code contains 61 triplet codons of three bases for the 20 L-proteinogenic amino acids (AAs), no D-AA should be found in natural products. This is not what is observed in the living world. D-AAs are found in numerous natural compounds produced by bacteria, algae, fungi, or marine animals, and even vertebrates. A review of the literature indicated the existence of at least 132 peptide natural compounds in which D-AAs are an essential part of their structure. All compounds are listed, numbered and described herein. The two biosynthetic routes leading to the presence of D-AA in natural products are: non-ribosomal peptide synthesis (NRPS), and ribosomally synthesized and post-translationally modified peptide (RiPP) synthesis which are described. The methods used to identify the AA chirality within naturally occurring peptides are briefly discussed. The biological activity of an all-L synthetic peptide is most often completely different from that of the D-containing natural compounds. Analyzing the selected natural compounds showed that D-Ala, D-Val, D-Leu and D-Ser are the most commonly encountered D-AAs closely followed by the non-proteinogenic D-allo-Thr. D-Lys and D-Met were the least prevalent D-AAs in naturally occurring compounds.

Biosynthesis and engineering of the nonribosomal peptides with a C-terminal putrescine

The broad bioactivities of nonribosomal peptides rely on increasing structural diversity. Genome mining of the Burkholderiales strain Schlegelella brevitalea DSM 7029 leads to the identification of a class of dodecapeptides, glidonins, that feature diverse N-terminal modifications and a uniform putrescine moiety at the C-terminus. The N-terminal diversity originates from the wide substrate selectivity of the initiation module. The C-terminal putrescine moiety is introduced by the unusual termination module 13, the condensation domain directly catalyzes the assembly of putrescine into the peptidyl backbone, and other domains are essential for stabilizing the protein structure. Swapping of this module to another two nonribosomal peptide synthetases leads to the addition of a putrescine to the C-terminus of related nonribosomal peptides, improving their hydrophilicity and bioactivity. This study elucidates the mechanism for putrescine addition and provides further insights to generate diverse and improved nonribosomal peptides by introducing a C-terminal putrescine.

Rapid Unambiguous Structure Elucidation of Streptnatamide A, a New Cyclic Peptide Isolated from A Marine-derived Streptomyces sp

Cyclic peptides have been excellent source of drug leads. With the advances in discovery platforms, the pharmaceutical industry has a growing interest in cyclic peptides and has pushed several into clinical trials. However, structural complexity of cyclic peptides brings extreme challenges for structure elucidation efforts. Isotopic fine structure analysis, Nuclear magnetic resonance (NMR), and detailed tandem mass spectrometry rapidly provided peptide sequence for streptnatamide A, a cyclic peptide isolated from a marine-derived Streptomyces sp. Marfey's analysis determined the stereochemistry of all amino acids, enabling the unambiguous structure determination of this compound. A non-ribosomal peptide synthetase biosynthetic gene cluster (stp) was tentatively identified and annotated for streptnatamide A based on the in silico analysis of whole genome sequencing data. These analytical tools will be powerful tools to overcome the challenges for cyclic peptide structure elucidation and accelerate the development of bioactive cyclic peptides.

The effects of daptomycin on cell wall biosynthesis in Enterococcal faecalis

Daptomycin is a cyclic lipodepsipeptide antibiotic reserved for the treatment of serious infections by multidrug-resistant Gram-positive pathogens. Its mode of action is considered to be multifaceted, encompassing the targeting and depolarization of bacterial cell membranes, alongside the inhibition of cell wall biosynthesis. To characterize the daptomycin mode of action, ¹⁵N cross-polarization at magic-angle spinning NMR measurements were performed on intact whole cells of Staphylococcus aureus grown in the presence of a sub-inhibitory concentration of daptomycin in a chemically defined media containing L-[ϵ-¹⁵N]Lys. Daptomycin-treated cells showed a reduction in the lysyl-ε-amide intensity that was consistent with cell wall thinning. However, the reduced lysyl-ε-amine intensity at 10 ppm indicated that the daptomycin-treated cells did not accumulate in Park's nucleotide, the cytoplasmic peptidoglycan (PG) precursor. Consequently, daptomycin did not inhibit the transglycosylation step of PG biosynthesis. To further elucidate the daptomycin mode of action, the PG composition of daptomycin-susceptible Enterococcus faecalis grown in the presence of daptomycin was analyzed using liquid chromatography-mass spectrometry. Sixty-nine muropeptide ions correspond to PG with varying degrees of modifications including crosslinking, acetylation, alanylation, and 1,6-anhydrous ring formation at MurNAc were quantified. Analysis showed that the cell walls of daptomycin-treated E. faecalis had a significant reduction in PG crosslinking which was accompanied by an increase in lytic transglycosylase activities and a decrease in PG-stem modifications by the carboxypeptidases. The changes in PG composition suggest that daptomycin inhibits cell wall biosynthesis by impeding the incorporation of nascent PG into the cell walls by transpeptidases and maturation by carboxypeptidases. As a result, the newly formed cell walls become highly susceptible to degradation by the autolysins, resulting in thinning of the cell wall.

Modern Approaches to the Genome Editing of Antibiotic Biosynthetic Clusters in Actinomycetes

Representatives of the phylum Actinomycetota are one of the main sources of secondary metabolites, including antibiotics of various classes. Modern studies using high-throughput sequencing techniques enable the detection of dozens of potential antibiotic biosynthetic genome clusters in many actinomycetes; however, under laboratory conditions, production of secondary metabolites amounts to less than 5% of the total coding potential of producer strains. However, many of these antibiotics have already been described. There is a continuous "rediscovery" of known antibiotics, and new molecules become almost invisible against the general background. The established approaches aimed at increasing the production of novel antibiotics include: selection of optimal cultivation conditions by modifying the composition of nutrient media; co-cultivation methods; microfluidics, and the use of various transcription factors to activate silent genes. Unfortunately, these tools are non-universal for various actinomycete strains, stochastic in nature, and therefore do not always lead to success. The use of genetic engineering technologies is much more efficient, because they allow for a directed and controlled change in the production of target metabolites. One example of such technologies is mutagenesis-based genome editing of antibiotic biosynthetic clusters. This targeted approach allows one to alter gene expression, suppressing the production of previously characterized molecules, and thereby promoting the synthesis of other unknown antibiotic variants. In addition, mutagenesis techniques can be successfully applied both to new producer strains and to the genes of known isolates to identify new compounds.

Structural diversity, biosynthesis, and biological functions of lipopeptides from Streptomyces

Covering: up to 2022Streptomyces are ubiquitous in terrestrial and marine environments, where they display a fascinating metabolic diversity. As a result, these bacteria are a prolific source of active natural products. One important class of these natural products is the nonribosomal lipopeptides, which have diverse biological activities and play important roles in the lifestyle of Streptomyces. The importance of this class is highlighted by the use of related antibiotics in the clinic, such as daptomycin (tradename Cubicin). By virtue of recent advances spanning chemistry and biology, significant progress has been made in biosynthetic studies on the lipopeptide antibiotics produced by Streptomyces. This review will serve as a comprehensive guide for researchers working in this multidisciplinary field, providing a summary of recent progress regarding the investigation of lipopeptides from Streptomyces. In particular, we highlight the structures, properties, biosynthetic mechanisms, chemical and chemoenzymatic synthesis, and biological functions of lipopeptides. In addition, the application of genome mining techniques to Streptomyces that have led to the discovery of many novel lipopeptides is discussed, further demonstrating the potential of lipopeptides from Streptomyces for future development in modern medicine.

Enhancement of herbicolin A production by integrated fermentation optimization and strain engineering in Pantoea agglomerans ZJU23

BACKGROUND

The lipopeptide herbicolin A (HA) secreted by the biocontrol agent Pantoea agglomerans ZJU23 is a promising antifungal drug to combat fungal pathogens by targeting lipid rafts, both in agricultural and clinical settings. Improvement of HA production would be of great significance in promoting its commercialization. This study aims to enhance the HA production in ZJU23 by combining fermentation optimization and strain engineering.

RESULTS

Based on the results in the single-factor experiments, corn steep liquor, temperature and initial pH were identified as the significant affecting factors by the Plackett-Burman design. The fermentation medium and conditions were further optimized using the Box-Behnken response surface method, and the HA production of the wild type strain ZJU23 was improved from ~ 87 mg/mL in King's B medium to ~ 211 mg/mL in HA induction (HAI) medium. A transposon library was constructed in ZJU23 to screen for mutants with higher HA production, and two transcriptional repressors for HA biosynthesis, LrhA and PurR, were identified. Disruption of the LrhA gene led to increased mRNA expression of HA biosynthetic genes, and subsequently improved about twofold HA production. Finally, the HA production reached ~ 471 mg/mL in the ΔLrhA mutant under optimized fermentation conditions, which is about 5.4 times higher than before (~ 87 mg/mL). The bacterial suspension of the ΔLrhA mutant fermented in HAI medium significantly enhanced its biocontrol efficacy against gray mold disease and Fusarium crown rot of wheat, showing equivalent control efficacies as the chemical fungicides used in this study. Furthermore, HA was effective against fungicide resistant Botrytis cinerea. Increased HA production substantially improved the control efficacy against gray mold disease caused by a pyrimethanil resistant strain.

CONCLUSIONS

This study reveals that the transcriptional repressor LrhA negatively regulates HA biosynthesis and the defined HAI medium is suitable for HA production. These findings provide an extended basis for large-scale production of HA and promote biofungicide development based on ZJU23 and HA in the future.

Three Bacterial DedA Subfamilies with Distinct Functions and Phylogenetic Distribution

Recent studies in bacteria have suggested that the broadly conserved but enigmatic DedA proteins function as undecaprenyl-phosphate (UndP) flippases, recycling this essential lipid carrier. To determine whether all DedA proteins have UndP flippase activity, we performed a phylogenetic analysis and correlated our findings to previously published experimental results and predicted structures. We uncovered three major DedA subfamilies: one contains UndP flippases, the second contains putative phospholipid flippases and is associated with aerobic metabolism, and the third is found only in specific Gram-negative phyla. IMPORTANCE DedA family proteins are highly conserved and nearly ubiquitous integral membrane proteins found in archaea, bacteria, and eukaryotes. Recent work revealed that eukaryotic DedA proteins are phospholipid scramblases and that some bacterial DedA proteins are undecaprenyl phosphate flippases. We performed a phylogenetic analysis of this protein family in bacteria that revealed 3 DedA subfamilies with distinct phylogenetic distributions, genomic contexts, and putative functions. Our bioinformatic analysis lays the groundwork for future experimental studies on the role of DedA proteins in maintaining and modifying the membrane.

Three bacterial DedA subfamilies with distinct functions and phylogenetic distribution

Recent studies in bacteria suggested that the broadly conserved but enigmatic DedA proteins function as undecaprenyl-phosphate (UndP) flippases, recycling this essential lipid carrier. To determine whether all DedA proteins have UndP flippase activity, we performed a phylogenetic analysis and correlated it to previously published experimental results and predicted structures. We uncovered three major DedA subfamilies: one contains UndP flippases, the second contains putative phospholipid flippases and is associated with aerobic metabolism, and the third is found only in specific Gram-negative phyla.

IMPORTANCE

DedA-family proteins are highly conserved and nearly ubiquitous integral membrane proteins found in Archaea, Bacteria, and Eukaryotes. Recent work revealed that eukaryotic DedA proteins are phospholipid scramblases and some bacterial DedA proteins are undecaprenyl phosphate flippases. We perform a phylogenetic analysis of this protein family in Bacteria revealing 3 DedA subfamilies with distinct phylogenetic distributions, genomic contexts, and putative functions. Our analysis lays the groundwork for a deeper understanding of DedA proteins and their role in maintaining and modifying the membrane.

Talaropeptins A and B, Tripeptides with an N-trans-Cinnamoyl Moiety from the Marine-Derived Fungus Talaromyces purpureogenus CX11

We report the discovery of talaropeptins A (1) and B (2), tripeptides with an unusual 5/6/5 heterocyclic scaffold and an N-trans-cinnamoyl moiety, which were identified from the marine-derived fungus Talaromyces purpureogenus CX11. A bioinformatic analysis of the genome of T. purpureogenus CX11 and gene inactivation revealed that the biosynthesis of talaropeptins involves a nonribosomal peptide synthase gene cluster. Their chemical structures were elucidated using a combination of 1D and 2D NMR spectroscopy and mass spectrometry. The absolute configurations of 1 and 2 were established by electronic circular dichroism calculations and Marfey's method. The plausible biosynthesis of 1 and 2 is also proposed on the basis of gene deletion, substrate feeding, and heterologous expression. Compounds 1 and 2 showed moderate antifungal activity against phytopathogenic fungus Fusarium oxysporum with MIC values of 12.5 and 25 μg/mL, respectively.

Underexplored bacteria as reservoirs of novel antimicrobial lipopeptides

Natural products derived from microorganisms play a prominent role in drug discovery as potential anti-infective agents. Over the past few decades, lipopeptides produced by particularly Bacillus, Pseudomonas, Streptomyces, Paenibacillus, and cyanobacteria species, have been extensively studied for their antimicrobial potential. Subsequently, daptomycin and polymyxin B were approved by the Food and Drug Administration as lipopeptide antibiotics. Recent studies have however, indicated that Serratia, Brevibacillus, and Burkholderia, as well as predatory bacteria such as Myxococcus, Lysobacter, and Cystobacter, hold promise as relatively underexplored sources of novel classes of lipopeptides. This review will thus highlight the structures and the newly discovered scaffolds of lipopeptide families produced by these bacterial genera, with potential antimicrobial activities. Additionally, insight into the mode of action and biosynthesis of these lipopeptides will be provided and the application of a genome mining approach, to ascertain the biosynthetic gene cluster potential of these bacterial genera (genomes available on the National Center for Biotechnology Information) for their future pharmaceutical exploitation, will be discussed.

Establishing the Structure-Activity Relationship between Phosphatidylglycerol and Daptomycin

Daptomycin is a clinical antibiotic used to treat serious infections caused by Gram-positive bacteria. Although there is debate about the action mechanism of daptomycin, it is known that daptomycin requires both calcium and phosphatidylglycerol (PG) to exert its antibacterial effect. Despite the importance and uniqueness of the interaction of daptomycin with PG, very little is known about this interaction or the nascent daptomycin-PG complex. In this work, we establish a structure-activity relationship between daptomycin and PG through the synthesis of PG analogues. In total, nine PGs were synthesized using a divergent approach employing phosphoramidite chemistry. The interaction between daptomycin and these PGs was studied using fluorescence, circular dichroism, and isothermal titration calorimetry. It was determined that daptomycin is highly sensitive to the modification of the headgroup of PG and both hydroxyl groups influence membrane binding, oligomerization, and backbone structure. Methylation of each hydroxyl in the headgroup suggests that the binding pocket envelops both hydroxyl groups. A PG acyl tail chain length of at least 7-8 carbons is required for stoichiometric binding at micromolar peptide concentrations. Daptomycin binds to PG having 8-carbon, linear, unsaturated acyl groups (C8PGs) at the micromolar concentration and interacts with C8PG in essentially the same manner as when the PG is incorporated into a liposome, and thus, preassembly of individual PG moieties is not a prerequisite for binding, structural transition, and oligomerization.

Antimicrobial Peptide Mimics for Clinical Use: Does Size Matter?

The search for efficient antimicrobial therapies that can alleviate suffering caused by infections from resistant bacteria is more urgent than ever before. Infections caused by multi-resistant pathogens represent a significant and increasing burden to healthcare and society and researcher are investigating new classes of bioactive compounds to slow down this development. Antimicrobial peptides from the innate immune system represent one promising class that offers a potential solution to the antibiotic resistance problem due to their mode of action on the microbial membranes. However, challenges associated with pharmacokinetics, bioavailability and off-target toxicity are slowing down the advancement and use of innate defensive peptides. Improving the therapeutic properties of these peptides is a strategy for reducing the clinical limitations and synthetic mimics of antimicrobial peptides are emerging as a promising class of molecules for a variety of antimicrobial applications. These compounds can be made significantly shorter while maintaining, or even improving antimicrobial properties, and several downsized synthetic mimics are now in clinical development for a range of infectious diseases. A variety of strategies can be employed to prepare these small compounds and this review describes the different compounds developed to date by adhering to a minimum pharmacophore based on an amphiphilic balance between cationic charge and hydrophobicity. These compounds can be made as small as dipeptides, circumventing the need for large compounds with elaborate three-dimensional structures to generate simplified and potent antimicrobial mimics for a range of medical applications. This review highlight key and recent development in the field of small antimicrobial peptide mimics as a promising class of antimicrobials, illustrating just how small you can go.

Microwave-assisted preparation of Ag/Fe magnetic biochar from clivia leaves for adsorbing daptomycin antibiotics

Novel clivia biochar adsorbing daptomycin (DAP) was prepared by microwave digestion–anaerobic carbonization in this work. Fe/Ag submicron particles were introduced to the biochar surface based on the reducibility of biochar to enhance its adsorption capacity. Characterization confirmed that modified biochar (AF-biochar) had a higher particle size (126 μm), larger specific surface area (521.692 m2 g−1), richer pore structure, and higher thermal stability. The effects of the main variables (e.g., the solution pH, contact time, initial DAP concentration, and temperature) were investigated during adsorption. The results showed that AF-biochar could reach the adsorption equilibrium at pH 4.8 for 85 min. Besides, the adsorption capacity was 48.25 mg g−1, and the adsorption efficiency was 96.50% when the concentration of DAP was 25 mg L−1. The pseudo-second-order kinetics (R 2 = 0.9997), Langmuir equation (R 2 = 0.9999), and thermodynamics (R 2 = 0.9631) of AF-biochar fit well, indicating that the main adsorption process of AF-biochar was spontaneous, exothermic, and monolayer. Their adsorption was analyzed by physical and chemical adsorption. The main adsorption mechanisms included the electron donor–acceptor interaction, electrostatic force interaction, Lewis acid–base interaction, and H-bond interaction.

Turbinaria ornata and its associated epiphytic Bacillus sp. A promising molecule supplier to discover new natural product approaches

Marine ecosystems are highly dependent on macroalgea in providing food and shelter for aquatic organisms, interacting with many bacteria and mostly producing secondary metabolites of potent therapeutic antibacterial property. Screening of marine microbial secondary metabolites of valuable biotechnological and therapeutical applications are now extensively studied. In this study, Bacillus spp. identified by DNA sequencing and found associated with Turbinaria ornata, was screened and characterized for its cell free supernatant (CFS) possible antimicrobial and antibiofilm applications. Among the 7 microbial isolates tested, CFS greatly affected Bacillus subitilis (12 mm) and inhibited equally the yeast isolates Candida albicans, Candida tropicalis and Candida glabrata (10 mm) and had no or negligible effect on S.aureus, E.coli, P. aeruginosa. As for the CFS antibiofilm activity, no difference was revealed from the positive control. Algal crude extracts (methanol, acetone and aqueous), on the other hand, were similarly tested for their antimicrobial activity against the seven microbial isolates, where highest activity was observed with the aqueous crude extract against Staphylococcus aureus(10 mm) and Pseudomonas aeruginosa (9 mm) compared to the negligible effects of methanol and acetone crude extracts. Chemical analysis was performed to reveal the major constituents of both crude algal extracts and Bacillus spp. CFS. FTIR spectrum of the bacterial CFS indicated the presence of bacteriocin as the major lipopeptide responsible for its biological activity. Whereas, methanol and water crude algal extract GC-MS spectra revealed different chemical groups of various combined therapeutical activity mainly Naphthalene, amino ethane-sulfonic acid, pyrlene, Biotin and mercury chloromethyl correspondingly. Thus, the present study, demonstrated the moderate activity of both crude algal extract and the bacterial CFS, however, further investigations are needed for a better biological activity.

Isolation and characterization of Streptomyces bacteriophages and Streptomyces strains encoding biosynthetic arsenals

The threat to public health posed by drug-resistant bacteria is rapidly increasing, as some of healthcare's most potent antibiotics are becoming obsolete. Approximately two-thirds of the world's antibiotics are derived from natural products produced by Streptomyces encoded biosynthetic gene clusters. Thus, to identify novel gene clusters, we sequenced the genomes of four bioactive Streptomyces strains isolated from the soil in San Diego County and used Bacterial Cytological Profiling adapted for agar plate culturing in order to examine the mechanisms of bacterial inhibition exhibited by these strains. In the four strains, we identified 104 biosynthetic gene clusters. Some of these clusters were predicted to produce previously studied antibiotics; however, the known mechanisms of these molecules could not fully account for the antibacterial activity exhibited by the strains, suggesting that novel clusters might encode antibiotics. When assessed for their ability to inhibit the growth of clinically isolated pathogens, three Streptomyces strains demonstrated activity against methicillin-resistant Staphylococcus aureus. Additionally, due to the utility of bacteriophages for genetically manipulating bacterial strains via transduction, we also isolated four new phages (BartholomewSD, IceWarrior, Shawty, and TrvxScott) against S. platensis. A genomic analysis of our phages revealed nearly 200 uncharacterized proteins, including a new site-specific serine integrase that could prove to be a useful genetic tool. Sequence analysis of the Streptomyces strains identified CRISPR-Cas systems and specific spacer sequences that allowed us to predict phage host ranges. Ultimately, this study identified Streptomyces strains with the potential to produce novel chemical matter as well as integrase-encoding phages that could potentially be used to manipulate these strains.

Recombineering-Mediated Genome Editing in Burkholderiales Strains

Red/ET recombineering is primarily mediated by the E. coli recombinase pair Redα/Redβ from λ phage or RecE/RecT from Rac prophage, which is applied in E. coli and also closely related Gram-negative bacteria for efficient genome editing. However, some distant bacterial species like Burkholderiales strains require host-specific Redα/Redβ recombinase pair for highly efficient genome editing. A pair of recombinases Redαβ7029 from the Burkholderiales strain DSM 7029, recently identified as Schlegelella brevitalea, were identified for efficient genetic manipulation in the native strain and several other Burkholderiales strains. In this chapter, we describe a detailed protocol for genome engineering in Burkholderiales strains via the Redγ-Redαβ7029 recombineering and Cre/loxP site-specific recombination.

Short Peptides and Their Mimetics as Potent Antibacterial Agents and Antibiotic Adjuvants

Antimicrobial resistance (AMR) has been increasing unrelentingly worldwide, thus negatively impacting human health. The discovery and development of novel antibiotics is an urgent unmet need of the hour. However, it has become more challenging, requiring increasingly time-consuming efforts with increased commercial risks. Hence, alternative strategies are urgently needed to potentiate the existing antibiotics. In this context, short cationic peptides or peptide-based antimicrobials that mimic the activity of naturally occurring antimicrobial peptides (AMPs) could overcome the disadvantages of AMPs having evolved as potent antibacterial agents. Besides their potent antibacterial efficacy, short peptide conjugates have also gained attention as potent adjuvants to conventional antibiotics. Such peptide antibiotic combinations have become an increasingly cost-effective therapeutic option to tackle AMR. This Review summarizes the recent progress for peptide-based small molecules as promising antimicrobials and as adjuvants for conventional antibiotics to counter multidrug resistant (MDR) pathogens.

Complex cyclic peptide synthesis via serine/threonine ligation chemistry

Non-ribosomal cyclic peptides are abundant in natural sources, exhibiting attractive bioactivities and favorable pharmacological properties. Furthermore, their structural complexity renders them as attractive synthetic targets. A general task for cyclic peptide synthesis is the peptide cyclization. Compared to the traditional dehydration-based peptide macrolactamization, chemoselective peptide ligation provides an alternative, sometimes advantageous, strategy to cyclize peptides. Herein, we provide a series of structurally complex cyclic peptide examples whose total syntheses were achieved via peptide ligation-mediated peptide cyclization. The special features of these strategies for achieving the total synthesis are highlighted.

Isolation and Purification of a Hydrophobic Non-Ribosomal Peptide from an Escherichia coli Fermentation Broth

Non-ribosomal peptide synthases (NRPSs) generate versatile bioactive peptides by incorporating non-proteinogenic amino acids and catalyzing diverse modifications. Here, we developed an efficient downstream process for the capture, intermediate purification and polishing of a rhabdopeptide (RXP) produced by the NRPS VietABC. Many typical unit operations were unsuitable due to the similar physical and chemical properties of the RXP and related byproducts. However, we were able to capture the RXP from a fermentation broth using a hydrophobic resin (XAD-16N), resulting in a 14-fold increase in concentration while removing salts as well as polar and weak non-polar impurities. We then used ultra-high-performance liquid chromatography (UHPLC) for intermediate purification, with optimized parameters determined using statistical experimental designs, resulting in the complete removal of hydrophobic impurities. Finally, the UHPLC eluents were removed by evaporation. Our three-step downstream process achieved an overall product recovery of 81.7 ± 8.4%.

Improving the bioremediation technology of contaminated wastewater using biosurfactants produced by novel bacillus isolates

Biosurfactants have many advantages outside chemical one, led for application it through different sectors. So, the present study aimed for improving the bioremediation technology of contaminated wastewater using biosurfactants produced by novel bacillus isolates. In this regard, Bacillus thuringiensis and Bacillus toyonensis strains were obtained as most producing isolates of highly active biosurfactants. The optimized conditions for high biosurfactants yield production were established. Also, the stability of the produced biosurfactants at various conditions, pH, temperature and salinity was studied. The biosurfactant has been reported up to 120 °C, pH 12 and 10% of NaCl. The identified biosurfactants, decanoic acid and oleamide were applied for wastewater remediation from oil residues and pathogens contamination. The biosurfactant was had high antibacterial activity compared with references antimicrobial drugs, as well as it is enhanced bioremediation technology for petroleum oil residues contaminating sites. Thus, we can say, these biosurfactants could achieve the objectives of sustainable development.

An Antibacterial Peptide with High Resistance to Trypsin Obtained by Substituting d-Amino Acids for Trypsin Cleavage Sites

The poor stability of antibacterial peptide to protease limits its clinical application. Among these limitations, trypsin mainly exists in digestive tract, which is an insurmountable obstacle to orally delivered peptides. OM19R is a random curly polyproline cationic antimicrobial peptide, which has high antibacterial activity against some gram-negative bacteria, but its stability against pancreatin is poor. According to the structure-activity relationship of OM19R, all cationic amino acid residues (l-arginine and l-lysine) at the trypsin cleavage sites were replaced with corresponding d-amino acid residues to obtain the designed peptide OM19D, which not only maintained its antibacterial activity but also enhanced the stability of trypsin. Proceeding high concentrations of trypsin and long-time (such as 10 mg/mL, 8 h) treatment, it still had high antibacterial activity (MIC = 16–32 µg/mL). In addition, OM19D also showed high stability to serum, plasma and other environmental factors. It is similar to its parent peptide in secondary structure and mechanism of action. Therefore, this strategy is beneficial to improve the protease stability of antibacterial peptides.

Antimicrobial Peptides: An Update on Classifications and Databases

Antimicrobial peptides (AMPs) are distributed across all kingdoms of life and are an indispensable component of host defenses. They consist of predominantly short cationic peptides with a wide variety of structures and targets. Given the ever-emerging resistance of various pathogens to existing antimicrobial therapies, AMPs have recently attracted extensive interest as potential therapeutic agents. As the discovery of new AMPs has increased, many databases specializing in AMPs have been developed to collect both fundamental and pharmacological information. In this review, we summarize the sources, structures, modes of action, and classifications of AMPs. Additionally, we examine current AMP databases, compare valuable computational tools used to predict antimicrobial activity and mechanisms of action, and highlight new machine learning approaches that can be employed to improve AMP activity to combat global antimicrobial resistance.

Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils

Heavy metal toxicity has become a pressing ecological problem that affects the ecosystems through bioaccumulation, representing a serious public health hazard. Many conventional strategies have been developed and applied to decontaminate and restore metal-contaminated areas. However, these conventional approaches are not very suitable and environmentally safe for heavy metal remediation because of their high operational costs, high energy requirements, post-waste disposal problems, and secondary pollutant generation. Thus, biosurfactant-based bioremediation of heavy metals is a sustainable and promising approach because of its biodegradation capability, economic effectiveness, and ecofriendly nature. Pseudomonas sp., Bacillus sp., Citrobacter freundii, and Candida tropicalis have been isolated as potential sources of biosurfactants and produce compounds such as surfactin, rhamnolipids, and sophorolipids. Owing to the severity of heavy metal pollution in certain parts of the environment, biosurfactants have garnered great interest and attention as an emerging multi-functional technology of the new century for successful removal of heavy metal pollutants. The present study describes the role of biosurfactants in the bioremediation of heavy metals from contaminated environments. Moreover, the interaction mechanism underlying biosurfactant-metal complexation and metal remediation are discussed. Based on the review of the literature, further research is warranted to elucidate the mechanistic roles and explore the structural characterization and gene regulation of biosurfactants to improve their productivity and expand their applicability in bioremediation.

Saccharochelins A-H, Cytotoxic Amphiphilic Siderophores from the Rare Marine Actinomycete Saccharothrix sp. D09

Siderophores are secreted by microorganisms to survive in iron-depleted conditions, and they also possess tremendous therapeutic potential. Genomic-inspired isolation facilitated the identification of eight amphiphilic siderophores, saccharochelins A-H (1-8), from a rare marine-derived Saccharothrix species. Saccharochelins feature a series of fatty acyl groups appended to the same tetrapeptide skeleton. With the help of gene disruption and heterologous expression, we identified the saccharochelin biosynthetic pathway. The diversity of saccharochelins originates from the flexible specificity of the starter condensation (C_S) domain at the beginning of the nonribosomal peptide synthetase (NRPS) toward various fatty acyl substrates. Saccharochelins showed cytotoxicity against several human tumor cell lines, with IC₅₀ values ranging from 2.3 to 17 μM. Additionally, the fatty acid side chains of the saccharochelins remarkably affected the cytotoxicity, suggesting changing the N-terminal acyl groups of lipopeptides may be a promising approach to produce more potent derivatives.

Synthetic Zippers as an Enabling Tool for Engineering of Non-Ribosomal Peptide Synthetases*

Non‐ribosomal peptide synthetases (NRPSs) are the origin of a wide range of natural products, including many clinically used drugs. Efficient engineering of these often giant biosynthetic machineries to produce novel non‐ribosomal peptides (NRPs) is an ongoing challenge. Here we describe a cloning and co‐expression strategy to functionally combine NRPS fragments of Gram‐negative and ‐positive origin, synthesising novel peptides at titres up to 220 mg L⁻¹. Extending from the recently introduced definition of eXchange Units (XUs), we inserted synthetic zippers (SZs) to split single protein NRPSs into independently expressed and translated polypeptide chains. These synthetic type of NRPS (type S) enables easier access to engineering, overcomes cloning limitations, and provides a simple and rapid approach to building peptide libraries via the combination of different NRPS subunits.

Comparative Transcriptome Analysis Demonstrates the Positive Effect of the Cyclic AMP Receptor Protein Crp on Daptomycin Biosynthesis in Streptomyces roseosporus

Daptomycin, which is produced by Streptomyces roseosporus, has been characterized as a novel cyclic lipopeptide antibiotic that is effective against Gram-positive bacteria. The biosynthesis of daptomycin is regulated by various factors. In the present study, we demonstrated that the cyclic AMP receptor protein (Crp) plays an important role in producing daptomycin in the S. roseosporus industrial strain. We found that daptomycin production from the crp deletion strain decreased drastically, whereas production from the crp overexpression strain increased by 22.1%. Transcriptome and qPCR analyses showed that some genes related to the daptomycin biosynthetic gene cluster (dpt) and the pleiotropic regulator (adpA) were significantly upregulated. RNA-seq also shows Crp to be a multifunctional regulator that modulates primary metabolism and enhances precursor flux to secondary metabolite biosynthesis. These results provide guidance for the development and improvement of potential natural products.

Sterol Sponge Mechanism Is Conserved for Glycosylated Polyene Macrolides

Amphotericin-like glycosylated polyene macrolides (GPMs) are a clinically and industrially important family of natural products, but the mechanisms by which they exert their extraordinary biological activities have remained unclear for more than half a century. Amphotericin B exerts fungicidal action primarily via self-assembly into an extramembranous sponge that rapidly extracts ergosterol from fungal membranes, but it has remained unclear whether this mechanism is applicable to other GPMs. Using a highly conserved polyene-hemiketal region of GPMs that we hypothesized to represent a conserved ergosterol-binding domain, we bioinformatically mapped the entirety of the GPM sequence-function space and expanded the number of GPM biosynthetic gene clusters (BGCs) by 10-fold. We further leveraged bioinformatic predictions and tetrazine-based reactivity screening targeting the electron-rich polyene region of GPMs to discover a first-in-class methyltetraene- and diepoxide-containing GPM, kineosporicin, and to assign BGCs to many new producers of previously reported members. Leveraging a range of structurally diverse known and newly discovered GPMs, we found that the sterol sponge mechanism of fungicidal action is conserved.

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.

An Ecological and Miniaturized Biological Method for the Analysis of Daptomycin Potency

BACKGROUND

Physicochemical and microbiological methods are found in the literature for the analysis of daptomycin, an antimicrobial.

OBJECTIVE

This paper brings a miniaturized turbidimetric microbiological method for analysis of daptomycin in lyophilized powder.

METHODS

The method was performed using 96-well microplates, 4-h incubation, 2, 4 and 8 μg/mL, 7% Staphylococcus aureus ATCC 6538 IAL 2082, and BHI broth.

RESULTS

Linearity was proven by obtaining analytical curves with a correlation coefficient greater than 0.99 and statistical evaluation by ANOVA. The method was also selective, since the standard and sample analytical curves were parallel, proving that the excipient does not interfere with daptomycin analysis. Intraday, interday and inter-analyst precision presented RSDs of 2, 2.27, and 1.08%, respectively. Accuracy was assessed by the recovery test, where known quantities of standard solution are added to the sample and an average recovery value of 100.73% (RSD = 0.71%) was obtained. The present method was robust when minor changes were made in the parameters of used antimicrobial volume, inoculum volume and incubation time.

CONCLUSIONS

This work is an innovative and ecological proposal and has advantages such as (i) less waste generation, (ii) miniaturized quantities of sample, culture media and inoculum, (iii) no need to use formaldehyde as in the traditional turbidimetric method, (iv) lower volume of glassware used and (v) shorter incubation time compared to other methods as agar diffusion requiring approximately 24 h.

HIGHLIGHTS

This work is focuses on a current, innovative and sustainable theme for pharmaceutical analysis around the world.

A novel substituted derivative of sterol from marine actinomycetes Nocardiopsis alba MCCB 110 antagonistic to the aquaculture pathogen Vibrio harveyi

In an attempt to screen antagonistic microorganisms from marine environment for the management of bacterial pathogens in aquaculture, an isolate of actinomycete MCCB 110 was segregated based on its comparatively higher inhibitory property on Vibrio harveyi (MCCB 111) and profound luminescent inhibition. Based on the culture characteristics, cell wall fatty acid profile and the nucleotide sequence of the 16S rRNA gene (1495 bp), the isolate was identified as Nocardiopsis alba. Solvent extraction of the fermentation broth followed by TLC and HPLC analyses resulted in the isolation of a major fraction active against luminescent Vibrio harveyi. Partial characterization of this bioactive fraction based on spectroscopic data obtained from FT-IR, UV, MS-MS and ¹H NMR analyses identified it as a substituted derivative of sterol, and was recognized to differ from those reportedly produced by the same genus. The fraction was not toxic to VERO cell line and shrimp haemocytes up to 1000 ppm tested. The study demonstrated the potential of the putative probiotic Nocardiopsis alba (MCCB 110) and its novel extra-cellular bioactive product in the management of Vibrio harveyi in aquaculture.

In vitro identification of antimicrobial hemolytic lipopeptide from halotolerant Bacillus by Zymogram, FTIR, and GC mass analysis

OBJECTIVES

The multi-drug resistant bacteria and clinical infections are some of the biggest global concerns, so new drugs are needed. Antimicrobial peptides and lipopeptides are new bioactive agents with great potential that can become a new strategy for clinical applications.

MATERIALS AND METHODS

Some Bacillus strains were isolated based on hemolytic antimicrobial production from the soil. The extracellular proteins were extracted by acidic precipitation and chloroform/methanol method and analyzed by SDS-PAGE electrophoresis and stained with Sudan black. The black fragment was purified and characterized by FTIR, GC/MS, and HPLC analysis to demonstrate the presence of lipids and proteins. The anti-microbial ability and stability of the purified lipopeptide were assayed by the Kirby-Bauer method. Also, it was examined for metal removal.

RESULTS

A new Bacillus halotolerans strain SCM 034 with hemolytic antimicrobial production was isolated. According to GC/MS (detecting C16, C17) and HPLC (detecting leucine, glutamic acid, valine, arginine, glycine, and aspartic acid) data, the black fragment was lipopeptide. Polyacrylamide hydrogel containing lipopeptide and gel purified lipopeptide showed anti-microbial activities against S. aureus and S. cerevisiae that were stable for a few months. Also, the lipopeptide was useful for cation removal and decreased cobalt, nickel, and calcium by 10.81 %, 24.39 %, and 34 %, respectively.

CONCLUSION

Production of antibacterial lipopeptide hemolysin from this strain is reported for the first time and according to the results, lipopeptides have unique properties with biomedical and pharmaceutical applications. Also, polyacrylamide hydrogel lipopeptide is a promising candidate for wound healing.

Discovery, Synthesis, and Optimization of Peptide-Based Antibiotics

The rise of multidrug resistant bacteria has significantly compromised our supply of antibiotics and poses an alarming medical and economic threat to society. To combat this problem, it is imperative that new antibiotics and treatment modalities be developed, especially those toward which bacteria are less capable of developing resistance. Peptide natural products stand as promising candidates to meet this need as bacterial resistance is typically slow in response to their unique modes of action. They also have additional benefits including favorable modulation of host immune responses and often possess broad-spectrum activity against notoriously treatment resistant bacterial biofilms. Moreover, nature has provided a wealth of peptide-based natural products from a range of sources, including bacteria and fungi, which can be hijacked in order to combat more dangerous clinically relevant infections.This Account highlights recent advances in the total synthesis and development of a range of peptide-based natural product antibiotics and details the medicinal chemistry approaches used to optimize their activity.In the context of antibiotics with potential to treat Gram-positive bacterial infections, this Account covers the synthesis and optimization of the natural products daptomycin, glycocin F, and alamethicin. In particular, the reported synthesis of daptomycin highlights the utility of on-resin ozonolysis for accessing a key kynurenine residue from the canonical amino acid tryptophan. Furthermore, the investigation into glycocin F analogues uncovered a potent lead compound against Lactobacillus plantarum that bears a non-native thioacetal linkage to a N-acetyl-d-glucosamine (GlcNAc) sugar, which is otherwise O-linked in its native form.For mycobacterial infections, this Account covers the synthesis and optimization of teixobactin, callyaerin A, lassomycin, and trichoderin A. The synthesis of callyaerin A, in particular, highlighted the importance of a (Z)-2,3-diaminoacrylamide motif for antimicrobial activity against Mycobacterium tuberculosis, while the synthesis of trichoderin A highlighted the importance of (R)-stereoconfiguration in a key 2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid (AHMOD) residue.Lastly, this Account covers lipopeptide antibiotics bearing activity toward Gram-negative bacterial infections, namely, battacin and paenipeptin C. In both cases, optimization of the N-terminal lipid tails led to the identification of analogues with potent activity toward Escherichia coli and Pseudomonas aeruginosa.

Genome mining for drug discovery: cyclic lipopeptides related to daptomycin

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

Molecular engineering of antimicrobial peptides: microbial targets, peptide motifs and translation opportunities

The global public health threat of antimicrobial resistance has led the scientific community to highly engage into research on alternative strategies to the traditional small molecule therapeutics. Here, we review one of the most popular alternatives amongst basic and applied research scientists, synthetic antimicrobial peptides. The ease of peptide chemical synthesis combined with emerging engineering principles and potent broad-spectrum activity, including against multidrug-resistant strains, has motivated intense scientific focus on these compounds for the past decade. This global effort has resulted in significant advances in our understanding of peptide antimicrobial activity at the molecular scale. Recent evidence of molecular targets other than the microbial lipid membrane, and efforts towards consensus antimicrobial peptide motifs, have supported the rise of molecular engineering approaches and design tools, including machine learning. Beyond molecular concepts, supramolecular chemistry has been lately added to the debate; and helped unravel the impact of peptide self-assembly on activity, including on biofilms and secondary targets, while providing new directions in pharmaceutical formulation through taking advantage of peptide self-assembled nanostructures. We argue that these basic research advances constitute a solid basis for promising industry translation of rationally designed synthetic peptide antimicrobials, not only as novel drugs against multidrug-resistant strains but also as components of emerging antimicrobial biomaterials. This perspective is supported by recent developments of innovative peptide-based and peptide-carrier nanobiomaterials that we also review.

Engineering and elucidation of the lipoinitiation process in nonribosomal peptide biosynthesis

Nonribosomal peptide synthetases containing starter condensation domains direct the biosynthesis of nonribosomal lipopeptides, which generally exhibit wide bioactivities. The acyl chain has strong impacts on bioactivity and toxicity, but the lack of an in-depth understanding of starter condensation domain-mediated lipoinitiation limits the bioengineering of NRPSs to obtain novel derivatives with desired acyl chains. Here, we show that the acyl chains of the lipopeptides rhizomide, holrhizin, and glidobactin were modified by engineering the starter condensation domain, suggesting a workable approach to change the acyl chain. Based on the structure of the mutated starter condensation domain of rhizomide biosynthetic enzyme RzmA in complex with octanoyl-CoA and related point mutation experiments, we identify a set of residues responsible for the selectivity of substrate acyl chains and extend the acyl chains from acetyl to palmitoyl. Furthermore, we illustrate three possible conformational states of starter condensation domains during the reaction cycle of the lipoinitiation process. Our studies provide further insights into the mechanism of lipoinitiation and the engineering of nonribosomal peptide synthetases.

Nonribosomal lipopeptides contain an acyl chain important for bioactivity, but its incorporation into the peptidyl backbone, mediated by the starter condensation (Cs) domain of nonribosomal peptide synthases, is not fully understood. Here, the authors show that acyl chains of different lengths can be obtained by engineering Cs domains and identify residues that determine the selectivity for acyl chains.