Pseudomycins, a family of novel peptides from Pseudomonas syringae possessing broad-spectrum antifungal activity

Antimicrobials from endophytes as novel therapeutics to counter drug-resistant pathogens

The rapid increase in antimicrobial resistance (AMR) projects a "global emergency" and necessitates a need to discover alternative resources for combating drug-resistant pathogens or "superbugs." One of the key themes in "One Health Concept" is based on the fact that the interconnected network of humans, the environment, and animal habitats majorly contribute to the rapid selection and spread of AMR. Moreover, the injudicious and overuse of antibiotics in healthcare, the environment, and associated disciplines, further aggravates the concern. The prevalence and persistence of AMR contribute to the global economic burden and are constantly witnessing an upsurge due to fewer therapeutic options, rising mortality statistics, and expensive healthcare. The present decade has witnessed the extensive exploration and utilization of bio-based resources in harnessing antibiotics of potential efficacies. The discovery and characterization of diverse chemical entities from endophytes as potent antimicrobials define an important yet less-explored area in natural product-mediated drug discovery. Endophytes-produced antimicrobials show potent efficacies in targeting microbial pathogens and synthetic biology (SB) mediated engineering of endophytes for yield enhancement, forms a prospective area of research. In keeping with the urgent requirements for new/novel antibiotics and growing concerns about pathogenic microbes and AMR, this paper comprehensively reviews emerging trends, prospects, and challenges of antimicrobials from endophytes and their effective production via SB. This literature review would serve as the platform for further exploration of novel bioactive entities from biological organisms as "novel therapeutics" to address AMR.

Isolation and characterization of a newly discovered plant growth-promoting endophytic fungal strain from the genus Talaromyces

In the Kandi zone of Punjab, India, root and rhizospheric soil samples were collected from the local vegetation near the Shivalik mountain foothills. Fifteen fungal colonies exhibiting distinct cultural morphology on Potato Dextrose Agar (PDA) plates were selected for plant-microbe interaction studies. Among these, the isolate HNB9 was identified as a nonpathogenic root colonizer. Morphological and molecular analyses confirmed HNB9 as Talaromyces albobiverticillius, characterized by the secretion of a red pigment as a secondary metabolite. Plants colonized with T. albobiverticillius HNB9 exhibited enhanced growth, manifesting in increased shoot and root length compared to untreated controls. This study unveiled the first evidence that a species from the Talaromyces genus, specifically T. albobiverticillius, possesses dual capabilities of root colonization and plant growth promotion. Moreover, HNB9 demonstrated the production of plant growth-regulating compounds like Indole Acetic Acid (IAA) and proficient solubilization of crucial nutrients (Phosphorous, Zinc, and Silica) through plate culture methods. This finding represents a significant contribution to the understanding of root-colonizing fungi with plant growth-promoting attributes, challenging the existing knowledge gap within the Talaromyces genus.

The Diversity of Lipopeptides in the Pseudomonas syringae Complex Parallels Phylogeny and Sheds Light on Structural Diversification during Evolutionary History

Pseudomonas spp. colonize diverse aquatic and terrestrial habitats and produce a wide variety of secondary metabolites, including lipopeptides. However, previous studies have often examined a limited number of lipopeptide-producing strains. In this study, we performed a systematic analysis of lipopeptide production across a wide data set of strains of the Pseudomonas syringae complex (724) by using a combined bioinformatics, mass spectrometry, and phylogenetics approach. The large P. syringae complex, which is composed of 13 phylogroups, is known to produce factins (including syringafactin-like lipopeptides), mycins (including syringomycin-like lipopeptides), and peptins (such as syringopeptins). We found that 80.8% of P. syringae strains produced lipopeptides and that factins were the most frequently produced (by 96% of the producing strains). P. syringae strains were either factin monoproducers or factin, mycin, and peptin coproducers or lipopeptide nonproducers in relation to their phylogenetic group. Our analyses led to the discovery of 42 new lipopeptides, bringing the number of lipopeptides identified in the P. syringae complex to 75. We also highlighted that factins have high structural resemblance and are widely distributed among the P. syringae complex, while mycins and peptins are highly structurally diverse and patchily distributed. IMPORTANCE This study provides an insight into the P. syringae metabolome that emphasizes the high diversity of lipopeptides produced within the P. syringae complex. The production profiles of strains are closely related to their phylogenetic classification, indicating that structural diversification of lipopeptides parallels the phylogeny of this bacterial complex, thereby further illustrating the inherent importance of lipopeptides in the ecology of this group of bacteria throughout its evolutionary history. Furthermore, this overview of P. syringae lipopeptides led us to propose a refined classification that could be extended to the lipopeptides produced by other bacterial groups.

Bioactive Lipodepsipeptides Produced by Bacteria and Fungi

Natural products are a vital source for agriculture, medicine, cosmetics and other fields. Lipodepsipeptides (LPDs) are a wide group of natural products distributed among living organisms such as bacteria, fungi, yeasts, virus, insects, plants and marine organisms. They are a group of compounds consisting of a lipid connected to a peptide, which are able to self-assemble into several different structures. They have shown different biological activities such as phytotoxic, antibiotic, antiviral, antiparasitic, antifungal, antibacterial, immunosuppressive, herbicidal, cytotoxic and hemolytic activities. Their biological activities seem to be due to their interactions with the plasma membrane (MP) because they are able to mimic the architecture of the native membranes interacting with their hydrophobic segment. LPDs also have surfactant properties. The review has been focused on the lipodepsipeptides isolated from fungal and bacterial sources, on their biological activity, on the structure–activity relationships of some selected LPD subgroups and on their potential application in agriculture and medicine. The chemical and biological characterization of lipodepsipeptides isolated in the last three decades and findings that resulted from SCI-FINDER research are reported. A critical evaluation of the most recent reviews dealing with the same argument has also been described.

Antimicrobial Compounds from Microorganisms

Antimicrobial resistance is an exigent public health concern owing to the emergence of novel strains of human resistant pathogens and the concurrent rise in multi-drug resistance. An influx of new antimicrobials is urgently required to improve the treatment outcomes of infectious diseases and save lives. Plant metabolites and bioactive compounds from chemical synthesis have found their efficacy to be dwindling, despite some of them being developed as drugs and used to treat human infections for several decades. Microorganisms are considered untapped reservoirs for promising biomolecules with varying structural and functional antimicrobial activity. The advent of cost-effective and convenient model organisms, state-of-the-art molecular biology, omics technology, and machine learning has enhanced the bioprospecting of novel antimicrobial drugs and the identification of new drug targets. This review summarizes antimicrobial compounds isolated from microorganisms and reports on the modern tools and strategies for exploiting promising antimicrobial drug candidates. The investigation identified a plethora of novel compounds from microbial sources with excellent antimicrobial activity against disease-causing human pathogens. Researchers could maximize the use of novel model systems and advanced biomolecular and computational tools in exploiting lead antimicrobials, consequently ameliorating antimicrobial resistance.

Isolation and characterization of new phenolic siderophores with antimicrobial properties from Pseudomonas sp. UIAU-6B

Five new phenolic siderophores 1–5 were isolated from the organic extract of a culture broth in a modified SGG medium of Pseudomonas sp. UIAU-6B, obtained from sediments collected from the Oyun river in North Central Nigeria. The structure of the new compounds, pseudomonin A–C (1–3) and pseudomobactin A and B (4 and 5) isolated alongside two known compounds, pseudomonine (6) and salicylic acid (7), were elucidated based on high-resolution mass spectrometry, 1D and 2D NMR analyses. The absolute configuration of the threonine residue in compounds 1–5 was determined by Marfey analysis. The antimicrobial evaluation of compound 4 exhibited the most potent activity against vancomycin-sensitive Enterococcus faecium VS144754, followed by 3 and 5, with MIC values ranging from 8 to 32 µg/mL. Compounds 2 and 3 exhibited moderate activity against Mycobacterium tuberculosis H37Rv, with MIC values of 7.8 and 15.6 µg/mL, respectively. Plausible biosynthetic hypotheses toward the new compounds 1–5 were proposed.

Discovery and engineering of an endophytic Pseudomonas strain from Taxus chinensis for efficient production of zeaxanthin diglucoside

BACKGROUND

Endophytic microorganisms are a rich source of bioactive natural products. They are considered as promising biofertilizers and biocontrol agents due to their growth-promoting interactions with the host plants and their bioactive secondary metabolites that can help manage plant pathogens. Identification of new endophytes may lead to the discovery of novel molecules or provide new strains for production of valuable compounds.

RESULTS

In this study, we isolated an endophytic bacterium from the leaves of Taxus chinensis, which was identified as Pseudomonas sp. 102515 based on the 16S rRNA gene sequence and physiological characteristics. Analysis of its secondary metabolites revealed that this endophytic strain produces a major product zeaxanthin diglucoside, a promising antioxidant natural product that belongs to the family of carotenoids. A carotenoid (Pscrt) biosynthetic gene cluster was amplified from this strain, and the functions of PsCrtI and PsCrtY in the biosynthesis of zeaxanthin diglucoside were characterized in Escherichia coli BL21(DE3). The entire Pscrt biosynthetic gene cluster was successfully reconstituted in E. coli BL21(DE3) and Pseudomonas putida KT2440. The production of zeaxanthin diglucoside in Pseudomonas sp. 102515 was improved through the optimization of fermentation conditions such as medium, cultivation temperature and culture time. The highest yield under the optimized conditions reached 206 mg/L. The engineered strain of P. putida KT2440 produced zeaxanthin diglucoside at 121 mg/L in SOC medium supplemented with 0.5% glycerol at 18 °C, while the yield of zeaxanthin diglucoside in E. coli BL21(DE3) was only 2 mg/L. To further enhance the production, we introduced an expression plasmid harboring the Pscrt biosynthetic gene cluster into Pseudomonas sp. 102515. The yield in this engineered strain reached 380 mg/L, 85% higher than the wild type. Through PCR, we also discovered the presence of a turnerbactin biosynthetic gene cluster in Pseudomonas sp. 102515. Because turnerbactin is involved in nitrogen fixation, this endophytic strain might have a role in promoting growth of the host plant.

CONCLUSIONS

We isolated and identified an endophytic strain of Pseudomonas from T. chinensis. A zeaxanthin diglucoside biosynthetic gene cluster was discovered and characterized in this bacterium. Through fermentation and genetic engineering, the engineered strain produced zeaxanthin diglucoside at 380 ± 12 mg/L, representing a promising strain for the production of this antioxidant natural product. Additionally, Pseudomonas sp. 102515 might also be utilized as a plant-promoting strain for agricultural applications.

Inside the plant: addressing bacterial endophytes in biotic stress alleviation

Bacterial endophytes are the internal association of bacteria with the plants, cherished whole or any part of their life cycle inside the plant. They are reported to improve plant health against the biotic stresses via de novo synthesis of structural compounds and stimulation of plant immunity. They are found to be vital in development of host resistance against phytopathogens and capable in reducing and elimination of deleterious effects of plant pathogens. Fungal-, bacterial-, viral-, insect- and nematode-associated negative effect can be reduced by the bacterial endophytes. They are also reported to control plant pathogens through several defense mechanisms such as by producing antimicrobial compounds and antibiotics, de novo synthesis of structural compounds, keeping out of pathogens by niche competition and induction of plant immunity or induced systemic resistance. In this review, an effort is made to summarize the exploitation of endophytic bacteria as a biological substitute to control biotic stresses in agricultural practices.

Cyclic Lipodepsipeptides From Pseudomonas spp. - Biological Swiss-Army Knives

Cyclic lipodepsipeptides produced by Pseudomonas spp. (Ps-CLPs) are biosurfactants that constitute a diverse class of versatile bioactive natural compounds with promising application potential. While chemically diverse, they obey a common structural blue-print, allowing the definition of 14 distinct groups with multiple structurally homologous members. In addition to antibacterial and antifungal properties the reported activity profile of Ps-CLPs includes their effect on bacterial motility, biofilm formation, induced defense responses in plants, their insecticidal activity and anti-proliferation effects on human cancer cell-lines. To further validate their status of potential bioactive substances, we assessed the results of 775 biological tests on 51 Ps-CLPs available from literature. From this, a fragmented view emerges. Taken as a group, Ps-CLPs present a broad activity profile. However, reports on individual Ps-CLPs are often much more limited in the scope of organisms that are challenged or activities that are explored. As a result, our analysis shows that the available data is currently too sparse to allow biological function to be correlated to a particular group of Ps-CLPs. Consequently, certain generalizations that appear in literature with respect to the biological activities of Ps-CLPs should be nuanced. This notwithstanding, the data for the two most extensively studied Ps-CLPs does indicate they can display activities against various biological targets. As the discovery of novel Ps-CLPs accelerates, current challenges to complete and maintain a useful overview of biological activity are discussed.

Antibiotics from Gram-negative bacteria: a comprehensive overview and selected biosynthetic highlights

Covering: up to 2017The overwhelming majority of antibiotics in clinical use originate from Gram-positive Actinobacteria. In recent years, however, Gram-negative bacteria have become increasingly recognised as a rich yet underexplored source of novel antimicrobials, with the potential to combat the looming health threat posed by antibiotic resistance. In this article, we have compiled a comprehensive list of natural products with antimicrobial activity from Gram-negative bacteria, including information on their biosynthetic origin(s) and molecular target(s), where known. We also provide a detailed discussion of several unusual pathways for antibiotic biosynthesis in Gram-negative bacteria, serving to highlight the exceptional biocatalytic repertoire of this group of microorganisms.

Homology modeling of target proteins and identification novel antifungal compounds against Candida tropicalis through structure based virtual screening

Candida tropicalis, the etiological agent of candidiasis evades the immune system and survive in the human host for decades. Currently there are not many drugs available in the market to treat these fungal infections. The increasing number of fungal infections necessitates the need for new drug candidates that can be used to treat fungal infections such as candida. Many natural products available in plants, animals and microorganisms exhibit potent anti-microbial activity; but they are not explored to their potential. Virtual screening of anti-microbials against known targets accelerates the process of drug discovery and development. In the present study, a total of 27 compounds of natural origin such as plants, microbes and marine sponges were evaluated for their ability to interact with four of the new targets. The study revealed the effectiveness of 3 compounds with improved binding affinity against the four target proteins; that could be used as lead compounds in designing new drug candidates.

Endophytic bacteria as a source of novel antibiotics: An overview

World human population is increasing with an alarming rate; and a variety of new types of health issues are popping up. For instance, increase in number of drug-resistant bacteria is a cause of concern. Research on antibiotics and other microbial natural products is pivotal in the global fight against the growing problem of antibiotic resistance. It is necessary to find new antibiotics to tackle this problem. The use of therapeutic plant species in traditional medicine is as old as mankind; and currently, it is strongly believed that all types of plant species across the plant kingdom do harbour endophytic bacteria (EB). The natural therapeutic compounds produced by EB do have several potential applications in pharmaceutical industry. The EB derived natural products such as Ecomycins, Pseudomycins, Munumbicins and Xiamycins are antibacterial, antimycotic and antiplasmodial. Some of these natural products have been reported to possess even antiviral (including Human Immunodeficiency Virus (HIV)) properties. Therefore, to deal with increasing number of drug-resistant pathogens EB could serve as a potential source of novel antibiotics.

Identification of bacterial endophytes associated with traditional medicinal plant Tridax procumbens Linn

BACKGROUND

In traditional medicine, Tridax procumbens Linn. is used in the treatment of injuries and wounds. The bacterial endophytes (BEs) of medicinal plants could produce medicinally important metabolites found in their hosts; and hence, the involvement of BEs in conferring wound healing properties to T. Procumbens cannot be ruled out. But, we do not know which types of BEs are associated with T. Procumbens.

OBJECTIVE

The objective of this study was to investigate the fast growing and cultivable BEs associated with T. procumbens.

MATERIALS AND METHODS

Leaves and stems of healthy T. Procumbens plants were collected and cultivable BEs were isolated from surface-sterilized leaf and stem tissue samples using Luria-Bertani (LB) agar (medium) at standard conditions. A polymerase chain reaction was employed to amplify 16S rRNA coding gene fragments from the isolates. Cultivable endophytic bacterial isolates (EBIs) were identified using 16S rRNA gene nucleotide sequence similarity based method of bacterial identification.

RESULTS

Altogether, 50 culturable EBIs were isolated. 16S rRNA gene nucleotide sequences analysis using the Basic Local Alignment Search Tool (BLAST) revealed identities of the EBIs. Analysis reveals that cultivable Bacillus spp., Cronobacter sakazakii, Enterobacter spp., Lysinibacillus sphaericus, Pantoea spp., Pseudomonas spp. and Terribacillus saccharophilus are associated with T. Procumbens.

CONCLUSION

Based on the results, we conclude that 24 different types of culturable BEs are associated with traditionally used medicinal plant, T. Procumbens, and require further study.

Asymmetric synthesis of γ-chloro-α,β-diamino- and β,γ-aziridino-α-aminoacylpyrrolidines and -piperidines via stereoselective Mannich-type additions of N-(diphenylmethylene)glycinamides across α-chloro-N-sulfinylimines

The asymmetric synthesis of new chiral γ-chloro-α,β-diaminocarboxylamide derivatives by highly diastereoselective Mannich-type reactions of N-(diphenylmethylene)glycinamides across chiral α-chloro-N-p-toluenesulfinylaldimines was developed. The resulting (S_S,2S,3S)-γ-chloro-α,β-diaminocarboxylamides were formed with the opposite enantiotopic face selectivity as compared to the (S_S,2R,3R)-γ-chloro-α,β-diaminocarboxyl esters obtained via Mannich-type addition of analogous N-(diphenylmethylene)glycine esters across a chiral α-chloro-N-p-toluenesulfinylaldimine. Selective deprotection under different acidic reaction conditions and ring closure of the γ-chloro-α,β-diaminocarboxylamides was optimized, which resulted in N^α-deprotected syn-γ-chloro-α,β-diaminocarboxylamides, N-sulfinyl-β,γ-aziridino-α-aminocarboxylamide derivatives, a trans-imidazolidine, and an N^α,N^β-deprotected syn-γ-chloro-α,β-diaminocarboxylamide.

Biological control of wood decay against fungal infection

Wood (timber) is an important raw material for various purposes, and having biological composition it is susceptible to deterioration by various agents. The history of wood protection by impregnation with synthetic chemicals is almost two hundred years old. However, the ever-increasing public concern and the new environmental regulations on the use of chemicals have created the need for the development and the use of alternative methods for wood protection. Biological wood protection by antagonistic microbes alone or in combination with (bio)chemicals, is one of the most promising ways for the environmentally sound wood protection. The most effective biocontrol antagonists belong to genera Trichoderma, Gliocladium, Bacillus, Pseudomonas and Streptomyces. They compete for an ecological niche by consuming available nutrients as well as by secreting a spectrum of biochemicals effective against various fungal pathogens. The biochemicals include cell wall-degrading enzymes, siderophores, chelating iron and a wide variety of volatile and non-volatile antibiotics. In this review, the nature and the function of the antagonistic microbes in wood protection are discussed.

Diversity of nonribosomal peptide synthetases involved in the biosynthesis of lipopeptide biosurfactants

Lipopeptide biosurfactants (LPBSs) consist of a hydrophobic fatty acid portion linked to a hydrophilic peptide chain in the molecule. With their complex and diverse structures, LPBSs exhibit various biological activities including surface activity as well as anti-cellular and anti-enzymatic activities. LPBSs are also involved in multi-cellular behaviors such as swarming motility and biofilm formation. Among the bacterial genera, Bacillus (Gram-positive) and Pseudomonas (Gram-negative) have received the most attention because they produce a wide range of effective LPBSs that are potentially useful for agricultural, chemical, food, and pharmaceutical industries. The biosynthetic mechanisms and gene regulation systems of LPBSs have been extensively analyzed over the last decade. LPBSs are generally synthesized in a ribosome-independent manner with megaenzymes called nonribosomal peptide synthetases (NRPSs). Production of active-form NRPSs requires not only transcriptional induction and translation but also post-translational modification and assemblage. The accumulated knowledge reveals the versatility and evolutionary lineage of the NRPSs system. This review provides an overview of the structural and functional diversity of LPBSs and their different biosynthetic mechanisms in Bacillus and Pseudomonas, including both typical and unique systems. Finally, successful genetic engineering of NRPSs for creating novel lipopeptides is also discussed.

PCR Detection of Cyclic Lipodepsinonapeptide-Producing Pseudomonas syringae pv. syringae and Similarity of Strains

Many strains of Pseudomonas syringae pv. syringae produce one of four classes of small cyclic lipodepsinonapeptides: syringomycins, syringostatins, syringotoxins, or pseudomycins. These metabolites are phytotoxic and growth inhibitory against a broad spectrum of fungi. Their production is dependent upon the expression of conserved biosynthesis and export genes syrB and syrD, respectively. PCR and oligonucleotide primers specific for a 752-bp fragment of syrB were used to identify cyclic lipodepsinonapeptide-producing strains of P. syringae pv. syringae. In contrast, PCR amplification with primers based on syrD did not always correlate with possession of the syrD gene, as indicated by Southern blot analysis, or with cyclic lipodepsinonapeptide production. Sequence comparisons of 400 nucleotides from the syrB PCR-amplified fragments showed 94% plot similarity among 27 strains. In a sequence phenogram, syringostatin and syringotoxin producers were grouped apart from syringomycin-producing strain B301D, with sequences that differed by eight and nine conserved base substitutions, respectively. PCR amplification of the 752-bp syrB fragment offers rapid and accurate detection of cyclic lipodepsinonapeptide-producing strains, and its sequence provides some predictive capabilities for identifying syringotoxin and syringostatin producers.

Peptide antibiotics: an alternative and effective antimicrobial strategy to circumvent fungal infections

Mycosis, caused by both filamentous fungi and pathogenic yeasts is a major concern nowadays especially in the immunocompromised patient population. The emergence of pathogenic fungi resistant to current therapies in the last few decades has intensified the search for new antifungals like cationic peptides, which are the key components of innate defense mechanism. The review provides an inventory of different peptides from a diverse array of organisms from bacteria to mammals with proven antifungal activity, their therapeutic options and also about those which are in various stages of preclinical development. Literature, on the total and semi-synthetic variants of the parent peptides that exhibit an improved antifungal activity is also reviewed.

Novel Natural Products From Rainforest Endophytes

Endophytic microorganisms are found in virtually every higher plant on earth. These organisms reside in the living tissues of the host plant and do so in a variety of relationships, ranging from symbiotic to pathogenic. Endophytes may contribute to their host plant by producing a plethora of substances that provide protection and survival value to the plant. Ultimately, these compounds, once isolated and characterized, may also have potential for use in modern medicine. Novel antibiotics, antimycotics, immunosuppressants, and anticancer compounds are only a few examples of what has been found after the isolation and culturing of individual endophytes followed by purification and characterization of some of their natural products. The potential of finding new drugs that may be effective candidates for treating newly developing diseases in humans is great.

Natural products from endophytic microorganisms

Endophytic microorganisms are to be found in virtually every plant on earth. These organisms reside in the living tissues of the host plant and do so in a variety of relationships ranging from symbiotic to pathogenic. Endophytes may contribute to their host plant by producing a plethora of substances that provide protection and ultimately survival value to the plant. Ultimately, these compounds, once isolated and characterized, may also have potential for use in modern medicine, agriculture, and industry. Novel antibiotics, antimycotics, immunosuppressants, and anticancer compounds are only a few examples of what has been found after the isolation and culturing of individual endophytes followed by purification and characterization of some of their natural products. The prospects of finding new drugs that may be effective candidates for treating newly developing diseases in humans, plants, and animals are great. Other applications in industry and agriculture may also be discovered among the novel products produced by endophytic microbes.

Bioprospecting for microbial endophytes and their natural products

Endophytic microorganisms are to be found in virtually every plant on earth. These organisms reside in the living tissues of the host plant and do so in a variety of relationships, ranging from symbiotic to slightly pathogenic. Because of what appears to be their contribution to the host plant, the endophytes may produce a plethora of substances of potential use to modern medicine, agriculture, and industry. Novel antibiotics, antimycotics, immunosuppressants, and anticancer compounds are only a few examples of what has been found after the isolation, culture, purification, and characterization of some choice endophytes in the recent past. The potential prospects of finding new drugs that may be effective candidates for treating newly developing diseases in humans, plants, and animals are great.

Rainforest endophytes and bioactive products

An increase in the number of people in the world having health problems caused by certain cancers, drug-resistant bacteria, parasitic protozoans, and fungi has caused alarm. An intensive search for newer and more effective agents to deal with these problems is now underway. Endophytes are a potential source of novel chemistry and biology to assist in helping solve not only human health, but plant and animal health problems also. Endophytes reside in the tissues between living plant cells. The relationship that they establish with the plant varies from symbiotic to bordering on pathogenic. Of all of the world's plants, it seems that only a few grass species have had their complete complement of endophytes studied. As a result, the opportunity to find new and interesting endophytes among the myriad of plants is great. Sometimes extremely unusual and valuable organic substances are produced by these endophytes. These compounds may contribute to the host-microbe relationship. The initial step in dealing with endophytic microorganisms is their successful isolation from plant materials. Then, the isolation and characterization of bioactive substances from culture filtrates is done using bioassay guided fractionation and spectroscopic methods. Some of the more interesting compounds produced by endophytic microbes with which we have dealt are taxol, cryptocin, cryptocandin, jesterone, oocydin, isopestacin, the pseudomycins and ambuic acid. This review discusses an approach for bio-prospecting the rainforests, not only to harvest their endophytic microorganisms, but to eventually build a better understanding of the importance and value they have to humankind.

Endophytes as sources of bioactive products

An increase in the number of people in the world having health problems caused by various cancers, drug-resistant bacteria, parasitic protozoans, and fungi is a cause for alarm. An intensive search for newer and more effective agents to deal with these disease problems is now under way and endophytes are a novel source of potentially useful medicinal compounds.

Pseudomonas lipodepsipeptides and fungal cell wall-degrading enzymes act synergistically in biological control

Pseudomonas syringae pv. syringae strain B359 secreted two main lipodepsipeptides (LDPs), syringomycin E (SRE) and syringopeptin 25A (SP25A), together with at least four types of cell wall-degrading enzymes (CWDEs). In antifungal bioassays, the purified toxins SRE and SP25A interacted synergistically with chitinolytic and glucanolytic enzymes purified from the same bacterial strain or from the biocontrol fungus Trichoderma atroviride strain P1. The synergism between LDPs and CWDEs occurred against all seven different fungal species tested and P. syringae itself, with a level dependent on the enzyme used to permeabilize the microbial cell wall. The antifungal activity of SP25A was much more increased by the CWDE action than was that of the smaller SRE, suggesting a stronger antifungal role for SP25A. In vivo biocontrol assays were performed by using P. syringae alone or in combination with T. atroviride, including a Trichoderma endochitinase knock-out mutant in place of the wild type and a chitinase-specific enzyme inhibitor. These experiments clearly indicate that the synergistic interaction LDPs-CWDEs is involved in the antagonistic mechanism of P. syringae, and they support the concept that a more effective disease control is given by the combined action of the two agents.

Synthesis and evaluation of oxodioxolenylmethyl carbamate prodrugs of pseudomycins

With the aim of increasing therapeutic indexes of novel cyclic depsinonapeptide pseudomycins, we synthesized and evaluated a series of mono-, di-, and trioxodioxolenylmethyl carbamate prodrugs (2 and 4) of pseudomycin B 1 and pseudomycin C' 3. It is rather encouraging to note that several members of the newly synthesized prodrugs described herein (e.g., 2a, 2e, and 4e) exhibited comparable in vivo efficacy to that achieved by the parent compounds, yet free of tail vein irritation and histamine induced toxicity in vivo.

N-acyloxymethyl carbamate linked prodrugs of pseudomycins are novel antifungal agents

We describe herein the synthesis, bioconversion, antifungal activity, and preliminary toxicology evaluation of a series of N-acyloxymethyl carbamate linked triprodrugs of pseudomycins. The syntheses of these prodrugs (3-6) were achieved via simple N-acylation of PSB (1) or PSC' (2) with various prodrug linkers (7-9). As expected, upon incubation with mouse and/or human plasma, many of these prodrugs (3, 5, and 6) were converted to the parent compound within a few hours. Of particular significance, two pseudomycin triprodrugs (5 and 6) showed excellent in vivo efficacy against systemic Candidiasis without tail vein irritation being observed.

The synthesis of pseudomycin C' via a novel acid promoted side-chain deacylation of pseudomycin A

The gamma hydroxyl present in the aliphatic side chain of the natural products pseudomycin A and C' provided a unique handle for the pH dependent side-chain deacylation. Low pH reaction conditions were used to cleave the side chain with minimal degradation of the peptide core. The pseudomycin nucleus intermediate obtained from the deacylation of pseudomycin A was pivotal in the synthesis of novel side-chain analogues. A practical synthesis of a minor fermentation factor pseudomycin C' and related analogues is reported.

8-Amido-Bearing pseudomycin B (PSB) analogue: novel antifungal agents

During the course of a structure-activity relationship (SAR) study on novel depsinonapeptide pseudomycin B, we synthesized a total of 12 8-amidopseudomycin analogues via standard two-step sequence from either ZPSB 2 or AllocPSB 3. A number of these amides exhibited good in vitro antifungal activities.

Syntheses and antifungal activity of pseudomycin side-chain analogues. Part 1

We have described herein the syntheses of three novel series of aromatic ring containing pseudomycin side-chain analogues. Preliminary biological evaluations of these analogues clearly indicate that it is possible to synthesize rigid pseudomycin side-chain analogues without compromising in vitro antifungal activity.

Serendipitous synthesis of novel dehydro- and dechloro-pseudomycin B (PSB) derivatives

The syntheses and preliminary investigation of antifungal activities of two dehydro PSB derivatives 8 and 10 as well as one 3-imido-9-dechloro PSB analogue 13 are described.

Emerging targets for the development of novel antifungal therapeutics

Invasive mycoses have become important causes of morbidity and mortality in immunocompromised patients. New approaches for antifungal therapy are required to meet the challenges imposed by these life-threatening infections. Such approaches are being developed through identification of novel biochemical and molecular targets of pathogenic fungi.

Identification and analysis of a gene encoding L-2,4-diaminobutyrate:2-ketoglutarate 4-aminotransferase involved in the 1,3-diaminopropane production pathway in Acinetobacter baumannii

The ca. 2.2-kbp region upstream of the ddc gene encoding L-2,4-diaminobutyrate decarboxylase in Acinetobacter baumannii was sequenced and found to contain another open reading frame of 1,338 nucleotides encoding a protein with a deduced molecular mass of 47,423 Da. Analysis of the homologies observed from the deduced amino acid sequence indicated that the gene product is an enzyme belonging to subgroup II of the aminotransferases. This was first verified when examination of the crude extracts from Escherichia coli transformants led to detection of a novel aminotransferase activity catalyzing the following reversible reactions: L-2,4-diaminobutyric acid + 2-ketoglutaric acid<-->L-glutamic acid + L-aspartic beta-semialdehyde. Further confirmation was obtained when the gene was overexpressed in E. coli and the corresponding protein was purified to homogeneity. It catalyzed the same reactions and its N-terminal amino acid sequence was consistent with that deduced from the nucleotide sequence. Therefore, the gene and its product were named dat and L-2,4-diaminobutyrate:2-ketoglutarate 4-aminotransferase (DABA AT), respectively. Feeding experiments of A. baumannii with L-[U-14C]aspartic acid resulted in the incorporation of the label into 1,3-diaminopropane. Apparent homologs of dat and DABA AT were detected in other Acinetobacter species by PCR amplification and Western blotting. These results indicate that the dat gene (as well as the ddc gene) participates in the synthesis of 1,3-diaminopropane, the only diamine found in this genus. However, the biological role, if one exists, of 1,3-diaminopropane synthesis is unknown.

Biological activities of pseudomycin A, a lipodepsinonapeptide from Pseudomonas syringae MSU 16H

Similarly to other Pseudomonas lipodepsinonapeptides, pseudomycin A inhibits proton extrusion from maize roots, promotes closure of stomata in Vicia faba, necrosis of tobacco leaves, haemolysis of human erythrocytes, affects H(+)-ATPase activity and proton translocation in plasma membrane vesicles, and stimulates succinate respiration in pea mitochondria. In general, the biological activities of pseudomycin A are lower than those of syringomycin-E, the prototype member of this family of bacterial metabolities. This difference might depend on the diverse number and distribution of charged residues in the peptide moiety of these compounds.

In vitro antifungal and fungicidal activities and erythrocyte toxicities of cyclic lipodepsinonapeptides produced by Pseudomonas syringae pv. syringae

Recent increases in fungal infections, the few available antifungal drugs, and increasing fungal resistance to the available antifungal drugs have resulted in a broadening of the search for new antifungal agents. Strains of Pseudomonas syringae pv. syringae produce cyclic lipodepsinonapeptides with antifungal activity. The in vitro antifungal and fungicidal activities of three cyclic lipodepsinonapeptides (syringomycin E, syringotoxin B, and syringostatin A) against medically important isolates were evaluated by a standard broth microdilution susceptibility method. Erythrocyte toxicities were also evaluated. All three compounds showed broad antifungal activities and fungicidal actions against most of the fungi tested. Overall, the cyclic lipodepsinonapeptides were more effective against yeasts than against the filamentous fungi. Syringomycin E and syringostatin A had very similar antifungal activities (2.5 to > 40 micrograms/ml) and erythrocyte toxicities. Syringotoxin B was generally less active (0.8 to 200 micrograms/ml) than syringomycin E and syringostatin A against most fungi and was less toxic to erythrocytes. With opportunities for modification, these compounds are potential lead compounds for improved antifungal agents.

Synthesis of fragments of the peptide component of pseudobactin

Pseudobactin is a structurally complex and physiologically important siderophore (microbial iron chelator] from Pseudomonas putida-fluorescens. Various fragments of the unusual peptide component of pseudobactin listed below were prepared by solution-phase peptide synthesis. L-Lys.D-threo-beta-OH Asp.L-Ala.D-allo-Thr.L-Ala L-Lys.D-threo-beta OH Asp.L-Ala.D-allo-Thr D-threo-beta-OH Asp.L-Ala.D-allo-Thr.L-Ala.D-N-OH-cycloOrn D-threo-beta-OH-Asp.L-Ala.D-allo-Thr.L-Ala L-Ala.D-allo-Thr.L-Ala.D-N-OH-cycloOrn A class of related peptides named pseudomycins have shown promising antifungal activity. To examine if these peptide fragments above would elicit similar activity, the fragments were tested and found to have no antifungal activity in limited bioassays.

Novel bioactive lipodepsipeptides from Pseudomonas syringae: the pseudomycins

The covalent structure and most of the stereochemistry of the pseudomycins, bioactive metabolites of a transposon-generated mutant of a Pseudomonas syringae wild-type strain proposed for the biological control of Dutch elm disease, have been determined. While two pseudomycins are identical to the known syringopeptins 25-A and 25-B, pseudomycins A, B, C, C' are new lipodepsinonapeptides. For all of these the peptide moiety corresponds to L-Ser-D-Dab-L-Asp-L-Lys-L-Dab-L-aThr-Z-Dhb-L-Asp(3-OH) -L-Thr (4-Cl) with the terminal carboxyl group closing a macrocyclic ring on the OH group of the N-terminal Ser. This is in turn N-acylated by 3,4-dihydroxytetradecanoate in pseudomycin A, by 3-hydroxytetradecanoate in pseudomycin B, by 3,4-dihydroxyhexadecanoate in pseudomycin C, and by 3-hydroxyhexadecanoate in pseudomycin C'. Some preliminary data on the biological activity of pseudomycin A are reported.