Isolation and characterization of Streptomyces sp KH-614 producing anti-VRE (vancomycin-resistant enterococci) antibiotics

The high-throughput solid-phase extraction of cis-cyclo(L-Leu-L-Pro) and cis-cyclo(L-Phe-L-Pro) from Lactobacillus plantarum demonstrates efficacy against multidrug-resistant bacteria and influenza A (H3N2) virus

Introduction: 2,5-diketopiperazines are the simplest forms of cyclic dipeptides (CDPs) and have diverse frameworks with chiral side chains that are useful for drug development. Previous research has investigated the antimicrobial properties of proline-linked CDPs and their combinations in the culture filtrate (CF) of Lactobacillus plantarum LBP-K10 using anion exchange chromatography (AEC). However, the quantity of CDPs showcasing notable anti-influenza virus activity derived from AECs was generally lower than those originating from Lactobacillus CF. Methods: To address this issue, the study aims to propose a more efficient method for isolating CDPs and to introduce the antiviral combinations of CDPs obtained using a new method. The study employed a novel technique entailing high-throughput C18-based solid-phase extraction with a methanol gradient (MeSPE). The MeSPE method involved increasing the methanol concentration from 5% to 50% in 5% increments. Results: The methanol SPE fractions (MeSPEfs) eluted with methanol concentrations between 35% and 45% evinced substantial efficacy in inhibiting the influenza A/H3N2 virus via plaque-forming assay. MeSPEf-45, the 45% MeSPEf, exhibited exceptional efficacy in preventing viral infections in Madin-Darby kidney cells, surpassing both individual CDPs and the entire set of MeSPEfs. To identify the specific antiviral components of MeSPEf-45, all MeSPEfs were further fractionated through preparative high-performance liquid chromatography (prep-HPLC). MeSPEf-45 fractions S8 and S11 presented the highest activity against multidrug-resistant bacteria and influenza A/H3N2 virus among all MeSPEfs, with 11 common fractions. Antiviral fractions S8 and S11 were identified as proline-based CDPs, specifically cis-cyclo(L-Leu-L-Pro) and cis-cyclo(L-Phe-L-Pro), using gas chromatography-mass spectrometry. The combination of MeSPEf-45 fractions S8 and S11 displayed superior antibacterial and anti-influenza virus effects compared to the individual fractions S8 and S11. Discussion: High-throughput MeSPE-derived MeSPEfs and subsequent HPLC-fractionated fractions presents an innovative approach to selectively purify large amounts of potent antimicrobial CDPs from bacterial CF. The findings also show the effectiveness of physiologically bioactive combinations that utilize fractions not containing CDP. This study provides the initial evidence demonstrating the antimicrobial properties of CDPs acquired through high-throughput SPE techniques.

Antimicrobial Cyclic Dipeptides from Japanese Quail (Coturnix japonica) Eggs Supplemented with Probiotic Lactobacillus plantarum

Fifteen cyclic dipeptides (CDPs) containing proline, one cyclo(Phe-Ala) without proline, and a non-peptidyl DL-3-phenyllactic acid were previously identified in the culture filtrates of Lactobacillus plantarum LBP-K10, an isolate from kimchi. In this study, we used Japanese quail (Coturnix japonica) eggs to examine the effects of probiotic supplementation on the antimicrobial CDPs extracted from quail eggs (QE). Eggshell-free QE were obtained from two distinct groups of quails. The first group (K10N) comprised eggs from unsupplemented quails. The second group (K10S) comprised eggs from quails supplemented with Lb. plantarum LBP-K10. The QE samples were extracted using methylene chloride through a liquid-liquid extraction process. The resulting extract was fractionated into 16 parts using semi-preparative high-performance liquid chromatography. Two fractions, Q6 and Q9, were isolated from K10S and identified as cis-cyclo(L-Ser-L-Pro) and cis-cyclo(L-Leu-L-Pro). The Q9 fraction, containing cis-cyclo(L-Leu-L-Pro), has shown significant inhibitory properties against the proliferation of highly pathogenic multidrug-resistant bacteria, as well as human-specific and phytopathogenic fungi. Some of the ten combinations between the remaining fourteen unidentified fractions and two fractions, Q6 and Q9, containing cis-cyclo(L-Ser-L-Pro) and cis-cyclo(L-Leu-L-Pro) respectively, demonstrated a significant increase in activity against multidrug-resistant bacteria only when combined with Q9. The activity was 7.17 times higher compared to a single cis-cyclo(L-Leu-L-Pro). This study presents new findings on the efficacy of proline-containing CDPs in avian eggs. These CDPs provide antimicrobial properties when specific probiotics are supplemented.

Epiphytic and endophytic microbiome of the seagrass Zostera marina: Do they contribute to pathogen reduction in seawater?

Seagrass meadows provide crucial ecosystem services for coastal environments and were shown to reduce the abundance of waterborne pathogens linked to infections in humans and marine organisms in their vicinity. Among potential drivers, seagrass phenolics released into seawater have been linked to pathogen suppression, but the potential involvement of the seagrass microbiome has not been investigated. We hypothesized that the microbiome of the eelgrass Zostera marina, especially the leaf epiphytes that are at direct interface between the seagrass host and the surrounding seawater, inhibit waterborne pathogens thereby contributing to their removal. Using a culture-dependent approach, we isolated 88 bacteria and fungi associated with the surfaces and inner tissues of the eelgrass leaves (healthy and decaying) and the roots. We assessed the antibiotic activity of microbial extracts against a large panel of common aquatic, human (fecal) and plant pathogens, and mined the metabolome of the most active extracts. The healthy leaf epibiotic bacteria, particularly Streptomyces sp. strain 131, displayed broad-spectrum antibiotic activity superior to some control drugs. Gram-negative bacteria abundant on healthy leaf surfaces, and few endosphere-associated bacteria and fungi also displayed remarkable activities. UPLC-MS/MS-based untargeted metabolomics analyses showed rich specialized metabolite repertoires with low annotation rates, indicating the presence of many undescribed antimicrobials in the extracts. This study contributes to our understanding on microbial and chemical ecology of seagrasses, implying potential involvement of the seagrass microbiome in suppression of pathogens in seawater. Such effect is beneficial for the health of ocean and human, especially in the context of climate change that is expected to exacerbate all infectious diseases. It may also assist future seagrass conservation and management strategies.

Identification of isomeric cyclo(leu-pro) produced by Pseudomonas sesami BC42 and its differential antifungal activities against Colletotrichum orbiculare

Pseudomonas spp. produce various antimicrobial substances, including cyclic peptides, which have been shown to suppress fungal pathogens. In a previous study, Pseudomonas sesami BC42 was selected to control anthracnose caused by Colletotrichum orbiculare in cucumber plants, and the bioactive extract of strain BC42 inhibited fungal growth and development. In this work, preparative thin-layer chromatography was conducted to identify the antifungal compounds in the extract of strain BC42, and the portion of the extract that exhibited antifungal activity was further analyzed by gas chromatography-mass spectrometry. Three different isomers of the cyclic dipeptide, cyclo(Leu-Pro), were identified: cyclo(l-Leu-l-Pro), cyclo(d-Leu-d-Pro), and cyclo(d-Leu-l-Pro). Among these, 100 μg/mL of cyclo(l-Leu-l-Pro) significantly and more effectively inhibited the germination of conidia and appressorium formation and reduced leaf lesion size caused by C. orbiculare, relative to the control; cyclo(d-Leu-d-Pro) significantly reduced conidia germination and lesion occurrence, however, cyclo(d-Leu-l-Pro) did not exhibit antifungal activity. Therefore, the cyclo(l-Leu-l-Pro) and cyclo(d-Leu-d-Pro) derived from P. sesami BC42 may be a promising candidate for biocontrol applications in agriculture.

A Novel Antimicrobial Metabolite Produced by Paenibacillus apiarius Isolated from Brackish Water of Lake Balkhash in Kazakhstan

Four aerobic bacteria with bacteriolytic capabilities were isolated from the brackish water site Strait Uzynaral of Lake Balkhash in Kazakhstan. The morphology and physiology of the bacterial isolates have subsequently been analyzed. Using matrix assisted laser desorption ionization-time of flight mass spectrum and partial 16S rRNA gene sequence analyses, three of the isolates have been identified as Pseudomonas veronii and one as Paenibacillus apiarius. We determined the capability of both species to lyse pre-grown cells of the Gram-negative strains Pseudomonas putida SBUG 24 and Escherichia coli SBUG 13 as well as the Gram-positive strains Micrococcus luteus SBUG 16 and Arthrobacter citreus SBUG 321 on solid media. The bacteriolysis process was analyzed by creating growth curves and electron micrographs of co-cultures with the bacteriolytic isolates and the lysis sensitive strain Arthrobacter citreus SBUG 321 in nutrient-poor liquid media. One metabolite of Paenibacillus apiarius was isolated and structurally characterized by various chemical structure determination methods. It is a novel antibiotic substance.

Probing Anti-Leukemic Metabolites from Marine-Derived Streptomyces sp. LY1209

The unmet need for specific anti-leukemic agents for the treatment of acute lymphoblastic leukemia led us to screen a variety of marine-derived bacteria. The fermentation broth extract of Streptomyces sp. LY1209 exhibited the most potent anti-proliferative effect against Molt 4 leukemia cells. A chromatographic anti-proliferative profiling approach was applied to characterize the metabolites with bioactive potential. Among all the metabolites, the major anti-leukemic constituents were staurosporine and a series of diketopiperazines (DKPs), including one novel and two known DKPs identified from nature for the first time. The structures of these compounds were identified using extensive spectroscopic analysis. The anti-proliferative potential of these metabolites against the Molt 4 cancer cell line was also determined. According to the in silico analysis utilizing a chemical global positioning system for natural products (ChemGPS-NP), it was suggested that these DKPs are potential anti-microtubule and alkylating agents, while staurosporine was proposed to be a tyrosine kinase inhibitor. Our findings not only identified a series of anti-proliferative metabolites, but also suggested a strategic workflow for the future discovery of natural product drug leads.

Cyclic Dipeptides: The Biological and Structural Landscape with Special Focus on the Anti-Cancer Proline-Based Scaffold

Cyclic dipeptides, also know as diketopiperazines (DKP), the simplest cyclic forms of peptides widespread in nature, are unsurpassed in their structural and bio-functional diversity. DKPs, especially those containing proline, due to their unique features such as, inter alia, extra-rigid conformation, high resistance to enzyme degradation, increased cell permeability, and expandable ability to bind a diverse of targets with better affinity, have emerged in the last years as biologically pre-validated platforms for the drug discovery. Recent advances have revealed their enormous potential in the development of next-generation theranostics, smart delivery systems, and biomaterials. Here, we present an updated review on the biological and structural profile of these appealing biomolecules, with a particular emphasis on those with anticancer properties, since cancers are the main cause of death all over the world. Additionally, we provide a consideration on supramolecular structuring and synthons, based on the proline-based DKP privileged scaffold, for inspiration in the design of compound libraries in search of ideal ligands, innovative self-assembled nanomaterials, and bio-functional architectures.

Chemical Investigation of Diketopiperazines and N-Phenethylacetamide Isolated from Aquimarina sp. MC085 and Their Effect on TGF-β-Induced Epithelial–Mesenchymal Transition

Chemical investigations of Aquimarina sp. MC085, which suppressed TGF-β-induced epithelial–mesenchymal transition (EMT) in A549 human lung cancer cells, led to the isolation of compounds 1–3. Structural characterization using spectroscopic data analyses in combination with Marfey’s analysis revealed that they were two diketopiperazines [cyclo(l-Pro-l-Leu) (1) and cyclo(l-Pro-l-Ile) (2)] and one N-phenethylacetamide (3). Cyclo(l-Pro-l-Leu) (1) and N-phenethylactamide (3) inhibited the TGF-β/Smad pathway and suppressed the metastasis of A549 cells by affecting TGF-β-induced EMT. However, cyclo(l-Pro-l-Ile) (2) downregulated mesenchymal factors via a non-Smad-mediated signaling pathway.

Taxonomic and Metabolomics Profiling of Actinobacteria Strains from Himalayan Collection Sites in Pakistan

Actinobacteria have proven themselves as the major producers of bioactive compounds with wide applications. In this study, 35 actinobacteria strains were isolated from soil samples collected from the Himalayan mountains region in Pakistan. The isolated strains were identified by polyphasic taxonomy and were prioritized based on biological and chemical screening to identify the strains with ability to produce inimitable metabolites. The biological screening included antimicrobial activity against Staphylococcus aureus, Micrococcus luteus, Salmonella enterica, Escherichia coli, Mycobacterium aurum, and Bacillus subtilis and anticancer activity using human cancer cell lines PC3 and A549. For chemical screening, methanolic extracts were investigated using TLC, HPLC-UV/MS. The actinobacteria strain PU-MM93 was selected for scale-up fermentation based on its unique chemical profile and cytotoxicity (50-60% growth inhibition) against PC3 and A549 cell lines. The scale-up fermentation of PU-MM93, followed by purification and structure elucidation of compounds revealed this strain as a promising producer of the cytotoxic anthracycline aranciamycin and aglycone SM-173-B along with the potent neuroprotective carboxamide oxachelin C. Other interesting metabolites produced include taurocholic acid as first report herein from microbial origin, pactamycate and cyclo(L-Pro-L-Leu). The study suggested exploring more bioactive microorganisms from the untapped Himalayan region in Pakistan, which can produce commercially significant compounds.

Induction of mitochondria-mediated apoptosis and suppression of tumor growth in zebrafish xenograft model by cyclic dipeptides identified from Exiguobacterium acetylicum

Colorectal cancer is the most common type of gastrointestinal cancers with poor survival and limited therapeutic options. In this study, four structurally different cyclic dipeptides (or diketopiperazine) were isolated and identified as cyclo (l-Pro-l-Leu), cyclo (l-Pro-l-Val), cyclo (l-Pro-l-Phe) and cyclo (l-Pro-l-Tyr) from the ethyl acetate extract in the cell-free filtrate of Exiguobacterium acetylicum S01. The anticancer potential of identified DKPs on colorectal cancer HT-29 cells in vitro and in vivo zebrafish xenograft model was evaluated. The MTT (3-(4, 5-dimethylthiazol-2yl)-2, 5-diphenyltetrazolium bromide)) assay showed that four DKPs exhibited significant inhibition of HT-29 cells viability in a dose-dependent manner whereas there were no cytotoxic effects on normal mouse fibroblast 3T3 cells. Also, we observed that all DKPs induce early and late apoptotic cell death in HT-29 cells. Moreover, the expression levels of apoptotic (cytochrome-c, caspase-3 and Bid) and anti-apoptotic (Bcl-2) markers were up- and down-regulated in HT-29 cells in response to DKPs treatments. Furthermore, these four DKPs remarkably inhibited the tumor progression in a zebrafish xenograft model within a nonlethal dose range. Overall, our findings suggest that cyclic dipeptides derived from E. acetylicum S01 could be promising chemopreventive/ therapeutic candidates against cancer.

Streptomyces sp. MUM256: A Source for Apoptosis Inducing and Cell Cycle-Arresting Bioactive Compounds against Colon Cancer Cells

New and effective anticancer compounds are much needed as the incidence of cancer continues to rise. Microorganisms from a variety of environments are promising sources of new drugs; Streptomyces sp. MUM256, which was isolated from mangrove soil in Malaysia as part of our ongoing efforts to study mangrove resources, was shown to produce bioactive metabolites with chemopreventive potential. This present study is a continuation of our previous efforts and aimed to investigate the underlying mechanisms of the ethyl acetate fraction of MUM256 crude extract (MUM256 EA) in inhibiting the proliferation of HCT116 cells. Our data showed that MUM256 EA reduced proliferation of HCT116 cells via induction of cell-cycle arrest. Molecular studies revealed that MUM256 EA regulated the expression level of several important cell-cycle regulatory proteins. The results also demonstrated that MUM256 EA induced apoptosis in HCT116 cells mediated through the intrinsic pathway. Gas chromatography-mass spectrometry (GC-MS) analysis detected several chemical compounds present in MUM256 EA, including cyclic dipeptides which previous literature has reported to demonstrate various pharmacological properties. The cyclic dipeptides were further shown to inhibit HCT116 cells while exerting little to no toxicity on normal colon cells in this study. Taken together, the findings of this project highlight the important role of exploring the mangrove microorganisms as a bioresource which hold tremendous promise for the development of chemopreventive drugs against colorectal cancer.

Marine natural compound cyclo(L-leucyl-L-prolyl) peptide inhibits migration of triple negative breast cancer cells by disrupting interaction of CD151 and EGFR signaling

Cyclo (L-Leucyl-L-Prolyl) peptide/CLP is a marine natural metabolite and well recognized as an antimicrobial and antioxidant agent with limited studies on anticancer activity. The current study aims to determine the effect of CLP on migration and growth of triple negative breast cancer cell lines. The anti-growth potential was evaluated by MTT, BrdU and TUNEL assays; DNA damage by γH2AX and Dead green assays; antimigration activity by Boyden chamber invasion and wound healing assays. Interaction of CLP with CD151 was resolved by PatchDock. Effect of CLP on the expression of transmembrane CD151 was evaluated by cell-based ELISA assay. The interaction between CD151 and EGFR was predicted by using FireDoc Web server. Impact of CLP on the interaction of CD151 with EGFR was evaluated by co-immunoprecipitation assay. The effect of CLP on the cell cycle and its controlling proteins was determined by Western blotting. CLP reduced the viability of MDA-MB-231 and MDA-MB-468 TNBC cell lines but not human breast healthy epithelial cell line (MCF-12A) similar to eribulin, standard. CLP also inhibited proliferation; cell cycle and migration. It induced DNA strand breaks, DNA damage, and cell death. It showed the most favorable interactions with CD151 in in silico docking and significantly reduced the expression of membrane-bound CD151 proteins. FireDoc Web study predicted the association between CD151 and EGFR with -29.13 kcal/mol of binding energy. CLP reduced the interaction of CD151 with EGFR along with the expression of cyclin D, CDK4, PAK, RAC1, and P27kiP1. This study concludes that CLP suppresses growth and migration by attenuating cell cycle of TNBC cell lines via EGFR and CD151 signaling. Thus, exploring the EGFR and CD151 signaling pathway targeted by CLP may provide a new approach in the treatment of TNBC.

Cyclo(l-Pro⁻l-Leu) of Pseudomonas putida MCCC 1A00316 Isolated from Antarctic Soil: Identification and Characterization of Activity against Meloidogyne incognita

Pseudomonas putida MCCC 1A00316 was originally isolated from an Antarctic soil and has demonstrated potential nematicidal activity. Thus, it has promising applications for the biological control of Meloidogyne incognita. The larval mortality and egg-hatching inhibition rates of M. incognita will increase with the rising concentration of culture filtrates of P. putida MCCC 1A00316 and the duration of exposure. Thus, this study aimed to separate, purify, and identify nematicidal compounds from P. putida MCCC 1A00316 and to validate their anti-M. incognita activities. Compounds were purified through silica gel column chromatography and thin-layer chromatography combined with high-performance liquid chromatography (HPLC). Structural identification was conducted through liquid chromatography time-of-flight mass spectrometry, ¹H nuclear magnetic resonance (NMR) spectroscopy, ¹³C-NMR, and Marfey’s method. The isolated compounds were identified as cyclo(l-Pro–l-Leu) on the basis of the results of the above analyses and previously reported data. The effects of various concentrations of cyclo(l-Pro–l-Leu) on the mortality rates of second-stage juveniles (J2) of M. incognita were investigated. Results showed that HPLC-purified cyclo(l-Pro–l-Leu) displayed nematicidal activities. The mortality rate of M. incognita J2 reached 84.3% after 72 h of exposure to 67.5 mg/L cyclo(l-Pro–l-Leu). The lowest egg-hatching rate (9.74%) was observed after 8 days of incubation with 2000 mg/L cyclo(l-Pro–l-Leu). An egg-hatching rate of 53.11% was obtained under the control treatment (sterile distilled water). However, cyclo(l-Pro–l-Leu) did not elicit chemotaxis activity to M. incognita. This is the first work to investigate the anti-M. incognita characteristics of cyclo(l-Pro–l-Leu).

Identification for the First Time of Cyclo(d-Pro-l-Leu) Produced by Bacillus amyloliquefaciens Y1 as a Nematocide for Control of Meloidogyne incognita

The aim of the current study was to describe the role and mechanism of Bacillus amyloliquefaciens Y1 against the root-knot nematode, Meloidogyne incognita, under in vitro and in vivo conditions. Initially, the exposure of the bacterial culture supernatant and crude extract of Y1 to M. incognita significantly inhibited the hatching of eggs and caused the mortality of second-stage juveniles (J2), with these inhibitory effects depending on the length of incubation time and concentration of the treatment. The dipeptide cyclo(d-Pro-l-Leu) was identified in B. amyloliquefaciens culture for the first time using chromatographic techniques and nuclear magnetic resonance (NMR ¹H, 13C, H-H COSY, HSQC, and HMBC) and recognized to have nematocidal activity. Various concentrations of cyclo(d-Pro-l-Leu) were investigated for their effect on the hatching of eggs and J2 mortality. Moreover, the in vivo nematocidal activity of the Y1 strain was investigated by conducting pot experiments in which tomato plants were inoculated with M. incognita. Each and every pot was amended 50 mL of fertilizer media (F), or Y1 culture, or nematicide (N) (only once), or fertilizer media with N (FN) at 1, 2, 3, 4 and 5 weeks after transplantation. The results of the pot experiments demonstrated the antagonistic effect of B. amyloliquefaciens Y1 against M. incognita as it significantly decreases the count of eggs and galls per root of the tomato plant as well as the population of J2 in the soil. Besides, the investigation into the growth parameters, such as the length of shoot, shoot fresh and dry weights of the tomato plants, showed that they were significantly higher in the Y1 strain Y1-treated plants compared to F-, FN- and N-treated plants. Therefore, the biocontrol repertoire of this bacterium opens a new insight into the applications in crop pest control.

Proline-Based Cyclic Dipeptides from Korean Fermented Vegetable Kimchi and from Leuconostoc mesenteroides LBP-K06 Have Activities against Multidrug-Resistant Bacteria

Lactobacillus plantarum and Leuconostoc mesenteroides play a prominent role as functional starters and predominant isolates in the production of various types of antimicrobial compound-containing fermented foods, especially including kimchi. In the case of the bioactive cyclic dipeptides, their racemic diastereomers inhibitory to bacteria and fungi have been suggested to come solely from Lactobacillus spp. of these strains. We previously demonstrated the antifungal and antiviral activities of proline-based cyclic dipeptides, which were fractionated from culture filtrates of Lb. plantarum LBP-K10 originated from kimchi. However, cyclic dipeptides have not been identified in the filtrates, either from cultures or fermented subject matter, driven by Ln. mesenteroides, which have been widely used as starter cultures for kimchi fermentation. Most importantly, the experimental verification of cyclic dipeptide-content changes during kimchi fermentation have also not been elucidated. Herein, the antibacterial fractions, including cyclo(Leu-Pro) and cyclo(Phe-Pro), from Ln. mesenteroides LBP-K06 culture filtrates, which exhibited a typical chromatographic retention behavior (t_R), were identified by using semi-preparative high-performance liquid chromatography and gas chromatography-mass spectrometry. Based on this finding, the proline-based cyclic dipeptides, including cyclo(Ser-Pro), cyclo(Tyr-Pro), and cyclo(Leu-Pro), were additionally identified in the filtrates only when fermenting Chinese cabbage produced with Ln. mesenteroides LBP-K06 starter cultures. The detection and isolation of cyclic dipeptides solely in controlled fermented cabbage were conducted under the control of fermentation-process parameters concomitantly with strong CDP selectivity by using a two-consecutive-purification strategy. Interestingly, cyclic dipeptides in the filtrates, when using this strain as a starter, increased with fermentation time. However, no cyclic dipeptides were observed in the filtrates of other fermented products, including other types of kimchi and fermented materials of plant and animal origin. This is the first report to conclusively demonstrate evidence for the existence of antimicrobial cyclic dipeptides produced by Ln. mesenteroides in kimchi. Through filtrates from lactic acid bacterial cultures and from fermented foods, we have also proved a method of combining chromatographic fractionation and mass spectrometry-based analysis for screening cyclic dipeptide profiling, which may allow evaluation of the fermented dairy foods from a new perspective.

2,5-diketopiperazines in food and beverages: Taste and bioactivity

2,5-Diketopiperazines (2,5-DKPs) have been found to occur in a wide range of food and beverages, and display an array of chemesthetic effects (bitter, astringent, metallic, and umami) that can contribute to the taste of a variety of foods. These smallest cyclic peptides also occur as natural products and have been found to display a variety of bioactivities from antibacterial, antifungal, to anthroprotective effects and have the potential to be used in the development of new functional foods. An overview of the synthesis of these small chiral molecules and their molecular properties is presented. The occurrence, taste, and bioactivity of all simple naturally occurring 2,5-DKPs to date have been reviewed and those found in food from yeasts, fungi, and bacteria that have been used in food preparation or contamination, as well as metabolites of sweeteners and antibiotics added to food are also reviewed.

In vivo antimalarial activity of the endophytic actinobacteria, Streptomyces SUK 10

Endophytic bacteria, such as Streptomyces, have the potential to act as a source for novel bioactive molecules with medicinal properties. The present study was aimed at assessing the antimalarial activity of crude extract isolated from various strains of actinobacteria living endophytically in some Malaysian medicinal plants. Using the four day suppression test method on male ICR strain mice, compounds produced from three strains of Streptomyces (SUK8, SUK10, and SUK27) were tested in vivo against Plasmodium berghei PZZ1/100 in an antimalarial screen using crude extracts at four different concentrations. One of these extracts, isolated from Streptomyces SUK10 obtained from the bark of Shorea ovalis tree, showed inhibition of the test organism and was further tested against P. berghei-infected mice for antimalarial activity at different concentrations. There was a positive relationship between the survival of the infected mouse group treated with 50 µg/kg body weight (bw) of ethyl acetate-SUK10 crude extract and the ability to inhibit the parasites growth. The parasite inhibition percentage for this group showed that 50% of the mice survived for more than 90 days after infection with the parasite. The nucleotide sequence and phylogenetic tree suggested that Streptomyces SUK10 may constitute a new species within the Streptomyces genus. As part of the drug discovery process, these promising finding may contribute to the medicinal and pharmaceutical field for malarial treatment.

Endophytic Streptomyces in the traditional medicinal plant Arnica montana L.: secondary metabolites and biological activity

Arnica montana L. is a medical plant of the Asteraceae family and grows preferably on nutrient poor soils in mountainous environments. Such surroundings are known to make plants dependent on symbiosis with other organisms. Up to now only arbuscular mycorrhizal fungi were found to act as endophytic symbiosis partners for A. montana. Here we identified five Streptomyces strains, microorganisms also known to occur as endophytes in plants and to produce a huge variety of active secondary metabolites, as inhabitants of A. montana. The secondary metabolite spectrum of these strains does not contain sesquiterpene lactones, but consists of the glutarimide antibiotics cycloheximide and actiphenol as well as the diketopiperazines cyclo-prolyl-valyl, cyclo-prolyl-isoleucyl, cyclo-prolyl-leucyl and cyclo-prolyl-phenylalanyl. Notably, genome analysis of one strain was performed and indicated a huge genome size with a high number of natural products gene clusters among which genes for cycloheximide production were detected. Only weak activity against the Gram-positive bacterium Staphylococcus aureus was revealed, but the extracts showed a marked cytotoxic activity as well as an antifungal activity against Candida parapsilosis and Fusarium verticillioides. Altogether, our results provide evidence that A. montana and its endophytic Streptomyces benefit from each other by completing their protection against competitors and pathogens and by exchanging plant growth promoting signals with nutrients.

2,5-Diketopiperazines produced by Bacillus pumilus during bacteriolysis of Arthrobacter citreus

We report the detection by gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry analyses of the secreted 2,5-diketopiperazines (DKPs) cyclo(-Ala-Pro), cyclo(-Gly-Pro), cyclo(-Val-Pro), cyclo(-Ile-Pro), cyclo(-Leu-Pro), cyclo(-Pro-Pro), cyclo(-HyP-Pro), cyclo(-Met-Pro), and cyclo(-Phe-Pro) produced by Bacillus pumilus. The study focuses on a marine isolate and a laboratory test strain of B. pumilus with capabilities to lyse pregrown living cell lawns of different bacterial species, among them Arthrobacter citreus. Chromatographic methods were used to analyze induced bioactive compounds. At least 13 different DKPs are produced by B. pumilus. Both strains respond with an increased production of the DKPs cyclo(-Gly-Pro), cyclo(-Ala-Pro), and cyclo(-Val-Pro) to the presence of pasteurized A. citreus cells after 4 h in a nutrient-poor liquid medium. In agar diffusion assays, these DKPs did not cause lysis zones in living cell lawns, but they did inhibit further growth of several pregrown test bacteria in microplates even at concentrations as low as 1 μg ml(-1). Antibiotic substances produced by B. pumilus after 20 h of cultivation in a special lysis medium showed lytic activity in cell-free extracts of B. pumilus culture supernatants.

Purification of an antifungal compound, cyclo(l-Pro-d-Leu) for cereals produced by Bacillus cereus subsp. thuringiensis associated with entomopathogenic nematode

Mold spoilage is the main cause of substantial economic loss in cereals and might also cause public health problems due to the production of mycotoxins. The aim of this study was to separate and purify and to identify antifungal compounds of bacterium associated with novel entomopathogenic nematode and check the antifungal property of identified compound in particular food model systems. The antifungal compound was purified using silica gel column chromatography, TLC and HPLC and its structure was elucidated using NMR (¹H NMR, ¹³C NMR, ¹H-¹H COSY, ¹H-¹³C HMBC), HRMS and Marfey's method. Based on the spectral data, the active compounds were identified as diketopiperazine [cyclo(l-Pro-d-Leu)]. The antifungal activity of cyclo(l-Pro-d-Leu) was studied by MIC and paper disk assay against Aspergillus flavus MTCC 277 and Aspergillus niger MTCC 282 and best MIC value of 8μg/ml was recorded against A. flavus. Cyclo(l-Pro-d-Leu) strongly inhibit mycelia growth of fungus and thereby affecting aflatoxin production. To investigate the potential application of the cyclo(l-Pro-d-Leu) and to eliminate fungal spoilage in food and feed, soybean and peanut were used as models. White mycelia and dark/pale green spores of A. flavus were observed in the control soybeans after 2-day incubation. However the fungal growth was not observed in soybeans treated with cyclo(l-Pro-d-Leu). Almost the same result was observed for peanuts treated with cyclo(l-Pro-d-Leu) for A. niger. The cyclo(l-Pro-d-Leu) was nontoxic to two normal human cell lines (FS normal fibroblast and L231 lung epithelial) up to 200μg/ml. Thus the diketopiperazine derivative identified in the study may be a promising alternative to chemical preservatives as a potential biopreservative which prevent fungal growth and mycotoxin formation in food and feed.

In vitro synergistic activity of diketopiperazines alone and in combination with amphotericin B or clotrimazole against Candida albicans

The synergistic anticandidal activity of three diketopiperazines [cyclo-(L-Pro-L-Leu) (1), cyclo-(D-Pro-L-Leu) (2), and cyclo-(D-Pro-L-Tyr) (3)] purified from a Bacillus sp. N strain associated with entomopathogenic nematode Rhabditis (Oscheius) in combination with amphotericin B and clotrimazole was investigated using the macrodilution method. The minimum inhibitory concentration and minimum fungicidal concentration of the diketopiperazines was compared with that of the standard antibiotics. The synergistic anticandidal activities of diketopiperazines with amphotericin B or clotrimazole were assessed using the checkerboard and time-kill methods. The results of the present study showed that the combined effects of diketopiperazines with amphotericin B or clotrimazole predominantly recorded synergistic (<0.5). Time-kill study showed that the growth of the Candida was completely attenuated after 12-24 h of treatment with 50:50 ratios of diketopiperazines and antibiotics. These results suggest that diketopiperazines combined with antibiotics may be microbiologically beneficial and not antagonistic. These findings have potential implications in delaying the development of resistance as the anticandidal effect is achieved with lower concentrations of both drugs (diketopiperazines and antibiotics). The cytotoxicity of diketopiperazines was also tested against two normal human cell lines (L231 lung epithelial and FS normal fibroblast) and no cytotoxicity was recorded for diketopiperazines up to 200 μg/mL. The in vitro synergistic activity of diketopiperazines with antibiotics against Candida albicans is reported here for the first time.

Photoelectron spectra and structures of three cyclic dipeptides: PhePhe, TyrPro, and HisGly

We have investigated the electronic structure of three cyclic dipeptides: cyclo(Histidyl-Glycyl) (cHisGly), cyclo(Tyrosyl-Prolyl) (cTyrPro), and cyclo(Phenylalanyl-Phenylalanyl) (cPhePhe) in the vapor phase, by means of photoemission spectroscopy and theoretical modeling. The last compound was evaporated from the solid linear dipeptide, but cyclised, losing water to form cPhePhe in the gas phase. The results are compared with our previous studies of three other cyclopeptides. Experimental valence and core level spectra have been interpreted in the light of calculations to identify the basic chemical properties associated with the central diketopiperazine ring, and with the additional functional groups. The valence spectra are generally characterized by a restricted set of outer valence orbitals separated by a gap from most other valence orbitals. The theoretically simulated core and valence spectra of all three cyclic dipeptides agree reasonably well with the experimental spectra. The central ring and the side chains act as independent chromophores whose spectra do not influence one another, except for prolyl dipeptides, where the pyrrole ring is fused with the central ring. In this case, significant changes in the valence and core level spectra were observed, and explained by stronger hybridization of the valence orbitals.

Taxonomy, purification and chemical characterization of four bioactive compounds from new Streptomyces sp. TN256 strain

A new actinomycete strain designated TN256, producing antimicrobial activity against pathogenic bacteria and fungi, was isolated from a Tunisian Saharan soil. Morphological and chemical studies indicated that strain TN256 belonged to the genus Streptomyces. Analysis of the 16S rDNA sequence of strain TN256 showed a similarity level ranging between 99.79 and 97.8% within Streptomyces microflavus DSM 40331(T) and Streptomyces griseorubiginosus DSM 40469(T) respectively. The comparison of its physiological characteristics showed significant differences with the nearest species. Combined analysis of the 16 S rRNA gene sequences (FN687758), fatty acids profile, and results of physiological and biochemical tests indicated that there were genotypic and phenotypic differentiations of that isolate from other Streptomyces species neighbours. These date strongly suggest that strain TN256 represents a novel species with the type strain Streptomyces TN256 (=CTM50228(T)). Experimental validation by DNA-DNA hybridization would be required for conclusive confirmation. Four active products (1-4) were isolated from the culture broth of Streptomyces TN256 using various separation and purification steps and procedures. 1: N-[2-(1H-indol-3-yl)-2 oxo-ethyl] acetamide 'alkaloid' derivative; 2: di-(2-ethylhexyl) phthalate, a phthalate derivative; 3: 1-Nonadecene and 4: Cyclo (L: -Pro-L: -Tyr) a diketopiperazine 'DKP' derivative. The chemical structure of these four active compounds was established on the basis of spectroscopic studies NMR and by comparing with data from the literature. According to our biological studies, we showed in this work that the pure compounds (1-4) possess antibacterial and antifungal activities.

Bioactive compounds from marine actinomycetes

Actinomycetes are one of the most efficient groups of secondary metabolite producers and are very important from an industrial point of view. Among its various genera, Streptomyces, Saccharopolyspora, Amycolatopsis, Micromonospora and Actinoplanes are the major producers of commercially important biomolecules. Several species have been isolated and screened from the soil in the past decades. Consequently the chance of isolating a novel actinomycete strain from a terrestrial habitat, which would produce new biologically active metabolites, has reduced. The most relevant reason for discovering novel secondary metabolites is to circumvent the problem of resistant pathogens, which are no longer susceptible to the currently used drugs. Existence of actinomycetes has been reported in the hitherto untapped marine ecosystem. Marine actinomycetes are efficient producers of new secondary metabolites that show a range of biological activities including antibacterial, antifungal, anticancer, insecticidal and enzyme inhibition. Bioactive compounds from marine actinomycetes possess distinct chemical structures that may form the basis for synthesis of new drugs that could be used to combat resistant pathogens.

Cyclo(L-leucyl-L-prolyl) produced by Achromobacter xylosoxidans inhibits aflatoxin production by Aspergillus parasiticus

Aflatoxins are potent carcinogenic and toxic substances that are produced primarily by Aspergillus flavus and Aspergillus parasiticus. We found that a bacterium remarkably inhibited production of norsolorinic acid, a precursor of aflatoxin, by A. parasiticus. This bacterium was identified as Achromobacter xylosoxidans based on its 16S ribosomal DNA sequence and was designated A. xylosoxidans NFRI-A1. A. xylosoxidans strains commonly showed similar inhibition. The inhibitory substance(s) was excreted into the medium and was stable after heat, acid, or alkaline treatment. Although the bacterium appeared to produce several inhibitory substances, we finally succeeded in purifying a major inhibitory substance from the culture medium using Diaion HP20 column chromatography, thin-layer chromatography, and high-performance liquid chromatography. The purified inhibitory substance was identified as cyclo(L-leucyl-L-prolyl) based on physicochemical methods. The 50% inhibitory concentration for aflatoxin production by A. parasiticus SYS-4 (= NRRL2999) was 0.20 mg ml(-1), as determined by the tip culture method. High concentrations (more than 6.0 mg ml(-1)) of cyclo(L-leucyl-L-prolyl) further inhibited fungal growth. Similar inhibitory activities were observed with cyclo(D-leucyl-D-prolyl) and cyclo(L-valyl-L-prolyl), whereas cyclo(D-prolyl-L-leucyl) and cyclo(L-prolyl-D-leucyl) showed weaker activities. Reverse transcription-PCR analyses showed that cyclo(L-leucyl-L-prolyl) repressed transcription of the aflatoxin-related genes aflR, hexB, pksL1, and dmtA. This is the first report of a cyclodipeptide that affects aflatoxin production.

Cyclic dipeptides exhibit synergistic, broad spectrum antimicrobial effects and have anti-mutagenic properties

Cyclic dipeptides are known to have antiviral, antibiotic and antitumour properties. The aim of this study was to determine the combined effects of cyclo(L-leucyl-L-prolyl) and cyclo(L-phenylalanyl-L-prolyl) on the growth of vancomycin-resistant enterococci (VRE) and pathogenic yeasts, as well as determining their anti-mutagenic effects. This drug combination was especially effective against five VRE strains: Enterococcus faecium (K-99-38), E. faecalis (K-99-17), E. faecalis (K-99-258), E. faecium (K-01-312) and E. faecalis (K-01-511) with MIC values of 0.25-1 mg/l. It was also effective against Escherichia coli, Staphylococcus aureus, Micrococcus luteus, Candida albicans and Cryptococcus neoformans with MIC values of 0.25-0.5 mg/l. In addition, the cyclic dipeptides exerted anti-mutagenic activity against Salmonella typhimurium TA98 and TA100 strains in a Salmonella mutation assay. The number of mutant colonies of S. typhimurium strains TA98 and TA100 induced by exposure to AF-2 (0.2 microg/plate) decreased in a concentration-dependent manner in the presence of the two cyclic dipeptides (correlation 0.72 and 0.78, respectively). Here, for the first time, we report synergistic effects of the cyclic dipeptides [cyclo(L-leu-L-pro) and cyclo(L-phe-L-pro)] in inhibiting the growth of pathogenic microorganisms, as well as their anti-mutagenic effects in Salmonella strains.