Purification and Characterization of a New Antifungal Compound 10-(2,2-dimethyl-cyclohexyl)-6,9-dihydroxy-4,9-dimethyl-dec-2-enoic Acid Methyl Ester from Streptomyces hydrogenans Strain DH16

Why Do We Need Alternative Methods for Fungal Disease Management in Plants?

Fungal pathogens pose a major threat to food production worldwide. Traditionally, chemical fungicides have been the primary means of controlling these pathogens, but many of these fungicides have recently come under increased scrutiny due to their negative effects on the health of humans, animals, and the environment. Furthermore, the use of chemical fungicides can result in the development of resistance in populations of phytopathogenic fungi. Therefore, new environmentally friendly alternatives that provide adequate levels of disease control are needed to replace chemical fungicides-if not completely, then at least partially. A number of alternatives to conventional chemical fungicides have been developed, including plant defence elicitors (PDEs); biological control agents (fungi, bacteria, and mycoviruses), either alone or as consortia; biochemical fungicides; natural products; RNA interference (RNAi) methods; and resistance breeding. This article reviews the conventional and alternative methods available to manage fungal pathogens, discusses their strengths and weaknesses, and identifies potential areas for future research.

Purification and biological analysis of antimicrobial compound produced by an endophytic Streptomyces sp

Fungal phytopathogens and drug-resistant bacteria are two significant challenges in agriculture and public health, respectively. As a result, new sources of antimicrobial compounds are urgently needed. Taking into consideration these aspects, the present study was carried out to explore the antimicrobial activity of Streptomyces sp. SP5 against drug-resistant bacteria, especially methicillin resistant Staphylococcus aureus (MRSA), vancomycin resistant Enterococcus  and fungal phytopathogens. MRSA and VRE are both types of antibiotic-resistant bacteria that pose significant challenges to public health. In vitro analysis of the metabolites of Streptomyces sp. SP5 exhibited broad-spectrum antimicrobial activity against drug-resistant bacteria and phytopathogenic fungi. Further chemical investigation of the diethyl ether extract led to the isolation and purification of an antimicrobial compound. The structure of the purified compound was elucidated by performing detailed spectroscopic analysis including MS, IR, and NMR. The compound was identified as plicacetin. Plicacetin is a nucleoside antibiotic that has been reported for antibacterial activity against Gram-positive bacterium Mycobacterium tuberculosis. According to our knowledge, the present study is the first to demonstrate the antimicrobial properties of plicacetin against Fusarium oxysporum, Alternaria brassicicola, Fusarium solani, VRE and Bacillus subtilis. The outcome of the current study endorses that compound produced by Streptomyces sp. SP5 can be used as an antimicrobial agent against fungal phytopathogens and drug-resistant bacteria.

Insecticidal potential of endophytic Streptomyces sp. against Zeugodacus cucurbitae (Coquillett) (Diptera: Tephritidae) and biosafety evaluation

Fruit flies of Tephritidae family pose a serious threat to cultivation of fruits and vegetables across the world. Among them, melon fruit fly, Zeugodacus cucurbitae (Coquillett) (Diptera: Tephritidae) is a devastating pest of plants from Cucurbitaceae family. In a rising concern about the harmful effects associated with the use of chemical insecticides and development of resistance in pest insects, safer pest management strategies such as, use of biopesticides of microbial origin are being contemplated. Therefore, the present study aimed to evaluate the insecticidal potential of Streptomyces sp. SP5 protein extract against Z. cucurbitae. MTT assay, Ames mutagenicity, DNA nicking, and comet assay were conducted to determine the biosafety of protein extract. Second instar larvae of Z. cucurbitae were treated with various concentrations (1, 100, 200, 300, 400, and 500 μg/ml) of Streptomyces sp. SP5 protein extract. The protein extract showed significant larvicidal effects with LC50 value of 308.92 μg/ml. The percentage of adults emerged declined with increase in concentration. There was significant prolongation in developmental durations of the larvae. Various morphological aberrations in the form of deformed adults and pupae and decline in pupal weight were also observed. The nutritional physiology of the treated larvae was also adversely affected. The results from biosafety evaluation revealed antimutagenic and non-toxic nature of Streptomyces sp. proteins. This study indicates that Streptomyces sp. SP5 has the potential to be used as an ecologically safe biocontrol agent against Z. cucurbitae.

Multifaceted plant growth-promoting traits of indigenous rhizospheric microbes against Phomopsis theae, a causal agent of stem canker in tea plants

Phomopsis canker is one of the major devastating stem diseases that occur in tea plants caused by the fungal pathogen Phomopsis theae. Rapid development of this disease leads to a capital loss in the tea industry which demands an ecofriendly disease management strategy to control this aggressive pathogen. A total of 245 isolates were recovered from the tea rhizosphere and screened for in vitro plant growth promoting (PGP) traits and antagonism against P. theae. Among them, twelve isolates exhibited multifarious PGP traits including phytohormones, siderophore, hydrogen cyanide, salicylic acid production, phosphate solubilization, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity, and antifungal activity. In vitro studies on morphological, biochemical, and phylogenetic analyses classified the selected isolates as Pseudomonas fluorescens (VPF5), Bacillus subtilis (VBS3), Streptomyces griseus (VSG4) and Trichoderma viride (VTV7). Specifically, P. fluorescens VPF5 and B. subtilis VBS3 strains showed the highest level of PGP activities. On the other hand, VBS3 and VTV7 strains showed higher biocontrol efficacy in inhibiting mycelia growth and spore germination of P. theae. A detailed investigation on hydrolytic enzymes produced by antagonistic strains, which degrade the fungus cell wall, revealed that highest amount of chitinase and β-1,3- glucanase in VTV7 and VBS3 strains. Further, the key antifungal secondary metabolites from these biocontrol agents associated with suppression of P. theae were identified using gas chromatography mass spectrometry. The above study clearly recognized the specific traits in the isolated microbes, which make them good candidates as plant growth-promoting rhizobacteria (PGPR) and biocontrol agents to improve plant growth and health. However, greenhouse trials and field application of these beneficial microbes is required to further confirm their efficacy for the management of stem canker in tea cultivation.

Streptomyces and their specialised metabolites for phytopathogen control - comparative in vitro and in planta metabolic approaches

In the search for new crop protection microbial biocontrol agents, isolates from the genus Streptomyces are commonly found with promising attributes. Streptomyces are natural soil dwellers and have evolved as plant symbionts producing specialised metabolites with antibiotic and antifungal activities. Streptomyces biocontrol strains can effectively suppress plant pathogens via direct antimicrobial activity, but also induce plant resistance through indirect biosynthetic pathways. The investigation of factors stimulating the production and release of Streptomyces bioactive compounds is commonly conducted in vitro, between Streptomyces sp. and a plant pathogen. However, recent research is starting to shed light on the behaviour of these biocontrol agents in planta, where the biotic and abiotic conditions share little similarity to those of controlled laboratory conditions. With a focus on specialised metabolites, this review details (i) the various methods by which Streptomyces biocontrol agents employ specialised metabolites as an additional line of defence against plant pathogens, (ii) the signals shared in the tripartite system of plant, pathogen and biocontrol agent, and (iii) an outlook on new approaches to expedite the identification and ecological understanding of these metabolites under a crop protection lens.

Suppression of Fusarium Wilt in Watermelon by Bacillus amyloliquefaciens DHA55 through Extracellular Production of Antifungal Lipopeptides

Fusarium wilt caused by Fusarium oxysporum f. sp. niveum is one of the most devastating fungal diseases affecting watermelon (Citrullus lanatus L.). The present study aimed to identify potent antagonistic bacterial strains with substantial antifungal activity against F. oxysporum f. sp. niveum and to explore their potential for biocontrol of Fusarium wilt in watermelon. Out of 77 isolates from watermelon rhizosphere, six bacterial strains—namely, DHA4, DHA6, DHA10, DHA12, DHA41, and DHA55—exhibited significant antifungal activity against F. oxysporum f. sp. niveum, as well as other phytopathogenic fungi, including Didymella bryoniae, Sclerotinia sclerotiorum, Fusarium graminearum, and Rhizoctonia solani. These Gram-positive, rod-shaped, antagonistic bacterial strains were able to produce exo-enzymes (e.g., catalase, protease, and cellulase), siderophore, and indole-3-acetic acid and had the ability to solubilize phosphate. In greenhouse experiments, these antagonistic bacterial strains not only promoted plant growth but also suppressed Fusarium wilt in watermelon. Among these strains, DHA55 was the most effective, achieving the highest disease suppression of 74.9%. Strain DHA55 was identified as Bacillus amyloliquefaciens based on physiological, biochemical, and molecular characterization. B. amyloliquefaciens DHA55 produced various antifungal lipopeptides, including iturin, surfactin, and fengycin, that showed significant antifungal activities against F. oxysporum f. sp. niveum. Microscopic observations revealed that B. amyloliquefaciens DHA55 exhibited an inhibitory effect against F. oxysporum f. sp. niveum on the root surface of watermelon plants. These results demonstrate that B. amyloliquefaciens DHA55 can effectively promote plant growth and suppress the development of watermelon Fusarium wilt, providing a promising agent for the biocontrol of Fusarium wilt in watermelon.

β-Sitosterol, a phytocompound from Parthenium hysterophorus, reveals anti-diabetic properties through α-Amylase inhibition: an in-silico and in-vitro analysis

The study aims to identify and validate a potential α-Amylase inhibitor from the leaf extract of the Parthenium hysterophorus. Molecular docking and dynamics analyses were performed to test the anti-diabetic efficacy of the compound by focusing on α-Amylase inhibition. The molecular docking study using AutoDock Vina (PyRx) and SeeSAR tools identified β-Sitosterol as an effective α-Amylase inhibitory compound. Among the analysed fifteen phytochemicals, β-Sitosterol demonstrated the most appreciable binding energy (-9.0 Kcal/mol) and is comparatively higher than the binding energy of the standard α-Amylase inhibitor, the Acarbose (-7.6 Kcal/mol). The significance of the interaction between β-Sitosterol and α-Amylase was further investigated using Molecular Dynamics Simulation (MDS) for 100 ns via GROMACS. The data reveals that the compound could exhibit the highest stability with α-Amylase regarding RMSD, RMSF, SASA and Potential Energy analysis. The key residue of α-Amylase (Asp -197) shows a significantly low fluctuation of 0.7 Å while interacting with β-Sitosterol. The data obtained from MDS results strongly suggested the potential inhibitory impact of β-Sitosterol on α-Amylase. In addition, the proposed phytochemical was purified from the leaf extracts of P.hysterophorus using the silica gel column chromatography and identified by GC-MS analysis. The purified β-Sitosterol demonstrated a significant 42.30% in-vitro α-Amylase enzyme inhibition property under 400 µg/ml concentration and thus supported the in-silico predictions. Further in-vivo investigations are necessary to analyse the efficiency of β-Sitosterol on α-Amylase inhibition to help the anti-diabetic potential of the phytocompound.Communicated by Ramaswamy H. Sarma.

Marine Natural Product Antimycin A Suppresses Wheat Blast Disease Caused by Magnaporthe oryzae Triticum

The application of chemical pesticides to protect agricultural crops from pests and diseases is discouraged due to their harmful effects on humans and the environment. Therefore, alternative approaches for crop protection through microbial or microbe-originated pesticides have been gaining momentum. Wheat blast is a destructive fungal disease caused by the Magnaporthe oryzae Triticum (MoT) pathotype, which poses a serious threat to global food security. Screening of secondary metabolites against MoT revealed that antimycin A isolated from a marine Streptomyces sp. had a significant inhibitory effect on mycelial growth in vitro. This study aimed to investigate the inhibitory effects of antimycin A on some critical life stages of MoT and evaluate the efficacy of wheat blast disease control using this natural product. A bioassay indicated that antimycin A suppressed mycelial growth (62.90%), conidiogenesis (100%), germination of conidia (42%), and the formation of appressoria in the germinated conidia (100%) of MoT at a 10 µg/mL concentration. Antimycin A suppressed MoT in a dose-dependent manner with a minimum inhibitory concentration of 0.005 μg/disk. If germinated, antimycin A induced abnormal germ tubes (4.8%) and suppressed the formation of appressoria. Interestingly, the application of antimycin A significantly suppressed wheat blast disease in both the seedling (100%) and heading stages (76.33%) of wheat at a 10 µg/mL concentration, supporting the results from in vitro study. This is the first report on the inhibition of mycelial growth, conidiogenesis, conidia germination, and detrimental morphological alterations in germinated conidia, and the suppression of wheat blast disease caused by a Triticum pathotype of M. Oryzae by antimycin A. Further study is required to unravel the precise mode of action of this promising natural compound for considering it as a biopesticide to combat wheat blast.

Antifungal compound, methyl hippurate from Bacillus velezensis CE 100 and its inhibitory effect on growth of Botrytis cinerea

Botrytis cinerea, the causal agent of gray mold is one of the major devastating fungal pathogens that occurs in strawberry cultivation and leads to massive losses. Due to the rapid emergence of resistant strains in recent years, an ecofriendly disease management strategy needs to be developed to control this aggressive pathogen. Bacillus velezensis CE 100 exhibited strong antagonistic activity with 53.05% against B. cinerea by dual culture method. In the present study, 50% of culture filtrate supplemented into PDA medium absolutely inhibited mycelial growth of B. cinerea whereas the highest concentration (960 mg/L) of different crude extracts including ethyl acetate, chloroform, and n-butanol crude extracts of B. velezensis CE 100, strongly inhibited mycelial growth of B. cinerea with the highest inhibition of 79.26%, 70.21% and 69.59% respectively, resulting in severe damage to hyphal structures with bulging and swellings. Hence, the antifungal compound responsible was progressively separated from ethyl acetate crude extract using medium pressure liquid chromatography. The purified compound was identified as methyl hippurate by nuclear magnetic resonance and mass spectrometry. The inhibitory effect of methyl hippurate on both spore germination and mycelial growth of B. cinerea was revealed by its dose-dependent pattern. The spore germination rate was completely restricted at a concentration of 3 mg/mL of methyl hippurate whereas no mycelial growth was observed in agar medium supplemented with 4 mg/mL and 6 mg/mL of methyl hippurate by poisoned food method. Microscopic imaging revealed that the morphologies of spores were severely altered by long-time exposure to methyl hippurate at concentrations of 1 mg/mL, 2 mg/mL and 3 mg/mL and hyphae of B. cinerea were severely deformed by exposure to methyl hippurate at concentrations of 2 mg/mL, 4 mg/mL and 6 mg/mL. No significant inhibition on tomato seed germination was observed in treatments with methyl hippurate (2 mg/mL) for both 6 h and 12 h soaking period as compared to the controls. Based on these results, B. velezensis CE 100 could be considered a potential agent for development of environmentally friendly disease control strategies as a consequence of the synergetic interactions of diverse crude metabolites and methyl hippurate.

Screening and Purification of Natural Products from Actinomycetes that Induce a "Rounded" Morphological Phenotype in Fission Yeast

This study was designed to identify and investigate bioactive natural product compounds that alter the cellular shape of the fission yeast Schizosaccharomyces pombe and induce a "rounded" or "small" cellular morphological phenotype. Bioassays using a range of antifungal agents against a multidrug-sensitive fission yeast strain, SAK950 showed that many induced a "rounded" phenotype. We then investigated whether 46 of the actinomycete strains identified in our previous study as inducing a similar phenotype produced antifungal agents of similar classes. We show that five of the strains produced streptothricin and that 26 strains produced polyenes, including fungichromin, filipin and candicidin, the last of which was produced by 24 strains. A taxonomic study of the strains indicated that the majority of the candicidin only producers were Streptomyces hydrogenans and S. albidoflavus whilst those that additionally produced streptothricin were related to S. enissocaesilis. A follow-up study to investigate the natural products made by related strains indicated that they followed a similar pattern. The identification of several compounds from the actinomycete strains similar to the antifungal agents initially tested confirm the validity of an approach using the S. pombe morphological phenotype and actinomycete taxonomy as a predictive tool for natural product identification.

Surfactin Like Broad Spectrum Antimicrobial Lipopeptide Co-produced With Sublancin From Bacillus subtilis Strain A52: Dual Reservoir of Bioactives

An antimicrobial substance producing strain designated as A52 was isolated from a marine sediment sample and identified as Bacillus sp., based on 16S rRNA gene sequence analysis. The ANI and dDDH analysis of the genome sequence displayed high identity with two strains of B. subtilis sub sp. subtilis. Strain A52 yielded two antimicrobial peptides (AMPs) that differed in activity spectrum. MALDI mass spectrometry analysis of HPLC purified fractions revealed mass of peptides as 3881.6 and 1061.9 Da. The antiSMASH analysis of genome sequence unraveled presence of identical biosynthetic cluster involved in production of sublancin from B. subtilis sub sp. subtilis strain 168, which yielded peptide with identical mass. The low molecular weight peptide is found to be a cyclic lipopeptide containing C₁₆ β-hydroxy fatty acid that resembled surfactin-like group of biosurfactants. However, it differed in fatty acid composition and antimicrobial spectrum in comparison to other surfactins produced by strains of B. subtilis. It exhibited broad spectrum antibacterial activity, inhibited growth of pathogenic strains of Candida and filamentous fungi. Further, it exhibited hemolytic activity, but did not show phytotoxic effect in seed germination experiment. The emulgel formulation of surfactin-like lipopeptide showed antimicrobial activity in vitro and did not show any irritation effects in animal studies using BALB/c mice. Moreover, surfactin-like lipopeptide displayed synergistic activity with fluconazole against Candida, indicating its potential for external therapeutic applications.

A Neuroinflammation Inhibitor, Hypoxylon xanthone A, from Soil Fungus Hypoxylon sp

:

Hypoxylon sp. was used to ferment at 25°C for 45 days. The solid culture of Hypoxylon sp. was extracted with 75% EtOH under ultrasonic for twice. And the dried combined extracts were then suspended in H2O and partitioned with ethyl acetate. EtOAc extracts were subjected to a silica gel column and eluted with petroleum ether - acetone to a.ord seven fractions. Sephadex LH-20 and RPHPLC were used subsequently to yield a novel xanthone metabolite (Hypoxylon xanthone A). Its structure was elucidated based on HR-ESI-MS, 1D-, 2D-NMR spectra, and the comparison of the experimental and calculated ECD spectra. The anti-neuroinflammatory assay of Hypoxylon xanthone A, as manifested by the inhibitory effect on LPS-induced NO production in BV-2 microglial cells, indicated almost the same inhibitory effect as minocycline in a dose-dependent manner within the concentration of 1-50 μM, suggesting that Hypoxylon xanthone A could be a new potential neuroinflammation inhibitor.

Antibiotic Activity of Actinobacteria from the Digestive Tract of Millipede Nedyopus dawydoffiae (Diplopoda)

Because of the spread of drug resistance, it is necessary to look for new antibiotics that are effective against pathogenic microorganisms. The purpose of this study was to analyse the species composition of actinobacteria isolated from the digestive tract of the millipedes Nedyopus dawydoffiae and to determine their antimicrobial properties. Species identification was carried out on the basis of the morphological and culture properties and the sequence of the 16S rRNA gene. Actinobacteria were grown in different liquid media. Antibiotic properties were determined against some Gram-positive and Gram-negative bacteria as well as fungi. Of the 15 isolated strains, 13 have antibiotic activity against Gram-positive bacteria (including methicillin-resistant Staphylococcus aureus—MRSA) and fungi, but there was no antibiotic activity against Gram-negative test strains Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853. It was established that antibiotic-producing actinobacteria belong to eight species of the genus Streptomyces. Depending on the nutrient medium, actinobacteria demonstrate different antimicrobial activities. As an example, S. hydrogenans shows that even strains selected in one population differ by the range of antimicrobial activity and the level of biosynthesis. Since the antibiotic production is considered as a feature for species competition in the microbiota community, the variability of antibiotic production among different strains of the same species is an adaptive characteristic for the competition in millipedes’ digestive tract community.

Biocontrol Potential of Streptomyces hydrogenans Strain DH16 toward Alternaria brassicicola to Control Damping Off and Black Leaf Spot of Raphanus sativus

Biocontrol agents and their bioactive metabolites provide one of the best alternatives to decrease the use of chemical pesticides. In light of this, the present investigation reports the biocontrol potential of Streptomyces hydrogenans DH16 and its metabolites towards Alternaria brassicicola, causal agent of black leaf spot and damping off of seedlings of crucifers. In vitro antibiosis of strain against pathogen revealed complete suppression of mycelial growth of pathogen, grown in potato dextrose broth supplemented with culture supernatant (20% v/v) of S. hydrogenans DH16. Microscopic examination of the fungal growth showed severe morphological abnormalities in the mycelium caused by antifungal metabolites. In vivo studies showed the efficacy of streptomycete cells and culture supernatant as seed dressings to control damping off of Raphanus sativus seedlings. Treatment of pathogen infested seeds with culture supernatant (10%) and streptomycete cells significantly improved seed germination (75-80%) and vigor index (1167-1538). Furthermore, potential of cells and culture supernatant as foliar treatment to control black leaf spot was also evaluated. Clearly visible symptoms of disease were observed in the control plants with 66.81% disease incidence and retarded growth of root system. However, disease incidence reduced to 6.78 and 1.47% in plants treated with antagonist and its metabolites, respectively. Additionally, treatment of seeds and plants with streptomycete stimulated various growth traits of plants over uninoculated control plants in the absence of pathogen challenge. These results indicate that S. hydrogenans and its culture metabolites can be developed as biofungicides as seed dressings to control seed borne pathogens, and as sprays to control black leaf spot of crucifers.