Antifungal activity of volatile organic compounds fromStreptomyces alboflavusTD-1

Dinactin from a new producer, Streptomyces badius gz-8, and its antifungal activity against the rubber anthracnose fungus Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is a main cause of rubber anthracnose, which results in very large losses for the natural rubber industry. In this study, an actinomycete strain gz-8 was isolated and had strong antagonistic activity against C. gloeosporioides, with an inhibition rate of 72.5 %. Strain gz-8 was identified as Streptomyces badius. Three active compounds were separated from S. badius gz-8 and identified as feigrisolide B, feigrisolide C and dinactin according to the mass spectrometry and NMR-spectra results. In the three compounds, dinactin exhibited the best antifungal activity against C. gloeosporioides, with an EC₅₀ value of 2.55 μg/mL, and its minimum inhibitory concentration was 44 μg/mL. Dinactin had broad inhibitory activities against nine other pathogenic fungi, and it also had an obvious control effect on rubber anthracnose comparable to that of chlorothalonil. Dinactin could inhibit the conidiogenesis and spore germination of C. gloeosporioides. This report will contribute to understanding the antifungal activity of dinactin against C. gloeosporioides.

Antifungal Effects of Volatiles Produced by Bacillus subtilis Against Alternaria solani in Potato

Antifungal activities of plant-beneficial Bacillus have been widely studied in recent years. Numerous studies have studied the antifungal mechanisms of soluble non-volatile bioactive compounds such as lipopeptides and proteins produced by Bacillus against soil-borne diseases. However, the antagonistic mechanisms of volatile organic compounds (VOCs) from Bacillus against airborne phytopathogens are still largely unknown, and the function of Alternaria solani pathogenic genes has not been well identified. Here, we first isolated a Bacillus strain with strong antifungal activity and finally identified it as B. subtilis ZD01. Then, the antagonistic mechanisms of VOCs produced by strain ZD01, against A. solani, an airborne fungal pathogen that can cause early blight diseases of potato, were studied. We showed that VOCs produced by strain ZD01 can reduce the colony size and mycelial penetration and can cause serious morphological changes of A. solani. Scanning electron microscope (SEM) observation showed that VOCs released by ZD01 could cause more flaccid and gapped hyphae of A. solani. Also, we found that VOCs produced by ZD01 can inhibit the conidia germination and reduce the lesion areas and number of A. solani in vivo significantly. Meanwhile, based on gas chromatography/mass spectrometry (GC/MS) analysis, 29 volatile compounds produced by strain ZD01 were identified. Out of 29 identified VOCs, 9 VOCs showed complete growth inhibition activities against A. solani. Moreover, we identified two virulence-associated genes (slt2 and sod) in A. solani. slt2 is a key gene that regulates the mycelial growth, penetration, sporulation, and virulence in vivo in A. solani. In addition, sod plays a significant role in the SOD synthetic pathway in A. solani. Results from qRT-PCR showed that the transcriptional expression of these two genes was down-regulated after being treated by VOCs produced by ZD01. These results are useful for a better understanding of the biocontrol mechanism of Bacillus and offer a potential method for potato early blight disease control.

Volatile organic compounds kill the white-nose syndrome fungus, Pseudogymnoascus destructans, in hibernaculum sediment

Pseudogymnoascus destructans, the fungal pathogen that causes white-nose syndrome, has killed millions of bats across eastern North America and continues to threaten new bat populations. The spread and persistence of P. destructans has likely been worsened by the ability of this fungus to grow as a saprotroph in the hibernaculum environment. Reducing the environmental growth of P. destructans may improve bat survival. Volatile organic compounds (VOCs) are attractive candidates to target environmental P. destructans, as they can permeate through textured environments that may be difficult to thoroughly contact with other control mechanisms. We tested in hibernaculum sediment the performance of VOCs that were previously shown to inhibit P. destructans growth in agar cultures and examined the inhibition kinetics and specificity of these compounds. Three VOCs, 2-methyl-1-butanol, 2-methyl-1-propanol, and 1-pentanol, were fungicidal towards P. destructans in hibernaculum sediment, fast-acting, and had greater effects against P. destructans than other Pseudogymnoascus species. Our results suggest that use of these VOCs may be considered further as an effective management strategy to reduce the environmental exposure of bats to P. destructans in hibernacula.

Volatile Organic Compounds of Endophytic Burkholderia pyrrocinia Strain JK-SH007 Promote Disease Resistance in Poplar

Volatile organic compounds (VOCs) play important roles in the regulation of plant growth and pathogen resistance. However, little is known about the influence of VOCs released from endophytic strains (Burkholderia pyrrocinia strain JK-SH007) on controlling pathogens or inducing systemic resistance in poplar. In this study, we found that VOCs produced by strain JK-SH007 inhibit three poplar canker pathogens (Cytospora chrysosperma, Phomopsis macrospora, and Fusicoccum aesculi) and promote defense enzyme activity and malondialdehyde (MDA) and total phenol (TP) accumulation. Thirteen kinds of VOC components were identified using the solid-phase microextraction combined with gas chromatography-mass spectrometry method. Dimethyl disulfide (DMDS) accounted for the largest proportion of these VOCs. Treatments of poplar seedlings with different volumes of VOC standards (DMDS, benzothiazole, dimethylthiomethane, and phenylacetone) showed that DMDS had the greatest effects on various defense enzyme activities and MDA and TP accumulation. We also found that the inhibitory effect of the VOCs on the three pathogens was gradually enhanced with increasing standard volume. Moreover, the treatment of samples with DMDS significantly reduced the severity and development of the disease caused by three poplar canker pathogens. Comparative transcriptomics analysis of poplar seedlings treated with DMDS showed that there were 1,586 differentially expressed genes in the leaves and stems, and quantitative PCR showed that the gene expression trends were highly consistent with the transcriptome sequencing results. The most significant transcriptomic changes induced by VOCs were related to hormone signal transduction, transcriptional regulation of plant-pathogen interactions, and energy metabolism. Moreover, 137 transcription factors, including members of the ethylene response factor, NAC, WRKY, G2-like, and basic helix-loop-helix protein families, were identified to be involved in the VOC-induced process. This study elucidates the resistance induced by Burkholderia pyrrocinia strain JK-SH007 to poplar canker at the molecular level and can make possible a new method for the comprehensive prevention and control of poplar disease.

A novel cyanobacterial geosmin producer, revising GeoA distribution and dispersion patterns in Bacteria

Cyanobacteria are ubiquitous organisms with a relevant contribution to primary production in all range of habitats. Cyanobacteria are well known for their part in worldwide occurrence of aquatic blooms while producing a myriad of natural compounds, some with toxic potential, but others of high economical impact, as geosmin. We performed an environmental survey of cyanobacterial soil colonies to identify interesting metabolic pathways and adaptation strategies used by these microorganisms and isolated, sequenced and assembled the genome of a cyanobacterium that displayed a distinctive earthy/musty smell, typical of geosmin, confirmed by GC-MS analysis of the culture's volatile extract. Morphological studies pointed to a new Oscillatoriales soil ecotype confirmed by phylogenetic analysis, which we named Microcoleus asticus sp. nov. Our studies of geosmin gene presence in Bacteria, revealed a scattered distribution among Cyanobacteria, Actinobacteria, Delta and Gammaproteobacteria, covering different niches. Careful analysis of the bacterial geosmin gene and gene tree suggests an ancient bacterial origin of the gene, that was probably successively lost in different time frames. The high sequence similarities in the cyanobacterial geosmin gene amidst freshwater and soil strains, reinforce the idea of an evolutionary history of geosmin, that is intimately connected to niche adaptation.

Biocontrol ability and volatile organic compounds production as a putative mode of action of yeast strains isolated from organic grapes and rye grains

The inhibiting activity of three yeast strains belonging to Pichia kudriavzevii, Pichia occidentalis, and Meyerozyma quilliermondii/Meyerozyma caribbica genera against common plant pathogens representing Mucor spp., Penicillium chrysogenum, Penicillium expansum, Aspergillus flavus, Fusarium cereals, Fusarium poae, as well as Botrytis cinerea genera was investigated. The yeast strains tested had a positive impact on growth inhibition of all target plant pathogens. The degree of inhibition was more than 50% and varied depending on both the yeast antagonist and the mold. Ethyl esters of medium-chain fatty acids, phenylethyl alcohol, and its acetate ester prevailed among the analyzed volatile organic compounds (VOCs) emitted by yeasts in the presence of the target plant pathogens. Due to the method used, assuming no contact between the antagonist and the pathogen, the antagonistic activity of the yeast strains studied resulted mainly from the production of biologically active VOCs. Moreover, the antagonistic activity was not only restricted to a single plant pathogen but effective towards molds of different genera, making the yeast strains studied very useful for potential application in biological control.

Genome Mining Revealed a High Biosynthetic Potential for Antifungal Streptomyces sp. S-2 Isolated from Black Soot

The increasing resistance of fungal pathogens has heightened the necessity of searching for new organisms and compounds to combat their spread. Streptomyces are bacteria that are well-known for the production of many antibiotics. To find novel antibiotic agents, researchers have turned to previously neglected and extreme environments. Here, we isolated a new strain, Streptomyces sp. S-2, for the first time, from black soot after hard coal combustion (collected from an in-use household chimney). We examined its antifungal properties against plant pathogens and against fungi that potentially pose threat to human health (Fusarium avenaceum, Aspergillus niger and the environmental isolates Trichoderma citrinoviridae Cin-9, Nigrospora oryzae sp. roseF7, and Curvularia coatesieae sp. junF9). Furthermore, we obtained the genome sequence of S-2 and examined its potential for secondary metabolites production using anti-SMASH software. The S-2 strain shows activity against all of the tested fungi. Genome mining elucidated a vast number of biosynthetic gene clusters (55), which distinguish this strain from closely related strains. The majority of the predicted clusters were assigned to non-ribosomal peptide synthetases or type 1 polyketide synthetases, groups known to produce compounds with antimicrobial activity. A high number of the gene clusters showed no, or low similarity to those in the database, raising the possibility that S-2 could be a producer of novel antibiotics. Future studies on Streptomyces sp. S-2 will elucidate its full biotechnological potential.

Airborne medicine: bacterial volatiles and their influence on plant health

Like most other eukaryotes, plants do not live alone but in close association with a diverse microflora. These plant-associated microbes contribute to plant health in many different ways, ranging from modulation of hormonal pathways to direct antibiosis of plant pathogens. Over the last 15 yr, the importance of volatile organic compounds as mediators of mutualistic interactions between plant-associated bacteria and their hosts has become evident. This review summarizes current knowledge concerning bacterial volatile-mediated plant protection against abiotic and biotic stresses. It then discusses the translational potential of such metabolites or of their emitters for sustainable crop protection, the possible ways to harness this potential, and the major challenges still preventing us from doing so. Finally, the review concludes with highlighting the most pressing scientific gaps that need to be filled in order to enable a better understanding of: the molecular mechanisms underlying the biosynthesis of bacterial volatiles; the complex regulation of bacterial volatile emission in natural communities; the perception of bacterial volatiles by plants; and the modes of actions of bacterial volatiles on their host.

Comprehensive profiling of the VOCs of Trichoderma longibrachiatum EF5 while interacting with Sclerotium rolfsii and Macrophomina phaseolina

Trichoderma longibrachiatum EF5 is an endophytic fungal antagonist of rice. It is used for the control of soil-borne fungal pathogens-Sclerotium rolfsii and Macrophomina phaseolina. We demonstrate that T. longibrachiatum EF5 inhibits the growth of these pathogens on direct interaction as well as via the production of the microbial volatile organic compounds (mVOCs). The mVOCs reduced mycelial growth and inhibited the production of sclerotia by altering the mycelial structure. We profiled 138 mVOCs, when T. longibrachiatum EF5 interacted with the two pathogens. During these interactions, several compounds are up- or downregulated by T. longibrachiatum EF5, including longifolene, caryophyllene,1-Butanol 2-methyl, cedrene, and cuprenene. These compounds are involved in the biosynthetic pathways of the sesquiterpenoid and alkane, and the degradation pathway of trimethylamine. We provide an insight into the multiple modes by which T. longibrachiatum EF5 exerts antagonistic actions, such as hyperparasitism, competitions, and antibiosis via mVOCs. In contrast to their antimicrobial properties, these metabolites could also promote plant growth.

Commercial Biocontrol Agents Reveal Contrasting Comportments Against Two Mycotoxigenic Fungi in Cereals: Fusarium Graminearum and Fusarium Verticillioides

The aim of this study was to investigate the impact of commercialized biological control agents (BCAs) against two major mycotoxigenic fungi in cereals, Fusarium graminearum and Fusarium verticillioides, which are trichothecene and fumonisin producers, respectively. With these objectives in mind, three commercial BCAs were selected with contrasting uses and microorganism types (T. asperellum, S. griseoviridis, P. oligandrum) and a culture medium was identified to develop an optimized dual culture bioassay method. Their comportment was examined in dual culture bioassay in vitro with both fusaria to determine growth and mycotoxin production kinetics. Antagonist activity and variable levels or patterns of mycotoxinogenesis inhibition were observed depending on the microorganism type of BCA or on the culture conditions (e.g., different nutritional sources), suggesting that contrasting biocontrol mechanisms are involved. S. griseoviridis leads to a growth inhibition zone where the pathogen mycelium structure is altered, suggesting the diffusion of antimicrobial compounds. In contrast, T. asperellum and P. oligandrum are able to grow faster than the pathogen. T. asperellum showed the capacity to degrade pathogenic mycelia, involving chitinolytic activities. In dual culture bioassay with F. graminearum, this BCA reduced the growth and mycotoxin concentration by 48% and 72%, respectively, and by 78% and 72% in dual culture bioassay against F. verticillioides. P. oligandrum progressed over the pathogen colony, suggesting a close type of interaction such as mycoparasitism, as confirmed by microscopic observation. In dual culture bioassay with F. graminearum, P. oligandrum reduced the growth and mycotoxin concentration by 79% and 93%, respectively. In the dual culture bioassay with F. verticillioides, P. oligandrum reduced the growth and mycotoxin concentration by 49% and 56%, respectively. In vitro dual culture bioassay with different culture media as well as the nutritional phenotyping of different microorganisms made it possible to explore the path of nutritional competition in order to explain part of the observed inhibition by BCAs.

Identification of Rhizospheric Actinomycete Streptomyces lavendulae SPS-33 and the Inhibitory Effect of its Volatile Organic Compounds against Ceratocystis Fimbriata in Postharvest Sweet Potato (Ipomoea Batatas (L.) Lam.)

Black spot disease, which is caused by the pathogenic fungal Ceratocystis fimbriata, seriously affects the production of sweet potato and its quality during postharvest storage. In this study, the preliminary identification of the rhizosphere actinomycete strain SPS-33, and its antifungal activity of volatiles in vitro and in vivo was investigated. Based on morphological identification and phylogenetic analysis of the 16S rRNA gene sequence, strain SPS-33 was identified as Streptomyces lavendulae. Volatile organic compounds (VOCs) emitted by SPS-33 inhibited mycelial growth and sporulation of C. fimbriatain vitro and also induced a series of observable hyphae morphological changes. In an in vivo pathogenicity assay, exposure to SPS-33 significantly decreased the lesion diameter and water loss rate in sweet potato tuberous roots (TRs) inoculated with C. fimbriata. It increased the antioxidant enzymes’ activities of peroxidase, catalase, and superoxide dismutase as well as decreased malondialdehyde and increased total soluble sugar. In the VOC profile of SPS-33 detected by a headspace solid-phase micro extraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS), heptadecane, tetradecane, and 3-methyl-1-butanol were the most abundant compounds. 2-Methyl-1-butanol, 3-methyl-1-butanol, pyridine, and phenylethyl alcohol showed strong antifungal effects against C. fimbriata. These findings suggest that VOCs from S. lavendulae SPS-33 have the potential for pathogen C. fimbriata control in sweet potato postharvest storage by fumigant action.

Scent of a Killer: Microbial Volatilome and Its Role in the Biological Control of Plant Pathogens

The use of synthetic fungicides represents the most common strategy to control plant pathogens. Excessive and/or long-term distribution of chemicals is responsible for increased levels of environmental pollution, as well as adverse health consequence to humans and animals. These issues are deeply influencing public perception, as reflected by the increasing demand for safer and eco-friendly agricultural commodities and their by-products. A steadily increasing number of research efforts is now devoted to explore the use of safer and innovative approaches to control plant pathogens. The use of microorganisms as biological control agents (BCAs) represents one of the most durable and promising strategies. Among the panoply of microbial mechanisms exerted by BCAs, the production of volatile organic compounds (VOCs) represents an intriguing issue, mostly exploitable in circumstances where a direct contact between the pathogen and its antagonist is not practicable. VOCs are potentially produced by all living microorganisms, and may be active in the biocontrol of phytopathogenic oomycetes, fungi, and bacteria by means of antimicrobial activity and/or other cross-talk interactions. Their biological effects, the reduced residuals in the environment and on agricultural commodities, and the ease of application in different agricultural systems make the use of VOCs a promising and sustainable approach to replace synthetic fungicides in the control of plant pathogens. In this review, we focus on VOCs produced by bacteria and fungi and on their role in the cross-talk existing between the plant pathogens and their host. Biologic systemic effect of the microbial volatile blends on both pathogen and host plant cells is also briefly reviewed.

High Efficacy of the Volatile Organic Compounds of Streptomyces yanglinensis 3-10 in Suppression of Aspergillus Contamination on Peanut Kernels

Aspergillus flavus and Aspergillus parasiticus are saprophytic fungi which can infect and contaminate preharvest and postharvest food/feed with production of aflatoxins (B1, B2, and G). They are also an opportunistic pathogen causing aspergillosis diseases of animals and humans. In this study, the volatile organic compounds (VOCs) from Streptomyces yanglinensis 3-10 were found to be able to inhibit mycelial growth, sporulation, conidial germination, and expression of aflatoxin biosynthesis genes in A. flavus and A. parasiticus in vitro. On peanut kernels, the VOCs can also reduce the disease severity and inhibit the aflatoxins production by A. flavus and A. parasiticus under the storage conditions. Scanning electron microscope (SEM) observation showed that high dosage of the VOCs can inhibit conidial germination and colonization by the two species of Aspergillus on peanut kernels. The VOCs also showed suppression of mycelial growth on 18 other plant pathogenic fungi and one Oomycetes organism. By using SPME-GC-MS, 19 major VOCs were detected, like in other Streptomyces, 2-MIB was found as the main volatile component among the detected VOCs. Three standard chemicals, including methyl 2-methylbutyrate (M2M), 2-phenylethanol (2-PE), and β-caryophyllene (β-CA), showed antifungal activity against A. flavus and A. parasiticus. Among them, M2M showed highest inhibitory effect than other two standard compounds against conidial germination of A. flavus and A. parasiticus. To date, this is the first record about the antifungal activity of M2M against A. flavus and A. parasiticus. The VOCs from S. yanglinensis 3-10 did not affect growth of peanut seedlings. In conclusion, our results indicate that S. yanglinensis 3-10 may has a potential to become a promising biofumigant in for control of A. flavus and A. parasiticus.

Efficacy of metabolites of a Streptomyces strain (AS1) to control growth and mycotoxin production by Penicillium verrucosum, Fusarium verticillioides and Aspergillus fumigatus in culture

The objectives of this study were to determine the efficacy of metabolites of a Streptomyces strain AS1 on (a) spore germination, (b) mycelial growth, (c) control of mycotoxins produced by Penicillium verrucosum (ochratoxin A, OTA), Fusarium verticillioides (fumonisins, FUMs) and Aspergillus fumigatus (gliotoxin) and (d) identify the predominant metabolites involved in control. Initial screening showed that the Streptomyces AS1 strain was able to inhibit the mycelial growth of the three species at a distance, due to the release of secondary metabolites. A macroscopic screening system showed that the overall Index of Dominance against all three toxigenic fungi was inhibition at a distance. Subsequent studies showed that the metabolite mixture from the Streptomyces AS1 strain was very effective at inhibiting conidial germination of P. verrucosum, but less so against conidia of A. fumigatus and F. verticillioides. The efficacy was confirmed in studies on a conducive semi-solid YES medium in BioScreen C assays. Using the BioScreen C and the criteria of Time to Detection (TTD) at an OD = 0.1 showed good efficacy against P. verrucosum when treated with the Streptomyces AS1 extract at 0.95 and 0.99 water activity (a_w) when compared to the other two species tested, indicating good efficacy. The effective dose for 50% control of growth (ED₅₀) at 0.95 and 0.99 a_w were approx. 0.005 ng/ml and 0.15 μg/ml, respectively, with the minimum inhibitory concentration (MIC) at both a_w levels requiring > 40 μg/ml. In addition, OTA production was completely inhibited by 2.5 μg/ml AS1 extract at both a_w levels in the in vitro assays. Ten metabolites were identified with four of these being predominant in concentrations > 2 μg/g dry weight biomass. These were identified as valinomycin, cyclo(L-Pro-L-Tyr), cyclo(L-Pro-L-Val) and brevianamide F.

Control of Aflatoxigenic Molds by Antagonistic Microorganisms: Inhibitory Behaviors, Bioactive Compounds, Related Mechanisms, and Influencing Factors

Aflatoxin contamination has been causing great concern worldwide due to the major economic impact on crop production and their toxicological effects to human and animals. Contamination can occur in the field, during transportation, and also in storage. Post-harvest contamination usually derives from the pre-harvest infection of aflatoxigenic molds, especially aflatoxin-producing Aspergilli such as Aspergillusflavus and A. parasiticus. Many strategies preventing aflatoxigenic molds from entering food and feed chains have been reported, among which biological control is becoming one of the most praised strategies. The objective of this article is to review the biocontrol strategy for inhibiting the growth of and aflatoxin production by aflatoxigenic fungi. This review focuses on comparing inhibitory behaviors of different antagonistic microorganisms including various bacteria, fungi and yeasts. We also reviewed the bioactive compounds produced by microorganisms and the mechanisms leading to inhibition. The key factors influencing antifungal activities of antagonists are also discussed in this review.

Critical Assessment of Streptomyces spp. Able to Control Toxigenic Fusaria in Cereals: A Literature and Patent Review

Mycotoxins produced by Fusarium species on cereals represent a major concern for food safety worldwide. Fusarium toxins that are currently under regulation for their content in food include trichothecenes, fumonisins, and zearalenone. Biological control of Fusarium spp. has been widely explored with the aim of limiting disease occurrence, but few efforts have focused so far on limiting toxin accumulation in grains. The bacterial genus Streptomyces is responsible for the production of numerous drug molecules and represents a huge resource for the discovery of new molecules. Streptomyces spp. are also efficient plant colonizers and able to employ different mechanisms of control against toxigenic fungi on cereals. This review describes the outcomes of research using Streptomyces strains and/or their derived molecules to limit toxin production and/or contamination of Fusarium species in cereals. Both the scientific and patent literature were analyzed, starting from the year 2000, and we highlight promising results as well as the current pitfalls and limitations of this approach.

Production of ammonia as a low-cost and long-distance antibiotic strategy by Streptomyces species

Soil-inhabiting streptomycetes are nature's medicine makers, producing over half of all known antibiotics and many other bioactive natural products. However, these bacteria also produce many volatiles, molecules that disperse through the soil matrix and may impact other (micro)organisms from a distance. Here, we show that soil- and surface-grown streptomycetes have the ability to kill bacteria over long distances via air-borne antibiosis. Our research shows that streptomycetes do so by producing surprisingly high amounts of the low-cost volatile ammonia, dispersing over long distances to inhibit the growth of Gram-positive and Gram-negative bacteria. Glycine is required as precursor to produce ammonia, and inactivation of the glycine cleavage system nullified ammonia biosynthesis and concomitantly air-borne antibiosis. Reduced expression of the porin master regulator OmpR and its cognate kinase EnvZ is used as a resistance strategy by E. coli cells to survive ammonia-mediated antibiosis. Finally, ammonia was shown to enhance the activity of canonical antibiotics, suggesting that streptomycetes adopt a low-cost strategy to sensitize competitors for antibiosis from a distance.

Antifungal, Plant Growth-Promoting, and Genomic Properties of an Endophytic Actinobacterium Streptomyces sp. NEAU-S7GS2

Diseases caused by Sclerotinia sclerotiorum have caused severe losses of many economically important crops worldwide. Due to the long-term persistence of sclerotia in soil and the production of air-borne ascospores, synthetic fungicides play limited roles in controlling the diseases. The application of antagonistic microorganisms can effectively reduce the number of sclerotia and eventually eradicate S. sclerotiorum from soil, and therefore considerable interest has been focused on biological control. Streptomyces sp. NEAU-S7GS2 was isolated from the root of Glycine max and its rhizosphere soil. It showed significant inhibitory activity against the mycelial growth of S. sclerotiorum (99.1%) and completely inhibited sclerotia germination. Compared to the control, in the pot experiment the application of NEAU-S7GS2 not only demonstrated excellent potential to control sclerotinia stem rot of soybean with 77 and 38% decrease in disease incidence and disease index, respectively, but could promote the growth of soybean. The light microscopy and scanning electron microscopy showed that co-culture of NEAU-S7GS2 with S. sclerotiorum on potato dextrose agar could lead to contorted and fragmented mycelia of S. sclerotiorum, which was associated with the secretion of hydrolytic glucanase and cellulase and the production of active secondary metabolites by NEAU-S7GS2. The plant growth promoting activity of NEAU-S7GS2 was related to the solubilization of inorganic phosphate, and production of 1-aminocyclopropane-1-carboxylate (ACC) deaminase and indole acetic acid (IAA). To further explore the plant growth promoting and antifungal mechanisms, the complete genome of strain NEAU-S7GS2 was sequenced. Several genes associated with ammonia assimilation, phosphate solubilization and IAA synthesis, together with genes encoding ACC deaminase, glucanase and α-amylase, were identified. AntiSMASH analysis led to the identification of four gene clusters responsible for the biosynthesis of siderophores including desferrioxamine B and enterobactin. Moreover, the biosynthetic gene clusters of lydicamycins, phenazines, and a glycosylated polyol macrolide showing 88% gene similarity to PM100117/PM100118 were identified. These results suggested that strain NEAU-S7GS2 may be a potential biocontrol agent and biofertilizer used in agriculture.

The antibiotic activity and mechanisms of active metabolites (Streptomyces alboflavus TD-1) against Ralstonia solanacearum

OBJECTIVES

In order to elucidate the antibacterial activity and mechanism of S. alboflavus TD-1 active metabolites, the minimal inhibitory concentration of R. solanacearum and other effects on cell wall, cell membrane, nucleic acid, protein and cell morphology were studied. Besides, based on LCMS-IT-TOF, the active metabolites of S. alboflavus TD-1 were preliminarily analyzed.

RESULTS

In this study, We found that the active metabolites had obvious inhibitory effect on R. solanacearum, and the minimal inhibitory concentration (MIC) of R. solanacearum was 3.125 mg/mL. And the treatment of 10 mg/mL active metabolites can increase the permeability of R. solanacearum membranes, destroy the cell wall integrity, inhibit the synthesis of bacterial nucleic acids and proteins, and cause leakage of bacterial nucleic acids and proteins, obstruct the normal expression of proteins and destroy their bacterial morphology. At the same time, We speculated the molecular weights corresponding to the six compounds were 618, 615, 615, 615, 646, 646, respectively among the active metabolites, and it was found that were highly unstable.

CONCLUSIONS

The active metabolites produced by S. alboflavus TD-1 liquid fermentation contain components that can significant inhibitory effects on R. solanacearum. It had the potential to develop biocontrol agents against bacterial wilt and be a kind potential sources for the preparation of functional anti-pathogenic microbial agents.

Molecular profiling of endophytic Streptomyces cavourensis MH16 inhabiting Millingtonia hortensis Linn. and influence of different culture media on biosynthesis of antimicrobial metabolites

Endophytic actinomycetes, a prolific source of natural products, are well known for their diverse metabolic versatility, and their association with medicinal plants and antimicrobial potential are well worth exploring. We isolated and identified the Streptomyces cavourensis strain MH16 inhabiting the tree Millingtonia hortensis Linn. using phylogenetic analysis based on a 16S rRNA molecular approach. We used the disc diffusion method to evaluate the impact of differences in the compositions of the media on the production of secondary metabolites from strain MH16. The production of antimicrobial metabolites was determined by the observation of inhibition zones on intensive bands when using a TLC-bioautography assay. Biosynthesis of secondary metabolites was optimal when the strain MH16 was cultured in ISP-2 medium as depicted by a zone of inhibition. Strain MH16 effectively inhibited methicillin-resistant Staphylococcus aureus, Escherichia coli, Candida albicans, and other multi drug-resistant pathogens. The minimum inhibitory concentration of the antimicrobial metabolites was 25-100 μg mL^-1. The study manifests the optimization and utilization of different fermentation media which best suits for increased production of the secondary metabolites from Streptomyces cavourensis. This research suggests that the antimicrobial metabolites of strain MH16 found in M. hortensis has great potential for the biodiscovery of new anti-infective drugs against a wide range of multidrug-resistant pathogens.

Enhancement of disease resistance, growth potential, and photosynthesis in tomato (Solanum lycopersicum) by inoculation with an endophytic actinobacterium, Streptomyces thermocarboxydus strain BPSAC147

Biotic stresses in plants have a significant impact on agricultural productivity. In the present study, in vivo experiments were conducted to determine the physiological responses of tomato (Solanum lycopersicum L.) seedlings by inoculation with an endophytic actinobacterium, Streptomyces thermocarboxydus isolate BPSAC147 under greenhouse conditions. Further, photochemical quantum yield of photosystem II (PSII) (F_v/F_m), photochemical quenching (qP) and non-photochemical (NPQ) were calculated in seedlings inoculated with S. thermocarboxydus (T1) and were compared with control (T0) plants. Furthermore, the electron transport rate (ETR) of PSII exhibited a significant increase in T1 plants, relative to T0 plants. These results indicate that inoculation of tomato seedlings with S. thermocarboxydus had a positive effect on the process of photosynthesis, resulting in enhanced chlorophyll fluorescence parameters due to increased ETR in the thylakoid membrane. GC-MS analysis showed significant differences in the volatile compounds in the different treatments performed under greenhouse conditions. The present study suggests that S. thermocarboxydus can be used as new biocontrol agent to control Fusarium wilt in tomato crops and enhance productivity by enhancing photosynthesis.

Antifungal potential of Lauraceae rhizobacteria from a tropical montane cloud forest against Fusarium spp

The occurrence of pests and diseases can affect plant health and productivity in ecosystems that are already at risk, such as tropical montane cloud forests. The use of naturally occurring microorganisms is a promising alternative to mitigate forest tree fungal pathogens. The objectives of this study were to isolate rhizobacteria associated with five Lauraceae species from a Mexican tropical montane cloud forest and to evaluate their antifungal activity against Fusarium solani and F. oxysporum. Fifty-six rhizobacterial isolates were assessed for mycelial growth inhibition of Fusarium spp. through dual culture assays. Thirty-three isolates significantly reduced the growth of F. solani, while 21 isolates inhibited that of F. oxysporum. The nine bacterial isolates that inhibited fungal growth by more than 20% were identified through 16S rDNA gene sequence analysis; they belonged to the genera Streptomyces, Arthrobacter, Pseudomonas, and Staphylococcus. The volatile organic compounds (VOC) produced by these nine isolates were evaluated for antifungal activity. Six isolates (Streptomyces sp., Arthrobacter sp., Pseudomonas sp., and Staphylococcus spp.) successfully inhibited F. solani mycelial growth by up to 37% through VOC emission, while only the isolate INECOL-21 (Pseudomonas sp.) inhibited F. oxysporum. This work provides information on the microbiota of Mexican Lauraceae and is one of the few studies identifying forest tree-associated microbes with inhibitory activity against tree pathogens.

Taxonomy and Broad-Spectrum Antifungal Activity of Streptomyces sp. SCA3-4 Isolated From Rhizosphere Soil of Opuntia stricta

Actinobacteria are important producers of bioactive compounds. Extreme ecosystems cause evolution of novel secondary metabolic pathways of Actinobacteria and increase the possible discovery of new biological functions of bioactive compounds. Here, we isolated 65 Actinobacteria from rhizosphere soil samples of Opuntia stricta. An Actinobacteria strain (named SCA3-4) was screened against Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4, ATCC 76255). The strain produced pink-white aerial mycelia and brown substrate mycelium on Gause No. 1 agar. Biverticillate chains of cylindrical spores were observed by scanning electron microscopy (SEM). Based on alignment of 16S rRNA sequences, a constructed phylogenetic tree showed that strain SCA3-4 shared a 99.54% similarity with Streptomyces lilacinus NRRL B-1968T. The morphological, biochemical, physiological, and molecular characteristics further indicated that strain SCA3-4 belongs to the Streptomyces sp. It can grow well on medium with the following antibiotics chloramphenicol, streptomycin, penicillin-G, gentamicin, erythromycin, nystatin or neomycin sulfate. The polymerase chain reaction (PCR) amplification of types I and II polyketide synthase genes (PKS-I and PKS-II) suggested its bioactive potential. Under treatment with 100 μg/ml of ethyl acetate extracts isolated from Streptomyces sp. SCA3-4, growth of Foc TR4 was inhibited and cell membrane was destroyed. Crude extracts also showed a broad-spectrum antifungal activity against 13 phytopathogenic fungi including Foc TR4 and displayed the lowest minimum inhibitory concentration (MIC) (0.781 μg/ml) against Colletotrichum fragariae (ATCC 58718). A total of 21 different compounds identified by gas chromatography-mass spectrometry (GC-MS) were composed of phenolic compound, pyrrolizidine, hydrocarbons, esters, and acids. Besides the known active compounds, Streptomyces sp. SCA3-4 possesses antimicrobial or other biological activities. Further attention will be paid on other compounds with no functional annotation, aiming at the discovery of new bioactive substances.

Biocontrol of Cereal Crop Diseases Using Streptomycetes

A growing world population and an increasing demand for greater food production requires that crop losses caused by pests and diseases are dramatically reduced. Concurrently, sustainability targets mean that alternatives to chemical pesticides are becoming increasingly desirable. Bacteria in the plant root microbiome can protect their plant host against pests and pathogenic infection. In particular, Streptomyces species are well-known to produce a range of secondary metabolites that can inhibit the growth of phytopathogens. Streptomyces are abundant in soils and are also enriched in the root microbiomes of many different plant species, including those grown as economically and nutritionally valuable cereal crops. In this review we discuss the potential of Streptomyces to protect against some of the most damaging cereal crop diseases, particularly those caused by fungal pathogens. We also explore factors that may improve the efficacy of these strains as biocontrol agents in situ, as well as the possibility of exploiting plant mechanisms, such as root exudation, that enable the recruitment of microbial species from the soil to the root microbiome. We argue that a greater understanding of these mechanisms may enable the development of protective plant root microbiomes with a greater abundance of beneficial bacteria, such as Streptomyces species.

Antifungal spectrum characterization and identification of strong volatile organic compounds produced by Bacillus pumilus TM-R

To obtain biocontrol agents for suppression of food-deteriorating fungi during storage of agricultural products, bacteria producing volatile organic compounds (VOCs) with strong antifungal activity were screened and isolated from various environmental samples. Among 136 bacterial isolates, strain TM-R showed the strongest and broadest antifungal activity. Based on physiological and genetical characterization, the bacterium was identified as Bacillus pumilus. The effects of VOCs produced by the bacterium, which was grown on four types of agar media (nutrient, Trypto-Soya, Luria-Bertani, and TM Enterprise), were examined against six species of fungi (Alternaria alternata, Aspergillus niger, Cladosporium cladosporioides, Curvularia lunata, Fusarium oxysporum, and Penicillium italicum) in both small- and large-scale tests (plate and 12-L tests, respectively). In the plate test, the bacterium markedly suppressed the mycelial growth of five fungi (Alternaria alternata, Cladosporium cladosporioides, Curvularia lunata, F. oxysporum, and P. italicum) but promoted the growth of Aspergillus niger. In the 12-L test, the degree of growth inhibitiondecreased; however, the bacterium grown on TMEA still exhibited the strongest inhibition, especially against P. italicum (growth inhibition rate of 93%). Surprisingly, the growth of Aspergillus niger was promoted even more strongly (-36%) by the bacterium on TMEA than in the plate test (-9%). Twenty-two of 32 VOCs detected by GC-MS were identified using three databases (NIST 2011, AromaOffice, and AroChemBase). The species and concentration of detected VOCs differed greatly among growth media. To identify causative antifungal VOCs, we estimated the correlation between growth inhibition of P. italicum by the bacterium grown on each of the four media vs. the relative abundance of individual VOCs. As a result, four VOCs (methyl isobutyl ketone, ethanol, 5-methyl-2-heptanone, and S-(-)-2-methylbutylamine) were determined to be the predominant antifungal VOCs. To the best of our knowledge, this study is the first to specify causative antifungal VOCs using such an approach.

Biocontrol activity of volatile organic compounds from Streptomyces alboflavus TD-1 against Aspergillus flavus growth and aflatoxin production

Aspergillus flavus is a saprophytic fungus that contaminates crops with carcinogenic aflatoxin. In the present work, the antifungal effects of volatile organic compounds (VOCs) from Streptomyces alboflavus TD-1 against A. flavus were investigated. VOCs from 8-day-old wheat bran culture of S. alboflavus TD-1 displayed strong inhibitory effects against mycelial growth, sporulation, and conidial germination of A. flavus. Severely misshapen conidia and hyphae of A. flavus were observed by scanning electron microscopy after exposure to VOCs for 6 and 12 h, respectively. Rhodamine 123 staining of mitochondria indicated that mitochondria may be a legitimate antifungal target of the VOCs from S. alboflavus TD-1. Furthermore, the VOCs effectively inhibited aflatoxin B1 production by downregulating genes involved in aflatoxin biosynthesis. Dimethyl trisulfide and benzenamine may play important roles in the suppression of A. flavus growth and production of aflatoxin. The results indicate that VOCs from S. alboflavus TD-1 have tremendous potential to be developed as a useful bio-pesticide for controlling A. flavus.

Volatile organic compounds of Bacillus atrophaeus HAB-5 inhibit the growth of Colletotrichum gloeosporioides

The use of fungicides to control plant diseases creates a potential health risk. One alternative to this problem is the biological control, which has been succesfully applied to control plant diseases. Bacillus atrophaeus HAB-5 exhibits a high inhibitory acitivities against different fungal pathogens and suppresses them. The aim of current studies is to produce and identify the antifungal compounds produced by the strain HAB-5. We found that the submerge fermentation harvested from Luria-Bertani (LB) medium had the highest activity against Colletotrichum gloeosporioides. The petroleum ether crude extract was strongly bioactive and its activity was stable after heat treatment, pH treatment, illuminated light as well as ultra violet exposition. The antifungal compounds were purified using gel chromatography column. Based on Gas Chromatography-Mass Spectrometry (GC-MS) analysis, nineteen different volatile organic compounds (VOCs) were identified included the range of alkanes, alkenes, alcohols, and organics acid. Among these identified compounds, Chloroacetic acid, tetradecyl esters followed by Octadecane and Hexadecanoic acid, methyl ester showed antifungal activity against C. gloeosporioides. Our results clearly showed Chloroacetic acid, tetradecyl esters; Octadecane and Hexadecanoic acid, methyl ester are key inhibitory compounds produced by Bacillus atrophaeus HAB-5 against C. gloeosporioides.

Mangrove derived Streptomyces sp. MUM265 as a potential source of antioxidant and anticolon-cancer agents

Background

Colon cancer is the third most commonly diagnosed cancer worldwide, with a commensurately high mortality rate. The search for novel antioxidants and specific anticancer agents which may inhibit, delay or reverse the development of colon cancer is thus an area of great interest; Streptomyces bacteria have been demonstrated to be a source of such agents.

Results

The extract from Streptomyces sp. MUM265— a strain which was isolated and identified from Kuala Selangor mangrove forest, Selangor, Malaysia— was analyzed and found to exhibit antioxidant properties as demonstrated via metal-chelating ability as well as superoxide anion, DPPH and ABTS radical scavenging activities. This study also showed that MUM265 extract demonstrated cytotoxicity against colon cancer cells as evidenced by the reduced cell viability of Caco-2 cell line. Treatment with MUM265 extract induced depolarization of mitochondrial membrane potential and accumulation of subG₁ cells in cell cycle analysis, suggesting that MUM265 exerted apoptosis-inducing effects on Caco-2 cells.

Conclusion

These findings indicate that mangrove derived Streptomyces sp. MUM265 represents a valuable bioresource of bioactive compounds for the future development of chemopreventive agents, with particular promise suggested for treatment of colon cancer.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1409-7) contains supplementary material, which is available to authorized users.

Inhibitory effects of acetophenone or phenylethyl alcohol as fumigant to protect soybean seeds against two aflatoxin-producing fungi

The antifungal activity of acetophenone and phenylethyl alcohol prevents seed contamination by Aspergillus flavus TISTR 3041 and A. parasiticus TISTR 3276. Their effects on seed germination were investigated. In vitro results showed that 100 µL L^-1 acetophenone completely inhibited (by 100%) growth, conidial germination, and sporulation of the two aflatoxin-producing fungi, while phenylethyl alcohol showed only weak inhibitory activity even at 1000 µL L^-1. Exposure to acetophenone at 100 µL L^-1 for 6 h could completely kill (100% death) both fungal strains, while phenylethyl alcohol showed much lower efficacy (53.12%). In vivo results revealed that fumigation with 100 µL L^-1 acetophenone for 24 h completely controlled (100%) A. flavus TISTR 3041 on soybean seeds during a 14-day test but exhibited weak efficacy on A. parasiticus TISTR 3276 (31.77%). Phenylethyl alcohol (1000 µL L^-1) demonstrated weak inhibitory effect against both strains. The two volatile compounds had no adverse effects on seed germination. SEM confirmed that acetophenone could completely inhibit conidia germination, and abnormal growth of both fungal strains was observed. Thus, acetophenone has high potential to protect soybean seeds against aflatoxin-producing fungi.

Screening and Whole-Genome Sequencing of Two Streptomyces Species from the Rhizosphere Soil of Peony Reveal Their Characteristics as Plant Growth-Promoting Rhizobacteria

Two bacteria, Streptomyces albireticuli MDJK11 and S. alboflavus MDJK44, which are potential plant growth-promoting rhizobacteria against pathogenic fungi were isolated from the rhizosphere soil of peony in Shandong, China. Their biological characteristics and complete genome sequences were reported in this study. The total genome size of MDJK11 was only 8.14 Mb with 6,550 protein-coding genes and a high GC content of 72.8 mol%. The MDJK44 genome comprises a 9.62 Mb chromosome with 72.1 mol% GC content, 7,285 protein-coding genes, and two plasmids. Some gene sequences in these two genomes were analyzed to be heterologously obtained by horizontal transfer. Gene or gene cluster candidates responding to secondary metabolites production, antimicrobial activities, and plant growth-promoting capacities were also analyzed in this paper. The genomic information and biological characteristics will facilitate the understanding and application of S. albireticuli and S. alboflavus species as biocontrol agents in future agriculture.

Evaluation of Antimicrobial, Enzyme Inhibitory, Antioxidant and Cytotoxic Activities of Partially Purified Volatile Metabolites of Marine Streptomyces sp.S2A

In the present study, marine actinobacteria Streptomyces sp.S2A was isolated from the Gulf of Mannar, India. Identification was carried out by 16S rRNA analysis. Bioactive metabolites were extracted by solvent extraction method. The metabolites were assayed for antagonistic activity against bacterial and fungal pathogens, inhibition of α-glucosidase and α-amylase enzymes, antioxidant activity and cytotoxic activity against various cell lines. The actinobacterial extract showed significant antagonistic activity against four gram-positive and two gram-negative pathogens. Excellent reduction in the growth of fungal pathogens was also observed. The minimum inhibitory concentration of the partially purified extract (PPE) was determined as 31.25 μg/mL against Klebsiella pneumoniae, 15.62 μg/mL against Staphylococcus epidermidis, Staphylococcus aureus and Bacillus cereus. The lowest MIC was observed against Micrococcus luteus as 7.8 μg/mL. MIC against fungal pathogens was determined as 62.5 μg/mL against Bipolaris maydis and 15.62 μg/mL against Fusarium moniliforme. The α-glucosidase and α-amylase inhibitory potential of the fractions were carried out by microtiter plate method. IC50 value of active fraction for α-glucosidase and α-amylase inhibition was found to be 21.17 μg/mL and 20.46 μg/mL respectively. The antioxidant activity of partially purified extract (PPE) (DPPH, ABTS, FRAP and Metal chelating activity) were observed and were also found to have significant cytotoxic activity against HT-29, MDA and U-87MG cell lines. The compound analysis was performed using gas chromatography-mass spectrometry (GC-MS) and resulted in three constituents; pyrrolo[1⁻a]pyrazine-1,4-dione,hexahydro-3-(2-methylpropyl)-, being the main component (80%). Overall, the strain possesses a wide spectrum of antimicrobial, enzyme inhibitory, antioxidant and cytotoxic activities which affords the production of significant bioactive metabolites as potential pharmacological agents.

Involvement of Burkholderiaceae and sulfurous volatiles in disease-suppressive soils

Disease-suppressive soils are ecosystems in which plants suffer less from root infections due to the activities of specific microbial consortia. The characteristics of soils suppressive to specific fungal root pathogens are comparable to those of adaptive immunity in animals, as reported by Raaijmakers and Mazzola (Science 352:1392-3, 2016), but the mechanisms and microbial species involved in the soil suppressiveness are largely unknown. Previous taxonomic and metatranscriptome analyses of a soil suppressive to the fungal root pathogen Rhizoctonia solani revealed that members of the Burkholderiaceae family were more abundant and more active in suppressive than in non-suppressive soils. Here, isolation, phylogeny, and soil bioassays revealed a significant disease-suppressive activity for representative isolates of Burkholderia pyrrocinia, Paraburkholderia caledonica, P. graminis, P. hospita, and P. terricola. In vitro antifungal activity was only observed for P. graminis. Comparative genomics and metabolite profiling further showed that the antifungal activity of P. graminis PHS1 was associated with the production of sulfurous volatile compounds encoded by genes not found in the other four genera. Site-directed mutagenesis of two of these genes, encoding a dimethyl sulfoxide reductase and a cysteine desulfurase, resulted in a loss of antifungal activity both in vitro and in situ. These results indicate that specific members of the Burkholderiaceae family contribute to soil suppressiveness via the production of sulfurous volatile compounds.

Bioprospection of actinobacteria derived from freshwater sediments for their potential to produce antimicrobial compounds

BACKGROUND

Actinobacteria from freshwater habitats have been explored less than from other habitats in the search for compounds of pharmaceutical value. This study highlighted the abundance of actinobacteria from freshwater sediments of two rivers and one lake, and the isolates were studied for their ability to produce antimicrobial bioactive compounds.

RESULTS

16S rRNA gene sequencing led to the identification of 84 actinobacterial isolates separated into a common genus (Streptomyces) and eight rare genera (Nocardiopsis, Saccharopolyspora, Rhodococcus, Prauserella, Amycolatopsis, Promicromonospora, Kocuria and Micrococcus). All strains that showed significant inhibition potentials were found against Gram-positive, Gram-negative and yeast pathogens. Further, three biosynthetic genes, polyketide synthases type II (PKS II), nonribosomal peptide synthetases (NRPS) and aminodeoxyisochorismate synthase (phzE), were detected in 38, 71 and 29% of the strains, respectively. Six isolates based on their antimicrobial potentials were selected for the detection and quantification of standard antibiotics using ultra performance liquid chromatography (UPLC-ESI-MS/MS) and volatile organic compounds (VOCs) using gas chromatography mass spectrometry (GC/MS). Four antibiotics (fluconazole, trimethoprim, ketoconazole and rifampicin) and 35 VOCs were quantified and determined from the methanolic crude extract of six selected Streptomyces strains.

CONCLUSION

Infectious diseases still remain one of the leading causes of death globally and bacterial infections caused millions of deaths annually. Culturable actinobacteria associated with freshwater lake and river sediments has the prospects for the production of bioactive secondary metabolites.

Biocontrol Activity of Volatile-Producing Bacillus megaterium and Pseudomonas protegens Against Aspergillus and Penicillium spp. Predominant in Stored Rice Grains: Study II

In our previous studies, Bacillus megaterium KU143, Microbacterium testaceum KU313, and Pseudomonas protegens AS15 have been shown to be antagonistic to Aspergillus flavus in stored rice grains. In this study, the biocontrol activities of these strains were evaluated against Aspergillus candidus, Aspergillus fumigatus, Penicillium fellutanum, and Penicillium islandicum, which are predominant in stored rice grains. In vitro and in vivo antifungal activities of the bacterial strains were evaluated against the fungi on media and rice grains, respectively. The antifungal activities of the volatiles produced by the strains against fungal development and population were also tested using I-plates. In in vitro tests, the strains produced secondary metabolites capable of reducing conidial germination, germ-tube elongation, and mycelial growth of all the tested fungi. In in vivo tests, the strains significantly inhibited the fungal growth in rice grains. Additionally, in I-plate tests, strains KU143 and AS15 produced volatiles that significantly inhibited not only mycelial growth, sporulation, and conidial germination of the fungi on media but also fungal populations on rice grains. GC-MS analysis of the volatiles by strains KU143 and AS15 identified 12 and 17 compounds, respectively. Among these, the antifungal compound, 5-methyl-2-phenyl-1H-indole, was produced by strain KU143 and the antimicrobial compounds, 2-butyl 1-octanal, dimethyl disulfide, 2-isopropyl-5-methyl-1-heptanol, and 4-trifluoroacetoxyhexadecane, were produced by strain AS15. These results suggest that the tested strains producing extracellular metabolites and/or volatiles may have a broad spectrum of antifungal activities against the grain fungi. In particular, B. megaterium KU143 and P. protegens AS15 may be potential biocontrol agents against Aspergillus and Penicillium spp. during rice grain storage.

Biomimicry of volatile-based microbial control for managing emerging fungal pathogens

Volatile organic compounds (VOCs) are known to be produced by a wide range of micro-organisms and for a number of purposes. Volatile-based microbial inhibition in environments such as soil is well-founded, with numerous antimicrobial VOCs having been identified. Inhibitory VOCs are of interest as microbial control agents, as low concentrations of gaseous VOCs can elicit significant antimicrobial effects. Volatile organic compounds are organic chemicals typically characterized as having low molecular weight, low solubility in water, and high vapour pressure. Consequently, VOCs readily evaporate to the gaseous phase at standard temperature and pressure. This contact-independent antagonism presents unique advantages over traditional, contact-dependent microbial control methods, including increased surface exposure and reduced environmental persistence. This approach has been the focus of our recent research, with positive results suggesting it may be particularly promising for the management of emerging fungal pathogens, such as the causative agents of white-nose syndrome of bats and snake fungal disease, which are difficult or impossible to treat using traditional approaches. Here, we review the history of volatile-based microbial control, discuss recent progress in formulations that mimic naturally antagonistic VOCs, outline the development of a novel treatment device, and highlight areas where further work is needed to successfully deploy VOCs against existing and emerging fungal pathogens.

Growth Promotion and Disease Suppression Ability of a Streptomyces sp. CB-75 from Banana Rhizosphere Soil

An actinomycete strain, CB-75, was isolated from the soil of a diseased banana plantation in Hainan, China. Based on phenotypic and molecular characteristics, and 99.93% sequence similarity with Streptomyces spectabilis NBRC 13424 (AB184393), the strain was identified as Streptomyces sp. This strain exhibited broad-spectrum antifungal activity against 11 plant pathogenic fungi. Type I polyketide synthase (PKS-I) and non-ribosomal peptide synthetase (NRPS) were detected, which were indicative of the antifungal compounds that Streptomyces sp. CB-75 could produce. An ethyl acetate extract from the strain exhibited the lowest minimum inhibitory concentration (MIC) against Colletotrichum musae (ATCC 96167) (0.78 μg/ml) and yielded the highest antifungal activity against Colletotrichum gloeosporioides (ATCC 16330) (50.0 μg/ml). Also, spore germination was significantly inhibited by the crude extract. After treatment with the crude extract of Streptomyces sp. CB-75 at the concentration 2 × MIC, the pathogenic fungi showed deformation, shrinkage, collapse, and tortuosity when observed by scanning electron microscopy (SEM). By gas chromatography-mass spectrometry (GC-MS) of the crude extract, 18 chemical constituents were identified; (Z)-13-docosenamide was the major constituent. Pot experiments showed that the incidence of banana seedlings was reduced after using Streptomyces sp. CB-75 treatment. The disease index was 10.23, and the prevention and control effect was 83.12%. Furthermore, Streptomyces sp. CB-75 had a growth-promoting effect on banana plants. The chlorophyll content showed 88.24% improvement, the leaf area, root length, root diameter, plant height, and stem showed 88.24, 90.49, 136.17, 61.78, and 50.98% improvement, respectively, and the shoot fresh weight, root fresh weight, shoot dry weight, and root dry weight showed 82.38, 72.01, 195.33, and 113.33% improvement, respectively, compared with treatment of fermentation broth without Streptomyces sp. CB-75. Thus, Streptomyces sp. CB-75 is an important microbial resource as a biological control against plant pathogenic fungi and for promoting banana growth.

Plant growth and resistance promoted by Streptomyces spp. in tomato

Plant Growth Promoting Rhizobacteria (PGPR) represent an alternative to improve plant growth and yield as well as to act as agents of biocontrol. This study characterized isolates of Streptomyces spp. (Stm) as PGPR, determined the antagonism of these isolates against Pectobacterium carotovorum subsp. brasiliensis (Pcb), evaluated the ability of Stm on promoting growth and modulating the defense-related metabolism of tomato plants, and the potential of Stm isolates on reducing soft rot disease in this species. The VOC profile of Stm was also verified. Promotion of plant growth was assessed indirectly through VOC emission and by direct interaction with Stm isolates in the roots. Evaluation of soft rot disease was performed in vitro on plants treated with Stm and challenged with Pcb. Enzymes related to plant defense were then analyzed in plants treated with three selected isolates of Stm, and PM1 was chosen for further Pcb-challenging experiment. Streptomyces spp. isolates displayed characteristics of PGPR. PM3 was the isolate with efficient antagonism against Pcb by dual-culture. Most of the isolates promoted growth of root and shoot of tomato plants by VOC, and PM5 was the isolate that most promoted growth by direct interaction with Stm. Soft rot disease and mortality of plants were significantly reduced when plants were treated with StmPM1. Modulation of secondary metabolism was observed with Stm treatment, and fast response of polyphenoloxidases was detected in plants pretreated with StmPM1 and challenged with Pcb. Peroxidase was significantly activated three days after infection with Pcb in plants pretreated with StmPM1. Results suggest that Streptomyces sp. PM1 and PM5 have the potential to act as PGPR.

Streptomyces sp. MUM212 as a Source of Antioxidants with Radical Scavenging and Metal Chelating Properties

Reactive oxygen species and other radicals potentially cause oxidative damage to proteins, lipids, and DNA which may ultimately lead to various complications including mutations, carcinogenesis, neurodegeneration, cardiovascular disease, aging, and inflammatory disease. Recent reports demonstrate that Streptomyces bacteria produce metabolites with potent antioxidant activity that may be developed into therapeutic drugs to combat oxidative stress. This study shows that Streptomyces sp. MUM212 which was isolated from mangrove soil in Kuala Selangor, Malaysia, could be a potential source of antioxidants. Strain MUM212 was characterized and determined as belonging to the genus Streptomyces using 16S rRNA gene phylogenetic analysis. The MUM212 extract demonstrated significant antioxidant activity through DPPH, ABTS and superoxide radical scavenging assays and also metal-chelating activity of 22.03 ± 3.01%, 61.52 ± 3.13%, 37.47 ± 1.79%, and 41.98 ± 0.73% at 4 mg/mL, respectively. Moreover, MUM212 extract was demonstrated to inhibit lipid peroxidation up to 16.72 ± 2.64% at 4 mg/mL and restore survival of Vero cells from H₂O₂-induced oxidative damages. The antioxidant activities from the MUM212 extract correlated well with its total phenolic contents; and this in turn was in keeping with the gas chromatography-mass spectrometry analysis which revealed the presence of phenolic compounds that could be responsible for the antioxidant properties of the extract. Other chemical constituents detected included hydrocarbons, alcohols and cyclic dipeptides which may have contributed to the overall antioxidant capacity of MUM212 extract. As a whole, strain MUM212 seems to have potential as a promising source of novel molecules for future development of antioxidative therapeutic agents against oxidative stress-related diseases.

Production of Potent Antimicrobial Compounds from Streptomyces cyaneofuscatus Associated with Fresh Water Sediment

The genus Streptomyces under phylum actinobacteria has been recognized as a prolific source for the production of bioactive secondary metabolites. An actinobacterial strain designated as DST103 isolated from a wetland fresh water sediment of Tamdil Lake, Mizoram, Northeast, India was identified as Streptomyces cyaneofuscatus (KY287599) using 16SrRNA gene sequencing which shares 99.87% sequence similarity with Streptomyces cyaneofuscatus NRRL B-2570^T. The strain showed broad spectrum antimicrobial activities against Gram negative bacteria (Escherichia coli MTCC 739 and Pseudomonas aeruginosa MTCC 2453), Gram positive bacteria (Micrococcus luteus NCIM 2170 and Staphylococcus aureus MTCC 96) and yeast pathogen Candida albicans MTCC 3017). The methanolic extract of the strain DST103 exhibited highest antimicrobial activity against E. coli (IC₅₀ = 2.10 μg/mL) and minimum activity against S. aureus (IC₅₀ = 43.63 μg/mL). Five antibiotics [trimethoprim (18 μg/g), fluconazole (6 μg/g), ketoconazole (18 μg/g), nalidixic acid (135 μg/g), and rifampicin (56 μg/g)] were detected and quantified using ultra-performance liquid chromatography (UPLC-ESI-MS/MS). Further, biosynthetic potential genes [polyketide synthases type II, non-ribosomal peptide synthetases, and aminodeoxyisochorismate synthase (phzE)] were also detected in strain DST103 which may possibly be responsible for the production of antimicrobial compounds. Additionally, gas chromatography-mass spectrometry analysis showed the presence of four volatile compounds which might be responsible for their diverse biological activity. The present study revealed the presence of bioactive compounds in strain DST103, which may be a promising resource for the discovery of novel bioactive metabolites against wide range of pathogens.

Streptomyces exploration is triggered by fungal interactions and volatile signals

It has long been thought that the life cycle of Streptomyces bacteria encompasses three developmental stages: vegetative hyphae, aerial hyphae and spores. Here, we show interactions between Streptomyces and fungi trigger a previously unobserved mode of Streptomyces development. We term these Streptomyces cells 'explorers', for their ability to adopt a non-branching vegetative hyphal conformation and rapidly transverse solid surfaces. Fungi trigger Streptomyces exploratory growth in part by altering the composition of the growth medium, and Streptomyces explorer cells can communicate this exploratory behaviour to other physically separated streptomycetes using an airborne volatile organic compound (VOC). These results reveal that interkingdom interactions can trigger novel developmental behaviours in bacteria, here, causing Streptomyces to deviate from its classically-defined life cycle. Furthermore, this work provides evidence that VOCs can act as long-range communication signals capable of propagating microbial morphological switches.

Streptomyces antioxidans sp. nov., a Novel Mangrove Soil Actinobacterium with Antioxidative and Neuroprotective Potentials

A novel strain, Streptomyces antioxidans MUSC 164^T was recovered from mangrove forest soil located at Tanjung Lumpur, Malaysia. The Gram-positive bacterium forms yellowish-white aerial and brilliant greenish yellow substrate mycelium on ISP 2 agar. A polyphasic approach was used to determine the taxonomy status of strain MUSC 164^T. The strain showed a spectrum of phylogenetic and chemotaxonomic properties consistent with those of the members of the genus Streptomyces. The cell wall peptidoglycan was determined to contain LL-diaminopimelic acid. The predominant menaquinones were identified as MK-9(H₆) and MK-9(H₈), while the identified polar lipids consisted of aminolipid, diphosphatidylglycerol, glycolipid, hydroxyphosphatidylethanolamine, phospholipid, phosphatidylinositol, phosphatidylethanolamine, phosphatidylglycerol and lipid. The cell wall sugars consist of galactose, glucose and ribose. The predominant cellular fatty acids (>10.0%) were identified as iso-C_15:0 (34.8%) and anteiso-C_15:0(14.0%). Phylogenetic analysis identified that closely related strains for MUSC 164^T as Streptomyces javensis NBRC 100777^T (99.6% sequence similarity), Streptomyces yogyakartensis NBRC 100779^T (99.6%) and Streptomyces violaceusniger NBRC 13459^T (99.6%). The DNA–DNA relatedness values between MUSC 164^T and closely related type strains ranged from 23.8 ± 0.3% to 53.1 ± 4.3%. BOX-PCR fingerprints comparison showed that MUSC 164^T exhibits a unique DNA profile, with DNA G + C content determined to be 71.6 mol%. Based on the polyphasic study of MUSC 164^T, it is concluded that this strain represents a novel species, for which the name Streptomyces antioxidans sp. nov. is proposed. The type strain is MUSC 164^T (=DSM 101523^T = MCCC 1K01590^T). The extract of MUSC 164^T showed potent antioxidative and neuroprotective activities against hydrogen peroxide. The chemical analysis of the extract revealed that the strain produces pyrazines and phenolic-related compounds that could explain for the observed bioactivities.

Streptomyces malaysiense sp. nov.: A novel Malaysian mangrove soil actinobacterium with antioxidative activity and cytotoxic potential against human cancer cell lines

Actinobacteria from the unique intertidal ecosystem of the mangroves are known to produce novel, bioactive secondary metabolites. A novel strain known as MUSC 136^T (=DSM 100712^T = MCCC 1K01246^T) which was isolated from Malaysian mangrove forest soil has proven to be no exception. Assessed by a polyphasic approach, its taxonomy showed a range of phylogenetic and chemotaxonomic properties consistent with the genus of Streptomyces. Phylogenetically, highest similarity was to Streptomyces misionensis NBRC 13063^T (99.6%) along with two other strains (>98.9% sequence similarities). The DNA–DNA relatedness between MUSC 136^T and these type strains ranged from 22.7 ± 0.5% to 46.5 ± 0.2%. Overall, polyphasic approach studies indicated this strain represents a novel species, for which the name Streptomyces malaysiense sp. nov. is proposed. The potential bioactivities of this strain were explored by means of antioxidant and cytotoxic assays. Intriguingly, MUSC 136^T exhibited strong antioxidative activities as evaluated by a panel of antioxidant assays. It was also found to possess high cytotoxic effect against HCT-116 cells, which probably mediated through altering p53 protein and intracellular glutathione levels. Chemical analysis of the extract using GC-MS further affirms that the strain produces chemopreventive related metabolites.

Volatiles in Inter-Specific Bacterial Interactions

The importance of volatile organic compounds for functioning of microbes is receiving increased research attention. However, to date very little is known on how inter-specific bacterial interactions effect volatiles production as most studies have been focused on volatiles produced by monocultures of well-described bacterial genera. In this study we aimed to understand how inter-specific bacterial interactions affect the composition, production and activity of volatiles. Four phylogenetically different bacterial species namely: Chryseobacterium, Dyella, Janthinobacterium, and Tsukamurella were selected. Earlier results had shown that pairwise combinations of these bacteria induced antimicrobial activity in agar media whereas this was not the case for monocultures. In the current study, we examined if these observations were also reflected by the production of antimicrobial volatiles. Thus, the identity and antimicrobial activity of volatiles produced by the bacteria were determined in monoculture as well in pairwise combinations. Antimicrobial activity of the volatiles was assessed against fungal, oomycetal, and bacterial model organisms. Our results revealed that inter-specific bacterial interactions affected volatiles blend composition. Fungi and oomycetes showed high sensitivity to bacterial volatiles whereas the effect of volatiles on bacteria varied between no effects, growth inhibition to growth promotion depending on the volatile blend composition. In total 35 volatile compounds were detected most of which were sulfur-containing compounds. Two commonly produced sulfur-containing volatile compounds (dimethyl disulfide and dimethyl trisulfide) were tested for their effect on three target bacteria. Here, we display the importance of inter-specific interactions on bacterial volatiles production and their antimicrobial activities.

Evaluation of Antioxidative and Cytotoxic Activities of Streptomyces pluripotens MUSC 137 Isolated from Mangrove Soil in Malaysia

Streptomyces pluripotens MUSC 137 was isolated from mangrove soil obtained from Tanjung Lumpur, Pahang, Malaysia. We investigated the phylogenetic, genomic, biochemical, and phenotypic characteristics of this strain. Uniquely adapted microorganisms from mangrove habitats have previously yielded compounds of biopharmaceutical interest. In order to examine the bioactivities possessed by the strain, fermentation extract was prepared through solvent extraction method prior to bioactivities screenings. Antioxidant activity was examined via DPPH assay while the cytotoxic effect was assessed by means of examining the activity of the extract against selected human cancer cell lines, namely colon cancer cells (HCT-116, Caco-2, SW480, and HT-29), breast cancer cell (MCF-7), lung cancer cell (A549), prostate cancer cell (DU145), and cervical cancer cell (Ca Ski). The results revealed MUSC 137 possesses significant antioxidant activity and demonstrates cytotoxic effect against several cancer cell lines tested. The results indicated MCF-7 cells were most susceptible to the extract with the lowest IC₅₀ (61.33 ± 17.10 μg/mL), followed by HCT-116 and A549. Additionally, selective index (SI) showed that MUSC 137 extract was less toxic against normal cell lines when compared to MCF-7 and HCT-116 cells. The extract was further subjected to chemical analysis using GC–MS and revealed the presence of deferoxamine and pyrrolizidines related compounds which may account for the antioxidant and cytotoxic properties observed.

Bioactivity of volatile organic compounds produced by Pseudomonas tolaasii

Pseudomonas tolaasii is the main bacterial pathogen of several mushroom species. In this paper we report that strains of P. tolaasii produce volatile substances inducing in vitro mycelia growth inhibition of Pleurotus ostreatus and P. eryngii, and Agaricus bisporus and P. ostreatus basidiome tissue blocks brown discoloration. P. tolaasii strains produced the volatile ammonia but not hydrogen cyanide. Among the volatiles detected by GC-MS, methanethiol, dimethyl disulfide (DMDS), and 1-undecene were identified. The latter, when assayed individually as pure compounds, led to similar effects noticed when P. tolaasii volatiles natural blend was used on mushrooms mycelia and basidiome tissue blocks. Furthermore, the natural volatile mixture resulted toxic toward lettuce and broccoli seedling growth. In contrast, pure volatiles showed different activity according to their nature and/or doses applied. Indeed, methanethiol resulted toxic at all the doses used, while DMDS toxicity was assessed till a quantity of 1.25 μg, below which it caused, together with 1-undecene (≥10 μg), broccoli growth increase.

Presence of antioxidative agent, Pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro- in newly isolated Streptomyces mangrovisoli sp. nov

A novel Streptomyces, strain MUSC 149(T) was isolated from mangrove soil. A polyphasic approach was used to study the taxonomy of MUSC 149(T), which shows a range of phylogenetic and chemotaxonomic properties consistent with those of the members of the genus Streptomyces. The diamino acid of the cell wall peptidoglycan was LL-diaminopimelic acid. The predominant menaquinones were identified as MK9(H8) and MK9(H6). Phylogenetic analysis indicated that closely related strains include Streptomyces rhizophilus NBRC 108885(T) (99.2% sequence similarity), S. gramineus NBRC 107863(T) (98.7%) and S. graminisoli NBRC 108883(T) (98.5%). The DNA-DNA relatedness values between MUSC 149(T) and closely related type strains ranged from 12.4 ± 3.3% to 27.3 ± 1.9%. The DNA G + C content was determined to be 72.7 mol%. The extract of MUSC 149(T) exhibited strong antioxidant activity and chemical analysis reported identification of an antioxidant agent, Pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-. These data showed that metabolites of MUSC 149(T) shall be useful as preventive agent against free-radical associated diseases. Based on the polyphasic study of MUSC 149(T), the strain merits assignment to a novel species, for which the name S. mangrovisoli sp. nov. is proposed. The type strain is MUSC 149(T) (=MCCC 1K00699(T)=DSM 100438(T)).