Functional Comparison of Three Chitinases from Symbiotic Bacteria of Entomopathogenic Nematodes

Xenorhabdus and Photorhabdus, bacterial symbionts of entomopathogenic nematodes Steinernema and Heterorhabditis, respectively, have several biological activities including insecticidal and antimicrobial activities. Thus, XnChi, XhChi, and PtChi, chitinases of X. nematophila, X. hominickii, and P. temperata isolated from Korean indigenous EPNs S. carpocapsae GJ1-2, S. monticolum GJ11-1, and H. megidis GJ1-2 were cloned and expressed in Escherichia coli BL21 to compare their biological activities. Chitinase proteins of these bacterial symbionts purified using the Ni-NTA system showed different chitobiosidase and endochitinase activities, but N-acetylglucosamidinase activities were not shown in the measuring of chitinolytic activity through N-acetyl-D-glucosarmine oligomers. In addition, the proteins showed different insecticidal and antifungal activities. XnChi showed the highest insecticidal activity against Galleria mellonella, followed by PtChi and XhChi. In antifungal activity, XhChi showed the highest half-maximal inhibitory concentration (IC50) against Fusarium oxysporum with 0.031 mg/mL, followed by PtChi with 0.046 mg/mL, and XnChi with 0.072 mg/mL. XhChi also showed the highest IC50 against F. graminearum with 0.040 mg/mL, but XnChi was more toxic than PtChi with 0.055 mg/mL and 0.133 mg/mL, respectively. This study provides an innovative approach to the biological control of insect pests and fungal diseases of plants with the biological activity of symbiotic bacterial chitinases of entomopathogenic nematodes.

HETERORHABDITIS BACTERIOPHORA NEMATODES ARE SENSITIVE TO THE BACTERIAL PATHOGEN PHOTORHABDUS ASYMBIOTICA

The entomopathogenic nematode (EPN) Heterorhabditis bacteriophora infects a wide range of insect hosts with the aid of its mutualistic bacteria Photorhabdus luminescens. While the mutualistic relationship between H. bacteriophora and P. luminescens and the infectivity of the nematode-bacteria complex have been characterized, how nematode fitness is affected by entomopathogenic bacteria existing in association with other EPN species remains poorly understood. In this study, the survival of H. bacteriophora infective juveniles containing or lacking P. luminescens was tested against the entomopathogenic bacteria Xenorhabdus nematophila and Photorhabdus asymbiotica as well as the non-pathogenic Escherichia coli. While X. nematophila and E. coli did not significantly affect the survival of H. bacteriophora, P. asymbiotica exerted a significant effect on nematode survival, particularly on those lacking P. luminescens. These results imply that P. asymbiotica encodes factors that are pathogenic to EPNs. Future efforts will focus on the identification of the bacterial molecular components that induce these effects. This study makes an important contribution to a growing body of research aimed at exploiting the full potential of nematode-bacterial complexes for eliminating noxious insect pests and treating infectious diseases caused by parasitic nematodes.

Comparing Crop Yield, Secondary Metabolite Contents, and Antifungal Activity of Extracts of Helichrysum odoratissimum Cultivated in Aquaponic, Hydroponic, and Field Systems

The overexploitation of wild plants for medicinal uses and conventional agricultural farming methods, which use high amounts of water, fertilizers, and pesticides, have had devastating environmental consequences. This study aimed to evaluate the prospects of using aquaponics and hydroponics as alternative approaches to soil cultivation by comparing the crop yields, secondary metabolite contents, and the antifungal activities of ethanol extracts of Helichrysum odoratissimum (L.) Sweet, a widely used medicinal plant species in Southern Africa. Six-week-old H. odoratissimum seedlings were grown in aquaponic and hydroponic systems. The growth parameters, secondary metabolite contents, and antifungal activity against Fusarium oxysporum were assessed. The results for crop yield (plant height, fresh and dry weights) and the tissue nutrient contents did not change substantially (p > 0.05) between aquaponic and hydroponic treatments. Gas chromatography−mass spectrometry (GC−MS) analysis showed that monoterpenes and sesquiterpenes were the most abundant compounds in H. odoratissimum; however, no statistical difference was observed among the field, hydroponic, and aquaponic plants (DF = 2; χ2 = 2.67; p > 0.05). While there was no significant difference in polyphenol contents among the three treatments, remarkably, the flavonol contents in the leaves varied significantly (DF = 2; χ2 = 6.23; p < 0.05) among the three treatments. A higher flavonol content occurred in leaves from the hydroponic system than in leaves from the aquaponic (p < 0.05) and field (p > 0.05) systems. The MIC results showed that the ethanolic extract of H. odoratissimum was fungistatic against F. oxysporum; however, this effect was more prominent in the ethanol extracts of plants grown in the aquaponic system, with a mean MIC value of 0.37 ± 0.00 mg/mL The key findings of this study are that aquaponically cultivated plants exhibited the best antifungal activity, while higher total flavonol contents occurred in the hydroponically cultivated plants. In conclusion, aquaponics and hydroponics performed better or similar to field cultivation and are viable alternative methods for cultivating H. odoratissimum plants.

Genome analysis of secondary metabolite‑biosynthetic gene clusters of Photorhabdus akhurstii subsp. akhurstii and its antibacterial activity against antibiotic-resistant bacteria

Xenorhabdus and Photorhabdus can produce a variety of secondary metabolites with broad spectrum bioactivity against microorganisms. We investigated the antibacterial activity of Xenorhabdus and Photorhabdus against 15 antibiotic-resistant bacteria strains. Photorhabdus extracts had strong inhibitory the growth of Methicillin-resistant Staphylococcus aureus (MRSA) by disk diffusion. The P. akhurstii s subsp. akhurstii (bNN168.5_TH) extract showed lower minimum inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC). The interaction between either P. akhurstii subsp. akhurstii (bNN141.3_TH) or P. akhurstii subsp. akhurstii (bNN168.5_TH) or P. hainanensis (bNN163.3_TH) extract in combination with oxacillin determined by checkerboard assay exhibited partially synergistic interaction with fractional inhibitory concentration index (FICI) of 0.53. Time-killing assay for P. akhurstii subsp. akhurstii (bNN168.5_TH) extract against S. aureus strain PB36 significantly decreased cell viability from 10⁵ CFU/ml to 10³ CFU/ml within 30 min (P < 0.001, t-test). Transmission electron microscopic investigation elucidated that the bNN168.5_TH extract caused treated S. aureus strain PB36 (MRSA) cell membrane damage. The biosynthetic gene clusters of the bNN168.5_TH contained non-ribosomal peptide synthetase cluster (NRPS), hybrid NRPS-type l polyketide synthase (PKS) and siderophore, which identified potentially interesting bioactive products: xenematide, luminmide, xenortide A-D, luminmycin A, putrebactin/avaroferrin and rhizomide A-C. This study demonstrates that bNN168.5_TH showed antibacterial activity by disrupting bacterial cytoplasmic membrane and the draft genome provided insights into the classes of bioactive products. This also provides a potential approach in developing a novel antibacterial agent.

Heterorhabditis and Photorhabdus Symbiosis: A Natural Mine of Bioactive Compounds

Phylum Nematoda is of great economic importance. It has been a focused area for various research activities in distinct domains across the globe. Among nematodes, there is a group called entomopathogenic nematodes, which has two families that live in symbiotic association with bacteria of genus Xenorhabdus and Photorhabdus, respectively. With the passing years, researchers have isolated a wide array of bioactive compounds from these symbiotically associated nematodes. In this article, we are encapsulating bioactive compounds isolated from members of the family Heterorhabditidae inhabiting Photorhabdus in its gut. Isolated bioactive compounds have shown a wide range of biological activity against deadly pathogens to both plants as well as animals. Some compounds exhibit lethal effects against fungi, bacteria, protozoan, insects, cancerous cell lines, neuroinflammation, etc., with great potency. The main aim of this article is to collect and analyze the importance of nematode and its associated bacteria, isolated secondary metabolites, and their biomedical potential, which can serve as potential leads for further drug discovery.

The Odilorhabdin Antibiotic Biosynthetic Cluster and Acetyltransferase Self-Resistance Locus Are Niche and Species Specific

Antibiotic resistance is an increasing threat to human health. A direct link has been established between antimicrobial self-resistance determinants of antibiotic producers, environmental bacteria, and clinical pathogens. Natural odilorhabdins (ODLs) constitute a new family of 10-mer linear cationic peptide antibiotics inhibiting bacterial translation by binding to the 30S subunit of the ribosome. These bioactive secondary metabolites are produced by entomopathogenic bacterial symbiont Xenorhabdus (Morganellaceae), vectored by the soil-dwelling nematodes. ODL-producing Xenorhabdus nematophila symbionts have mechanisms of self-protection. In this study, we cloned the 44.5-kb odl biosynthetic gene cluster (odl-BGC) of the symbiont by recombineering and showed that the N-acetyltransferase-encoding gene, oatA, is responsible for ODL resistance. In vitro acetylation and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses showed that OatA targeted the side chain amino group of ODL rare amino acids, leading to a loss of translation inhibition and antibacterial properties. Functional, genomic, and phylogenetic analyses of oatA revealed an exclusive cis-link to the odilorhabdin BGC, found only in X. nematophila and a specific phylogenetic clade of Photorhabdus. This work highlights the coevolution of antibiotic production and self-resistance as ancient features of this unique tripartite complex of host-vector-symbiont interactions without odl-BGC dissemination by lateral gene transfer. IMPORTANCE Odilorhabdins (ODLs) constitute a novel antibiotic family with promising properties for treating problematic multidrug-resistant Gram-negative bacterial infections. ODLs are 10-mer linear cationic peptides inhibiting bacterial translation by binding to the small subunit of the ribosome. These natural peptides are produced by Xenorhabdus nematophila, a bacterial symbiont of entomopathogenic nematodes well known to produce large amounts of specialized secondary metabolites. Like other antimicrobial producers, ODL-producing Xenorhabdus nematophila has mechanisms of self-protection. In this study, we cloned the ODL-biosynthetic gene cluster of the symbiont by recombineering and showed that the N-acetyltransferase-encoding gene, oatA, is responsible for ODL resistance. In vitro acetylation and LC-MS/MS analyses showed that OatA targeted the side chain amino group of ODL rare amino acids, leading to a loss of translation inhibition and antibacterial properties. Functional, genomic, and phylogenetic analyses of oatA revealed the coevolution of antibiotic production and self-resistance as ancient feature of this particular niche in soil invertebrates without resistance dissemination.

Overview and future research needs for development of effective biocontrol strategies for management of Bactrocera dorsalis Hendel (Diptera: Tephritidae) in sub-Saharan Africa

Infestation of fruits by native and invasive fruit flies causes significant economic losses. In most cases, incidence of 'regulated' dangerous fruit flies in orchards results in restrictions on export of fruits from such places to international markets. Unfortunately, use of insecticides applied on foliage and fruits does not kill the fruit-to-soil stages of fruit flies. However, diverse biological control agents (BCAs) do so. Thus, prevalence of native and invasive fruit flies in orchards will require that a combination of BCAs is included in integrated pest management (IPM) programmes. In the case of Bactrocera dorsalis Hendel and other economically important fruit flies found in sub-Saharan Africa (SSA), use of classical biocontrol approach involves concomitant releases of two exotic parasitoids (Fopius arisanus Sonan and Diachasmimorpha longicaudata Ashmead). These non-native wasps may have complemented the indigenous parasitoids in combination with application of entomopathogenic fungi (EPFs) and conservation of predatory ants (Oecophylla longinoda Latreille, with O. smaragdina) in fruit fly IPM plans. Consequently, some levels of decline in fruit infestation have been observed. Although interspecific interactions between BCAs against several insect pests have produced varying results, including threatening the survival of other BCAs, the prevalence of B. dorsalis in orchards across SSA requires further research to investigate effects of coalescing biocontrol approaches in IPM strategies. Therefore, future research into combining parasitoids, EPFs and entomopathogenic nematodes, in addition to conservation of predatory ants (O. longinoda, O. smaragdina and others) in IPM plans may improve the effectiveness of currently used strategies for the management of fruit-infesting tephritids. © 2021 Society of Chemical Industry.

CRAGE-CRISPR facilitates rapid activation of secondary metabolite biosynthetic gene clusters in bacteria

With the advent of genome sequencing and mining technologies, secondary metabolite biosynthetic gene clusters (BGCs) within bacterial genomes are becoming easier to predict. For subsequent BGC characterization, clustered regularly interspaced short palindromic repeats (CRISPR) has contributed to knocking out target genes and/or modulating their expression; however, CRISPR is limited to strains for which robust genetic tools are available. Here we present a strategy that combines CRISPR with chassis-independent recombinase-assisted genome engineering (CRAGE), which enables CRISPR systems in diverse bacteria. To demonstrate CRAGE-CRISPR, we select 10 polyketide/non-ribosomal peptide BGCs in Photorhabdus luminescens as models and create their deletion and activation mutants. Subsequent loss- and gain-of-function studies confirm 22 secondary metabolites associated with the BGCs, including a metabolite from a previously uncharacterized BGC. These results demonstrate that the CRAGE-CRISPR system is a simple yet powerful approach to rapidly perturb expression of defined BGCs and to profile genotype-phenotype relationships in bacteria.

Photorhabdus spp.: An Overview of the Beneficial Aspects of Mutualistic Bacteria of Insecticidal Nematodes

The current approaches to sustainable agricultural development aspire to use safer means to control pests and pathogens. Photorhabdus bacteria that are insecticidal symbionts of entomopathogenic nematodes in the genus Heterorhabditis can provide such a service with a treasure trove of insecticidal compounds and an ability to cope with the insect immune system. This review highlights the need of Photorhabdus-derived insecticidal, fungicidal, pharmaceutical, parasiticidal, antimicrobial, and toxic materials to fit into current, or emerging, holistic strategies, mainly for managing plant pests and pathogens. The widespread use of these bacteria, however, has been slow, due to cost, natural presence within the uneven distribution of their nematode partners, and problems with trait stability during in vitro culture. Yet, progress has been made, showing an ability to overcome these obstacles via offering affordable mass production and mastered genome sequencing, while detecting more of their beneficial bacterial species/strains. Their high pathogenicity to a wide range of arthropods, efficiency against diseases, and versatility, suggest future promising industrial products. The many useful properties of these bacteria can facilitate their integration with other pest/disease management tactics for crop protection.

Susceptibility of Various Developmental Stages of the Fall Armyworm, Spodoptera frugiperda, to Entomopathogenic Nematodes

Simple Summary

The fall armyworm, Spodoptera frugiperda, native to Central and South America, has recently invaded Africa and Asia, causing serious economic damage to various crops. The chemical control of fall armyworm is not effective due to the development of pesticide resistance as well as environmental contamination. Alternatively, biological control using natural enemies can be used successfully in a sustainable way. Entomopathogenic nematodes are soil-dwelling natural enemies for many underground pest insects including lepidopteran caterpillars. This study evaluates the virulence of several entomopathogenic nematodes on different developmental stages of fall armyworm. We found that all the larval and pupal stages of fall armyworm were highly susceptible to the entomopathogenic nematodes. Our study provides important information of entomopathogenic nematodes for the practical application of biological control of fall armyworm.

Abstract

The fall armyworm, Spodoptera frugiperda, which is native to Central and South America, has recently invaded Africa and Asia, causing serious damage to various crops. Although management to date has been largely unsuccessful, entomopathogenic nematodes (EPNs) are a potential biological control agent that could be used to control the late larval and pupal stages of S. frugiperda that dwell under the ground. Here, we compared the virulence of seven EPNs against larval and pupal stages of S. frugiperda. In a Petri dish assay, both Heterorhabditis indica and Steinernema carpocapsae were highly virulent against younger larvae, whereas S. arenarium and S. longicaudum were highly virulent against older larvae. In contrast, H. bacteriophora, Heterorhabditis sp., and S. kushidai showed low virulence against all larval stages. In soil column and pot assays, H. indica, S. carpocapsae, and S. longicaudum were highly virulent against late larval and pupal stages compared with the other EPN species. Thus, H. indica, S. carpocapsae, and S. longicaudum are recommended for the biological control of S. frugiperda. Our study provides important information of EPNs for the practical application of biological control of fall armyworm.

Tapinarof in the treatment of psoriasis: A review of the unique mechanism of action of a novel therapeutic aryl hydrocarbon receptor-modulating agent

Tapinarof, a novel, first-in-class, small-molecule topical therapeutic aryl hydrocarbon receptor (AhR)-modulating agent, is in clinical development for the treatment of psoriasis and atopic dermatitis. The efficacy of tapinarof in psoriasis is attributed to its specific binding and activation of AhR, a ligand-dependent transcription factor, leading to the downregulation of proinflammatory cytokines, including interleukin 17, and regulation of skin barrier protein expression to promote skin barrier normalization. AhR signaling regulates gene expression in immune cells and skin cells and has critical roles in the regulation of skin homeostasis. Tapinarof-mediated AhR signaling underlies the mechanistic basis for the significant efficacy and acceptable tolerability observed in early-phase clinical trials of tapinarof cream in the treatment of psoriasis.

Xenorhabdus bovienii strain jolietti uses a type 6 secretion system to kill closely related Xenorhabdus strains

Xenorhabdus bovienii strain jolietti (XBJ) is a Gram-negative bacterium that interacts with several organisms as a part of its life cycle. It is a beneficial symbiont of nematodes, a potent pathogen of a wide range of soil-dwelling insects and also has the ability to kill soil- and insect-associated microbes. Entomopathogenic Steinernema nematodes vector XBJ into insects, releasing the bacteria into the insect body cavity. There, XBJ produce a variety of insecticidal toxins and antimicrobials. XBJ's genome also encodes two separate Type Six Secretion Systems (T6SSs), structures that allow bacteria to inject specific proteins directly into other cells, but their roles in the XBJ life cycle are mostly unknown. To probe the function of these T6SSs, we generated mutant strains lacking the key structural protein Hcp from each T6SS and assessed phenotypes related to different parts of XBJ's life cycle. Here we demonstrate that one of the T6SSs is more highly expressed in in vitro growth conditions and has antibacterial activity against other Xenorhabdus strains, and that the two T6SSs have a redundant role in biofilm formation.

Evolution and taxonomy of nematode-associated entomopathogenic bacteria of the genera Xenorhabdus and Photorhabdus: an overview

Entomopathogenic bacteria from the genera Photorhabdus and Xenorhabdus are closely related Gram-negative bacilli from the family Enterobacteriaceae (γ-Proteobacteria). They establish obligate mutualistic associations with soil nematodes from the genera Steinernema and Heterorhabditis to facilitate insect pathogenesis. The research of these two bacterial genera is focused mainly on their unique interactions with two different animal hosts, i.e. nematodes and insects. So far, studies of the mutualistic bacteria of nematodes collected from around the world have contributed to an increase in the number of the described Xenorhabdus and Photorhabdus species. Recently, the classification system of entomopatogenic nematode microsymbionts has undergone profound revision and now 26 species of the genus Xenorhabdus and 19 species of the genus Photorhabdus have been identified. Despite their similar life style and close phylogenetic origin, Photorhabdus and Xenorhabdus bacterial species differ significantly in e.g. the nematode host range, symbiotic strategies for parasite success, and arrays of released antibiotics and insecticidal toxins. As the knowledge of the diversity of entomopathogenic nematode microsymbionts helps to enable the use thereof, assessment of the phylogenetic relationships of these astounding bacterial genera is now a major challenge for researchers. The present article summarizes the main information on the taxonomy and evolutionary history of Xenorhabdus and Photorhabdus, entomopathogenic nematode symbionts.

CRAGE enables rapid activation of biosynthetic gene clusters in undomesticated bacteria

It is generally believed that exchange of secondary metabolite biosynthetic gene clusters (BGCs) among closely related bacteria is an important driver of BGC evolution and diversification. Applying this idea may help researchers efficiently connect many BGCs to their products and characterize the products' roles in various environments. However, existing genetic tools support only a small fraction of these efforts. Here, we present the development of chassis-independent recombinase-assisted genome engineering (CRAGE), which enables single-step integration of large, complex BGC constructs directly into the chromosomes of diverse bacteria with high accuracy and efficiency. To demonstrate the efficacy of CRAGE, we expressed three known and six previously identified but experimentally elusive non-ribosomal peptide synthetase (NRPS) and NRPS-polyketide synthase (PKS) hybrid BGCs from Photorhabdus luminescens in 25 diverse γ-Proteobacteria species. Successful activation of six BGCs identified 22 products for which diversity and yield were greater when the BGCs were expressed in strains closely related to the native strain than when they were expressed in either native or more distantly related strains. Activation of these BGCs demonstrates the feasibility of exploiting their underlying catalytic activity and plasticity, and provides evidence that systematic approaches based on CRAGE will be useful for discovering and identifying previously uncharacterized metabolites.

Isolation of Klebsiella pneumoniae and Pseudomonas aeruginosa from entomopathogenic nematode-insect host relationship to examine bacterial pathogenicity on Trichoplusia ni

Klebsiella pneumoniae was isolated from infected pupae of Galleria mellonella and Pseudomonas aeruginosa was isolated from the entomopathogenic nematode Heterorhabditis bacteriophora hosted within the pupae of G. mellonella. Insect consumption and surface application of P. aeruginosa resulted in 83.33% and 81.66% mortality of Trichoplusia ni larvae, respectively. In contrast, 50% mortality was shown when T. ni larvae were fed with K. pneumoniae, and no larvae were killed when applying the bacterium to the larval cuticle. This report shows that two opportunistic human pathogens found in the insect-nematode ecosystem could kill insect pests.

Secretion Systems and Secreted Proteins in Gram-Negative Entomopathogenic Bacteria: Their Roles in Insect Virulence and Beyond

Many Gram-negative bacteria have evolved insect pathogenic lifestyles. In all cases, the ability to cause disease in insects involves specific bacterial proteins exported either to the surface, the extracellular environment, or the cytoplasm of the host cell. They also have several distinct mechanisms for secreting such proteins. In this review, we summarize the major protein secretion systems and discuss examples of secreted proteins that contribute to the virulence of a variety of Gram-negative entomopathogenic bacteria, including Photorhabdus, Xenorhabdus, Serratia, Yersinia, and Pseudomonas species. We also briefly summarize two classes of exported protein complexes, the PVC-like elements, and the Tc toxin complexes that were first described in entomopathogenic bacteria.

Responses of Anastrepha suspensa, Diachasmimorpha longicaudata, and Sensitivity of Guava Production to Heterorhabditis bacteriophora in Fruit Fly Integrated Pest Management

Caribbean fruit fly, also known as Caribfly or Anastrepha suspensa , is a major tephritid pest of guavas. A virulent entomopathogenic nematode (EPN) species was investigated to suppress the fruit-to-soil stages of Caribflies, which are also attacked by the koinobiont parasitoid Diachasmimorpha longicaudata in south Florida. The main objective was to develop a feasible and cost-effective EPN-application method for integrated pest management (IPM) of Caribfly to improve guava production. Naturally infested guavas were treated with increasing Heterorhabditis bacteriophora infective juvenile (IJ) concentration or rate (0, 25, 50, …, 1,600 IJs cm -2 ) in field trials to measure the optimum IJ rate and then examine sensitivity of producing guavas to inclusion of Heterorhabditis bacteriophora in Caribfly IPM plans. Relative survival of Caribfly in treatments significantly decreased with increasing IJ rate from 0 to 100 IJs cm -2 . Similarly, probability of observing large numbers of parasitoid wasps ( Diachasmimorpha longicaudata ) in EPN treatments significantly declined with increasing IJ rate (0-100 IJs cm -2 ), even though the non-target effects of Heterorhabditis bacteriophora on relative survival of Diachasmimorpha longicaudata could not be determined because of few emerging parasitoid wasps. Optimum suppression (⩾ 60%) of Caribfly was consistently achieved at 100 IJs cm -2 or 17,500 IJs fruit -1 . Profitability analysis showed that Heterorhabditis bacteriophora can be included in Caribfly IPM tactics to produce guavas. Costs of EPNs in Caribfly IPM are minimized if Heterorhabditis bacteriophora is strategically applied by spot treatment of fruit. Repayment of costs of EPN-augmentation by spot treatments appears achievable by recovering 5.71% of the annual yield losses (⩾1,963 kg ha -1 ≈ US$ 8,650 ha -1 ), which are largely due to Caribfly infestation. Hectare-wide EPN-augmentation (or broadcasting) method requires more fruit recovery than the total annual yield losses to repay its high costs. Profitability of guava production in south Florida will not be very sensitive to marginal costs of the spot treatment method, when compared to the field-wide broadcasting of Heterorhabditis bacteriophora .