Insects: True Pioneers in Anti-Infective Therapy and What We Can Learn from Them

Discovery and Biosynthesis of Glycosylated Cycloheximide from a Millipede-Associated Actinomycete

Chemical redundancy of microbial natural products (NPs) underscores the importance to exploit new resources of microorganisms. Insect-associated microbes are prolific but largely underexplored sources of diverse NPs. Herein, we discovered the new compound α-l-rhamnosyl-actiphenol (1) from a millipede-associated Streptomyces sp. ML6, which is the first glycosylated cycloheximide-class natural product. Interestingly, bioinformatics analysis of the ML6 genome revealed that the biosynthesis of 1 involves a cooperation between two gene clusters (chx and rml) located distantly on the genome of ML6. We also carried out in vitro enzymatic glycosylation of cycloheximide using an exotic promiscuous glycosyltransferase BsGT-1, which resulted in the production of an additional cycloheximide glycoside cycloheximide 7-O-β-d-glucoside (5). Although the antifungal and cytotoxic activities of the new compounds 1 and 5 were attenuated relative to those of cycloheximide, our work not only enriches the chemical repertoire of the cycloheximide family but also provides new insights into the structure-activity relationship optimization and ecological roles of cycloheximide.

Antibiotics from Insect-Associated Actinobacteria

Actinobacteria are involved into multilateral relationships between insects, their food sources, infectious agents, etc. Antibiotics and related natural products play an essential role in such systems. The literature from the January 2016-August 2022 period devoted to insect-associated actinomycetes with antagonistic and/or enzyme-inhibiting activity was selected. Recent progress in multidisciplinary studies of insect-actinobacterial interactions mediated by antibiotics is summarized and discussed.

Identification of Volatile Organic Compounds Produced by Xenorhabdus indica Strain AB and Investigation of Their Antifungal Activities

Xenorhabdus spp. are symbiotic bacteria associated with entomopathogenic nematodes to form a model complex that is used for the biological control of insect pests. These bacteria also produce secondary metabolites that have commercial potential in the pharmaceutical and agroforestry industries. Volatile organic compounds (VOCs) produced by the Xenorhabdus indica "strain AB" have been shown to have significant antifungal activity against Fusarium oxysporum f. sp. cucumerinum. Using gas chromatography-mass spectrometry, we identified 61 volatiles in the mixture of VOCs emitted by strain AB compared to a control strain, 6 of which were investigated for their antifungal activities. Of these, methyl anthranilate exhibited the highest mycelial growth suppression toward F. oxysporum, with a minimum inhibitory volume (MIV) of 50 μL/plate. Fluorescence assays, scanning electron microscopy, and measurements of the leakage of intracellular components revealed that the use of methyl anthranilate changed cell wall and cell membrane integrity as well as the permeability of the plasma membrane. Furthermore, methyl anthranilate treatment upregulated the transcription level of target genes related to redox reactions and the cell wall integrity pathway. The results suggest a novel mechanism used by Xenorhabdus spp. to overcome competitors during its life cycle and open up a new approach to using these bacteria in biological control. IMPORTANCE Fungal phytopathogens, particularly Fusarium oxysporum, are a major problem worldwide, especially in the postharvest of vital economic crops. Concerns about negative effects on the environment and human health have led to increasing restrictions on the use of chemical fungicides, and therefore, biological control agents are now being considered alternatives. It is in this context that we investigated the antifungal activity of VOCs produced by X. indica strain AB against F. oxysporum. We found that AB VOCs have a strong effect on the growth of the fungal phytopathogen. In addition, 85% of the identified volatile compounds were determined to be new compounds, opening up new lines of research to discover their properties, effects, and potential for pharmaceutical and agricultural applications. Antifungal assays proved that four of the six compounds with a high concentration in the GC-MS profile had a significant inhibitory effect on pathogen growth. Accordingly, this study opens up a new approach for the use of these bacteria in biocontrol.

The bacterial and fungal nest microbiomes in populations of the social spider Stegodyphus dumicola

Social spiders of the species Stegodyphus dumicola live in communal nests with hundreds of individuals and are characterized by extremely low species-wide genetic diversity. The lack of genetic diversity in combination with group living imposes a potential threat for infection by pathogens. We therefore proposed that specific microbial symbionts inhabiting the spider nests may provide antimicrobial defense. To compare the bacterial and fungal diversity in 17 nests from three different locations in Namibia, we used 16S rRNA gene and internal transcribed spacer (ITS2) sequencing. The nest microbiomes differed between geographically distinct spider populations and appeared largely determined by the local environment. Nevertheless, we identified a core microbiome consisting of four bacterial genera (Curtobacterium, Modestobacter, Sphingomonas, Massilia) and four fungal genera (Aureobasidium, Didymella, Alternaria, Ascochyta), which likely are selected from surrounding soil and plants by the nest environment. We did not find indications for a strain- or species-specific symbiosis in the nests. Isolation of bacteria and fungi from nest material retrieved a few bacterial strains with antimicrobial activity but a number of antimicrobial fungi, including members of the fungal core microbiome. The significance of antimicrobial taxa in the nest microbiome for host protection remains to be shown.

Azetidine-Bearing Non-Ribosomal Peptides, Bonnevillamides D and E, Isolated from a Carrion Beetle-Associated Actinomycete

Two new nonribosomal peptides, bonnevillamides D and E (1 and 2), have been discovered in Streptomyces sp. UTZ13 isolated from the carrion beetle, Nicrophorus concolor. Combinational analysis of the UV, MS, and NMR spectroscopic data revealed that their planar structures were comprised of dichlorinated linear peptides containing nonproteinogenic amino acid residues, such as 4-methylazetidinecarboxylic acid and 4-O-acetyl-5-methylproline. The configurations of bonnevillamides D and E (1 and 2) were determined based on ROESY correlations, the advanced Marfey's method, phenylglycine methyl ester derivatization, molecular modeling, and circular dichroism spectroscopy. The nonribosomal peptide synthetase biosynthetic pathway of bonnevillamides D and E has been proposed using bioinformatic analysis of the whole-genome sequence data of Streptomyces sp. UTZ13. Their biological activity toward the aggregation of amyloid-β, which is one of the key pathogenic proteins in Alzheimer's disease, was evaluated using a thioflavin T assay and gel electrophoresis. Bonnevillamides D and E reversed the fibril formation by inducing the monomerization of amyloid-β aggregates.

Transcriptome Analysis of Myzus persicae to UV-B Stress

As an environmental stress factor, ultraviolet-B (UV-B) radiation directly affects the growth and development of Myzus persicae (Sulzer) (Homoptera: Aphididae). How M. persicae responds to UV-B stress and the molecular mechanisms underlying this adaptation remain unknown. Here, we analyzed transcriptome data for M. persicae following exposure to UV-B radiation for 30 min. We identified 758 significant differentially expressed genes (DEGs) following exposure to UV-B stress, including 423 upregulated and 335 downregulated genes. In addition, enrichment analysis using the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes databases illustrated that these DEGs are associated with antioxidation and detoxification, metabolic and protein turnover, immune response, and stress signal transduction. Simultaneously, these DEGs are closely related to the adaptability to UV-B stress. Our research can raise awareness of the mechanisms of insect responses to UV-B stress.

Antibacterial Potential of Termite-Associated Streptomyces spp

Twenty-one strains of termite-associated actinomycetes were tested for their activities against three bacteria. The results showed that nine strains showed bacteriostatic activities against at least one tested bacterium, and the actinomycete YH01, which was isolated from the body surface of the queen of Odontotermes formosanus, had potent antibacterial activity. The YH01 was further identified as Streptomyces davaonensis. Two metabolites roseoflavin (1) and 8-methylamino-8-demethyl-d-riboflavin (2) were isolated and purified from S. davaonensis YH01. Their structures were determined by NMR, MS, and the related literature. The metabolite 1 showed strong inhibition activities against Bacillus subtilis (MIC = 1.56 μg/mL) and Staphylococcus aureus (MIC = 3.125 μg/mL), which were comparable to referenced gentamycin sulfate, with MIC values of 1.56 and 1.56 μg/mL, respectively. Furthermore, the anti-MRSA potential of compound 1 was determined against nine kinds of MRSA strains, with inhibition zones in the ranges of 12.7-19.7 mm under a concentration of 15 μg/6 mm discs and 18.3-22.7 mm under a concentration of 30 μg/6 mm discs. However, metabolite 1 had no inhibitory effect on Gram-negative bacteria. These results suggested that roseoflavin produced by YH01 holds promise for use against Gram-positive bacteria, especially to MRSA.

Antibacterial and cytotoxic metabolites of termite-associated Streptomyces sp. BYF63

Four anthraquinone derivatives, termstrin A, B, C and D (1-4), were isolated and purified from termite-associated Streptomyces sp. BYF63. Their structures were elucidated on the basis of extensive spectroscopic analyses (HR-ESI-MS, 1D and 2D NMR). Compounds 1 and 4 were found to possess potent antibacterial activities against Staphylococcus aureus, with the zone of inhibition (ZOI) values of 12.85 and 11.17 mm, respectively, which were comparable to that of penicillin sodium with ZOI of 13.15 mm. Furthermore, metabolite 1 showed moderate cytotoxicities against melanoma cell line A375 and gastric cancer cell line MGC-803, with IC₅₀ values of 22.76 and 36.65 μM, respectively, which were less than those of referenced adriamycin.

Digital gene expression profiling in larvae of Tribolium castaneum at different periods post UV-B exposure

UV-B radiation is an important environmental factor. Exposure to excess UV-B radiation can cause serious effects on the development, survival, and reproduction of different organisms. Plants and animals have developed many different strategies to cope with UV-B-induced damage, but the physiological response of insects to UV-B remains unclear. In the present study, the red flour beetle Tribolium castaneum (Herbst) was used to assess the stress response of UV-B. The underlying molecular mechanisms were explored using RNA sequencing. We investigated the transcriptomic profile of T. castaneum larvae at 4 and 24 h after treatment with UV-B radiation via digital gene expression analysis. The 310 and 996 differentially expressed genes were detected at 4 and 24 h, respectively. Then the biological functions and associated metabolic processes of these genes were determined by Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis. The reliability of the data was verified using qRT-PCR. The results indicated that several differentially expressed genes are involved in antioxidation, DNA repair, protein folding, carbon flux diversion, and the extracellular matrix to protect against UV-B-induced damage. This study will increase our understanding of the molecular mechanism underlying insect response to UV-B radiation.

Studies toward the synthesis of macrotermycin C: stereoselective construction of the acyclic skeleton of the aglycon

The first asymmetric synthesis of the acyclic skeleton of the aglycon of macrotermycin C has been achieved in 17 linear steps with 5.7% overall yield following a convergent approach.

Mucorolactone, a Macrolactone from Mucor sp. SNB-VECD13A, a Fungus Isolated from the Cuticle of a Vespidae Species

The newly discovered macrolactone, mucorolactone, along with eight known compounds, was isolated from an ethyl acetate extract of the insect-borne fungus Mucor sp. All structures were elucidated using 1D and 2D NMR and MS spectroscopic experiments. Relative and absolute configurations of the original skeleton of mucorolactone was deduced from NOESY experiments, from the ¹³C NMR chemical shift calculation based on the DP4 probability method, and from the comparison of experimental and calculated electronic circular dichroism spectra.

Nicrophorusamides A and B, Antibacterial Chlorinated Cyclic Peptides from a Gut Bacterium of the Carrion Beetle Nicrophorus concolor

Nicrophorusamides A and B (1 and 2) were discovered from a rare actinomycete, Microbacterium sp., which was isolated from the gut of the carrion beetle Nicrophorus concolor. The structures of the nicrophorusamides were established as new chlorinated cyclic hexapeptides bearing uncommon amino acid units mainly based on 1D and 2D NMR spectroscopic analysis. The absolute configurations of the amino acid residues 5-chloro-l-tryptophan, d-threo-β-hydroxyasparagine/d-asparagine, l-ornithine, l-allo-isoleucine, d-leucine, and d-valine were determined using Marfey's method and chemical derivatization with 2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanate followed by LC/MS analysis. Nicrophorusamide A (1) showed antibacterial activity against several Gram-positive bacteria.

Isolation, Biosynthesis and Chemical Modifications of Rubterolones A-F: Rare Tropolone Alkaloids from Actinomadura sp. 5-2

The discovery of six new, highly substituted tropolone alkaloids, rubterolones A-F, from Actinomadura sp. 5-2, isolated from the gut of the fungus-growing termite Macrotermes natalensis is reported. Rubterolones were identified by using fungus-bacteria challenge assays and a HRMS-based dereplication strategy, and characterised by NMR and HRMS analyses and by X-ray crystallography. Feeding experiments and subsequent chemical derivatisation led to a first library of rubterolone derivatives (A-L). Genome sequencing and comparative analyses revealed their putative biosynthetic pathway, which was supported by feeding experiments. This study highlights how gut microbes can present a prolific source of secondary metabolites.

Macrotermycins A-D, Glycosylated Macrolactams from a Termite-Associated Amycolatopsis sp. M39

Bioassay-guided metabolomic analyses led to the characterization of four new 20-membered glycosylated polyketide macrolactams, macrotermycins A-D, from a termite-associated actinomycete, Amycolatopsis sp. M39. M39's sequenced genome revealed the macrotermycin's putative biosynthetic gene cluster. Macrotermycins A and C had antibacterial activity against human-pathogenic Staphylococcus aureus and, of greater ecological relevance, they also had selective antifungal activity against a fungal parasite of the termite fungal garden.

Biology of Paenibacillus larvae, a deadly pathogen of honey bee larvae

The gram-positive bacterium Paenibacillus larvae is the etiological agent of American Foulbrood of honey bees, a notifiable disease in many countries. Hence, P. larvae can be considered as an entomopathogen of considerable relevance in veterinary medicine. P. larvae is a highly specialized pathogen with only one established host, the honey bee larva. No other natural environment supporting germination and proliferation of P. larvae is known. Over the last decade, tremendous progress in the understanding of P. larvae and its interactions with honey bee larvae at a molecular level has been made. In this review, we will present the recent highlights and developments in P. larvae research and discuss the impact of some of the findings in a broader context to demonstrate what we can learn from studying "exotic" pathogens.

Natural products from microbes associated with insects

Here we review discoveries of secondary metabolites from microbes associated with insects. We mainly focus on natural products, where the ecological role has been at least partially elucidated, and/or the pharmaceutical properties evaluated, and on compounds with unique structural features. We demonstrate that the exploration of specific microbial–host interactions, in combination with multidisciplinary dereplication processes, has emerged as a successful strategy to identify novel chemical entities and to shed light on the ecology and evolution of defensive associations.

Animals in a bacterial world: opportunities for chemical ecology

This Viewpoint article provides a brief and selective summary of research on the chemical ecology underlying symbioses between bacteria and animals. Animals engage in multiple highly specialized interactions with bacteria that reflect their long coevolutionary history. The article focuses on a few illustrative but hardly exhaustive examples in which bacterially produced small molecules initiate a developmental step with important implications for the evolution of animals, provide signals for the maturation of mammalian immune systems, and furnish chemical defenses against microbial pathogens.

Metabolomics in the natural products field--a gateway to novel antibiotics

Metabolomics is a high throughput analytical technique used to globally measure low molecular weight metabolites, allowing simultaneous metabolic comparison of different biological samples and thus highlighting differentially produced compounds as potential biomarkers. Although microbes are renowned as prolific sources of antibiotics, the traditional approach for new anti-infectives discovery is time-consuming and labor-intensive. In this review, the use of NMR- or MS-based metabolomics is proposed as an efficient approach to find antimicrobials in microbial single- or co-cultures.

Natalamycin A, an Ansamycin from a Termite-Associated Streptomyces sp

We report a preliminary functional and complete structural characterization of a highly unusual geldanamycin analog, natalamycin A, that was isolated from Streptomyces strain M56 recovered from a South African nest of Macrotermes natalensis termites. Bioassay-guided fractionation based on antifungal activity led to the isolation of natalamycin A, and a combination of NMR spectroscopy and X-ray crystallographic analysis, including highly-accurate quantum-chemical NMR calculations on the largest and most conformationally-flexible system to date, revealed natalamycin A's three-dimensional solid- and solution-state structure. This structure along with the structures of related compounds isolated from the same source suggest a geldanamycin-like biosynthetic pathway with unusual post-PKS modifications.

Eco-taxonomic insights into actinomycete symbionts of termites for discovery of novel bioactive compounds

Termites play a major role in foraging and degradation of plant biomass as well as cultivating bioactive microorganisms for their defense. Current advances in "omics" sciences are revealing insights into function-related presence of these symbionts, and their related biosynthetic activities and genes identified in gut symbiotic bacteria might offer a significant potential for biotechnology and biodiscovery. Actinomycetes have been the major producers of bioactive compounds with an extraordinary range of biological activities. These metabolites have been in use as anticancer agents, immune suppressants, and most notably, as antibiotics. Insect-associated actinomycetes have also been reported to produce a range of antibiotics such as dentigerumycin and mycangimycin. Advances in genomics targeting a single species of the unculturable microbial members are currently aiding an improved understanding of the symbiotic interrelationships among the gut microorganisms as well as revealing the taxonomical identity and functions of the complex multilayered symbiotic actinofloral layers. If combined with target-directed approaches, these molecular advances can provide guidance towards the design of highly selective culturing methods to generate further information related to the physiology and growth requirements of these bioactive actinomycetes associated with the termite guts. This chapter provides an overview on the termite gut symbiotic actinoflora in the light of current advances in the "omics" science, with examples of their detection and selective isolation from the guts of the Sunshine Coast regional termite Coptotermes lacteus in Queensland, Australia.

Structure and biosynthesis of xenoamicins from entomopathogenic Xenorhabdus

During the search for novel natural products from entomopathogenic Xenorhabdus doucetiae DSM17909 and X. mauleonii DSM17908 novel peptides named xenoamicins were identified in addition to the already known antibiotics xenocoumacin and xenorhabdin. Xenoamicins are acylated tridecadepsipeptides consisting of mainly hydrophobic amino acids. The main derivative xenoamicin A (1) was isolated from X. mauleonii DSM17908, and its structure elucidated by detailed 1D and 2D NMR experiments. Detailed MS experiments, also in combination with labeling experiments, confirmed the determined structure and allowed structure elucidation of additional derivatives. Moreover, the xenoamicin biosynthesis gene cluster was identified and analyzed in X. doucetiae DSM17909, and its participation in xenoamicin biosynthesis was confirmed by mutagenesis. Advanced Marfey's analysis of 1 showed that the absolute configuration of the amino acids is in agreement with the predicted stereochemistry deduced from the nonribosomal peptide synthetase XabABCD. Biological testing revealed activity of 1 against Plasmodium falciparum and other neglected tropical diseases but no antibacterial activity.

Bacterial symbionts in agricultural systems provide a strategic source for antibiotic discovery

As increased antibiotic resistance erodes the efficacy of currently used drugs, the need for new candidates with therapeutic potential grows. Although the majority of antibiotics in clinical use originated from natural products, mostly from environmental actinomycetes, high rediscovery rates, among other factors, have diminished the enthusiasm for continued exploration of this historically important source. Several well-studied insect agricultural systems have bacterial symbionts that have evolved to produce small molecules that suppress environmental pathogens. These molecules represent an underexplored reservoir of potentially useful antibiotics. This report describes the multilateral symbioses common to insect agricultural systems, the general strategy used for antibiotic discovery and pertinent examples from three farming systems: fungus-farming ants, southern pine beetles (SPBs) and fungus-growing termites.

Actinomycetes with antimicrobial activity isolated from paper wasp (Hymenoptera: Vespidae: Polistinae) nests

Actinomycetes-a group of antimicrobial producing bacteria-have been successfully cultured and characterized from the nest material of diverse arthropods. Some are symbionts that produce antimicrobial chemicals found to protect nest brood and resources from pathogenic microbes. Others have no known fitness relationship with their associated insects, but have been found to produce antimicrobials in vitro. Consequently, insect nest material is being investigated as a new source of novel antimicrobial producing actinomycetes, which could be harnessed for therapeutic potential. To extend studies of actinomycete-insect associations beyond soil-substrate dwelling insects and wood boring excavators, we conducted a preliminary assessment of the actinomycetes within the nests of the paper wasp, Polistes dominulus (Christ). We found that actinomycetes were readily cultured from nest material across multiple invasive P. dominulus populations-including members of the genera Streptomyces, Micromonospora, and Actinoplanes. Thirty of these isolates were assayed for antimicrobial activity against the challenge bacteria Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, Serratia marcescens, and Bacillus subtilis. Sixty percent of isolates inhibited the growth of at least one challenge strain. This study provides the first assessment of bacteria associated with nests of P. dominulus, and the first record of antimicrobial producing actinomycetes isolated from social wasps. We provide a new system to explore nest associated actinomycetes from a ubiquitous and cosmopolitan group of insects.

Actinotetraoses I-K: tetrasaccharide metabolites produced by an insect-derived actinobacteria, Amycolatopsis sp. HCa1

An isolate of rare actinobacteria strain Amycolatopsis sp. HCa1 obtained from the gut of grasshopper produced seven different metabolites in vitro. The metabolites isolated from its mycelia cakes were characterized by NMR and MS analyses. Actinotetraose hexatiglate (or tigloside; 1) with nonreducing glucotetraose skeleton was isolated as a major constituent; three new tetrasaccharide derivatives actinotetraoses I-K (2-4, resp.) and three known actinotetraoses A-C (5-7, resp.) were also isolated.

Microtermolides A and B from termite-associated Streptomyces sp. and structural revision of vinylamycin

Microtermolides A (1) and B (2) were isolated from a Streptomyces sp. strain associated with fungus-growing termites. The structures of 1 and 2 were determined by 1D- and 2D-NMR spectroscopy and high-resolution mass spectrometry. Structural elucidation of 1 led to the re-examination of the structure originally proposed for vinylamycin (3). Based on a comparison of predicted and experimental ¹H and ¹³C NMR chemical shifts, we propose that vinylamycin’s structure be revised from 3 to 4.

An ecological perspective of microbial secondary metabolism

Bacteria and fungi produce a remarkable array of bioactive small molecules. Many of these have found use in medicine as chemotherapies to treat diseases ranging from infection and cancer to hyperlipidemia and autoimmune disorders. The applications may or may not reflect the actual targets for these compounds. Through careful studies of microbes, their associated molecules and their targets, a growing understanding of the ecology of microbial secondary metabolism is emerging that exposes the central role of secondary metabolites in many complex biological systems.

Common biosynthetic origins for polycyclic tetramate macrolactams from phylogenetically diverse bacteria

A combination of small molecule chemistry, biosynthetic analysis, and genome mining has revealed the unexpected conservation of polycyclic tetramate macrolactam biosynthetic loci in diverse bacteria. Initially our chemical analysis of a Streptomyces strain associated with the southern pine beetle led to the discovery of frontalamides A and B, two previously undescribed members of this antibiotic family. Genome analyses and genetic manipulation of the producing organism led to the identification of the frontalamide biosynthetic gene cluster and several biosynthetic intermediates. The biosynthetic locus for the frontalamides' mixed polyketide/amino acid structure encodes a hybrid polyketide synthase nonribosomal peptide synthetase (PKS-NRPS), which resembles iterative enzymes known in fungi. No such mixed iterative PKS-NRPS enzymes have been characterized in bacteria. Genome-mining efforts revealed strikingly conserved frontalamide-like biosynthetic clusters in the genomes of phylogenetically diverse bacteria ranging from proteobacteria to actinomycetes. Screens for environmental actinomycete isolates carrying frontalamide-like biosynthetic loci led to the isolation of a number of positive strains, the majority of which produced candidate frontalamide-like compounds under suitable growth conditions. These results establish the prevalence of frontalamide-like gene clusters in diverse bacterial types, with medicinally important Streptomyces species being particularly enriched.