The use of Phasmarhabditis nematodes and metabolites of Xenorhabdus bacteria in slug control

Many species of slugs are considered serious pests in agriculture and horticulture around the world. In Europe, slugs of the genera Arion and Deroceras are the most harmful pests in agriculture. Therefore, the main goal of this study was to evaluate the effect of the whole-cell metabolites of 10 strains of five Xenorhabdus and three slug-parasitic nematodes (Phasmarhabditis hermaphrodita, Phasmarhabditis bohemica, and Phasmarhabditis apuliae) on the feeding behaviour and repellent effect on target slugs and evaluate a new possible means of biocontrol of these pests. The repellent and anti-feedant effects of nematode-killed insects, metabolites, slug-parasitic nematodes and a combination of metabolites and nematodes were studied through experimental designs: sand-filled plastic boxes divided into two parts in several modifications: with dead Galleria mellonella killed by nematodes, lettuce treated with bacterial metabolites and lettuce placed on the treated sand. We found that slugs avoid eating G. mellonella killed by nematodes, while they eat freeze-killed G. mellonella. Similarly, they avoid the consumption of lettuce in areas treated with bacterial metabolites (the most effective strains being Xenorhabus bovienii NFUST, Xenorhabdus kozodoii SLOV and JEGOR) with zero feeding in the treated side. All three Phasmarhabditis species also provided a significant anti-feedant/repellent effect. Our study is the first to show the repellent and anti-feedant effects of metabolites of Xenorhabdus bacteria against Arion vulgaris, and the results suggest that these substances have great potential for biocontrol. Our study is also the first to demonstrate the repellent effect of P. apuliae and P. bohemica. KEY POINTS: • Slugs avoid eating G. mellonella killed by entomopathogenic nematodes. • Bacterial metabolites have a strong repellent and antifeedant effect on slugs. • Presence of slug parasitic nematodes increases the repellent effect of metabolites.

Biosynthesis of selenium nanoparticles using cell-free extract of Xenorhabdus cabanillasii GU480990 and their potential mosquito larvicidal properties against yellow fever mosquito Aedes aegypti

Nanomaterials are successful due to their numerous applications in various domains such as cancer treatment, environmental applications, drug and gene delivery. Selenium is a metalloid element with broad biological activities and low toxicity especially at the nanoscale. Several studies have shown that nanoparticles synthesized from microbial and plant extracts are effective against important pests and pathogens. This study describes the bio fabrication of selenium nanoparticles using cell free extract of Xenorhabdus cabanillasii (XC-SeNPs) and assessed their mosquito larvicidal properties. Crystallographic structure and size of XC-SeNPs were determined with UV-a spectrophotometer, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), Energy-dispersive X-ray spectroscopy (EDAX), Zeta potential and Transmission electron microscopy (TEM). The significant surface plasmon resonance at 275 nm indicated the synthesis of XC-SeNPs from the pure cell-free extract of X. cabanillasii. The XRD result exhibits the crystalline nature of XC-SeNPs. The Zeta potential analysis confirmed that the surface charge of XC-SeNPs was -24.17 mV. TEM analysis revealed that synthesized XC-SeNPs were monodispersed, spherically shaped, and sized about 80-200 nm range. In addition, the larvicidal potentials of the bio-fabricated XC-SeNPs were assessed against the 4th-instar Ae. aegypti. XC-SeNPs displayed a dose-dependent larvicidal effect; the larval mortality was 13.3 % at the minimum evaluated concentration and increased to 72 % at higher dose treatments. The LC50 and LC90 concentration of XC-SeNPs against mosquito larvae were 79.4 and 722.4 ppm, respectively.

The effect of Xenorhabdus bacteria metabolites on Colorado potato beetle (Leptinotarsa decemlineata) adult feeding and larval survival

Colorado Potato Beetle (CPB) is one of the most destructive potato pests that can quickly develop resistance to insecticides. Therefore, new safe and effective control strategies that are less susceptible to the development of resistance by CPB are urgently needed. Due to their complex mode of action, the likelihood of resistance development by target pests is generally low with antifeedants. In the present study, we assessed the effect of secondary metabolites of various Xenorhabdus bacteria species and strains on CPB adult feeding and on larval development. The metabolites were applied in the form of cell free supernatants (CFSs) from Xenorhabdus cultures. In bioassay 1, leaves treated with ten Xenorhabdus cultures were fed to CPB adults, and their feeding was assessed daily for one week. In bioassay 2, CPB egg masses were placed on the leaves treated with five bacterial cultures, and larval development to pupae was monitored. Out of the ten Xenorhabdus cultures tested, two strains exhibited a significant reduction in the feeding behavior of Colorado Potato Beetle adults, with reductions of up to 70% compared to the control. The effect of CFSs on larval development was variable, and when treated with X. khoisanae SGI 197, over 90% of larvae died in the first few days before reaching the 2nd instar, and complete mortality was achieved on the 8th day of the experiment. Our study is the first study to demonstrate the antifeedant effect of Xenorhabdus cultures towards herbivorous beetles, and the metabolites of these bacteria may have potential for CPB control. Clearly, the metabolites produced by X. khoisanae SGI-197 may be a promising tool for CPB larvae control with the potential to significantly decrease damage to potato plants.

STEINERNEMA ADAMSI N. SP. (RHABDITIDA: STEINERNEMATIDAE), A NEW ENTOMOPATHOGENIC NEMATODE FROM THAILAND

A new species of entomopathogenic nematode, Steinernema adamsi n. sp., was recovered from the soil of a longan tree (Dimocarpus sp.) in Mueang Lamphun District, Thailand, using baiting techniques. Upon analysis of the nematode's morphological traits, we found it to be a new species of Steinernema and a member of the Longicaudatum clade. Molecular analyses of the ITS rDNA and D2D3 of 28S rDNA sequences further confirmed that S. adamsi n. sp. is a new species of the Longicaudatum clade, which is closely related to Steinernema guangdongense and Steinernema longicaudam. Using morphometric analysis, the infective juveniles measure between 774.69 and 956.96 μm, males have a size range of 905.44 to 1,281.98 μm, and females are within the range of 1,628.21 to 2,803.64 μm. We also identified the symbiotic bacteria associated with the nematode based on 16S sequences as Xenorhabdus spp. closely related toXenorhabdus griffiniae. Furthermore, we have successfully assessed a cryopreservation method for the long-term preservation of S. adamsi n. sp. Successful cryopreservation of this new species will allow for the longer preservation of its traits and will be valuable for its future use. The discovery of this new species has significant implications for the development of effective biological control agents in Thailand, and our work contributes to our understanding of the diversity and evolution of entomopathogenic nematodes.

Identification of fabclavine derivatives, Fcl-7 and Fcl-8, from Xenorhabdus budapestensis as major antifungal natural products against Rhizoctonia solani

AIMS

Black scurf disease, caused by Rhizoctonia solani, is a severe soil-borne and tuber-borne disease, which occurs and spreads in potato growing areas worldwide and poses a serious threat to potato production. New biofungicide is highly desirable for addressing the issue, and natural products (NPs) from Xenorhabdus spp. provide prolific resources for biofungicide development. In this study, we aim to identify antifungal NPs from Xenorhabdus spp. for the management of this disease.

METHODS AND RESULTS

Out of the 22 Xenorhabdus strains investigated, Xenorhabdus budapestensis 8 (XBD8) was determined to be the most promising candidate with the measured IC50 value of its cell-free supernatant against R. solani as low as 0.19 ml l-1. The major antifungal compound in XBD8 started to be synthesized in the middle logarithmic phase and reached a stable level at stationary phase. Core gene deletion coupled with high-resolution mass spectrometry analysis determined the major antifungal NPs as fabclavine derivatives, Fcl-7 and 8, which showed broad-spectrum bioactivity against important pathogenic fungi. Impressively, the identified fabclavine derivatives effectively controlled black scurf disease in both greenhouse and field experiments, significantly improving tuber quality and increasing with marketable tuber yield from 29 300 to 35 494 kg ha-1, comparable with chemical fungicide fludioxonil.

CONCLUSIONS

The fabclavine derivatives Fcl-7 and 8 were determined as the major antifungal NPs in XBD8, which demonstrated a bright prospect for the management of black scurf disease.

nov., Isolated from Steinernema Entomopathogenic Nematodes

Three bacterial strains, XENO-2T, XENO-7T, and XENO-10T, isolated from Steinernema entomopathogenic nematodes, were found to represent novel Xenorhabdus species. In this study, we describe these new species by whole-genome and whole-proteome phylogenomic reconstructions, by calculating sequence identity scores using core genome sequences, and by phenotypic characterization. Phylogenomic reconstructions using ribosomal and house-keeping genes, and whole-genome and whole-proteome sequences show that XENO-2T and XENO-10T are closely related to Xenorhabdus japonica DSM 16522T and that XENO-7T is closely related to Xenorhabdus bovienii subsp. africana XENO-1T and to X. bovienii subsp. bovienii T228T. The dDDH values between XENO-2T and XENO-10T and between XENO-2T and X. japonica DSM 16522T are 56.4 and 51.8%, respectively. The dDDH value between XENO-10T and X. japonica DSM 16522T is 53.4%. The dDDH values between XENO-7T and X. bovienii subsp. africana XENO-1T and between XENO-7T and X. bovienii subsp. bovienii T228T are 63.6 and 69.4%, respectively. These dDDH values are below the 70% divergence threshold for prokaryotic species delineation. The newly described species are highly pathogenic to G. mellonella larvae, grow at pH between 5 and 9 (optimum 5-7), at salt concentrations of 1-3% (optimum 1-2%), and temperatures between 20 and 37 °C (optimum 28-30 °C). Biochemical tests such as lysine decarboxylase, ornithine decarboxylase, urease, gelatinase, citrate utilization, indole and acetoin production, and cytochrome oxidase tests allow to differentiate the novel species from their more closely related species. Considering these genetic and phenotypic divergencies, we propose the following new species: Xenorhabdus aichiensis sp. nov. with XENO-7T (= CCM 9233T = CCOS 2024T) as the type strain, Xenorhabdus anantnagensis sp. nov., with XENO-2T (= CCM 9237T = CCOS 2023T) as the type strain, and Xenorhabdus yunnanensis sp. nov., with XENO-10T (= CCM 9322T = CCOS 2071T) as the type strain. Our study contributes to a better understanding of the biodiversity and phylogenetic relationships of entomopathogenic bacteria associated with insect parasitic nematodes.

Photorhabdus and Xenorhabdus as Symbiotic Bacteria for Bio-Control Housefly (Musca domestica L.)

<b>Background and Objective:</b> The housefly poses a threat to the public health of humans and domestic animals since it can carry and transmit pathogens. Despite there are many attempts to control this insect, most of them depend on conventional pesticides. Thus, the current study aimed to evaluate the efficacy of whole-cell suspension, cell-free supernatant and crude cells of the symbiotic bacteria <i>Photorhabdus</i> sp. and <i>Xenorhabdus</i> sp., as bio-control agents for housefly stages. <b>Materials and Methods:</b> The <i>Photorhabdus</i> sp. and <i>Xenorhabdus</i> sp., were isolated from the entomopathogenic nematodes, <i>Heterorhabditis indica</i> and <i>Steinernema feltiae</i>, respectively. The phenotypic, as well as the enzymatic characterizations of both bacteria, were determined. In addition, histopathological changes of the alimentary canal of <i>M. domestica</i> adults treated with whole-cell suspensions (at 3×10<sup>8 </sup>cells mL<sup></sup><sup>1</sup>) of both bacteria were carefully examined using transmission electron microscopy. <b>Results:</b> The results showed that both symbiotic bacteria significantly suppressed larvae, pupae and adults of <i>M. domestica</i>, particularly when they were applied as whole-cell suspensions. For example, the highest concentration of whole-cell suspension, cell-free supernatant and crude cells of <i>Photorhabdus</i> sp., induced larval mortalities by 94.7, 64.0 and 45.3%, while those of <i>Xenorhabdus</i> sp., induced larval mortalities by 58.7, 46.7 and 30.7% at 96 hrs, respectively. The results also showed that whole-cell suspensions of both symbiotic bacteria caused severe histopathological changes in the ultrastructure of the treated adults' alimentary canal. <b>Conclusion:</b> Both symbiotic bacteria can be effectively used, particularly the whole-cell suspension, as bio-control agents against the housefly either in the larval or adult stage.

Studies on antibacterial and chemotaxis properties of Pseudomonas aeruginosa TEN01 biomass-derived sustainable biosurfactant

Biosurfactant producing bacterial strains were isolated from oil-contaminated sites at Chennai Petroleum Corporation Limited, Chennai, the potential strain was selected and identified as Pseudomonas aeruginosa TEN01 by 16 S rRNA sequencing technique. Biosurfactant was produced from cassava solid waste from the sago industry. Further, it was extracted by solvent extraction and partially purified by column chromatography. The partially purified biosurfactant was qualitatively analyzed by Thin Layer Chromatography (TLC), quantitatively analyzed by anthrone assay and characterized by Fourier Transform Infra-Red Spectroscopy (FT-IR) and Gas Chromatography-Mass Spectrometry (GC-MS). R_f value and chemical groups confirm the presence of glycolipid in the partially purified biosurfactant. GC-MS results confirmed the presence of long-chain fatty acids and carbohydrate which is found to be mainly present in glycolipids. Biosurfactants are surface-active molecules which have been found to be the best alternative to chemical-based surfactants. The present study focuses on modifying the cell surface using a biosurfactant from P. aeruginosa TEN01 to enhance membrane permeabilization. Antibacterial and chemotaxis properties of biosurfactant from P. aeruginosa TEN01 were found to be better towards Xenorhabdus poinarii, a bio-pesticide producing microbial strain, X. poinarii exhibited 81.7% adhesion to hydrocarbons upon biosurfactant treatment as analyzed by Bacterial Adhesion to Hydrocarbon (BATH) assay. The alteration in the membrane permeability was tested in X. poinarii using biosurfactant and chemical surfactants viz. Sodium dodecyl sulfate (SDS) and toluene by estimating the amount of intracellular protein released. High protein recovery (51.55%) was achieved with a biosurfactant. Cell viability in the biosurfactant-treated cells was also high (93.98%) in comparison to cells treated with chemical surfactants. Increased recovery of intracellular protein along with high cell viability makes the biosurfactant a potential candidate for application in numerous environmental fields.

NMR resonance assignments for a docking domain pair with an attached thiolation domain from the PAX peptide-producing NRPS from Xenorhabdus cabanillasii

Non-ribosomal peptide synthetases (NRPSs) are large multienzyme machineries. They synthesize numerous important natural products starting from amino acids. For peptide synthesis functionally specialized NRPS modules interact in a defined manner. Individual modules are either located on a single or on multiple different polypeptide chains. The "peptide-antimicrobial-Xenorhabdus" (PAX) peptide producing NRPS PaxS from Xenorhabdus bacteria consists of the three proteins PaxA, PaxB and PaxC. Different docking domains (DDs) located at the N-termini of PaxB and PaxC and at the C-termini of PaxA and BaxB mediate specific non-covalent interactions between them. The N-terminal docking domains precede condensation domains while the C-terminal docking domains follow thiolation domains. The binding specificity of individual DDs is important for the correct assembly of multi-protein NRPS systems. In many multi-protein NRPS systems the docking domains are sufficient to mediate the necessary interactions between individual protein chains. However, it remains unclear if this is a general feature for all types of structurally different docking domains or if the neighboring domains in some cases support the function of the docking domains. Here, we report the ¹H, ¹³C and ¹⁵ N NMR resonance assignments for a C-terminal di-domain construct containing a thiolation (T) domain followed by a C-terminal docking domain (^CDD) from PaxA and for its binding partner - the N-terminal docking domain (^NDD) from PaxB from the Gram-negative entomopathogenic bacterium Xenorhabdus cabanillasii JM26 in their free states and for a 1:1 complex formed by the two proteins. These NMR resonance assignments will facilitate further structural and dynamic studies of this protein complex.

Genetically modified entomopathogenic bacteria, recent developments, benefits and impacts: A review

Entomopathogenic bacteria (EPBs), insect pathogens that produce pest-specific toxins, are environmentally-friendly alternatives to chemical insecticides. However, the most important problem with EPBs application is their limited field stability. Moreover, environmental factors such as solar radiation, leaf temperature, and vapor pressure can affect the pathogenicity of these pathogens and their toxins. Scientists have conducted intensive research to overcome such problems. Genetic engineering has great potential for the development of new engineered entomopathogens with more resistance to adverse environmental factors. Genetically modified entomopathogenic bacteria (GM-EPBs) have many advantages over wild EPBs, such as higher pathogenicity, lower spraying requirements and longer-term persistence. Genetic manipulations have been mostly applied to members of the bacterial genera Bacillus, Lysinibacillus, Pseudomonas, Serratia, Photorhabdus and Xenorhabdus. Although many researchers have found that GM-EPBs can be used safely as plant protection bioproducts, limited attention has been paid to their potential ecological impacts. The main concerns about GM-EPBs and their products are their potential unintended effects on beneficial insects (predators, parasitoids, pollinators, etc.) and rhizospheric bacteria. This review address recent update on the significant role of GM-EPBs in biological control, examining them through different perspectives in an attempt to generate critical discussion and aid in the understanding of their potential ecological impacts.

Effects of Nanoparticles of Metal Oxides on the Survival of the Entomopathogenic Nematode: Steinernema carpocapsae

Nanoparticles (NPs) are technological engineered materials with unique physical and chemical properties, and dimension of less than 100 nm. Nanotechnology has developed at a rapid pace, resulting into tremendous wide application that has resulted into concerns and ecotoxicological consequences. The antimicrobial potentials of the nanoparticles have been extensively studied, however, little has been done on the allied health and environmental toxicity assessments. Thus, the current work evaluated the toxicity effects of the ZnO, TiO₂ and Fe₃O₄ NPs on the survival of the entomopathogenic nematodes (Steinernema carpocapsae), as well as their growth inhibition effects on the nematode symbiotic bacteria (Xenorhabdus nematophila). The metal oxides NPs were characterized by scanning electron microscope and transmission electron microscope. Their toxicity effects were evaluated at various concentrations with the consideration of the media on the toxicity influence. All metal oxides had less influence on the survival of the entomopathogenic nematode and growth of the nematode symbiotic bacterial partner in a concentration dependant manner NPs. The observed toxicity was in the order of Fe₃O₄ < TiO₂ < ZnO NPs respectively, with no significant difference between the NPs. The less toxic effect of the NPs noted may be associated with the ability of entomopathogenic nematodes and their bacterial partner to tolerate toxicants. Nonetheless, other toxicity parameter of NPs on the beneficial nematodes needs to be evaluated for consideration of the compatibility potential of the nematodes and NPs for pest management.

Specificity and putative mode of action of a mosquito larvicidal toxin from the bacterium Xenorhabdus innexi

Reduction of mosquito-borne diseases relies, in part, on the use of synthetic pesticides to control pest mosquitoes. This reliance has led to genetic resistance, environmental contamination and the nondiscriminatory elimination of both pest and non-pest species. To expand our options for control, we screened entomopathogenic bacteria for potential larvicidal activity. A lipopeptide from the bacterium, Xenorhabdus innexi, was discovered that displayed potent larvicidal activity. The LC₅₀s of the lipopeptide towards Aedes aegypti, Culex pipiens and Anopheles gambiae larvae were 1.81, 1.25 and 1.86 parts-per-million, respectively. No mortality was observed in other insect species tested. The putative mode of action of the lipopeptide suggested that after orally ingestion, it bound to the apical membrane of anterior midgut cells and created pores in the cellular membranes. The rapid neutralization of midgut pH suggested the pores disabled the H⁺-V-ATPase on the basal membrane and led to epithelial cell death. Specificity and toxicity towards mosquito larvae and the unique mode of action makes this lipopeptide a potentially attractive bacterial insecticide for control of mosquitoes.

Life history trait analysis of the entomopathogenic nematode Steinernema feltiae provides the basis for prediction of dauer juvenile yields in monoxenic liquid culture

Entomopathogenic nematodes (Steinernema spp.) are used in integrated pest management to control insect pests in cryptic environments. The nematodes are mass produced in monoxenic liquid culture with their symbiotic bacteria Xenorhabdus spp. For a better understanding of nematode population dynamics, the life history traits (LHTs) of the entomopathogenic nematode Steinernema feltiae were assessed at 25 °C by observing single pairs of male and female nematodes using a hanging drop technique. To investigate the influence of different food supplies on nematode reproduction, the LHTs were assessed with a daily supply of 5 ×, 10 × and 20 × 10(9) cells ml(-1) of the nematode's bacterial symbiont Xenorhabdus bovienii in semi-solid nematode growth gelrite (NGG) medium. Increasing bacterial density had a significant positive influence on the average number of offspring produced, which ranged from 359 to 813 per female. The intrinsic rate of natural increase r m, which ranges from 1.10 to 1.19 day(-1), was neither influenced by the bacterial density, nor was the mean generation time T (5.12-5.25 days) and population doubling time (PDT) (0.64-0.59 days). The average lifespan of reproductive females, which ranged from 6.7 to 7.3 days, was positively correlated with bacterial density. A positive correlation between female body volume and bacterial density was recorded (R = 0.67) as well as a significant positive correlation between female body size and offspring production (R = 0.89) in hanging drops. Whether these data can be used to predict nematode yields in liquid culture was tested. The total female body volume calculated as the average female body volume × total number of parental females per millilitre 3 days after nematode inoculation was positively correlated (R = 0.72) with nematode yields. The total female body volume on process day 3 is thus a good indicator for the estimation of nematode yield at the end of the process (12-15 days post dauer juvenile (DJ) inoculation) in both Erlenmeyer flasks and bioreactors. With a mean deviation of 9467 DJs ml(-1), the error resembles approximately 5 % of the final DJ yields.

Innate hemocyte responses of Malacosoma disstria larvae (C. Insecta) to antigens are modulated by intracellular cyclic AMP

Invertebrate intracellular hemocyte signaling pathways affecting cellular-antigen responses, although defined for molluscs and some arthropods including dipteran insects, is less known for lepidopterans. Hemocytic-antigen responses of the arboreal pest lepidopteran Malacosoma disstria are linked to cAMP-dependent protein kinase A implicating cAMP in cellular hemocyte immune responses. The purpose in the present study was to determine intracellular cAMP effects on larval M. disstria hemocytes adhering to slides and bacteria. Altering adenylate cyclase and phosphodiesterase activities as well as cAMP levels in vitro and in vivo changed hemocyte responses to antigens. Quiescent hemocytes had high cAMP levels due to adenylate cyclase activity and possibly low phosphodiesterase (type 4) activity. Antigen contact diminished hemocytic cAMP levels. Inhibiting adenylate cyclase increased hemocyte-antigen and hemocyte-hemocyte adhesion, the latter producing nodules in vivo without bacterial antigens. Inhibiting phosphodiesterase type 4 produced the reverse effects. Pharmacologically increasing intracellular cAMP in attached hemocytes caused many of the cells to detach. Diminished intracellular cAMP changed hemograms in vivo in bacteria-free larvae comparable to changes induced by the bacterium, Bacillus subtilis, by producing nodules. Lowering cAMP enhanced also the removal of Xenorhabdus nematophila and B. subtilisin vivo.

Monoxenic production of the entomopathogenic nematode Steinernema carpocapsae using culture media containing agave juice (aguamiel) from Mexican maguey-pulquero (Agave spp). Effects of the contents of nitrogen, carbohydrates and fat on infective juvenile production

The production of infective juvenile stages (IJ) of the entomopathogenic nematode Steinernema carpocapsae in the presence of its symbiotic bacterium Xenorhabdus nematophilus was carried out in orbitally agitated bottles. Four complex culture media (M1-M4) were used, containing from 8% to 28% (by vol.) agave juice (aguamiel) from Mexican maguey-pulquero (Agave spp) as the main carbohydrate source. After 20 days of fermentation, a maximum viable IJ concentration of 249,000 IJ/ml and an initial nematode population multiplication factor of x620 were achieved when medium M4 was used (aguamiel concentration in this medium was 28% by vol.). M4 medium contained (w/v): 0.3% total nitrogen, 3.2% total carbohydrates and 3.0% total fat. According to the results obtained, total carbohydrates concentration appeared to be of great importance in obtaining high IJ concentrations.

Insecticidal pilin subunit from the insect pathogen Xenorhabdus nematophila

Xenorhabdus nematophila is an insect pathogen and produces protein toxins which kill the larval host. Previously, we characterized an orally toxic, large, outer membrane-associated protein complex from the culture medium of X. nematophila. Here, we describe the cloning, expression, and characterization of a 17-kDa pilin subunit of X. nematophila isolated from that protein complex. The gene was amplified by PCR, cloned, and expressed in Escherichia coli. The recombinant protein was refolded in vitro in the absence of its cognate chaperone by using a urea gradient. The protein oligomerized during in vitro refolding, forming multimers. Point mutations in the conserved N-terminal residues of the pilin protein greatly destabilized its oligomeric organization, demonstrating the importance of the N terminus in refolding and oligomerization of the pilin subunit by donor strand complementation. The recombinant protein was cytotoxic to cultured Helicoverpa armigera larval hemocytes, causing agglutination and subsequent release of the cytoplasmic enzyme lactate dehydrogenase. The agglutination of larval cells by the 17-kDa protein was inhibited by several sugar derivatives. The biological activity of the purified recombinant protein indicated that it has a conformation similar to that of the native protein. The 17-kDa pilin subunit was found to be orally toxic to fourth- or fifth-instar larvae of an important crop pest, H. armigera, causing extensive damage to the midgut epithelial membrane. To our knowledge, this is first report describing an insecticidal pilin subunit of a bacterium.

Spatial structuring and frequency distribution of the nematode Steinernema feltiae Filipjev

The frequency distribution of first generation, Steinernema feltiae Filipjev parasitic stages was over-dispersed with the majority of hosts containing few or no parasitic stages, whilst a few hosts contained a great many. Because of high extraction efficiency, the frequency distributions of the parasitic stages and the infective stages in the soil were assumed to be directly related. To explain the frequency distribution of the parasites it was therefore necessary to account for the frequency distribution of the S. feltiae infective stages in the soil. The infective stages were spatially aggregated into 30 cm diameter patches at the site of host death. These patches were randomly distributed approximately 1 m apart. At the 1 m scale, the pooled counts of infective stages were randomly distributed. Thus, in contrast to the frequency distributions, the spatial structuring of S. feltiae changed with the spatial scale of the interaction. This dynamic spatial structuring means that the majority of samples taken would contain few or no infective stages, whilst a few soil samples would contain a great many. Thus, the spatial structuring of the infective stages generates the over-dispersed frequency distribution of the S. feltiae in the soil. Hosts, encountering infective stages from this spatial distribution will, therefore, show an over-dispersed frequency distribution of S. feltiae parasitic stages.

Pathogenicity, development, and reproduction of Heterorhabditis bacteriophora and Steinernema carpocapsae under axenic in vivo conditions

Galleria mellonella larvae cultured axenically were treated with axenic dauer juveniles of Heterorhabditis bacteriophora and Steinernema carpocapsae. After 3 days S. carpocapsae had killed all insects, with 9.4 +/- 4.3 nematodes per larva. H. bacteriophora were unable to kill G. mellonella, although 13.3 +/- 6.4 nematodes per Galleria were found in the hemocoel. Invading nematodes of both strains recovered from the dauer stage. H. bacteriophora developed into hermaphrodites with eggs and J1 in the uterus and in the hemolymph of the living insects. Development beyond the J1 stage was not recorded. An injection of supernatants from different Photorhabdus luminescens cultures killed the insects but could not provide nutrients to support a further development. Only the injection of bacterial cells supported production of dauers in the axenic insects. Axenic S. carpocapsae developed to adults and produced offspring. After 3 weeks an average of 5275 nematodes per larva were counted, of which 6.7% were dauer juveniles, 39.2% other juvenile stages, 11.9% males, and 42.2% females. Compared to in vivo reproduction in the presence of the symbiotic bacterium Xenorhabdus nematophilus the dauer juvenile yields were low. Even after 5 weeks the percentage of dauer juveniles did not surpass 10%.