Molecular diversity of Photorhabdus and Xenorhabdus bacteria, symbionts of Heterorhabditis and Steinernema nematodes retrieved from soil in Benin

Systematics and phylogeny of the entomopathogenic nematobacterial complexes Steinernema-Xenorhabdus and Heterorhabditis-Photorhabdus

Entomopathogenic nematodes of the genera Steinernema and Heterorhabditis, along with their bacterial symbionts from the genera Xenorhabdus and Photorhabdus, respectively, are important biological control agents against agricultural pests. Rapid progress in the development of genomic tools has catalyzed a transformation of the systematics of these organisms, reshaping our understanding of their phylogenetic and cophlylogenetic relationships. In this review, we discuss the major historical events in the taxonomy and systematics of this group of organisms, highlighting the latest advancements in these fields. Additionally, we synthesize information on nematode-bacteria associations and assess the existing evidence regarding their cophylogenetic relationships.

Diversity and functional analysis of Chinese bumblebee gut microbiota reveal the metabolic niche and antibiotic resistance variation of Gilliamella

Bumblebees play an important role in maintaining the balance of natural and agricultural ecosystems, and the characteristic gut microbiota of bumblebees exhibit significant mutualistic functions. China has the highest diversity of bumblebees; however, gut microbiota of Chinese bumblebees have mostly been investigated through culture-independent studies. Here, we analyzed the gut communities of bumblebees from Sichuan, Yunnan, and Shaanxi provinces in China through 16S ribosomal RNA amplicon sequencing and bacterial isolation. It revealed that the bumblebees examined in this study harbored two gut enterotypes as previously reported: one is dominated by Gilliamella and Snodgrassella, and the other is distinguished by prevalent environmental species. The gut compositions obviously varied among different individual bees. We then isolated 325 bacterial strains and the comparative genomic analysis of Gilliamella strains revealed that galactose and pectin digestion pathways were conserved in strains from bumblebees, while genes for the utilization of arabinose, mannose, xylose, and rhamnose were mostly lost. Only two strains from the Chinese bumblebees possess the multidrug-resistant gene emrB, which is phylogenetically closely related to that from the symbionts of soil entomopathogenic nematode. In contrast, tetracycline-resistant genes were uniquely present in three strains from the USA. Our results illustrate the prevalence of strain-level variations in the metabolic potentials and the distributions of antibiotic-resistant genes in Chinese bumblebee gut bacteria.

Reclaimed desert habitats favor entomopathogenic nematode and microarthropod abundance compared to ancient farmlands in the Nile Basin

Characterizing entomopathogenic nematode (EPN) biogeography with a goal of augmentation and conservation biological control requires fine-scale taxonomic resolution, because closely related EPN species can exhibit divergent phenotypes for key properties such as habitat adaptation and insect host specificity. Consequently, we employed high throughput genome sequencing (HTS) to identify and compare EPNs and natural enemies of EPNs in 58 citrus orchards in 2 ecoregions in Egypt (El Beheira and Al Qalyubia governorates). We designed improved primers targeting the ITS2 rDNA to discriminate EPN species and used pre-reported primers targeting D2-D3 region for soil microarthropods. Five EPN species (Heterorhabditis bacteriophora, H. indica, H. taysearae, Steinernema glaseri, and S. scapterisci) and one steinernematid not represented in Genbank databases were detected. This is the first report of S. scapterisci and possibly the unknown (perhaps undescribed) species in Egypt. Only heterorhabditid species, dominated by H. indica, were detected in the reclaimed, sandy desert soils of El Beheira governorate. In the fine textured, ancient farming lands of the Nile delta all six species were detected, but at lower frequency and abundance. Microarthropod family richness (P = 0.01) and abundance (P = 0.001) was higher in the reclaimed lands than in the Nile Delta. Soil clay content, pH and elevation explained significant variation in the mite community structure. Population density of H. indica, the only EPN found consistently and at high abundance in El-Beheira, was inversely related to abundance of species in the nematophagous mite family Rhodacaridae.

Advances in Entomopathogen Isolation: A Case of Bacteria and Fungi

Entomopathogenic bacteria and fungi are quite frequently found in soils and insect cadavers. The first step in utilizing these microbes as biopesticides is to isolate them, and several culture media and insect baiting procedures have been tested in this direction. In this work, the authors review the current techniques that have been developed so far, in the last five decades, and display brief protocols which can be adopted for the isolations of these entomopathogens. Among bacteria, this review focuses on Serratia spp. and bacteria from the class Bacilli. Among fungi, the review focuses those from the order Hypocreales, for example, genera Beauveria, Clonostachys, Lecanicillium, Metarhizium, and Purpureocillium. The authors chose these groups of entomopathogenic bacteria and fungi based on their importance in the microbial biopesticide market.

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.

Steinernema kandii n. sp. (Rhabditida: Steinernematidae), a new entomopathogenic nematode from northern Benin

Two nematode isolates from the genus Steinernema were collected in northern Benin. Morphological, morphometric, molecular and cross-hybridisation studies placed these nematodes into a new species, Steinernema kandii n. sp., within the bicornutum-group. Phylogenetic analyses based on both ITS and D2-D3 regions of 28S rDNA revealed that S. kandii n. sp. is different from all known Steinernema species and sister to S. abbasi (97.3-97.6% ITS nucleotide similarity) and S. bifurcatum (98.3-98.4% D2-D3 similarity). Steinernema kandii n. sp. can be separated from other members of the bicornutum-group by the greater infective juvenile (IJ) max. body diam. of 35 (27-48) μm (type isolate). It differs from S. abbasi by the greater IJ body length 707 (632-833) μm (type isolate), EP distance 55 (52-60) μm (type isolate), spicule length 67 (57-75) μm (type isolate) and the occurrence of one pair of genital papillae at the cloacal aperture.

Enhanced entomopathogenic nematode yield and fitness via addition of pulverized insect powder to solid media

Beneficial nematodes are used as biological control agents. Low-cost mass production of entomopathogenic nematodes (EPNs) is an important prerequisite toward their successful commercialization. EPNs can be grown via in vivo methods or in sold or liquid fermentation. For solid and liquid approaches, media optimization is paramount to maximizing EPN yield and quality. In solid media, the authors investigated the effects of incorporating pulverized insect powder from larvae of three insects (Galleria mellonella, Tenebrio molitor, and Lucillia sericata) at three dose levels (1, 3, and 5%). The impact of insect powder was assessed on infective juvenile (IJ) yield in solid media. Additionally, IJs produced in solid culture were subsequently assessed for virulence, and progeny production in a target insect, Spodoptera litura. The dose level of larval powder had a significant effect on IJ yield in both trials, whereas insect type had significant effect on IJ yield in trial 1 but not in trial 2. The maximum solid culture yield was observed in T. molitor powder at the highest dose in both trials. Moreover, the time-to-death in S. litura was substantially shortened in trial 1 and in trial 2 when IJs from the T. molitor powder treatment were applied. There was no significant effect of combining two insect powders relative to addition of powder from a single insect species. These findings indicate that addition of insect powder to solid media leads to high mass production yields, and the fitness of the IJs produced (e.g., in virulence and reproductive capacity) can be enhanced as well.

Beneficial nematodes are used as biological control agents. Low-cost mass production of entomopathogenic nematodes (EPNs) is an important prerequisite toward their successful commercialization. EPNs can be grown via in vivo methods or in sold or liquid fermentation. For solid and liquid approaches, media optimization is paramount to maximizing EPN yield and quality. In solid media, the authors investigated the effects of incorporating pulverized insect powder from larvae of three insects (Galleria mellonella, Tenebrio molitor, and Lucillia sericata) at three dose levels (1, 3, and 5%). The impact of insect powder was assessed on infective juvenile (IJ) yield in solid media. Additionally, IJs produced in solid culture were subsequently assessed for virulence, and progeny production in a target insect, Spodoptera litura. The dose level of larval powder had a significant effect on IJ yield in both trials, whereas insect type had significant effect on IJ yield in trial 1 but not in trial 2. The maximum solid culture yield was observed in T. molitor powder at the highest dose in both trials. Moreover, the time-to-death in S. litura was substantially shortened in trial 1 and in trial 2 when IJs from the T. molitor powder treatment were applied. There was no significant effect of combining two insect powders relative to addition of powder from a single insect species. These findings indicate that addition of insect powder to solid media leads to high mass production yields, and the fitness of the IJs produced (e.g., in virulence and reproductive capacity) can be enhanced as well.