Isolation and identification of entomopathogenic nematodes and their symbiotic bacteria from Hérault and Gard (Southern France)

Biocontrol potential of six Heterorhabditis bacteriophora strains isolated in the Azores Archipelago

Entomopathogenic nematodes (EPNs) are closely associated with Popillia japonica and potentially used as their biological control agents, although field results proved inconsistent and evoked a continual pursuit of native EPNs more adapted to the environment. Therefore, we surveyed the Azorean Archipelago to isolate new strains of Heterorhabditis bacteriophora and to evaluate their virulence against the model organism Galleria mellonella under laboratory conditions. Six strains were obtained from pasture and coastal environments and both nematode and symbiont bacteria were molecularly identified. The bioassays revealed that Az172, Az186, and Az171 presented high virulence across the determination of a lethal dose (LD50) and short exposure time experiments with a comparable performance to Az29. After 72 hours, these virulent strains presented a mean determination of a lethal dose of 11 infective juveniles cm-2, a lethal time (LT50) of 34 hours, and achieved 40% mortality after an initial exposure time of only 60 minutes. Az170 exhibited an intermediate performance, whereas Az179 and Az180 were classified as low virulent strains. However, both strains presented the highest reproductive potential with means of 1700 infective juveniles/mg of larvae. The bioassays of the native EPNs obtained revealed that these strains hold the potential to be used in biological control initiatives targeting P. japonica because of their high virulence and locally adapted to environmental conditions.

Natural occurrence of entomopathogenic nematodes (Steinernema and Heterorhabditis) and Pristionchus nematodes in black truffle soils from Spain

The European truffle beetle Leiodes cinnamomeus is the most important pest in black truffle (Tuber melanosporum) plantations. Current control methods against it are inefficient, so entomopathogenic nematodes (EPNs) could play an important role in their population regulation due to their efficacy against many soil-dwelling insect pests. A survey of EPNs and Pristionchus nematodes was conducted in truffle soils of Spain, considering environmental and physical-chemical soil factors. A total of 164 soil samples were collected from forests, productive plantations and null-low productive plantations, representing three distinct black truffle-growing habitat types. EPNs were isolated from seven soil samples (4.3%); four nematodes were identified as Steinernema feltiae and three as Heterorhabditis bacteriophora. Both species were sampled in three types of soil texture (loam, sandy loam or sandy clay loam), characterized by alkaline pH (7.5 to 8.5) and high organic matter (2.1-11.04%). The presence of these EPNs was influenced by habitat type and organic matter content. Pristionchus nematodes were isolated from truffle soil, around truffle fruit bodies and under the elytra of L. cinnamomeus, with Pristionchus maupasi being the most commonly identified species. No significant associations were found between environmental and soil factors and the occurrence of Pristionchus nematodes. These nematodes were found in alkaline soils (pH 7.75 to 8.7), across all seven sampled soil textures, with variable organic matter content (0.73%-5.92%). The ecological trends and the presence of Pristionchus may affect the occurrence of EPNs and their prospective use as biological control agents against L. cinnamomeus in black truffle plantations.

Genotypic characterization of soil bacteria in the Umm Al-Namil Island, Kuwait

Microflora is an integral part of soil ecosystem, in which bacteria are the largest group of soil microbes. This is a pioneer study for establishing baseline data on the diversity of soil bacteria among different regions in Kuwait. The aim is to understand biodiversity in different settings, how bacteria adapt to different niches in the environment as well as in different hosts. The identification of bacterial 16S rRNA molecules from environmental soil samples was investigated. Genomic Deoxyribonucleic acid DNA was extracted from 25 soil samples derived from five different test regions in the Umm Al-Namil Island, Kuwait. After amplification of bacterial 16S rRNA molecules by the Polymerase chain reaction PCR, the products were characterized and complex band patterns were obtained, indicating high bacterial diversity. A sample of the 16 s rRNA amplicons were sequenced in order to identify the species. The spatial distribution of bacterial taxa in the different soil samples was homogeneous, suggesting a stable and widespread community. Forty-nine isolates from Umm Al-Namil island were identified by comparative analysis of partial 16S rRNA gene sequences. Phylogenetic analysis was carried out in order to study the connection between the isolates to identify species. A large proportion of these isolates represent correspond to known or novel species within the Pseudomonus and Bacillus genera, which are common soil bacteria. Our results provided a reference for future studies to facilitate bacterial identification and ecological research in Kuwait.

Bacterial community profile after the lethal infection of Steinernema-Xenorhabdus pairs into soil-reared Tenebrio molitor larvae

The host microbiota may have an impact on pathogens. This is often studied in laboratory-reared hosts but rarely in individuals whose microbiota looks like that of wild animals. In this study, we modified the gut microbiota of the insect Tenebrio molitor by rearing larvae in soil sampled from the field. We showed by high throughput sequencing methods that this treatment modifies the gut microbiota so that it is more diversified than that of laboratory-reared insects, and closely resembled the one of soil-dwelling insects. To describe what the entomopathogenic bacterial symbiont Xenorhabdus (Enterobacteriaceae), vectored by the soil-dwelling nematode Steinernema, might experience in natural conditions, we studied the infestation of the soil-reared T. molitor larvae with three Steinernema-Xenorhabdus pairs. We performed the infestation at 18°C, which delays the emergence of new infective juveniles (IJs), the soil-dwelling nematode forms, but which is a temperature compatible with natural infestation. We analyzed by high throughput sequencing methods the composition of the bacterial community within the insect cadavers before the first emergences of IJs. These bacterial communities were generally characterized by one or two non-symbiont taxa. Even for highly lethal Steinernema-Xenorhabdus pairs, the symbiont does not dominate the bacterial community within the insect cadaver.

Ecological characterization of Heterorhabditis sonorensis (Caborca strain) (Nematoda: Heterorhabditidae), an entomopathogenic nematode from the Sonoran Desert

Heterorhabditis nematodes are parasites of a wide range of soil-dwelling insect species. Although these nematodes have been exploited as biological control agents since the last half of the 20th century, much research remains to be done to understand how these organisms function in agricultural and other ecosystems. In this study, we present some ecological traits of Heterorhabditis sonorensis, a natural parasite of the cicada Diceroprocta ornea, from the Sonoran Desert. Specifically, we evaluated its infectivity across a diverse panel of insect groups and assessed its fitness (infectivity and reproduction) considering different temperatures, and soil moisture levels. Three other Heterorhabditis species served as points of comparison for temperature and soil moisture assays. Host range experiments indicate that H. sonorensis, although isolated from seasonal cicada nymphs, is more virulent and reproductively fit in the lepidopteran hosts tested. This nematode has an optimum temperature range at 25-30 °C but can also successfully reproduce at temperatures ranging from 15 to 35 °C. Additionally, this nematode is adapted to a variety of soil moisture conditions with successful infections across the tested moisture range (3%-20%). Finally, we demonstrate that H. sonorensis infective juveniles have a high survival rate (over 80%) at various storage temperatures (10-25 °C) after 24 weeks of storage and remain infective as revealed by the post-storage infection assays.

Steinernema poinari (Nematoda: Steinernematidae): a new symbiotic host of entomopathogenic bacteria Xenorhabdus bovienii

Three strains of symbiotic bacteria were isolated from an entomopathogenic nematode Steinernema poinari retrieved from soil in eastern Poland. Using 16S rDNA, recA, gltX, gyrB, and dnaN gene sequences for phylogenetic analysis, these strains were shown to belong to the species Xenorhabdus bovienii. The nucleotide identity between the studied S. poinari microsymbionts and other X. bovienii strains calculated for 16S rDNA and concatenated sequences of four protein-coding genes was 98.7-100% and 97.9-99.5%, respectively. The phenotypic properties of the isolates also supported their close phylogenetic relationship with X. bovienii. All three tested X. bovienii strains of different Steinernema clade origin supported the recovery of infective juveniles and subsequent development of the nematode population. However, the colonization degree of new infective juvenile generations was significantly affected by the bacterial host donor/recipient. The colonization degree of infective juveniles reared on bacterial symbionts deriving from a non-cognate clade of nematodes was extremely low, but proved the possible host-switching between non-related Steinernema species.

Symbiont-mediated competition: Xenorhabdus bovienii confer an advantage to their nematode host Steinernema affine by killing competitor Steinernema feltiae

Bacterial symbionts can affect several biotic interactions of their hosts, including their competition with other species. Nematodes in the genus Steinernema utilize Xenorhabdus bacterial symbionts for insect host killing and nutritional bioconversion. Here, we establish that the Xenorhabdus bovienii bacterial symbiont (Xb-Sa-78) of Steinernema affine nematodes can impact competition between S. affine and S. feltiae by a novel mechanism, directly attacking its nematode competitor. Through co-injection and natural infection assays we demonstrate the causal role of Xb-Sa-78 in the superiority of S. affine over S. feltiae nematodes during competition. Survival assays revealed that Xb-Sa-78 bacteria kill reproductive life stages of S. feltiae. Microscopy and timed infection assays indicate that Xb-Sa-78 bacteria colonize S. feltiae nematode intestines, which alters morphology of the intestine. These data suggest that Xb-Sa-78 may be an intestinal pathogen of the non-native S. feltiae nematode, although it is a nonharmful colonizer of the native nematode host, S. affine. Screening additional X. bovienii isolates revealed that intestinal infection and killing of S. feltiae is conserved among isolates from nematodes closely related to S. affine, although the underlying killing mechanisms may vary. Together, these data demonstrate that bacterial symbionts can modulate competition between their hosts, and reinforce specificity in mutualistic interactions.

New record of Steinernema arenarium (Artyukhovsky) (Rhabditida: Steinernematidae) from Ukraine and a note on its distribution

During a survey of the biodiversity of entomopathogenic nematodes in Ukraine, a population of Steinernema arenarium, strain Ch, was recovered in the sensitive Chornobyl Exclusion Zone. In the present work, this strain was morphologically and molecularly characterised using light microscopy and the sequences of the ITS and D2-D3 region of the 28S rDNA. In addition, we sequenced the ITS and D2-D3 regions of four populations of S. arenarium from a laboratory collection. Phylogenetic analyses were performed and the phylogenetic structure and geographic distribution of S. arenarium are discussed.

An Entomopathogenic Nematode Extends Its Niche by Associating with Different Symbionts

Bacterial symbionts are increasingly recognised as mediators of ecologically important traits of their animal hosts, with acquisition of new traits possible by uptake of novel symbionts. The entomopathogenic nematode Heterorhabditis downesi associates with two bacterial symbionts, Photorhabdus temperata subsp. temperata and P. temperata subsp. cinerea. At one intensively studied coastal dune site, P. temperata subsp. cinerea is consistently more frequently isolated than P. temperata subsp. temperata in H. downesi recovered from under the bare sand/Ammophila arrenaria of the front dunes (where harsh conditions, including drought, prevail). This is not the case in the more permissive closed dune grassland further from the sea. No differences were detected in ITS1 (internal transcribed spacer) sequence between nematode lines carrying either of the two symbiont subspecies, nor did they differ in their ability to utilise insects from three orders. The two symbionts could be readily swapped between lines, and both were carried in equal numbers within infective juveniles. In laboratory experiments, we tested whether the symbionts differentially affected nematode survival in insect cadavers that were allowed to dry. We assessed numbers of nematode infective juveniles emerging from insects that had been infected with H. downesi carrying either symbiont subspecies and then allowed to desiccate for up to 62 days. In moist conditions, cadavers produced similar numbers of nematodes, irrespective of the symbiont subspecies present, while under desiccating conditions, P. temperata subsp. cinerea cadavers yielded more nematode progeny than P. temperata subsp. temperata cadavers. Desiccating cadavers with the same nematode isolates, carrying either one or the other symbiont subspecies, confirmed that the symbiont was responsible for differences in nematode survival. Moreover, cadavers harbouring P. temperata subsp. cinerea had a reduced rate of drying relative to cadavers harbouring P. temperata subsp. temperata. Our experiments support the hypothesis that H. downesi can extend its niche into harsher conditions by associating with P. temperata subsp. cinerea.

Molecular and phenotypic characterization of Xenorhabdus bovienii symbiotically associated with Steinernema silvaticum

Steinernema silvaticum is a common entomopathogenic nematode in soil of Europe; however, little is known about the bacteria living in symbiosis with this animal. In this study, we have isolated four bacterial strains from S. silvaticum and identified them as members of the species Xenorhabdus bovienii. This study was based on 16S rRNA and concatenated recA, dnaN, gltX, and gyrB gene sequence analysis. In addition, phenotypic traits have been considered, indicating that the tested strains are the most similar to those of X. bovienii. The phylogenetic relationships between the isolated strains and other strains of X. bovienii derived from various nematode hosts were analyzed and discussed. This is the first report confirming the symbiotic association of X. bovienii with S. silvaticum.

Xenorhabdus bovienii CS03, the bacterial symbiont of the entomopathogenic nematode Steinernema weiseri, is a non-virulent strain against lepidopteran insects

Xenorhabdus bacteria (γ-proteobacteria: Enterobacteriaceae) have dual lifestyles. They have a mutualistic relationship with Steinernema nematodes (Nematoda: Steinernematidae) and are pathogenic to a wide range of insects. Each Steinernema nematode associates with a specific Xenorhabdus species. However, a Xenorhabdus species can have multiple nematode hosts. For example, Xenorhabdus bovienii (Xb) colonizes at least nine Steinernema species from two different phylogenetic clades. The Steinernema-Xb partnership has been found in association with different insect hosts. Biological and molecular data on the Steinernema jollieti-Xb strain SS-2004 pair have recently been described. In particular, the Xb SS-2004 bacteria are virulent alone after direct injection into insect, making this strain a model for studying Xb virulence. In this study, we searched for Xb strains attenuated in virulence. For this purpose, we underwent infection assays with five Steinernema spp.-Xb pairs with two insects, Galleria mellonella (Lepidoptera: Pyralidae) and Spodoptera littoralis (Lepidoptera: Noctuidae). The S. weiseri-Xb CS03 pair showed attenuated virulence and lower fitness in S. littoralis in comparison to the other nematode-bacteria pairs. Furthermore, when injected alone into the hemolymph of G. mellonella or S. littoralis, the Xb CS03 bacterial strain was the only non-virulent strain. By comparison with the virulent Xb SS-2004 strain, Xb CS03 showed an increased sensitivity to the insect antimicrobial peptides, suggesting an attenuated response to the insect humoral immunity. To our current knowledge, Xb CS03 is the first non-virulent Xb strain identified. We propose this strain as a new model for studying the Xenorhabdus virulence.

Biodiversity of entomopathogenic nematodes in Italy

An investigation was carried out on the distribution and biodiversity of steinernematid and heterorhabdtid entomopathogenic nematodes (EPN) in nine regions of Italy in the period 1990–2010. More than 2000 samples were collected from 580 localities and 133 of them yielded EPN specimens. A mapping of EPN distribution in Italy showed 133 indigenous EPN strains belonging to 12 species: 43 isolates of Heterorhabditis bacteriophora, 1 of H. downesi, 1 of H. megidis, 51 of Steinernema feltiae, 12 of S. affine, 4 of S. kraussei, 8 of S. apuliae, 5 of S. ichnusae, 3 of S. carpocapsae, 1 of S. vulcanicum, 3 of Steinernema ‘isolate S.sp.MY7’ of ‘S. intermedium group’ and 1 of S.arenarium. Steinernematids are more widespread than heterorhabditids and S. feltiae and H. bacteriophora are the most commonly encountered species. Sampling sites were grouped into 11 habitats: uncultivated land, orchard, field, sea coast, pinewood, broadleaf wood, grasslands, river and lake borders, caves, salt pan and moist zones; the soil texture of each site was defined and the preferences of habitat and soil texture of each species was assessed. Except for the two dominant species, S. feltiae and H. bacteriophora, EPN occurrence tends to be correlated with a specific vegetation habitat. Steinernema kraussei, H. downesi and H. megidis were collected only in Sicily and three of the species recently described – S. apuliae, S. ichnusae and S. vulcanicum – are known only from Italy and seem to be endemic.

A survey of entomopathogenic nematode species in continental Portugal

Entomopathogenic nematodes (EPN) are lethal parasites of insects, used as biocontrol agents. The objectives of this work were to survey the presence of EPN in continental Portugal and to characterize the different species. Of the 791 soil samples collected throughout continental Portugal, 53 were positive for EPN. Steinernema feltiae and Heterorhabditis bacteriophora were the two most abundant species. Analysis of EPN geographical distribution revealed an association between nematode species and vegetation type. Heterorhabditis bacteriophora was mostly found in the Alentejo region while S. feltiae was present in land occupied by agriculture with natural vegetation, broadleaved forest, mixed forest and transitional woodland-shrub, agro-forestry areas, complex cultivated patterns and non-irrigated arable land. Although no clear association was found between species and soil type, S. feltiae was typically recovered from cambisols and H. bacteriophora was more abundant in lithosols. Sequencing of the internal transcribed spacer (ITS) region indicated that S. feltiae was the most abundant species, followed by H. bacteriophora. Steinernema intermedium and S. kraussei were each isolated from one site and Steinernema sp. from two sites. Phylogenetic analyses of ITS, D2D3 expansion region of the 28S rRNA gene, as well as mitochondrial cytochrome c oxidase subunit I (COXI) and cytochrome b (cytb) genes, was performed to evaluate the genetic diversity of S. feltiae and H. bacteriophora. No significant genetic diversity was found among H. bacteriophora isolates. However, COXI seems to be the best marker to study genetic diversity of S. feltiae. This survey contributes to the understanding of EPN distribution in Europe.

Phenotypic variation and host interactions of Xenorhabdus bovienii SS-2004, the entomopathogenic symbiont of Steinernema jollieti nematodes

Xenorhabdus bovienii (SS-2004) bacteria reside in the intestine of the infective-juvenile (IJ) stage of the entomopathogenic nematode, Steinernema jollieti. The recent sequencing of the X. bovienii genome facilitates its use as a model to understand host - symbiont interactions. To provide a biological foundation for such studies, we characterized X. bovienii in vitro and host interaction phenotypes. Within the nematode host X. bovienii was contained within a membrane bound envelope that also enclosed the nematode-derived intravesicular structure. Steinernema jollieti nematodes cultivated on mixed lawns of X. bovienii expressing green or DsRed fluorescent proteins were predominantly colonized by one or the other strain, suggesting the colonizing population is founded by a few cells. Xenorhabdus bovienii exhibits phenotypic variation between orange-pigmented primary form and cream-pigmented secondary form. Each form can colonize IJ nematodes when cultured in vitro on agar. However, IJs did not develop or emerge from Galleria mellonella insects infected with secondary form. Unlike primary-form infected insects that were soft and flexible, secondary-form infected insects retained a rigid exoskeleton structure. Xenorhabdus bovienii primary and secondary form isolates are virulent towards Manduca sexta and several other insects. However, primary form stocks present attenuated virulence, suggesting that X. bovienii, like Xenorhabdus nematophila may undergo virulence modulation.

The first record of entomopathogenic nematodes (Rhabiditiae: Steinernematidae and Heterorhabditidae) in natural ecosystems in Lebanon: A biogeographic approach in the Mediterranean region

A survey of entomopathogenic nematodes in Lebanon was conducted for the first time during 2008-2009. Samples were collected on the coastal strip and in nine vegetation types extending from the coastal line to 3088m above sea level. Wooded and herbaceous ecosystems were considered for sampling purposes. A total of 570 samples were taken, out of which 1% were positive for entomopathogenic nematodes. Approximately, 15.8% out of the 19 sites sampled revealed entomopathogenic nematodes presence (representing three samples). Two entomopathogenic nematodes species Heterorhabditis bacteriophora and Steinernema feltiae were recovered, and identification of their symbiotic bacteria revealed the presence of a Xenorhabdus bovienii, Photorhabdus temperata subsp. thracensis, Photorhabdus luminescens subsp. kayaii and Photorhabdus luminescens subsp. Laumondii.

Phylogeny of Photorhabdus and Xenorhabdus based on universally conserved protein-coding sequences and implications for the taxonomy of these two genera. Proposal of new taxa: X. vietnamensis sp. nov., P. luminescens subsp. caribbeanensis subsp. nov., P. luminescens subsp. hainanensis subsp. nov., P. temperata subsp. khanii subsp. nov., P. temperata subsp. tasmaniensis subsp. nov., and the reclassification of P. luminescens subsp. thracensis as P. temperata subsp. thracensis comb. nov

We used the information from a set of concatenated sequences from four genes (recA, gyrB, dnaN and gltX) to investigate the phylogeny of the genera Photorhabdus and Xenorhabdus (entomopathogenic bacteria associated with nematodes of the genera Heterorhabditis and Steinernema, respectively). The robustness of the phylogenetic tree obtained by this multigene approach was significantly better than that of the tree obtained by a single gene approach. The comparison of the topologies of single gene phylogenetic trees highlighted discrepancies which have implications for the classification of strains and new isolates; in particular, we propose the transfer of Photorhabdus luminescens subsp. thracensis to Photorhabdus temperata subsp. thracensis comb. nov. (type strain CIP 108426T =DSM 15199T). We found that, within the genus Xenorhabdus, strains or isolates that shared less than 97 % nucleotide identity (NI), calculated on the concatenated sequences of the four gene fragments (recA, gyrB, dnaN and gltX) encompassing 3395 nucleotides, did not belong to the same species. Thus, at the 97% NI cutoff, we confirm the current 20 species of the genus Xenorhabdus and propose the description of a novel species, Xenorhabdus vietnamensis sp. nov. (type strain VN01T =CIP 109945T =DSM 22392T). Within each of the three current species of the genus Photorhabdus, P. asymbiotica, P. luminescens and P. temperata, strains or isolates which shared less than 97% NI did not belong to the same subspecies. Comparisons of the four gene fragments plus the rplB gene fragment analysed separately led us to propose four novel subspecies: Photorhabdus luminescens subsp. caribbeanensis subsp. nov. (type strain HG29T =CIP 109949T =DSM 22391T), P. luminescens subsp. hainanensis subsp. nov. (type strain C8404T = CIP 109946T =DSM 22397T), P. temperata subsp. khanii subsp. nov. (type strain C1T =NC19(T) =CIP 109947T =DSM 3369T), and P. temperata subsp. tasmaniensis subsp. nov. (type strain T327T =CIP 109948T =DSM 22387T).