Xenorhabdus japonicus sp. nov. associated with the nematode Steinernema kushidai

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.

Xenorhabdus aichiensis sp. nov., Xenorhabdus anantnagensis sp. nov., and Xenorhabdus yunnanensis sp. 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.

Draft genomes, phylogenomic reconstruction and comparative genome analysis of three Xenorhabdus strains isolated from soil-dwelling nematodes in Kenya

As a proven source of potent and selective antimicrobials, Xenorhabdus bacteria are important to an age plagued with difficult-to-treat microbial infections. Yet, only 27 species have been described to date. In this study, a novel Xenorhabdus species was discovered through genomic studies on three isolates from Kenyan soils. Soils in Western Kenya were surveyed for steinernematids and Steinernema isolates VH1 and BG5 were recovered from red volcanic loam soils from cultivated land in Vihiga and clay soils from riverine land in Bungoma respectively. From the two nematode isolates, Xenorhabdus sp. BG5 and Xenorhabdus sp. VH1 were isolated. The genomes of these two, plus that of X. griffiniae XN45 - this was previously isolated from Steinernema sp. scarpo that also originated from Kenyan soils - were sequenced and assembled. Nascent genome assemblies of the three isolates were of good quality with over 70 % of their proteome having known functions. These three isolates formed the X. griffiniae clade in a phylogenomic reconstruction of the genus. Their species were delineated using three overall genome relatedness indices: an unnamed species of the genus, Xenorhabdus sp. BG5, X. griffiniae VH1 and X. griffiniae XN45. A pangenome analysis of this clade revealed that over 70 % of species-specific genes encoded unknown functions. Transposases were linked to genomic islands in Xenorhabdus sp. BG5. Thus, overall genome-related indices sufficiently delineated species of two new Xenorhabdus isolates from Kenya, both of which were closely related to X. griffiniae . The functions encoded by most species-specific genes in the X. griffiniae clade remain unknown.

Nematophilic bacteria associated with entomopathogenic nematodes and drug development of their biomolecules

The importance of Xenorhabdus and Photorhabdus symbionts to their respective Steinernema and Heterorhabditis nematode hosts is that they not only contribute to their entomopathogenicity but also to their fecundity through the production of small molecules. Thus, this mini-review gives a brief introductory overview of these nematophilic bacteria. Specifically, their type species, nematode hosts, and geographic region of isolations are tabulated. The use of nucleotide sequence-based techniques for their species delineation and how pangenomes can improve this are highlighted. Using the Steinernema–Xenorhabdus association as an example, the bacterium-nematode lifecycle is visualized with an emphasis on the role of bacterial biomolecules. Those currently in drug development are discussed, and two potential antimalarial lead compounds are highlighted. Thus, this mini-review tabulates forty-eight significant nematophilic bacteria and visualizes the ecological importance of their biomolecules. It further discusses three of these biomolecules that are currently in drug development. Through it, one is introduced to Xenorhabdus and Photorhabdus bacteria, their natural production of biomolecules in the nematode-bacterium lifecycle, and how these molecules are useful in developing novel therapies.

Larvicidal activity of Photorhabdus and Xenorhabdus bacteria isolated from insect parasitic nematodes against Aedes aegypti and Aedes albopictus

Aedes aegypti and Aedes albopictus are important vectors for several arboviruses such as the dengue virus. The chemical control of Aedes spp., which is usually implemented, affects both humans and the environment. The biological control of Aedes spp. with entomopathogenic bacteria such as Photorhabdus and Xenorhabdus may be an alternative method that can overcome such issues. This study aimed to isolate and identify Photorhabdus and Xenorhabdus bacteria from entomopathogenic nematodes (EPNs) collected in Thailand and evaluate their larvicidal properties in controlling A. aegypti and A. albopictus. Colony morphology and recA sequencing of the 118 symbiotic isolated bacteria indicated that most were P. luminescens subsp. akhurstii and X. stockiae with minor prevalence of P. luminescens subsp. hainanensis, P. asymbiotica subsp. australis, X. indica, X. griffiniae, X. japonica, X. thuongxuanensis, and X. eapokensis . The larvicidal bioassay with the third- and fourth-instar mosquito larvae suggested that a whole-cell suspension of X. griffiniae (bMSN3.3_TH) had the highest efficiency in eradicating A. aegypti and A. albopictus, with 90 ± 3.71% and 81 ± 2.13% mortality, respectively, after 96 h exposure. In contrast, 1% of ethyl acetate extracted from X. indica (bSNK8.5_TH) showed reduced mortality for A. aegypti of only 50 ± 3.66% after 96 h exposure. The results indicate that both X. griffiniae (bMSN3.3_TH) and X. indica (bSNK8.5_TH) could be used as biocontrol agents against Aedes larvae.

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.

Bacteria of the Genus Xenorhabdus, a Novel Source of Bioactive Compounds

The genus Xenorhabdus of the family Enterobacteriaceae, are mutualistically associated with entomopathogenic nematodes of the genus Steinernema. Although most of the associations are species-specific, a specific Xenorhabdus sp. may infect more than one Steinernema sp. During the Xenorhabdus-Steinernema life cycle, insect larvae are infected and killed, while both mutualists produce bioactive compounds. These compounds act synergistically to ensure reproduction and proliferation of the nematodes and bacteria. A single strain of Xenorhabdus may produce a variety of antibacterial and antifungal compounds, some of which are also active against insects, nematodes, protozoa, and cancer cells. Antimicrobial compounds produced by Xenorhabdus spp. have not been researched to the same extent as other soil bacteria and they may hold the answer to novel antibacterial and antifungal compounds. This review summarizes the bioactive secondary metabolites produced by Xenorhabdus spp. and their application in disease control. Gene regulation and increasing the production of a few of these antimicrobial compounds are discussed. Aspects limiting future development of these novel bioactive compounds are also pointed out.

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.

Screening of the Antimicrobial Activity against Drug Resistant Bacteria of Photorhabdus and Xenorhabdus Associated with Entomopathogenic Nematodes from Mae Wong National Park, Thailand

Photorhabdus and Xenorhabdus are symbiotic with entomopathogenic nematodes (EPNs) of the genera Heterorhabditis and Steinernema, respectively. These bacteria produce several secondary metabolites including antimicrobial compounds. The objectives of this study were to isolate and identify EPNs and their symbiotic bacteria from Mae Wong National Park, Thailand and to evaluate the antibacterial activities of symbiont extracts against drug resistant bacteria. A total of 550 soil samples from 110 sites were collected between August 2014 and July 2015. A total of EPN isolates were obtained through baiting and White trap methods, which yielded 21 Heterorhabditis and 3 Steinernema isolates. Based on molecular identification and phylogenetic analysis, the most common species found in the present study was P. luminescens subsp. akhurstii associated with H. indica. Notably, two species of EPNs, H. zealandica and S. kushidai, and two species of symbiotic bacteria, X. japonica and P. temperata subsp. temperata represented new recorded organisms in Thailand. Furthermore, the association between P. temperata subsp. temperata and H. zealandica has not previously been reported worldwide. Disk diffusion, minimal inhibitory concentration, and minimal bactericidal concentration analyses demonstrated that the crude compound extracted by ethyl acetate from P. temperata subsp. temperata could inhibit the growth of up to 10 strains of drug resistant bacteria. Based on HPLC-MS analysis, compound classes in bacterial extracts were identified as GameXPeptide, xenoamicin, xenocoumacin, mevalagmapeptide phurealipids derivatives, and isopropylstilbene. Together, the results of this study provide evidence for the diversity of EPNs and their symbiotic bacteria in Mae Wong National Park, Thailand and demonstrate their novel associations. These findings also provide an important foundation for further research regarding the antimicrobial activity of Photorhabdus bacteria.

Xenorhabdus thuongxuanensis sp. nov. and Xenorhabdus eapokensis sp. nov., isolated from Steinernema species

Two slightly yellowish-pigmented, oxidase-negative, rod-shaped and Gram-stain-negative bacterial strains (30TX1T and DL20T), isolated from Steinernema sangi and Steinernema eapokense, respectively, during soil sampling in Vietnam were studied using a polyphasic taxonomic approach. Strain 30TX1T showed highest 16S rRNA gene sequence similarity to the type strain of Xenorhabdus ehlersii (98.9 %) and strain DL20T to that of Xenorhabdus ishibashii (98.7 %). Sequence similarities to all other Xenorhabdus species were lower (<98.4 %). The two strains shared 98 % 16S rRNA gene sequence similarity. Multilocus sequence analysis (MLSA) based on concatenated partial recA, dnaN, gltX, gyrB and infB gene sequences showed a clear distinction of strains 30TX1T and DL20T among each other and to the closest related type strains. DNA-DNA hybridizations between strain DL20T and the type strain of X. ishibashii resulted in a relatedness value of 53 %. Genome-to-genome-based comparisons gave average nucleotide identities of 93.6 % (reciprocal 93.5 %) for strain 30TX1T and X. ehlersii DSM 16337T, of 92.8 % (reciprocal 93 %) for strain DL20T and X. ishibashiiDSM 22670Tand of 93.0 % (reciprocal 93.2 %) for the two novel strains. The fatty acid profile of the strains consisted of the major fatty acids C14 : 0, C16 : 0, C17 : 0 cyclo, C16 : 1ω7c and/or iso-C15 : 0 2-OH, and C18 : 1ω7c. Genome-to-genome comparison and MLSA results together with the differential biochemical and chemotaxonomic properties showed that strains 30TX1T and DL20T represent novel Xenorhabdus species, for which the names Xenorhabdus thuongxuanensis sp. nov. (type strain 30TX1T=CCM 8727T=LMG 29916T) and Xenorhabdus eapokensis sp. nov. (type strain DL20T=CCM 8728T=LMG 29917T) are proposed, respectively.

Xenorhabdus ishibashii sp. nov., isolated from the entomopathogenic nematode Steinernema aciari

Gram-negative bacteria of the genus Xenorhabdus exhibit a mutualistic association with steinernematid entomopathogenic nematodes and a pathogenic relationship with insects. Here we describe two isolates of the entomopathogenic nematode Steinernema aciari collected from China and Japan. 16S rRNA gene sequence similarity and phylogenetic analysis indicated that the isolates obtained from S. aciari belonged to the genus Xenorhabdus . Multilocus sequence analysis based on five universal protein-coding gene sequences revealed that the isolates were closely related to Xenorhabdus ehlersii DSM 16337T and Xenorhabdus griffiniae ID10T but that they exhibited <97 % sequence similarity with these reference strains, which indicated that the isolates were distinct from previously described species. Based on these genetic differences and several differential phenotypic traits, we propose that the isolates represent a novel species of the genus Xenorhabdus , for which we propose the name Xenorhabdus ishibashii sp. nov. The type strain is GDh7T ( = DSM 22670T  = CGMCC 1.9166T).

Description of Xenorhabdus khoisanae sp. nov., the symbiont of the entomopathogenic nematode Steinernema khoisanae

Bacterial strain SF87(T), and additional strains SF80, SF362 and 106-C, isolated from the nematode Steinernema khoisanae, are non-bioluminescent Gram-reaction-negative bacteria that share many of the carbohydrate fermentation reactions recorded for the type strains of recognized Xenorhabdus species. Based on 16S rRNA gene sequence data, strain SF87(T) is shown to be closely related (98% similarity) to Xenorhabdus hominickii DSM 17903(T). Nucleotide sequences of strain SF87 obtained from the recA, dnaN, gltX, gyrB and infB genes showed 96-97% similarity with Xenorhabdus miraniensis DSM 17902(T). However, strain SF87 shares only 52.7% DNA-DNA relatedness with the type strain of X. miraniensis, confirming that it belongs to a different species. Strains SF87(T), SF80, SF362 and 106-C are phenotypically similar to X. miraniensis and X. beddingii, except that they do not produce acid from aesculin. These strains are thus considered to represent a novel species of the genus Xenorhabdus, for which the name Xenorhabdus khoisanae sp. nov. is proposed. The type strain is SF87(T) ( =DSM 25463(T) =ATCC BAA-2406(T)).

Description of Xenorhabdus magdalenensis sp. nov., the symbiotic bacterium associated with Steinernema australe

A symbiotic bacterium, strain IMI 397775(T), was isolated from the insect-pathogenic nematode Steinernema australe. On the basis of 16S rRNA gene sequence similarity, this bacterial isolate was shown to belong to the genus Xenorhabdus, in agreement with the genus of its nematode host. The accurate phylogenetic position of this new isolate was defined using a multigene approach and showed that isolate IMI 397775(T) shares a common ancestor with Xenorhabdus doucetiae FRM16(T) and Xenorhabdus romanii PR06-A(T), the symbiotic bacteria associated with Steinernema diaprepesi and Steinernema puertoricense, respectively. The nucleotide identity (less than 97%) between isolate IMI 397775(T), X. doucetiae FRM16(T) and X. romanii PR06-A(T) calculated for the concatenated sequences of five gene fragments encompassing 4275 nt, several phenotypic traits and the difference between the upper temperatures that limit growth of these three bacteria allowed genetic and phenotypic differentiation of isolate IMI 397775(T) from the two closely related species. Strain IMI 397775(T) therefore represents a novel species, for which the name Xenorhabdus magdalenensis sp. nov. is proposed, with the type strain IMI 397775(T) ( = DSM 24915(T)).

New insight into diversity in the genus Xenorhabdus, including the description of ten novel species

We investigated the diversity of a collection of 76 Xenorhabdus strains, isolated from at least 27 species of Steinernema nematodes and collected in 32 countries, using three complementary approaches: 16S rRNA gene sequencing, molecular typing and phenotypic characterization. The 16S rRNA gene sequences of the Xenorhabdus strains were highly conserved (similarity coefficient >95 %), suggesting that the common ancestor of the genus probably emerged between 250 and 500 million years ago. Based on comparisons of the 16S rRNA gene sequences, we identified 13 groups and seven unique sequences. This classification was confirmed by analysis of molecular typing profiles of the strains, leading to the classification of new isolates into the Xenorhabdus species described previously and the description of ten novel Xenorhabdus species: Xenorhabdus cabanillasii sp. nov. (type strain USTX62(T)=CIP 109066(T)=DSM 17905(T)), Xenorhabdus doucetiae sp. nov. (type strain FRM16(T)=CIP 109074(T)=DSM 17909(T)), Xenorhabdus griffiniae sp. nov. (type strain ID10(T)=CIP 109073(T)=DSM 17911(T)), Xenorhabdus hominickii sp. nov. (type strain KE01(T)=CIP 109072(T)=DSM 17903(T)), Xenorhabdus koppenhoeferi sp. nov. (type strain USNJ01(T)=CIP 109199(T)=DSM 18168(T)), Xenorhabdus kozodoii sp. nov. (type strain SaV(T)=CIP 109068(T)=DSM 17907(T)), Xenorhabdus mauleonii sp. nov. (type strain VC01(T)=CIP 109075(T)=DSM 17908(T)), Xenorhabdus miraniensis sp. nov. (type strain Q1(T)=CIP 109069(T)=DSM 17902(T)), Xenorhabdus romanii sp. nov. (type strain PR06-A(T)=CIP 109070(T)=DSM 17910(T)) and Xenorhabdus stockiae sp. nov. (type strain TH01(T)=CIP 109067(T)=DSM 17904(T)). The Xenorhabdus strains studied here had very similar phenotypic patterns, but phenotypic features nonetheless differentiated the following species: X. bovienii, X. cabanillasii, X. hominickii, X. kozodoii, X. nematophila, X. poinarii and X. szentirmaii. Based on phenotypic analysis, we identified two major groups of strains. Phenotypic group G(A) comprised strains able to grow at temperatures of 35-42 degrees C, whereas phenotypic group G(B) comprised strains that grew at temperatures below 35 degrees C, suggesting that some Xenorhabdus species may be adapted to tropical or temperate regions and/or influenced by the growth and development temperature of their nematode host.

A novel species of Xenorhabdus, family Enterobacteriaceae: Xenorhabdus indica sp. nov., symbiotically associated with entomopathogenic nematode Steinernema thermophilum Ganguly and Singh, 2000

In the search for novel Xenorhabdus strains in a recently described nematode species, Steinernema thermophilum, three strains (strain 28(T) = DSM 17382(T), strain 42 = DSM 17383 and strain 43 = DSM 17384) were isolated from three independent isolation approaches from crushed mixture of infective juveniles. 16S rRNA gene sequence comparison of strains 28(T) and DSM 17383 indicated identity and the phylogenetic position pointed towards an individual taxon within the phylogenetic dendrogram of Xenorhabdus type strains. The nearest phylogenetic relatives of strain 28(T) were Xenorhabdus poinarii and Xenorhabdus szentirmaii (97.7% each). The three isolates were almost identical in reaction towards the API and BIOLOG substrate panels but differed in their reactions from those of the established type strains of the genus Xenorhabdus. These clear genomic and metabolic differences let us propose a new species, Xenorhabdus indica sp. nov. for the three clones. The type strain is strain 28(T), DSM 17382(T), CIP 108830(T).

Identification and phylogenetic analysis of Arsenophonus- and Photorhabdus-type bacteria from adult Hippoboscidae and Streblidae (Hippoboscoidea)

This is the first report of Arsenophonus- and Photorhabdus-type bacteria from Streblidae (bat flies) and Hippoboscidae (louse flies, keds). Strains were detected by means of polymerase chain reaction of 16S rDNA, and phylogenetic analysis determined the relationship of the obtained sequences to previously reported sequences in GenBank. Phylogenetic analysis by means of maximum parsimony revealed that all isolated Arsenophonus spp. 16S rDNA sequences formed a monophyletic sub-clade within other insect Arsenophonus spp., while the Photorhabdus spp. sequences are part of a monophyletic clade including Photorhabdus spp., Xenorhabdus spp. and Proteus spp.

Description of four novel species of Xenorhabdus, family Enterobacteriaceae: Xenorhabdus budapestensis sp. nov., Xenorhabdus ehlersii sp. nov., Xenorhabdus innexi sp. nov., and Xenorhabdus szentirmaii sp. nov

The taxonomic affiliation was determined for four Xenorhabdus strains isolated from four Steinernema hosts from different countries. As compared to the five validly described Xenorhabdus species, i.e., X. nematophila, X. japonica, X. beddingii, X. bovienii and X. poinarii, these isolates represented novel species on the basis of 16S rRNA gene sequences and riboprint patterns, as well as by physiological and metabolic properties. They were named Xenorhabdus budapestensis sp. nov., type strain DSM 16342T, isolated from Steinernema bicornutam; Xenorhabdus ehlersii sp. nov., type strain DSM 16337T, isolated from Steinernema serratum; Xenorhabdus innexi sp. nov., type strain DSM 16336T isolated from Steinernema scapterisci; and Xenorhabdus szentirmaii sp. nov., type strain DSM 16338T, isolated from Steinernema rarum.

Two novel insect defensins from larvae of the cupreous chafer, Anomala cuprea: purification, amino acid sequences and antibacterial activity

A humoral immune response in larvae of the coleopteran insect, Anomala cuprea has been examined for exploring the molecular basis of host-pathogen interactions. The antibacterial activity against the Gram-positive strain, Micrococcus luteus was detected at a low level in absence of injection. The activity increased strikingly in the hemolymph of the larvae challenged with Escherichia coli, showing the fluctuating profile through a time course, which consists of the static induction phase, the production phase rising to a maximum level, and the reduction phase extending over a long duration. Two peptides were purified and characterized by reverse-phase HPLC, Edman degradation and mass spectrometry. They were isoforms, composed of similar sequences with two amino acid substitutions in 43 residues, and novel members of the insect defensins, cysteine-rich antibacterial peptides. Anomala defensins A and B showed potent activity against Gram-positive bacteria, with slight differences in activity against a few strains of tested bacteria. Anomala defensin B was active at high concentration of 40 microM against the Gram-negative strain, Xenorhabdus japonicus, a pathogen toward the host, A. cuprea larvae.

Phenotypic characterization of the Xenorhabdus bacterial symbiont of a Texas strain of the entomopathogenic nematode Steinernema riobrave, and characterization of the Xenorhabdus bovienii bacterial symbiont of a Newfoundland strain of Steinernema feltiae

Two bacterial symbionts of entomopathogenic nematodes, one of which originated from Texas, U.S.A., and the other from Newfoundland, Canada, were characterized phenotypically. These strains belonged to the genus Xenorhabdus. The Newfoundland (NF) strain was shown to be X. bovienii but the Texas (TX) strain was not identified at the species level. Four additional cultures of Xenorhabdus were included in the study. These were a strain of X. bovienii (Umeå), which was from a nematode of European origin, and strains of X. nematophilus, X. beddingii, and X. poinarii. The tests used in this study indicated identical properties for the NF (North American) and Umeå (European) strains of X. bovienii. These could be differentiated from the other strains studied by their failure to grow at 34 degrees C and resistance to low concentrations of a mixture of amoxilline and clavulanic acid. The Xenorhabdus TX strain could be differentiated from the other strains studied by its failure to grow at 10 degrees C. Of the tests done, approximately 30 were useful in distinguishing between the strains and species studied.

Novel application of PhastSystem polyacrylamide gel electrophoresis using restriction fragment length polymorphism--internal transcribed spacer patterns of individuals for molecular identification of entomopathogenic nematodes

différences! [editorial] [editorial]onomic way of identifying and assigning nematodes to taxons, which had already been determined either by comparative sequence analysis of nuclear rDNA internal transcribed spacer (ITS) region or by other methods of molecular or conventional taxonomy, is provided. Molecular identification of entomopathogenic nematodes (EPN) can be upgraded by basing it on PhastSystem polyacrylamide gel electrophoresis (PAGE) analysis of restriction fragment length polymorphism (RFLP) patterns of polymerase chain reaction (PCR)-amplified DNA derived from single nematodes of Steinernema or Heterorhabditis spp. Although analysis from single worms has previously been made on agarose gel, the resolution on PhastSystem PAGE gel is much higher. The DNA sequences selected for analysis were those constituting the internal transcribed spacer region between the 18S and 26S rDNA genes within the rRNA operon. RFLP analysis was carried out by gel electrophoresis on the PhastSystem (Pharmacia) as detailed elsewhere (Triga et al., Electrophoresis 1999, 20, 1272-1277. The downscaling from conventional agarose to PhastSystem gels resulted in pattern of DNA fragments differing from those obtained with agarose gel electrophoresis under conventional conditions by increasing the number of detected fragments. The approach supported previous species identifications and was able to identify several unclassified isolates, such as those from Hungary and Ireland, and provides a method for identification of previously unclassified strains. We confirmed that Heterorhabditis "Irish Type", represented by two strains of different geographical origin, comprise a species different from H. megidis. We also confirmed that strain IS5 belongs to the species H. indicus rather than to H. bacteriophora, as had been suggested previously.

PCR-ribotyping of Xenorhabdus and Photorhabdus isolates from the Caribbean region in relation to the taxonomy and geographic distribution of their nematode hosts

The genetic diversity of symbiotic Xenorhabdus and Photorhabdus bacteria associated with entomopathogenic nematodes was examined by a restriction fragment length polymorphism analysis of PCR-amplified 16S rRNA genes (rDNAs). A total of 117 strains were studied, most of which were isolated from the Caribbean basin after an exhaustive soil sampling. The collection consisted of 77 isolates recovered from entomopathogenic nematodes in 14 Caribbean islands and of 40 reference strains belonging to Xenorhabdus and Photorhabdus spp. collected at various localities worldwide. Thirty distinctive 16S rDNA genotypes were identified, and cluster analysis was used to distinguish the genus Xenorhabdus from the genus Photorhabdus. The genus Xenorhabdus appears more diverse than the genus Photorhabdus, and for both genera the bacterial genotype diversity is in congruence with the host-nematode taxonomy. The occurrence of symbiotic bacterial genotypes was related to the ecological distribution of host nematodes.

Xenorhabdus and Photorhabdus spp.: bugs that kill bugs

Xenorhabdus and Photorhabdus spp. are gram negative gamma proteobacteria that form entomopathogenic symbioses with soil nematodes. They undergo a complex life cycle that involves a symbiotic stage, in which the bacteria are carried in the gut of the nematodes, and a pathogenic stage, in which susceptible insect prey are killed by the combined action of the nematode and the bacteria. Both bacteria produce antibiotics, intracellular protein crystals, and numerous other products. These traits change in phase variants, which arise when the bacteria are maintained under stationary phase conditions in the laboratory. Molecular biological studies suggest that Xenorhabdus and Photorhabdus spp. may serve as valuable model systems for studying signal transduction and transcriptional and posttranscriptional regulation of gene expression. Such studies also indicate that these bacterial groups, which had been previously considered to be very similar, may actually be quite different at the molecular level.

Fatty acid composition of neutral lipid energy reserves in infective juveniles of entomopathogenic nematodes

The fatty acid composition of neutral lipids from infective juveniles (IJs) of Steinernema carpocapsae strain All, S. riobravis strain Biosys 355, S. feltiae strain UK76, and S. glaseri strain NC stored in distilled water at 25 degrees C was determined. Newly emerged IJs of all four species had similar neutral lipid fatty acid profiles and of the 18 fatty acids identified, C18:1n-9 (43-49 mol %), C16:0 (18-23%), C18:2n-6 (8-14%) and C18:0 (4-8%) were the most abundant. Unsaturated fatty acids predominated, with about 50% being monoene and 14-22% polyene; the unsaturation index ranged from 91.6 in S. glaseri to 111.6 in S. carpocapsae. The fatty acid composition of the total lipid and the free fatty acid fraction mirrored that of the neutral lipids. During storage, the relative levels (%) of C16:0, C18:0, and C18:ln-9 in the neutral lipids declined significantly, suggesting they were preferentially utilised.

Phylogenetic relationships of entomopathogenic nematophilic bacteria: Xenorhabdus spp. and Photorhabdus sp

Phylogenetic relationships of Xenorhabdus spp. and Photorhabdus sp. were investigated on the basis of 16S rRNA gene sequences. Xenorhabdus spp. and Photorhabdus sp. were grouped together with Proteus vulgaris and Arsenophonus nasoniae. This group was distant from other members of the family Enterobacteriaceae. Xenorhabdus japonicus, previously proposed as a new species, was nearly located to Xenorhabdus nematophilus. Signature nucleotides of X. japonicus were identified that distinguish it other members of the family Enterobacteriaceae.