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Phytophthora pseudosyringae is a self-fertile pathogen of woody plants, particularly associated with tree species from the genera Fagus, Notholithocarpus, Nothofagus and Quercus, which is found across Europe and in parts of North America and Chile. It can behave as a soil pathogen infecting roots and the stem collar region, as well as an aerial pathogen infecting leaves, twigs and stem barks, causing particular damage in the United Kingdom and western North America. The population structure, migration and potential outcrossing of a worldwide collection of isolates were investigated using genotyping-by-sequencing. Coalescent-based migration analysis revealed that the North American population originated from Europe. Historical gene flow has occurred between the continents in both directions to some extent, yet contemporary migration is overwhelmingly from Europe to North America. Two broad population clusters dominate the global population of the pathogen, with a subgroup derived from one of the main clusters found only in western North America. Index of association and network analyses indicate an influential level of outcrossing has occurred in this preferentially inbreeding, homothallic oomycete. Outcrossing between the two main population clusters has created distinct subgroups of admixed individuals that are, however, less common than the main population clusters. Differences in life history traits between the two main population clusters should be further investigated together with virulence and host range tests to evaluate the risk each population poses to natural environments worldwide., (© 2024 The Authors. Molecular Plant Pathology published by British Society for Plant Pathology and John Wiley & Sons Ltd.)

Fusarium oxysporum f.sp. lactucae (Fol) causes a vascular disease in lettuce that results in significant yield losses. Race-specific and sensitive real-time PCR assays were developed for Fol races 1 and 4, which are prevalent in Europe. Using genotyping-by-sequencing, unique DNA loci specific to each race were identified and subsequently used for the design of primers and hydrolysis probes. Two assays per race were developed to ensure specificity. The two assays of each race could be run in duplex format, while still giving a sensitivity of 100 fg genomic DNA for all assays. Sample preparation methods were developed for plant tissue, soil, and surfaces, with an extra enrichment step when additional sensitivity was required. By controlling the incubation conditions during the enrichment step, the real-time PCR signal could be matched to the number of spore equivalents in the original sample. When enriching naturally infested soil, down to six conidiospore equivalents L -1 soil could be detected. As enrichment ensures sensitive detection and focuses on living Fol propagules, it facilitates the evaluation of control measures. The developed detection methods for soil and surfaces were applied to samples from commercial lettuce farms and confirmed the prevalence of Fol race 4 in Belgium. Monitoring of soil disinfestation events revealed that despite a dramatic decrease in quantity, the pathogen could still be detected either immediately after sheet steaming or after harvesting the first new crop. The detection method for plant tissue was successfully used to quantify Fol in lettuce inoculated with race 1, race 4 or a combination of both. Under the temperature conditions used, race 4 was more aggressive than race 1, as reflected in larger amounts of DNA of race 4 detected in the roots. These newly developed assays are a promising tool for epidemiological research as well as for the evaluation of control measures., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision., (Copyright © 2023 Mestdagh, Van Poucke, Haegeman, Dockx, Vandevelde, Dendauw, Decombel, Höfte and Heungens.)

Invasive, exotic plant pathogens pose a major threat to native and agricultural ecosystems. Phytophthora × cambivora is an invasive, destructive pathogen of forest and fruit trees causing severe damage worldwide to chestnuts (Castanea), apricots, peaches, plums, almonds and cherries (Prunus), apples (Malus), oaks (Quercus), and beech (Fagus). It was one of the first damaging invasive Phytophthora species to be introduced to Europe and North America, although its origin is unknown. We determined its population genetic history in Europe, North and South America, Australia and East Asia (mainly Japan) using genotyping-by-sequencing. Populations in Europe and Australia appear clonal, those in North America are highly clonal yet show some degree of sexual reproduction, and those in East Asia are partially sexual. Two clonal lineages, each of opposite mating type, and a hybrid lineage derived from these two lineages, dominated the populations in Europe and were predominantly found on fagaceous forest hosts (Castanea, Quercus, Fagus). Isolates from fruit trees (Prunus and Malus) belonged to a separate lineage found in Australia, North America, Europe and East Asia, indicating the disease on fruit trees could be caused by a distinct lineage of P. × cambivora, which may potentially be a separate sister species and has likely been moved with live plants. The highest genetic diversity was found in Japan, suggesting that East Asia is the centre of origin of the pathogen. Further surveys in unsampled, temperate regions of East Asia are needed to more precisely identify the location and range of the centre of diversity., (© 2023. The Author(s).)

Ceratocystis platani (CP), an ascomycetous fungus, is the agent of canker stain, a lethal vascular disease of Platanus species. Ceratocystis platani has been listed as a quarantine pest (EPPO A2 list) due to extensive damage caused in Southern Europe and the Mediterranean region. As traditional diagnostic assays are ineffective, a Real-Time PCR detection method based on EvaGreen, SYBR Green, and Taqman assays was previously developed, validated in-house, and included in the official EPPO standard PM7/14 (2). Here, we describe the results of a test performance study performed by nine European laboratories for the purpose of an interlaboratory validation. Verification of the DNA extracted from biological samples guaranteed the high quality of preparations, and the stability and the homogeneity of the aliquots intended for the laboratories. All of the laboratories reproduced nearly identical standard curves with efficiencies close to 100%. Testing of blind-coded DNA extracted from wood samples revealed that all performance parameters-diagnostic sensitivity, diagnostic specificity, accuracy and reproducibility-were best fit in most cases both at the laboratory and at the assay level. The previously established limit of detection, 3 fg per PCR reaction, was also validated with similar excellent results. The high interlaboratory performance of this Real-Time PCR method confirms its value as a primary tool to safeguard C. platani -free countries by way of an accurate monitoring, and to investigate the resistance level of potentially canker stain-resistant Platanus genotypes.

The genus Phytophthora comprises many economically and ecologically important plant pathogens. Hybrid species have previously been identified in at least six of the 12 phylogenetic clades. These hybrids can potentially infect a wider host range and display enhanced vigour compared to their progenitors. Phytophthora hybrids therefore pose a serious threat to agriculture as well as to natural ecosystems. Early and correct identification of hybrids is therefore essential for adequate plant protection but this is hampered by the limitations of morphological and traditional molecular methods. Identification of hybrids is also important in evolutionary studies as the positioning of hybrids in a phylogenetic tree can lead to suboptimal topologies. To improve the identification of hybrids we have combined genotyping-by-sequencing (GBS) and genome size estimation on a genus-wide collection of 614 Phytophthora isolates. Analyses based on locus- and allele counts and especially on the combination of species-specific loci and genome size estimations allowed us to confirm and characterize 27 previously described hybrid species and discover 16 new hybrid species. Our method was also valuable for species identification at an unprecedented resolution and further allowed correct naming of misidentified isolates. We used both a concatenation- and a coalescent-based phylogenomic method to construct a reliable phylogeny using the GBS data of 140 non-hybrid Phytophthora isolates. Hybrid species were subsequently connected to their progenitors in this phylogenetic tree. In this study we demonstrate the application of two validated techniques (GBS and flow cytometry) for relatively low cost but high resolution identification of hybrids and their phylogenetic relations.

Boxwood blight is a disease threat to natural and managed landscapes worldwide. To determine mating potential of the fungi responsible for the disease, Calonectria pseudonaviculata and C. henricotiae, we characterized their mating-type (MAT) loci. Genomes of C. henricotiae, C. pseudonaviculata and two other Calonectria species (C. leucothoes, C. naviculata) were sequenced and used to design PCR tests for mating-type from 268 isolates collected from four continents. All four Calonectria species have a MAT locus that is structurally consistent with the organization found in heterothallic ascomycetes, with just one idiomorph per individual isolate. Mating type was subdivided by species: all C. henricotiae isolates possessed the MAT1-1 idiomorph, whereas all C. pseudonaviculata isolates possessed the MAT1-2 idiomorph. To determine the potential for divergence at the MAT1 locus to present a barrier to interspecific hybridization, evolutionary analysis was conducted. Phylogenomic estimates showed that C. henricotiae and C. pseudonaviculata diverged approximately 2.1 Mya. However, syntenic comparisons, phylogenetic analyses, and estimates of nucleotide divergence across the MAT1 locus and proximal genes identified minimal divergence in this region of the genome. These results show that in North America and parts of Europe, where only C. pseudonaviculata resides, mating is constrained by the absence of MAT1-1. In regions of Europe where C. henricotiae and C. pseudonaviculata currently share the same host and geographic range, it remains to be determined whether or not these two recently diverged species are able to overcome species barriers to mate., (Copyright © 2019 Elsevier Inc. All rights reserved.)

An international test performance study (TPS) was organised to generate validation data for three molecular Synchytrium endobioticum tests: van den Boogert et al. ( European Journal of Plant Pathology 113, 47-57, 2005), and van Gent-Pelzer et al. ( European Journal of Plant Pathology, 126, 129-133, 2010) for the detection of S. endobioticum , and the pathotype 1(D1) identification test described by Bonants et al. ( European Journal of Plant Pathology, 143 , 495-506, 2015). Two TPS rounds were organised focussing on different test matrices, i.e. round 1: warted potato tissue, and round 2: resting spore suspensions. When using the tests for detection and identification of S. endobioticum in warted potato tissue, no significant differences were observed for diagnostic sensitivity, diagnostic specificity, overall accuracy, analytical sensitivity and robustness. When using the tests for detection and identification of S. endobioticum in resting spore suspensions, the van den Boogert and van Gent-Pelzer tests significantly outperform the Bonants test for diagnostic sensitivity and diagnostic specificity. For overall accuracy and analytical sensitivity, the van Gent-Pelzer significantly outperforms the van den Boogert and Bonants tests and is regarded as the test of choice when identifying S. endobioticum from resting spores. Tests regarded fit for purpose for routine testing of wart material and resting spore suspensions are proposed for the update of EPPO standard PM7/28(1) Synchytrium endobioticum ., Competing Interests: The authors declare that they have no conflict of interest.This article does not contain any studies with human participants or animals performed by any of the authors. This research project was performed within the Euphresco II framework: Diagnostic methods for Synchytrium endobioticum, especially for pathotype identification (SENDO), via a non-competitive.

Phytophthora ramorum is a recently introduced, aggressive Phytophthora species that has caused extensive mortality of oak and tanoak trees in the western USA and Japanese larch trees in the UK. P. ramorum is also present on Rhododendron, Camellia, and Viburnum in the nursery industry, which is thought to have been the pathway for its spread into new geographic regions including forests and natural ecosystems. Three lineages of P. ramorum have been described, informally designated EU1, NA1, and NA2, and each lineage is believed to originate from an as yet unknown exotic centre of origin. Preliminary SSR and sequence analysis of isolates from a UK P. ramorum survey revealed seven isolates with profiles that did not match the previously known lineages. Detailed SSR and multilocus sequence analysis of these isolates are presented, allowing us to assign these isolates to a new P. ramorum lineage, designated EU2. Although the known geographical origin of these isolates is currently limited to Northern Ireland and western Scotland, the EU2 lineage isolates have been obtained from four different host plants, including Japanese larch. All isolates are of A1 compatibility type, which implies that this finding does not increase the risk of outcrossing with the EU1 lineage isolates already present in the UK. The oldest EU2 strain was isolated in 2007 but no SSR-based intraEU2 lineage genotypic diversity was detected. The combination of these elements points to a recent introduction, despite emergency phytosanitary measures to control introduction and spread. A PCR-RFLP method for the rapid identification of EU2 lineage isolates is presented., (Crown Copyright © 2012. Published by Elsevier Ltd. All rights reserved.)

The recently emerged plant pathogen Phytophthora ramorum is responsible for causing the sudden oak death epidemic. This review documents the emergence of P. ramorum based on evolutionary and population genetic analyses. Currently infection by P. ramorum occurs only in Europe and North America and three clonal lineages are distinguished: EU1, NA1 and NA2. Ancient divergence of these lineages supports a scenario in which P. ramorum originated from reproductively isolated populations and underwent at least four global migration events. This recent work sheds new light on mechanisms of emergence of exotic pathogens and provides crucial insights into migration pathways., (Published by Elsevier Ltd.)

An internal fruit rot disease of sweet peppers was first detected in Belgium in 2003. Research conducted mostly in Canada indicates that this disease is primarily caused by Fusarium lactis Pirotta. Ninety-eight Fusarium isolates obtained from diseased sweet peppers from Belgium, as well as from other countries (Canada, the Netherlands and the United Kingdom) were identified by sequencing the translation elongation factor 1α (EF). Of these 98 isolates, 13 were identified as F. oxysporum Schltdl., nine as F. proliferatum (Matsush.) Nirenberg and two belonged to clade 3 of the F. solani species complex. Of the 74 remaining isolates, the EF sequence showed 97% to 98% similarity to F. lactis. Of these isolates, the β-tubulin (TUB), calmodulin (CAM) and the second largest subunit of RNA polymerase II (RPB2) genes were also sequenced. Analysis of the combined sequences revealed that the 74 isolates share nine combined sequences that correspond to nine multilocus sequence types (STs), while the F. lactis neotype strain and one other strain, both isolated from figs, form a separate ST. Together, these 10 STs represent a monophyletic F. lactis species complex (FLASC). An unusually high level of genetic diversity was observed between (groups of) these STs. Two of them (ST5 and ST6) fulfilled the criteria for species recognition based on genealogical exclusivity and together represent a new monophyletic species lineage (FLASC-1). The seven other STs, together with the F. lactis neotype ST, form a paraphyletic species lineage in the African clade of the Gibberella fujikuroi species complex (GFSC). From each of the 10 STs, the mycotoxin production was assessed using a multi-mycotoxin liquid chromatography mass spectrometry method. Out of the 27 analyzed mycotoxins, beauvericin and fumonisins were detected in sweet pepper tissue and in maize kernels. The 10 STs clearly differed in the amount of mycotoxin produced, but there was only limited congruence between the production profile and the phylogenetic analysis. Furthermore, the morphological characterization (based on mycelial growth rate and the length of macroconidia) showed distinct differences between the 10 STs, but again there was limited congruence with the phylogenetic results. In conclusion, the data presented in this study demonstrate that 75% of the isolates obtained from sweet pepper with internal fruit rot belong to a F. lactis species complex (FLASC), including a new FLASC-1 monophyletic species, and that the members of this complex display great genetic and phenotypic diversity., (Copyright © 2011. Published by Elsevier B.V.)

Phytophthora ramorum is a recently introduced pathogen in Europe and North America consisting of three clonal lineages. Due to the limited intralineage genetic variation, only a few polymorphic markers are available for use in studies involving the epidemiology and evolution of P. ramorum. A total of 159 primer pairs for candidate polymorphic SSR loci were tested with universal labeling. Four polymorphic microsatellite loci were identified within the NA1 lineage and one within the NA2 lineage, demonstrating the power and flexibility of the screening technique. The markers may significantly increase the number of genotypes that can be identified and as such can help better characterize the North American lineages of P. ramorum.

The functionality of the sexual cycle in the heterothallic pathogen Phytophthora ramorum, causal agent of Sudden Oak Death, has recently been demonstrated. Sexual reproduction could create genotypic variation and increase the pathogen's ability to adapt to other host plants or changing environments. Genetic characterization using co-dominant microsatellite markers and flow cytometry of single-oospore progeny of crosses between a European A1 isolate and North American or European A2 isolates revealed a considerable number of non-Mendelian inheritance events. This includes inheritance of more than two alleles at a locus and non-inheritance of alleles from one parent at another locus. The progenies were mitotically unstable: zoospore and hyphal tip derivatives of the progenies showed genotypic rearrangements and phenotypic variation. Flow cytometry confirmed variation and instability in DNA content of the single-oospore progenies. This indicates that single-oospore progenies not only display aberrant genomic and phenotypic variation due to meiotic irregularities, but also extra variation as a result of post-meiotic genomic rearrangements., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Phytophthora ramorum, the cause of sudden oak death on oak and ramorum blight on woody ornamentals, has been reported in ornamental nurseries on the West Coast of North America from British Columbia to California. Long-distance migration of P. ramorum has occurred via the nursery trade, and shipments of host plants are known to have crossed the U.S.-Canadian border. We investigated the genotypic diversity of P. ramorum in Canadian nurseries and compared the Canadian population with U.S. and European nursery isolates for evidence of migration among populations. All three of the P. ramorum clonal lineages were found in Canada but, unexpectedly, the most common was the NA2 lineage. The NA1 clonal lineage, which has been the most common lineage in U.S. nurseries, was found relatively infrequently in Canada, and these isolates may have been the result of migration from the United States to Canada. The EU1 lineage was observed almost every year and shared multilocus genotypes with isolates from Europe and the United States. Estimation of migration rates between Europe and North America indicated that migration was higher from Europe to North America than vice versa, and that unidirectional migration from Europe to North America was more likely than bidirectional migration.

In this work the presence and migration behavior of mycotoxins formed in sweet pepper, inoculated by Fusarium species involved in internal fruit rot, were investigated. Two different commercial sweet pepper cultivars were inoculated with two different Fusarium proliferatum isolates that were sampled from diseased peppers. After 10 days of incubation at 20 °C in a closed container, the lesion caused by the fungal infection was dissected. Around the lesion, up to three concentric rings of pepper fruit tissue with a width of 5 mm were cut out and analyzed using a multimycotoxin LC-MS/MS method. The analyses resulted in the detection of beauvericin and fumonisins B(1), B(2), and B(3). Beauvericin was detected only in the lesions (95%), and the levels varied between 67 and 73800 μg/kg. Fumonisins B(1), B(2), and B(3) were detected in the lesions and in the surrounding tissue, indicating migration of these toxins into healthy parts of the sweet pepper. In the lesion the fumonisin B(1) level varied between 690 and 104000 μg/kg. Even in the outer ring fumonisin B(1) was still present. Mostly it was present at a lower level than in the lesion, with a maximum level of 556 μg/kg. A similar migration behavior was obtained for fumonisins B(2) and B(3), but lower levels were detected in the lesions, up to 10900 and 1287 μg/kg, respectively. The analysis of 20 pepper samples resulted in the detection of beauvericin or alternariol. Seven samples were contaminated, and the level of beauvericin was 124 μg/kg (N = 1), whereas the level of alternariol varied from below the LOQ (6.6 μg/kg) to 101 μg/kg (N = 6).

Isolates of the most important Puccinia species that have been reported on Chrysanthemum x morifolium were collected and the sequences of their ribosomal DNA internal transcribed spacers ITS1 and ITS2 were determined and used as phylogenetic markers. The focus of this study was on Puccinia horiana, due to its quarantine status and its impact in commercial chrysanthemum production. Three technical adjustments were needed to reliably obtain the nucleotide sequences starting from fresh or dried samples. The complete rDNA ITS nucleotide sequences of P. horiana, Puccinia chrysanthemi, and Puccinia tanaceti isolates of varying age and geographic origin were determined. We also identified an as yet undescribed Puccinia species on six old herbarium samples from chrysanthemum. This new species is morphologically similar to P. chrysanthemi and near identical to recent rust samples from Artemisia tridentata. P. tanaceti could not be confirmed as a pathogen of chrysanthemum. Different rDNA ITS sequences were present in P. horiana, with intra-isolate and inter-isolate variability in the length of three nucleotide repeat regions in the different rDNA tandem copies. We also identified three ITS types within P. horiana, with the rarer types displaying up to 67 bp nucleotide sequence differences. These rarer ITS types were detected at low copy number in all isolates. In general, very little rDNA ITS sequence variation was observed between P. horiana isolates from 1903 and 2003, and among isolates from different continents. Phylogenetic analyses using distance, Maximum Likelihood and Bayesian methods confirmed P. horiana, P. chrysanthemi, and the new Puccinia sp. as well-resolved groups, with P. horiana clustering in the clade where the economically important rust species of the Poaceae are located, and P. chrysanthemi and the new Puccinia sp. clustering in the clade where the majority of the rust fungi with hosts in the Asteraceae is located.

Puccinia horiana Henn. is a quarantine organism and one of the most important fungal pathogens of Chrysanthemum x morifolium cultivars grown for cut flower or potted plant production (florist's chrysanthemum) in several regions of the world. Highly specific primer pairs were identified for conventional, nested, and real-time PCR detection of P. horiana based on the specific and sensitive PCR amplification of selected regions in the internal transcribed spacers (ITS1 and ITS2) of the nuclear ribosomal DNA (rDNA). Using these different PCR versions, 10 pg, 10 fg, and 5 fg genomic DNA could be detected, respectively. When using cloned target DNA as template, the detection limits were 5000, 50, and 5 target copies, respectively. These detection limits were not affected by a background of chrysanthemum plant DNA. The DNA extraction method was optimized to maximize the recoverability of the pathogen from infected plant tissue. A CTAB extraction protocol or a selection of commercial DNA extraction methods allowed the use of 10 ng total (plant+pathogen) DNA without interference of PCR inhibitors. Due to the specificity of the primers, SYBR Green I technology enabled reliable real time PCR signal detection. However, an efficient TaqMan probe is available. The lowest proportion of infected plant material that could still be detected when mixed with healthy plant material was 0.001%. The real-time PCR assay could detect as few as eight pure P. horiana basidiospores, demonstrating the potential of the technique for aerial detection of the pathogen. The amount of P. horiana DNA in plant tissue was determined at various time points after basidiospore inoculation. Using the real-time PCR protocol, it was possible to detect the pathogen immediately after the inoculation period, even though the accumulation of pathogen DNA was most pronounced near the end of the latent period. The detection system proved to be accurate and sensitive and could help not only in pathogen diagnosis but also in pathogen monitoring and disease forecasting systems.

A multi-mycotoxin method was developed for the simultaneous determination of trichothecenes (nivalenol, deoxynivalenol, 3-acetyldeoxynivalenol, 15-acetyldeoxynivalenol, neosolaniol, fusarenon-X, diacetoxyscirpenol, HT-2 toxin, T-2 toxin), aflatoxins (aflatoxin-B(1), aflatoxin-B(2), aflatoxin-G(1) and aflatoxin-G(2)), Alternaria toxins (alternariol, alternariol methyl ether and altenuene), fumonisins (fumonisin-B(1), fumonisin-B(2) and fumonisin-B(3)), ochratoxin A, zearalenone, beauvericin and sterigmatocystin in sweet pepper. Sweet pepper was extracted with ethyl acetate/formic acid (99:1, v/v). After splitting up the extract, two-thirds of the extract was cleaned up using an aminopropyl column followed by an octadecyl column. The remaining part was cleaned up using a strong anion-exchange column. After recombination of both cleaned parts of the sample extract, the combined solvents were evaporated and the residue was dissolved in mobile phase; 20 microL was injected into the chromatographic system, so only one run was used to separate and detect the mycotoxins in positive electrospray ionization using selected reaction monitoring. The samples were analyzed with a Micromass Quattro Micro triple quadrupole mass spectrometer (Waters, Milford, MA, USA). The mobile phase consisted of variable mixtures of water and methanol, 1% acetic acid and 5 mM ammonium acetate. The limits of detection of the multi-mycotoxin method varied from 0.32 microg kg(-1) to 42.48 microg kg(-1). The multi-mycotoxin liquid chromatography/tandem mass spectrometry (LC/MS/MS) method fulfilled the method performance criteria required by the Commission Regulation (EC) No 401/2006. Sweet peppers inoculated by Fusarium species were analyzed using the developed method. Beauvericin (9-484 microg kg(-1)) and fumonisins (fumonisin-B(1) up to 4330 microg kg(-1), fumonisin-B(2) up to 4900 microg kg(-1), and fumonisin-B(3) up to 299 microg kg(-1)) were detected., ((c) 2008 John Wiley & Sons, Ltd.)

The bacterium Xenorhabdus nematophila is a mutualist of the entomopathogenic nematode Steinernema carpocapsae. During its life cycle, the bacterium exists both separately from the nematode and as an intestinal resident of a nonfeeding nematode form, the infective juvenile (IJ). The progression of X. nematophila from an ex vivo existence to a specific and persistent colonization of IJs is a model to understand the mechanisms mediating the initiation and maintenance of benign host-microbe interactions. To help characterize this process, we constructed an X. nematophila strain that constitutively expresses green fluorescent protein, which allowed its presence to be monitored within IJs. Using this strain, we showed that few bacterial cells initiate colonization of an individual IJ and that these grow inside the lumen of the IJ intestine in a reproducible polyphasic pattern during colonization. In accordance with these two observations, we demonstrated that the final population of bacteria in a nematode is of predominantly monoclonal origin, suggesting that only one or two bacterial clones initiate or persist during colonization of an individual nematode. These data suggest that X. nematophila initiates IJ colonization by competing for limited colonization sites or resources within the nematode intestine. This report represents the first description of the biological interactions occurring between X. nematophila and S. carpocapsae during the early stages of the colonization process, provides insights into the physiology of X. nematophila in its host niche, and will facilitate interpretation of future data regarding the molecular events mediating this process.

The production of an ant-deterrent factor(s) (ADF) by Xenorhabdus nematophila and Photorhabdus luminescens, the symbiotic bacteria of the nematodes Steinernema carpocapsae and Heterorhabditis bacteriophora, respectively, was examined. In addition to an in vivo assay in which bacteria were tested for their ability to produce ADF within insect cadavers (M.E. Baur, H. K. Kaya, and D. R. Strong, Biol. Control 12:231-236, 1998), an in vitro microtiter dish assay was developed to monitor ADF activity produced by bacteria grown in cultures. Using these methods, we show that ADF activity is present in the supernatants of bacterial cultures, is filterable, heat stable, and acid sensitive, and passes through a 10-kDa-pore-size membrane. Thus, ADF appears to be comprised of a small, extracellular, and possibly nonproteinaceous compound(s). The amount of ADF repellency detected depends on the ant species being tested, the sucrose concentration (in vitro assays), and the strain, form, and age of the ADF-producing bacteria. These findings demonstrate that the symbiotic bacteria of some species of entomopathogenic nematodes produce a compound(s) that deters scavengers such as ants and thus could protect nematodes from being eaten during reproduction within insect cadavers.

One stage in the symbiotic interaction between the bacterium Xenorhabdus nematophila and its nematode host, Steinernema carpocapsae, involves the species-specific colonization of the nematode intestinal vesicle by the bacterium. To characterize the bacterial molecular determinants that are essential for vesicle colonization, we adapted and applied a signature-tagged mutagenesis (STM) screen to this system. We identified 15 out of 3000 transposon mutants of X. nematophila with at least a 15-fold reduction in average vesicle colonization. These 15 mutants harbour disruptions in nine separate loci. Three of these loci have predicted open reading frames (ORFs) with similarity to genes (rpoS, rpoE, lrp) encoding regulatory proteins; two have predicted ORFs with similarity to genes (aroA, serC) encoding amino acid biosynthetic enzymes; one, designated nilB (nematode intestine localization), has an ORF with similarity to a gene encoding a putative outer membrane protein (OmpU) in Neisseria; and three, nilA, nilC and nilD, have no apparent homologues in the public database. nilA, nilB and nilC are linked on a single 4 kb locus. nilB and nilC are > 104-fold reduced in their ability to colonize the nematode vesicle and are predicted to encode membrane-localized proteins. The nilD locus contains an extensive repeat region and several small putative ORFs. Other than reduced colonization, the nilB, nilC and nilD mutants did not display alterations in any other phenotype tested, suggesting a specific role for these genes in allowing X. nematophila to associate with the nematode host.