Xenorhabdus nematophila (enterobacteriacea) secretes a cation-selective calcium-independent porin which causes vacuolation of the rough endoplasmic reticulum and cell lysis

Discovery of a new highly pathogenic toxin involved in insect sepsis

IMPORTANCE

As a current biocontrol resource, entomopathogenic nematodes and their symbiotic bacterium can produce many toxin factors to trigger insect sepsis, having the potential to promote sustainable pest management. In this study, we found Steinernema feltiae and Xenorhabdus bovienii were highly virulent against the insects. After infective juvenile injection, Galleria mellonella quickly turned black and softened with increasing esterase activity. Simultaneously, X. bovienii attacked hemocytes and released toxic components, resulting in extensive hemolysis and sepsis. Then, we applied high-resolution mass spectrometry-based metabolomics and found multiple substances were upregulated in the host hemolymph. We found extremely hazardous actinomycin D produced via 3-hydroxyanthranilic acid metabolites. Moreover, a combined transcriptomic analysis revealed that gene expression of proteins associated with actinomycin D was upregulated. Our research revealed actinomycin D might be responsible for the infestation activity of X. bovienii, indicating a new direction for exploring the sepsis mechanism and developing novel biotic pesticides.

Influence of exposure Heterorhabditis bacteriophora HP88, (Rhabditida: Heterorhabditidae) on biological and physiological parameters of Pseudosuccinea columella (Basommatophora: Lymnaeidae)

Many studies about fasciolosis control have been carried out, whether acting on the adult parasite or in Pseudosuccinea columella, compromising the development of the larval stages. The present study aimed to evaluate, under laboratory conditions, the susceptibility of P. columella to Heterorhabditis bacteriophora HP88, during for 24 and 48 hours of exposure. The snails were evaluated for 21 days for accumulated mortality; number of eggs laid; hatchability rate; biochemical changes; and histopathological analysis. We found that exposure induced a reduction in glucose and glycogen levels, characterizing a negative energy balance, due to the depletion of energy reserves as a result of the direct competition established by the nematode/endosymbiont bacteria complex in such substrates. A mortality rate of 48.25% and 65.52% was observed in the group exposed for 24 h and 48 h, respectively, along with significant impairment of reproductive biology in both exposed groups in relation to the respective controls. The results presented here show that P. columella is susceptible to the nematode H. bacteriophora, with the potential to be used as an alternative bioagent in the control of this mollusk, especially in areas considered endemic for fascioliasis, in line with the position expressed by the World Health Organization Health.

Cl- induces endothelium-dependent mesenteric arteriolar vasorelaxation through the NKCC1/TRPV4/NCX axis

AIMS

Although absorbed NaCl increases intestinal blood flow to facilitate absorption and transportation, it is unclear if it can directly mediate mesenteric arterial relaxation. We aimed to investigate and test our hypothesis that Cl- induces mesenteric arterial vasorelaxation via endothelium-dependent hyperpolarization (EDH).

MAIN METHODS

We used wire myograph to study NaCl-induced vasorelaxation of mesenteric arteries isolated from mice. Cl-, Ca2+ and K+ imaging was performed in human vascular endothelial cells pre-treated with pharmacological agents.

KEY FINDINGS

The Cl- concentration-dependently induced vasorelaxation of mesenteric arteries likely through EDH. The Cl--induced vasorelaxation was attenuated in TRPV4 KO mice and inhibited by selective blockers of Na+-K+-2Cl- cotransporter 1 (NKCC1) (bumetanide, 10 μM), transient receptor potential vanilloid 4 (TRPV4) (RN-1734, 40 μM), and small conductance Ca2+-activated K+ channels (SKCa) (apamin, 3 μM)/ intermediate conductance Ca2+-activated K+ channels (IKCa) (TRAM-34, 10 μM) and myoendothelial gap junction (18α-glycyrrhetinic acid, 10 μM), but enhanced by a selective activator of IKCa/SKCa (SKA-31, 0.3 μM). Cl- decreased intracellular K+ concentrations in endothelial cells, which was reversed by apamin (200 nM) plus TRAM-34 (500 nM). Extracellular Cl- raised intracellular Cl- concentrations in endothelial cells, which was attenuated by bumetanide (10 μM). Finally, Cl- induced a transient Ca2+ signaling via TRPV4 in endothelial cells, which became sustained when the Ca2+ exit mode of Na+-Ca2+ exchanger (NCX) was blocked.

SIGNIFICANCE

Cl- induces a pure EDH-mediated vasorelaxation of mesenteric arteries through activation of endothelial NKCC1/TRPV4/NCX axis. We have provided a novel insight into the role of Cl--induced vasorelaxation via EDH mechanism.

Heterorhabditis bacteriophora (Rhabditida: Heterorhabditidae), isolate HP88, induces reproductive and physiological alterations in Biomphalaria glabrata (Gastropoda: Planorbidae): an alternative for biological control of schistosomiasis

Heterorhabditis bacteriophora is an entomopathogenic nematode (EPN) that is mutually associated with Photorhabdus luminescens, utilized globally for biological control of numerous organisms. Freshwater snails of the species Biomphalaria glabrata have been incriminated as the main intermediate hosts of Schistosoma mansoni in Brazil, but virtually nothing is known about the susceptibility of these gastropod to EPNs. Information in this respect is relevant for control of these intermediate hosts, and thus of the helminthiases they transmit. This paper for the first time reports the susceptibility of B. glabrata to infective juveniles of H. bacteriophora (isolate HP88) under laboratory conditions. For that purpose, six groups were formed: three Control groups (not exposed) and three Treated groups, in which the snails were exposed to 300 juveniles infecting the nematode over three weeks. The entire experiment was conducted in triplicate, using a total of 270 snails. Significant physiological alterations in B. glabrata were observed in response to the infection by H. bacteriophora HP88, characterized by decreased levels of hemolymphatic glucose as well as reduced contents of glycogen stored in the host's digestive gland. In parallel, the hemolymphatic activity of lactate dehydrogenase increased in the infected snails, indicating that the infection induces breakdown of carbohydrate homeostasis in B. glabrata. Additionally, all the reproductive parameters analyzed were reduced as a consequence of the infection. The results indicate the occurrence of the phenomenon of parasitic castration in the B. glabrata/H. bacteriophora HP88 interface, probably due to the depletion of galactogen in the parasitized organism. Although the infection did not cause lethality in the population of infected snails, H. bacteriophora HP88 compromised the reproductive performance of B. glabrata, suggesting its applicability in programs for biological control of this planorbid.

Single-molecule imaging of pore-forming toxin dynamics in droplet interface bilayers

Single-channel recording from pore-forming toxins (PFTs) provides a clear and direct molecular readout of toxin action. However to complete any mechanistic understanding of PFT behavior, this functional kinetic readout must be linked to the underlying changes in toxin structure, binding, conformation, or stoichiometry. Here we review how single-molecule imaging methods might be used to further our understanding of PFTs, and provide detailed practical guidance on the use of droplet interface bilayers as a method capable of examining both single-molecule fluorescence and single-channel electrical signals from PFTs.

Partner-specific induction of Spodoptera frugiperda immune genes in response to the entomopathogenic nematobacterial complex Steinernema carpocapsae-Xenorhabdus nematophila

The Steinernema carpocapsae-Xenorhabdus nematophila association is a nematobacterial complex used in biological control of insect crop pests. The infection success of this dual pathogen strongly depends on its interactions with the host's immune system. Here, we used the lepidopteran pest Spodoptera frugiperda to analyze the respective impact of each partner in the induction of its immune responses. First, we used previously obtained RNAseq data to construct the immunome of S. frugiperda and analyze its induction. We then selected representative genes to study by RT-qPCR their induction kinetics and specificity after independent injections of each partner. We showed that both X. nematophila and S. carpocapsae participate in the induction of stable immune responses to the complex. While X. nematophila mainly induces genes classically involved in antibacterial responses, S. carpocapsae induces lectins and genes involved in melanization and encapsulation. We discuss putative relationships between these differential inductions and the pathogen immunosuppressive strategies.

Membrane insertion of α-xenorhabdolysin in near-atomic detail

α-Xenorhabdolysins (Xax) are α-pore-forming toxins (α-PFT) that form 1–1.3 MDa large pore complexes to perforate the host cell membrane. PFTs are used by a variety of bacterial pathogens to attack host cells. Due to the lack of structural information, the molecular mechanism of action of Xax toxins is poorly understood. Here, we report the cryo-EM structure of the XaxAB pore complex from Xenorhabdus nematophila and the crystal structures of the soluble monomers of XaxA and XaxB. The structures reveal that XaxA and XaxB are built similarly and appear as heterodimers in the 12–15 subunits containing pore, classifying XaxAB as bi-component α-PFT. Major conformational changes in XaxB, including the swinging out of an amphipathic helix are responsible for membrane insertion. XaxA acts as an activator and stabilizer for XaxB that forms the actual transmembrane pore. Based on our results, we propose a novel structural model for the mechanism of Xax intoxication.

Some bacteria make toxins that punch large holes into the membranes of host cells, destroying them like a puncture destroys a football. These “pore-forming toxins” allow many bacterial species to infect a variety of organisms, from insects to humans. Some sophisticated pore-forming toxins, such as the anthrax toxin, do not only form a pore but also use it to flood lethal toxins into the cell to kill it.

One bacterium called Xenorhabdus nematophila punctures the membranes of insect cells, using the same type of pore-forming toxins that other bacteria use to infect humans. Previous research has shown that two proteins – components A and B – form these pore-forming toxins. Given this two-protein formation, some scientists predicted these pore-forming toxins might act like those of the anthrax bacterium: one component forms the pore; the other component poisons the cell. But without detailed images of this pore-forming toxin’s structure, understanding exactly how these two components work together is almost impossible.

To explore how components A and B operate within X. nematophila, Schubert et al. captured images of the molecular structure of the two proteins. Common methods reliant on X-rays and electron microscopes revealed the layouts of both components. By visualizing the proteins at different stages, Schubert et al. observed key structural changes that enable them to form the pore and puncture a host cell.

Component A binds to component B’s back, forming a subunit – twelve to fifteen of which then conjoin as the pore-forming toxin. Schubert et al. conclude that component A stabilizes each subunit on the membrane and activates component B, which then punctures the membrane by swinging out its lower end. Unlike the anthrax pore-forming toxin, both components collaborate to form the pore complex and puncture the membrane. These results provide a foundation of knowledge about what these toxins look like and how they operate. More research building upon this structural analysis may help scientists develop antibiotics that prevent bacteria from destroying human cells.

Differential immunosuppression by inhibiting PLA2 affects virulence of Xenorhabdus hominickii and Photorhabdus temperata temperata

Immunity negatively influences bacterial pathogenicity. Eicosanoids mediate both cellular and humoral immune responses in insects. This study tested a hypothesis that differential bacterial virulence of Xenorhabdus/Photorhabdus is dependent on their inhibitory activity against phospholipase A₂ (PLA₂) activity. P. temperata subsp. temperata ('Ptt') was more than 40 times more potent than X. hominickii ('Xh'). Although both bacteria suppressed cellular immune responses, Ptt infection suppressed hemocyte nodule formation much more than Xh infection. Their differential immunosuppression appeared to be induced by their secondary metabolites because organic extracts of Ptt-cultured broth exhibited higher inhibitory activities against cellular immune responses than Xn-cultured broth extracts. Humoral immune responses were analyzed by measuring expression levels of 11 antimicrobial peptide (AMP) genes. Among inducible AMPs in hemocytes and fat body, higher number and more kinds of AMPs exhibited lower expression levels in Ptt infection than those in Xh infection. Suppressed immune responses induced by Ptt or Xh infection were significantly rescued by the addition of a catalytic product of PLA₂, suggesting that PLA₂ was a common inhibitory target. In fact, Ptt infection inhibited PLA₂ activity more strongly than Xh infection. RNA interference of a PLA₂ gene decreased its expression and significantly increased bacterial virulence. Moreover, addition of PLA₂ inhibitor to Xh infection enhanced its virulence, similar to virulence level of Ptt infection. These results suggest that variation in Xenorhabdus/Photorhabdus bacterial virulence can be explained by their differential inhibitory activities against host insect PLA₂.

Cytoplasmic vacuolization in cell death and survival

Cytoplasmic vacuolization (also called cytoplasmic vacuolation) is a well-known morphological phenomenon observed in mammalian cells after exposure to bacterial or viral pathogens as well as to various natural and artificial low-molecular-weight compounds. Vacuolization often accompanies cell death; however, its role in cell death processes remains unclear. This can be attributed to studying vacuolization at the level of morphology for many years. At the same time, new data on the molecular mechanisms of the vacuole formation and structure have become available. In addition, numerous examples of the association between vacuolization and previously unknown cell death types have been reported. Here, we review these data to make a deeper insight into the role of cytoplasmic vacuolization in cell death and survival.

Role of Endosymbionts in Insect-Parasitic Nematode Interactions

Endosymbiotic bacteria exist in many animals where they develop relationships that affect certain physiological processes in the host. Insects and their nematode parasites form great models for understanding the genetic and molecular basis of immune and parasitic processes. Both organisms contain endosymbionts that possess the ability to interfere with certain mechanisms of immune function and pathogenicity. This review summarizes recent information on the involvement of insect endosymbionts in the response to parasitic nematode infections, and the influence of nematode endosymbionts on specific aspects of the insect immune system. Analyzing this information will be particularly useful for devising endosymbiont-based strategies to intervene in insect immunity or nematode parasitism for the efficient management of noxious insects in the field.

The role of pilin protein of Xenorhabdus nematophila against immune defense reactions of insects

Xenorhabdus nematophila is a symbiotic bacterium of the entomopathogenic nematode Steinernema carpocapsae (Weiser). It produces several toxic proteins which interfere with the immune system of insects. The current study shows that purified pilin protein could be a virulence trait of X. nematophila. The fifth instar larvae of Spodoptera exigua (Hübner) was injected with purified pilin. Changes in the cellular defenses in terms of total haemocyte counts and granulocyte percentage and humoral factors including total protease, phospholipase A₂, and phenoloxidase activities (humoral defense) as well as the expression of the three main antimicrobial peptides attacin, cecropin, and spodoptericin were measured at specific times. The level of THC and granulocytes in larvae with different concentrations of pilin protein were less than the negative control. Also agglutination of haemocytes was observed 8-16h post-injection. The pilin protein activated phenoloxidase in the initial hour post-injection, by 2hpi, activity was stable. The activities of phospholipase A2 and protease activities reached maximum levels at 12 and 4hpi, respectively, and then decreased. The expressions of attacin, cecropin, and spodoptericin in larvae treated with pilin protein were up-regulated above that of the normal sample. The overexpression of cecropin was greater than the other antimicrobial protein mRNA transcripts. The spodoptericin expression had an irregular trend while expressions of attacin and cecropin reached maximum levels at 4hpi and then decreased. Generally, after the injection of pilin protein, the cellular and humoral immune system of S. exigua is activated but this toxin was able to inhibit them. This is the first report of the role of pilin protein when the bacterial symbiont of S. carpocapsae encounters the humoral defense of an insect.

Haemocoel injection of PirA1B1 to Galleria mellonella larvae leads to disruption of the haemocyte immune functions

The bacterium Photorhabdus luminescens produces a number of insecticidal proteins to kill its larval prey. In this study, we cloned the gene coding for a binary toxin PirA₁B₁ and purified the recombinant protein using affinity chromatography combined with desalination technology. Furthermore, the cytotoxicity of the recombinant protein against the haemocytes of Galleria mellonella larvae was investigated. We found that the protein had haemocoel insecticidal activity against G. mellonella with an LD50 of 131.5 ng/larva. Intrahaemocoelic injection of PirA₁B₁ into G. mellonella resulted in significant decreases in haemocyte number and phagocytic ability. In in vitro experiments, PirA₁B₁ inhibited the spreading behaviour of the haemocytes of G. mellonella larvae and even caused haemocyte degeneration. Fluorescence microscope analysis and visualization of haemocyte F-actin stained with phalloidin-FITC showed that the PirA₁B₁ toxin disrupted the organization of the haemocyte cytoskeleton. Our results demonstrated that the PirA₁B₁ toxin disarmed the insect cellular immune system.

Purification and identification of an antibacterial protein from the symbiotic bacteria associated with novel entomopathogenic nematode, Rhabditis (Oscheius) sp

Entomopathogenic nematodes (EPN) belonging to the families steinernematidae and heterorhabditidae and their symbiotic bacteria Xenorhabdus and Photorhabdus are well-known as biological control agents and are found to produce a wide range of bioactive secondary metabolites. Studies carried out at the Central Tuber Crops Research Institute (CTCRI) on entomopathogenic nematodes resulted in the identification of novel EPN belonging to the family Rhabditidae. This study reports the purification of a high molecular weight antibacterial protein from culture filtrates of a bacterium (Bacillus cereus) symbiotically associated with a novel entomopathogenic nematode Rhabditis (Oscheius) species, maintained at CTCRI laboratory. Fermentation conditions were standardized and optimum antibacterial activity was observed in tryptic soy broth after 48 h incubation at 30 °C. The aqueous extracts yielded antibacterial proteins which were purified by ammonium sulfate precipitation followed by ion exchange chromatography and size exclusion chromatography. Native gel electrophoresis indicated an active protein of molecular mass 220KDa which resolved into a major band of 90 kDa and a minor band of about 40 kDa on SDS-PAGE. The 90 kDa protein showed antibacterial activity and was further analysed by MALDI TOF-MS/MS. The protein was identified as a TQXA (Threonine-glutamine dipeptide) domain containing protein from Bacillus cereus. The protein was found to be active against Bacillus subtilis MTCC2756, Staphylococus aureus MTCC902 and Escherichia coli MTCC 2622 and was thermally stable.

An insecticidal protein from Xenorhabdus ehlersii triggers prophenoloxidase activation and hemocyte decrease in Galleria mellonella

The bacteria Xenorhabdus spp. are entomopathogenic symbionts that can produce several toxic proteins that interfere the immune system of insects. We purified an insecticidal protein from Xenorhabdus ehlersii, and designated it as XeGroEL with an estimated molecular mass of ~58 kDa. Galleria mellonella larva injected with XeGroEL presented prophenoloxidase activation and hemocyte decrease. XeGroEL can kill G. mellonella larva in 48 h with an LD(50) of 0.76 ± 0.08 μg/larva. Our results demonstrate that X. ehlersii possesses a toxic XeGroEL protein acting as a potential factor to activate proPO in host insect, which also provides a meaningful hypothesis to understand the interaction between nematode-symbiotic bacteria and host.

Imd pathway is involved in the interaction of Drosophila melanogaster with the entomopathogenic bacteria, Xenorhabdus nematophila and Photorhabdus luminescens

Xenorhabdus nematophila/Steinernema carpocapsae and Photorhabdus luminescens/Heterorhabditis bacteriophora are nemato-bacterial complexes highly pathogenic for insects. Using a syringe as artificial vector, we have analyzed the effects of the two bacteria, X. nematophila and P. luminescens on the genetic tool insect, Drosophila melanogaster. Both bacteria were found to kill adult flies in a dose dependent manner with X. nematophila being the fastest. On the other hand, when an injection of non-pathogenic bacteria, Escherichia coli, is performed 1 day before challenge with the entomopathogenic bacteria, then the survival of Drosophila flies was prolonged by at least 20h. After injection of entomopathogenic bacteria, Drosophila mutant Dif(1), affected on the Toll pathway, showed a similar phenotype than wild-type flies whereas Drosophila mutant Dredd(D55), affected on the imd pathway, was not protected by a prior injection of E. coli. This suggested that members of the imd pathway might be targets of these entomopathogenic bacteria albeit synthesis of antimicrobial peptides through this signaling pathway was induced by X. nematophila as well as P. luminescens. Finally, P. luminescens phoP mutant, an avirulent mutant in the Lepidopteran insect, Spodoptera littoralis, was found poorly virulent for D. melanogaster. phoP mutant partially protected D. melanogaster flies if injected 1 day before the injection of P. luminescens wild-type TT01 to the same extent than the E. coli-induced protection. However, phoP recovered a level of pathogenicity comparable to P. luminescens wild-type TT01 when injected to Drosophila flies affected on the imd pathway.

Spodoptera frugiperda X-tox protein, an immune related defensin rosary, has lost the function of ancestral defensins

Background

X-tox proteins are a family of immune-related proteins only found in Lepidoptera and characterized by imperfectly conserved tandem repeats of several defensin-like motifs. Previous phylogenetic analysis of X-tox genes supported the hypothesis that X-tox have evolved from defensins in a lineage-specific gene evolution restricted to Lepidoptera. In this paper, we performed a protein study in which we asked whether X-tox proteins have conserved the antimicrobial functions of their ancestral defensins and have evolved as defensin reservoirs.

Methodology/Principal Findings

We followed the outcome of Spod-11-tox, an X-tox protein characterized in Spodoptera frugiperda, in bacteria-challenged larvae using both immunochemistry and antimicrobial assays. Three hours post infection, the Spod-11-tox protein was expressed in 80% of the two main classes of circulating hemocytes (granulocytes and plasmatocytes). Located in secretory granules of hemocytes, Spod-11-tox was never observed in contact with microorganisms entrapped within phagolyzosomes showing that Spod-11-tox is not involved in intracellular pathogen killing. In fact, the Spod-11-tox protein was found to be secreted into the hemolymph of experimentally challenged larvae. In order to determine antimicrobial properties of the Spod-11-tox protein, it was consequently fractionated according to a protocol frequently used for antimicrobial peptide purification. Over the course of purification, the anti-Spod-11-tox immunoreactivity was found to be dissociated from the antimicrobial activity. This indicates that Spod-11-tox is not processed into bioactive defensins in response to a microbial challenge.

Conclusions/Significance

Altogether, our results show that X-tox proteins have not evolved as defensin reservoirs and have lost the antimicrobial properties of the ancestral insect defensins. The lepidopteran X-tox protein family will provide a valuable and tractable model to improve our knowledge on the molecular evolution of defensins, a class of innate immune effectors largely distributed over the three eukaryotic kingdoms.

Type III secretion system 1 of Vibrio parahaemolyticus induces oncosis in both epithelial and monocytic cell lines

The Vibrio parahaemolyticus type III secretion system 1 (T3SS1) induces cytotoxicity in mammalian epithelial cells. We characterized the cell death phenotype in both epithelial (HeLa) and monocytic (U937) cell lines following infection with V. parahaemolyticus. Using a combination of the wild-type strain and gene knockouts, we confirmed that V. parahaemolyticus strain NY-4 was able to induce cell death in both cell lines via a T3SS1-dependent mechanism. Bacterial contact, but not internalization, was required for T3SS1-induced cytotoxicity. The mechanism of cell death involves formation of a pore structure on the surface of infected HeLa and U937 cells, as demonstrated by cellular swelling, uptake of cell membrane-impermeable dye and protection of cytotoxicity by osmoprotectant (PEG3350). Western blot analysis showed that poly ADP ribose polymerase (PARP) was not cleaved and remained in its full-length active form. This result was evident for seven different V. parahaemolyticus strains. V. parahaemolyticus-induced cytotoxicity was not inhibited by addition of the pan-caspase inhibitor carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone (Z-VAD-FMK) or the caspase-1 inhibitor N-acetyl-tyrosyl-valyl-alanyl-aspartyl-aldehyde (Ac-YVAD-CHO); thus, caspases were not involved in T3SS1-induced cytotoxicity. DNA fragmentation was not evident following infection and autophagic vacuoles were not observed after monodansylcadaverine staining. We conclude that T3SS1 of V. parahaemolyticus strain NY-4 induces a host cell death primarily via oncosis rather than apoptosis, pyroptosis or autophagy.

FliZ, a flagellar regulator, is at the crossroads between motility, haemolysin expression and virulence in the insect pathogenic bacterium Xenorhabdus

There is a complex interplay between the regulation of flagellar motility and the expression of virulence factors in many bacterial pathogens. We investigated the role of FliZ in the regulation of flagellar and virulence genes in Xenorhabdus nematophila, an insect pathogen. The fliZ gene is the second gene in the fliAZ operon in X. nematophila. In vivo transcription analysis revealed a positive feedback loop of fliAZ transcription in which FliZ activates flhDC, the master operon of flagellar regulon in X. nematophila, leading to an increased transcription of the FlhDC-dependent promoter of fliAZ. We also showed that fliAZ and flhDC mutants lacked motility, had no haemolysin or Tween lipase activity and displayed an attenuated virulence phenotype in insects. Lipase activity is controlled by FliA, whereas haemolysin production and full virulence phenotype have been reported to be FliZ-dependent. Transcriptional analysis revealed that FliZ directly controlled expression of the xhlBA and xaxAB operons, which encode haemolysins from the two-partner secretion system and the binary XaxAB toxin family respectively. We suggest that this regulatory pathway may also occur in other pathogenic enterobacteria with genes encoding members of these two growing families of haemolysins.

Type 1 fimbriae of insecticidal bacterium Xenorhabdus nematophila is necessary for growth and colonization of its symbiotic host nematode Steinernema carpocapsiae

Xenorhabdus nematophila produces type 1 fimbriae on the surface of Phase I cells. Fimbriae mediate recognition and adhesion of the bacteria to its target cell. To investigate the role of fimbriae in the biology of X. nematophila, we have produced a fimbrial mutant strain by insertional inactivation of the mrxA gene, encoding the structural subunit of type 1 fimbriae. Phenotypic characterization of the mutant revealed loss of fimbriae on the cell surface. Cell surface characteristics like dye absorption, biofilm formation, red blood cell agglutination remained unaltered. The mrxA mutant was defective in swarming on soft agar, although swimming motility was not affected. Flagellar expression was suppressed in the mrxA strain under swarming conditions, but not swimming conditions. Agglutination and cytotoxicity of the mutant to larval haemocytes was also reduced. When the mutant cells were injected in the haemocoel of the fourth instar larvae of Helicoverpa armigera, an increase in the LT(50) of 9-12 h was observed relative to the wild-type strain. The nematode growth was slow on the lawn of the fimbrial mutant. The mrxA negative strain was unable to colonize the nematode gut efficiently. This study demonstrates importance of type 1 fimbriae in establishment of bacteria-nematode symbiosis, a key to successful pest management program.

Friend and foe: the two faces of Xenorhabdus nematophila

Comparisons of mutualistic and pathogenic relationships are necessary to decipher the common language of microorganism-host interactions, as well as the subtle differences in dialect that distinguish types of symbiosis. One avenue towards making such comparisons is to study a single organism that speaks both dialects, such as the gamma-proteobacterium Xenorhabdus nematophila. X. nematophila inhabits and influences the lives of two host animals, helping one to reproduce optimally while killing the other.

Clonal variation in Xenorhabdus nematophila virulence and suppression of Manduca sexta immunity

Virulence of the insect pathogen Xenorhabdus nematophila is attributed in part to its ability to suppress immunity. For example, X. nematophila suppresses transcripts encoding several antimicrobial proteins, even in the presence of Salmonella enterica, an inducer of these transcripts. We show here that virulence and immune suppression phenotypes can be lost in a subpopulation of X. nematophila. Cells that have undergone 'virulence modulation' (vmo) have attenuated virulence and fail to suppress antimicrobial transcript levels, haemocyte aggregation and nodulation in Manduca sexta insects. When plated on certain media, vmo cells have a higher proportion of translucent (versus opaque) colonies compared with non-vmo cells. Like vmo strains, translucent colony isolates are defective in virulence and immune suppression. The X. nematophila genome encodes two 'opacity' genes with similarity to the Ail/PagC/Rck family of outer membrane proteins involved in adherence, invasion and serum resistance. Quantitative polymerase chain reaction analysis shows that RNA levels of one of these opacity genes, opaB, are higher in opaque relative to translucent colonies. We propose that in X. nematophila opaB may be one of several factors involved in immune suppression during infection, and expression of these factors can be co-ordinately eliminated in a subpopulation, possibly through a phase variation mechanism.

The xaxAB Genes Encoding a New Apoptotic Toxin from the Insect Pathogen Xenorhabdus nematophila Are Present in Plant and Human Pathogens

Xenorhabdus nematophila, a member of the Enterobacteriaceae, kills many species of insects by strongly depressing the immune system and colonizing the entire body. A peptide cytotoxin has been purified from X. nematophila broth growth, and the cytolytic effect on insect immunocytes and hemolytic effect on mammalian red blood cells of this toxin have been described (Ribeiro, C., Vignes, M., and Brehélin, M. (2003) J. Biol. Chem. 278, 3030-3039). We show here that this toxin, Xenorhabdus alpha-xenorhabdolysin (Xax), triggers apoptosis in both insect and mammalian cells. We also report the cloning and sequencing of two genes, xaxAB, encoding this toxin in X. nematophila. The expression of both genes in recombinant Escherichia coli led to the production of active cytotoxin/hemolysin. However, hemolytic activity was observed only if the two peptides were added in the appropriate order. Furthermore, we report here that inactivation of xaxAB genes in X. nematophila abolished the major cytotoxic activity present in broth growth, called C1. We also show that these genes are present in various entomopathogenic bacteria of the genera Xenorhabdus and Photorhabdus, in Pseudomonas entomophila, in the human pathogens Yersinia enterocolitica and Proteus mirabilis, and in the plant pathogen Pseudomonas syringae. This toxin cannot be classified in any known family of cytotoxins on the basis of amino acid sequences, locus organization, and activity features. It is, therefore, probably the prototype of a new family of binary toxins.

The cytotoxic fimbrial structural subunit of Xenorhabdus nematophila is a pore-forming toxin

We have purified a fimbrial shaft protein (MrxA) of Xenorhabdus nematophila. The soluble monomeric protein lysed larval hemocytes of Helicoverpa armigera. Osmotic protection of the cells with polyethylene glycol suggested that the 17-kDa MrxA subunit makes pores in the target cell membrane. The internal diameter of the pores was estimated to be >2.9 nm. Electron microscopy confirmed the formation of pores by the fimbrial subunit. MrxA protein oligomerized in the presence of liposomes. Electrophysiological studies demonstrated that MrxA formed large, voltage-gated passive-diffusion channels in lipid bilayers.

Co-regulation of motility, exoenzyme and antibiotic production by the EnvZ-OmpR-FlhDC-FliA pathway in Xenorhabdus nematophila

Xenorhabdus nematophila is an emerging model for both mutualism and pathogenicity in different invertebrate hosts. Here we conduct a mutant study of the EnvZ-OmpR two-component system and the flagella sigma factor, FliA (sigma28). Both ompR and envZ strains displayed precocious swarming behaviour, elevated flhD and fliA mRNA levels and early production of lipase, protease, haemolysin and antibiotic activity. Inactivation of fliA eliminated exoenzyme production which was restored by complementation with the fliAZ operon. Inactivation of flhA, a gene encoding a component of the flagella export apparatus, eliminated lipase but not protease or haemolysin production indicating these enzymes are secreted by different export pathways. FliA-regulated lipase (xlpA) and protease (xrtA) genes were identified. Their expression and level of production were elevated in the ompR and envZ strains and markedly reduced in the fliA strain while both were expressed normally in the flhA strain. We also found that expression of nrps1 which encodes a non-ribosomal peptide synthetase was elevated in the ompR and envZ strains. The fliA strain was pathogenic towards the insect host indicating that motility and FliA-regulated exoenzyme production were not essential for virulence. These findings support a model in which the EnvZ-OmpR-FlhDC-FliA regulatory network co-ordinately controls flagella synthesis, and exoenzyme and antibiotic production in X. nematophila.

Stages of infection during the tripartite interaction between Xenorhabdus nematophila, its nematode vector, and insect hosts

Bacteria of the genus Xenorhabdus are mutually associated with entomopathogenic nematodes of the genus Steinernema and are pathogenic to a broad spectrum of insects. The nematodes act as vectors, transmitting the bacteria to insect larvae, which die within a few days of infection. We characterized the early stages of bacterial infection in the insects by constructing a constitutive green fluorescent protein (GFP)-labeled Xenorhabdus nematophila strain. We injected the GFP-labeled bacteria into insects and monitored infection. We found that the bacteria had an extracellular life cycle in the hemolymph and rapidly colonized the anterior midgut region in Spodoptera littoralis larvae. Electron microscopy showed that the bacteria occupied the extracellular matrix of connective tissues within the muscle layers of the Spodoptera midgut. We confirmed the existence of such a specific infection site in the natural route of infection by infesting Spodoptera littoralis larvae with nematodes harboring GFP-labeled Xenorhabdus. When the infective juvenile (IJ) nematodes reached the insect gut, the bacterial cells were rapidly released from the intestinal vesicle into the nematode intestine. Xenorhabdus began to escape from the anus of the nematodes when IJs were wedged in the insect intestinal wall toward the insect hemolymph. Following their release into the insect hemocoel, GFP-labeled bacteria were found only in the anterior midgut region and hemolymph of Spodoptera larvae. Comparative infection assays conducted with another insect, Locusta migratoria, also showed early bacterial colonization of connective tissues. This work shows that the extracellular matrix acts as a particular colonization site for X. nematophila within insects.