Photorhabdus luminescens is an enterobacterium that is symbiotic with soil entomopathogenic nematodes and pathogenic to a wide range of insects. P. luminescens promotes its own transmission among susceptible insect populations using its nematode host as vector 1 . Its life cycle comprises a symbiotic stage in the nematode's gut and a virulent stage in the insect larvae, which it kills through toxemia and septicemia. After the nematode attacks a prey insect and P. luminescens is released, the bacterium produces a wide variety of virulence factors ensuring rapid insect killing. Bioconversion of the insect cadaver by exoenzymes produced by the bacteria allows the bacteria to multiply and the nematode to reproduce. During this process P. luminescens produces antibiotics to prevent invasion of the insect cadaver by bacterial or fungal competitors. Finally, elimination of competitors allows P. luminescens and the nematode to reassociate specifically before leaving the insect cadaver 2,3 .To better understand this complex life style, we determined the genome sequence of P. luminescens subspecies laumondii strain TT01 4 , a symbiont of the nematode Heterorhabditis bacteriophora isolated on Trinidad and Tobago. RESULTS General featuresStrain TT01 possesses a single circular chromosome of 5,688,987 bp with an average GC content of 42.8%. No plasmid replicon was found.A total of 4,839 protein-coding genes, including 157 pseudogenes, seven complete sets (23S, 5S and 16S) of ribosomal RNA operons and 85 tRNA genes, were predicted ( Fig. 1; Supplementary Table 1 online). Toxins against insectsMore toxin genes were predicted in the P. luminescens genome than in any other bacterial genome sequenced yet. A large number of these toxins may be involved in the killing of a wide variety of insects. Some may act synergistically or use redundancy for 'overkill' 5 , ensuring a quick death of the host. In addition, some may kill insects by interfering with their development. In the TT01 genome, two paralogs, plu4092 and plu4436, encode proteins similar to juvenile hormone esterases (JHEs) of the insect Leptinotarsa decemlineata 6 . Juvenile hormone maintains the insect in a larval state. Its inactivation by JHE allows metamorphosis to proceed. JHEs may be used to trigger the insect endocrine machinery at an inappropriate time and thus represents a promising approach for insect control 7 . These genes are located downstream of highly related orphan genes (plu4093 and plu4437), suggesting a locus duplication.The toxicity of the proteins encoded by these two loci was verified experimentally. Two Escherichia coli clones, containing the recombinant BAC1A02 and BAC8C11, were shown to be toxic toward insects. BAC1A02, which contains the locus plu4093-plu4092, exhibited substantial oral toxicity toward three mosquito species, Aedes aegypti,

Members of the genus Xenorhabdus are entomopathogenic bacteria that associate with nematodes. The nematode-bacteria pair infects and kills insects, with both partners contributing to insect pathogenesis and the bacteria providing nutrition to the nematode from available insect-derived nutrients. The nematode provides the bacteria with protection from predators, access to nutrients, and a mechanism of dispersal. Members of the bacterial genus Photorhabdus also associate with nematodes to kill insects, and both genera of bacteria provide similar services to their different nematode hosts through unique physiological and metabolic mechanisms. We posited that these differences would be reflected in their respective genomes. To test this, we sequenced to completion the genomes of Xenorhabdus nematophila ATCC 19061 and Xenorhabdus bovienii SS-2004. As expected, both Xenorhabdus genomes encode many anti-insecticidal compounds, commensurate with their entomopathogenic lifestyle. Despite the similarities in lifestyle between Xenorhabdus and Photorhabdus bacteria, a comparative analysis of the Xenorhabdus, Photorhabdus luminescens, and P. asymbiotica genomes suggests genomic divergence. These findings indicate that evolutionary changes shaped by symbiotic interactions can follow different routes to achieve similar end points.

Xenorhabdus is a major insect pathogen symbiotically associated with nematodes of the family Steinernematidae. This motile bacterium displays swarming behavior on suitable media, but a spontaneous loss of motility is observed as part of a phenomenon designated phase variation which involves the loss of stationaryphase products active as antibiotics and potential virulence factors. To investigate the role of one of the transcriptional activators of flagellar genes, FlhDC, in motility and virulence, the Xenorhabdus nematophilus flhDC locus was identified by functional complementation of an Escherichia coli flhD null mutant and DNA sequencing. Construction of X. nematophilus flhD null mutants confirmed that the flhDC operon controls flagellin expression but also revealed that lipolytic and extracellular hemolysin activity is flhDC dependent. We also showed that the flhD null mutant displayed a slightly attenuated virulence phenotype in Spodoptera littoralis compared to that of the wild-type strain. Thus, these data indicated that motility, lipase, hemolysin, or unknown functions controlled by the flhDC operon are involved in the infectious process in insects. Our investigation expands the view of the flagellar regulon as a checkpoint coupled to a major network involving bacterial physiological aspects as well as motility.The genus Xenorhabdus (Enterobacteriaceae) consists of the specific bacterial symbionts of the entomopathogenic nematodes of the family Steinernematidae (57) and was separated from the genus Photorhabdus (10) containing the symbionts of the entomopathogenic nematodes of the family Heterorhabditidae. Both genera are entomopathogenic gram-negative bacteria belonging to the family Enterobacteriaceae. The nematodes of the Steinernematidae carry their bacterial symbionts monoxenically in a special vesicle in the intestines of the infective stage (L3 juveniles), while the nematodes of the Heterorhabditidae carry their bacterial symbionts throughout their intestines (22). These bacteria are transported by their nematode hosts into the hemocoel of the insect prey which is killed, probably via a combination of toxin action and septicemia. The bacterial symbionts also contribute to the symbiotic relationship by establishing and maintaining suitable conditions for nematode reproduction (46). Recently, a novel toxin complex with both oral and injectable activities against a wide range of insects was identified in Photorhabdus luminescens (11).The form of the bacterium that is normally isolated from symbiotic infective-stage nematodes is referred to as phase I. During in vitro culture or mass rearing of nematodes, Xenorhabdus and Photorhabdus strains spontaneously produce colonial variants which have been called phase II variants (9). The two variants of the bacteria have generally been shown to be equally pathogenic for the larvae of the greater wax moth, Galleria mellonella (4). Recently, Volgyi et al. (58) described for the first time a phase II variant that displayed reduced virulence in the Manduca sexta v...

Insects are the largest group of animals on earth. Like mammals, virus, fungi, bacteria and parasites infect them. Several tissue barriers and defense mechanisms are common for vertebrates and invertebrates. Therefore some insects, notably the fly Drosophila and the caterpillar Galleria mellonella, have been used as models to study host-pathogen interactions for several insect and mammal pathogens. They are excellent tools to identify pathogen determinants and host tissue cell responses. We focus here on the comparison of effectors used by two different groups of bacterial insect pathogens to accomplish the infection process in their lepidopteran larval host: Bacillus thuringiensis and the nematode-associated bacteria, Photorhabdus and Xenorhabdus. The comparison reveals similarities in function and expression profiles for some genes, which suggest that such factors are conserved during evolution in order to attack the tissue encountered during the infection process.

Growing resistance of pathogenic bacteria and shortage of antibiotic discovery platforms challenge the use of antibiotics in the clinic. This threat calls for exploration of unconventional sources of antibiotics and identification of inhibitors able to eradicate resistant bacteria. Here we describe a different class of antibiotics, odilorhabdins (ODLs), produced by the enzymes of the non-ribosomal peptide synthetase gene cluster of the nematode-symbiotic bacterium Xenorhabdus nematophila. ODLs show activity against Gram-positive and Gram-negative pathogens, including carbapenem-resistant Enterobacteriaceae, and can eradicate infections in animal models. We demonstrate that the bactericidal ODLs interfere with protein synthesis. Genetic and structural analyses reveal that ODLs bind to the small ribosomal subunit at a site not exploited by current antibiotics. ODLs induce miscoding and promote hungry codon readthrough, amino acid misincorporation, and premature stop codon bypass. We propose that ODLs' miscoding activity reflects their ability to increase the affinity of non-cognate aminoacyl-tRNAs to the ribosome.

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.

Vibrio splendidus, strain LGP32, is an oyster pathogen associated with the summer mortalities affecting the production of Crassostrea gigas oysters worldwide. Vibrio splendidus LGP32 was shown to resist to up to 10 microM Cg-Def defensin and Cg-BPI bactericidal permeability increasing protein, two antimicrobial peptides/proteins (AMPs) involved in C. gigas immunity. The resistance to both oyster Cg-Def and Cg-BPI and standard AMPs (polymyxin B, protegrin, human BPI) was dependent on the ompU gene. Indeed, upon ompU inactivation, minimal bactericidal concentrations decreased by up to fourfold. AMP resistance was restored upon ectopic expression of ompU. The susceptibility of bacterial membranes to AMP-induced damages was independent of the ompU-mediated AMP resistance. Besides its role in AMP resistance, ompU proved to be essential for the adherence of V. splendidus LGP32 to fibronectin. Interestingly, in vivo, ompU was identified as a major determinant of V. splendidus pathogenicity in oyster experimental infections. Indeed, the V. splendidus-induced oyster mortalities dropped from 56% to 11% upon ompU mutation (Kaplan-Meier survival curves, P < 0.01). Moreover, in co-infection assays, the ompU mutant was out competed by the wild-type strain with competitive indexes in the range of 0.1-0.2. From this study, ompU is required for virulence of V. splendidus. Contributing to AMP resistance, conferring adhesive properties to V. splendidus, and being essential for in vivo fitness, the OmpU porin appears as an essential effector of the C. gigas/V. splendidus interaction.

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 ␣-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.