Protein inclusions produced by the entomopathogenic bacterium Xenorhabdus nematophilus subsp. nematophilus

Characterization of the pixB gene in Xenorhabdus nematophila and discovery of a new gene family

Xenorhabdus nematophila are Gram-negative bacteria that engage in mutualistic associations with entomopathogenic nematodes. To reproduce, the nematodes invade insects and release X. nematophila into the haemolymph where it functions as an insect pathogen. In complex medium, X. nematophila cells produce two distinct types of intracellular crystalline inclusions, one composed of the methionine-rich PixA protein and the other composed of the PixB protein. Here we show that PixB crystalline inclusions were neither apparent in X. nematophila cells grown in medium that mimics insect haemolymph (Grace's medium) nor in cells grown directly in the insect haemocoel. The identified pixB gene was regulated by a conserved σ70 promoter while the pixA promoter was less well conserved. Expression of pixA and pixB under biological conditions was analysed using GFP promoter reporters. Microplate fluorescence detection and flow cytometry analyses revealed that pixB was expressed at high levels in Grace's medium and in insect haemolymph and at lower levels in complex medium, while pixA was expressed at lower levels under all conditions. Although pixB was highly expressed in Grace's medium, PixB crystalline inclusions were not present, suggesting that under biological conditions PixB production may be controlled post-transcriptionally. Although a pixB-minus strain was constructed, the function of PixB remains unresolved. The pixB gene was present in few Xenorhabdus species and pixB-type genes were identified in some Proteobacteria and Gram-positive species, while pixA was only present in Xenorhabdus species. Two conserved sequences were identified in PixB-type proteins that characterize this previously unrecognized gene family.

The Global Transcription Factor Lrp Controls Virulence Modulation in Xenorhabdus nematophila

The bacterium Xenorhabdus nematophila engages in phenotypic variation with respect to pathogenicity against insect larvae, yielding both virulent and attenuated subpopulations of cells from an isogenic culture. The global regulatory protein Lrp is necessary for X. nematophila virulence and immunosuppression in insects, as well as colonization of the mutualistic host nematode Steinernema carpocapsae, and mediates expression of numerous genes implicated in each of these phenotypes. Given the central role of Lrp in X. nematophila host associations, as well as its involvement in regulating phenotypic variation pathways in other bacteria, we assessed its function in virulence modulation. We discovered that expression of lrp varies within an isogenic population, in a manner that correlates with modulation of virulence. Unexpectedly, although Lrp is necessary for optimal virulence and immunosuppression, cells expressing high levels of lrp were attenuated in these processes relative to those with low to intermediate lrp expression. Furthermore, fixed expression of lrp at high and low levels resulted in attenuated and normal virulence and immunosuppression, respectively, and eliminated population variability of these phenotypes. These data suggest that fluctuating lrp expression levels are sufficient to drive phenotypic variation in X. nematophila.

IMPORTANCE

Many bacteria use cell-to-cell phenotypic variation, characterized by distinct phenotypic subpopulations within an isogenic population, to cope with environmental change. Pathogenic bacteria utilize this strategy to vary antigen or virulence factor expression. Our work establishes that the global transcription factor Lrp regulates phenotypic variation in the insect pathogen Xenorhabdus nematophila, leading to attenuation of virulence and immunosuppression in insect hosts. Unexpectedly, we found an inverse correlation between Lrp expression levels and virulence: high levels of expression of Lrp-dependent putative virulence genes are detrimental for virulence but may have an adaptive advantage in other aspects of the life cycle. Investigation of X. nematophila phenotypic variation facilitates dissection of this phenomenon in the context of a naturally occurring symbiosis.

Expression, Purification, and Functional Characterization of Atypical Xenocin, Its Immunity Protein, and Their Domains from Xenorhabdus nematophila

Xenorhabdus nematophila, a gram-negative bacterium belonging to the family Enterobacteriaceae is a natural symbiont of a soil nematode from the family Steinernematidae. In this study cloning, expression, and purification of broad range iron regulated multidomain bacteriocin called xenocin from X. nematophila (66 kDa, encoded by xcinA gene) and its multidomain immunity protein (42 kDa, encoded by ximB gene) have been done. xcinA-ximB (N' terminal 270 bp), translocation, and translocation-receptor domain of xcinA, ximB, and its hemolysin domain were cloned, expressed, and purified by single step Ni-NTA chromatography under native conditions. In the functional characterization, neutralization of xcinA toxicity by immunity domain of ximB gene was determined by endogenous assay. Exogenous toxic assays results showed that only the purified recombinant xenocin-immunity domain (10 kDa) protein complex had toxic activity. Atypical cognate immunity protein (42 kDa) of xenocin was fusion of immunity domain (10 kDa) and hemolysin domain (32 kDa). In silico analysis of immunity protein revealed its similarity with hemolysin and purine NTPase like proteins. Hemolytic activity was not observed in immunity protein or in its various domains; however, full-length immunity protein lacking Walker motif showed ATPase activity. Finally, using circular dichroism performed secondary structural analyses of all the recombinant proteins/protein complexes.

The hmsHFRS operon of Xenorhabdus nematophila is required for biofilm attachment to Caenorhabditis elegans

The bacterium Xenorhabdus nematophila is an insect pathogen and an obligate symbiont of the nematode Steinernema carpocapsae. X. nematophila makes a biofilm that adheres to the head of the model nematode Caenorhabditis elegans, a capability X. nematophila shares with the biofilms made by Yersinia pestis and Yersinia pseudotuberculosis. As in Yersinia spp., the X. nematophila biofilm requires a 4-gene operon, hmsHFRS. Also like its Yersinia counterparts, the X. nematophila biofilm is bound by the lectin wheat germ agglutinin, suggesting that beta-linked N-acetyl-D-glucosamine or N-acetylneuraminic acid is a component of the extracellular matrix. C. elegans mutants with aberrant surfaces that do not permit Yersinia biofilm attachment also are resistant to X. nematophila biofilms. An X. nematophila hmsH mutant that failed to make biofilms on C. elegans had no detectable defect in symbiotic association with S. carpocapsae, nor was virulence reduced against the insect Manduca sexta.

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.

Stability of entomopathogenic bacteria, Xenorhabdus nematophila and Photorhabdus luminescens, during in vitro culture

The entomopathogenic nematode-bacteria complexes Heterorhabditis bacteriophora/Photorhabdus luminescens and Steinernema carpocapsae/Xenorhabdus nematophila are mass produced for use as biological insecticides. Stability of the bacterial partner in culture is essential for maintaining traits important for both biological control and production. Two geographically distinct strains of each bacterial species were isolated from their nematode partners and serially subcultured on in vitro media to assess trait stability. Subculturing resulted in a shift to secondary cell production in one P. luminescens strain and both X. nematophila strains within ten in vitro culture cycles. However, when cell phenotypic variation was controlled in X. nematophila strains by regular selection for primary variants, no trait change was detected in the primary variant after prolonged subculture. When P. luminescens cell phenotypic variation was controlled by selection for primary variants, changes in the primary variant of both strains were noted including reductions in cell and inclusion body size and inclusion body prevalence. Bacterial ability to cause lethal infections following injection into the hemocoel of Tenebrio molitor larvae declined by more than half in primary variants of one P. luminescens strain. Conversely, yield was enhanced, with the subcultured P. luminescens strains showing 53.5 and 75.8% increases in primary cell density. Field adapted traits of primary variant P. luminescens strains tend to deteriorate during in vitro culture as tradeoffs for gains in yield. In vitro producers of the P. luminescens/H. bacteriophora complex must weigh the need for superior bacterial yield against the need to preserve traits important for biological control.

Analysis of the PixA inclusion body protein of Xenorhabdus nematophila

The symbiotic pathogenic bacterium Xenorhabdus nematophila produces two distinct intracellular inclusion bodies. The pixA gene, which encodes the 185-residue methionine-rich PixA inclusion body protein, was analyzed in the present study. The pixA gene was optimally expressed under stationary-phase conditions but its expression did not require RpoS. Analysis of a pixA mutant strain showed that PixA was not required for virulence towards the insect host or for colonization of or survival within the nematode host, and was not essential for nematode reproduction. The pixA gene was not present in the genome of Xenorhabdus bovienii, which also produces proteinaceous inclusions, indicating that PixA is specifically produced in X. nematophila.

Nutritive significance of crystalline inclusion proteins of Photorhabdus luminescens in Steinernema nematodes

Phase I cells of Photorhabdus luminescens produce two types of intracellular crystalline inclusion proteins designated CipA and CipB. The genes encoding CipA and CipB proteins from P. luminescens H06 were expressed respectively in Escherichia coli and these cells were used to feed the axenic first juveniles (J1) of three Steinernema nematode isolates in liquid cultures and on agar plates. In liquid cultures, the axenic J1 juveniles of all three test Steinernema nematode isolates were able to produce next dauer juveniles (DJs) in the E. coli cultures with at least one of the expressed Cip proteins, but unable to develop beyond the next J1 stage without expressed Cip proteins. For each target nematode isolate, addition of the supernatant of the bacterial culture of its Xenorhabdus symbiont to the tested liquid cultures did not induce the formation of DJs. However, on LB agar plates with different test E. coli cultures, all J1 juveniles of the three Steinernema strains finally developed into next DJs. It seemed that the metabolite pathway of the test bacteria in both culture systems was different. The presence of the Cip proteins has a significant influence on the DJ formation of the Steinernema nematodes in liquid culture system.

Analysis of Xenorhabdus nematophila metabolic mutants yields insight into stages of Steinernema carpocapsae nematode intestinal colonization

Xenorhabdus nematophila colonizes the intestinal tract of infective-juvenile (IJ) stage Steinernema carpocapsae nematodes. During colonization, X. nematophila multiplies within the lumen of a discrete region of the IJ intestine termed the vesicle. To begin to understand bacterial nutritional requirements during multiplication in the IJ vesicle, we analysed the colonization behaviour of several X. nematophila metabolic mutants, including amino acid and vitamin auxotrophs. X. nematophila mutants defective for para-aminobenzoate, pyridoxine or l-threonine biosynthesis exhibit substantially decreased colonization of IJs (0.1-50% of wild-type colonization). Analysis of gfp-labelled variants revealed that those mutant cells that can colonize the IJ vesicle differ noticeably from wild-type X. nematophila. One aberrant colonization phenotype exhibited by the metabolic mutants tested, but not wild-type X. nematophila, is a spherical shape indicative of apparently non-viable X. nematophila cells within the vesicle. Because these spherical cells appear to have initiated colonization but failed to proliferate, we term this type of colonization 'abortive'. In a portion of IJs grown on para-aminobenzoate auxotrophs, X. nematophila does not exhibit abortive colonization but rather reduced growth and filamentous cell morphology. Several mutants with defects in other amino acid, vitamin and nutrient metabolism pathways colonize IJs to wild-type levels suggesting that the IJ vesicle is replete with respect to a number of nutrients.

Response of ants to a deterrent factor(s) produced by the symbiotic bacteria of entomopathogenic nematodes

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.

Isolation and characterization of intracellular protein inclusions produced by the entomopathogenic bacterium Photorhabdus luminescens

Cells of the entomopathogenic bacterium Photorhabdus luminescens contain two types of morphologically distinct crystalline inclusion proteins. The larger rectangular inclusion (type 1) and a smaller bipyramid-shaped inclusion (type 2) were purified from cell lysates by differential centrifugation and isopycnic density gradient centrifugation. Both structures are composed of protein and are readily soluble at pH 11 and 4 in 1% sodium dodecyl sulfate (SDS) and in 8 M urea. Electrophoretic analysis reveals that each inclusion is composed of a single protein subunit with a molecular mass of 11,000 Da. The proteins differ in amino acid composition, protease digestion pattern, and immunological cross-reactivity. The protein inclusions are first visible in the cells at the time of late exponential growth. Western blot analyses showed that the proteins appeared in cells during mid- to late exponential growth. When at maximum size in stationary-phase cells, the proteins constitute 40% of the total cellular protein. The protein inclusions are not used during long-term starvation of the cells and were not toxic when injected into or fed to Galleria mellonella larvae.

Xenorhabdus bovienii T228 phase variation and virulence are independent of RecA function

Colony pleomorphism, or phase variation, expressed by entomopathogenic bacteria belonging to the genus Xenorhabdus, is an important factor which determines the association of the bacteria with their nematode symbiont and the outcome of infection of susceptible insect larvae by the bacterium- nematode parasitic complex. The mechanism underlying phase variation is unknown. To determine whether RecA-mediated processes are linked to phase variation, the recA gene of Xenorhabdus bovienii was cloned and sequenced. When expressed in a recA-deleted strain of Escherichia coli, the X. bovienii recA clone was able to complement the loss of RecA function. X. bovienii chromosomal recA insertion mutants showed increased sensitivity to UV. Phase 1 forms did not show altered ability to convert to phase 2 and no significant differences in expression of other phase-dependent characteristics, including phospholipase C, haemolysin, protease, antibiotic activity and Congo Red binding, were noted. Furthermore, the LD(50) of the X. bovienii recA insertion mutant for Galleria mellonella larvae was not significantly different from that of wild-type strains. From these data the authors conclude that recA is unlikely to be involved in phase variation, the expression of phase-dependent characteristics, or virulence factors involved in killing of susceptible larvae.

Inactivation of a novel gene produces a phenotypic variant cell and affects the symbiotic behavior of Xenorhabdus nematophilus

Xenorhabdus nematophilus is an insect pathogen that lives in a symbiotic association with a specific entomopathogenic nematode. During prolonged culturing, variant cells arise that are deficient in numerous properties. To understand the genetic mechanism underlying variant cell formation, a transposon mutagenesis approach was taken. Three phenotypically similar variant strains of X. nematophilus, each of which contained a single transposon insertion, were isolated. The insertions occurred at different locations in the chromosome. The variant strain, ANV2, was further characterized. It was deficient in several properties, including the ability to produce antibiotics and the stationary-phase-induced outer membrane protein, OpnB. Unlike wild-type cells, ANV2 produced lecithinase. The emergence of ANV2 from the nematode host was delayed relative to the emergence of the parental strain. The transposon in ANV2 had inserted in a gene designated var1, which encodes a novel protein composed of 121 amino acid residues. Complementation analysis confirmed that the pleiotropic phenotype of the ANV2 strain was produced by inactivation of var1. Other variant strains were not complemented by var1. These results indicate that inactivation of a single gene was sufficient to promote variant cell formation in X. nematophilus and that disruption of genetic loci other than var1 can result in the same pleiotropic phenotype.

flhDC, the flagellar master operon of Xenorhabdus nematophilus: requirement for motility, lipolysis, extracellular hemolysis, and full virulence in insects

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 stationary-phase 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.

Photorhabdus luminescens W-14 insecticidal activity consists of at least two similar but distinct proteins. Purification and characterization of toxin A and toxin B

Both the bacterium Photorhabdus luminescens alone and its symbiotic Photorhabdus-nematode complex are known to be highly pathogenic to insects. The nature of the insecticidal activity of Photorhabdus bacteria was investigated for its potential application as an insect control agent. It was found that in the fermentation broth of P. luminescens strain W-14, at least two proteins, toxin A and toxin B, independently contributed to the oral insecticidal activity against Southern corn rootworm. Purified toxin A and toxin B exhibited single bands on native polyacrylamide gel electrophoresis and two peptides of 208 and 63 kDa on SDS-polyacrylamide gel electrophoresis. The native molecular weight of both the toxin A and toxin B was determined to be approximately 860 kDa, suggesting that they are tetrameric. NH2-terminal amino acid sequencing and Western analysis using monospecific antibodies to each toxin demonstrated that the two toxins were distinct but homologous. The oral potency (LD50) of toxin A and toxin B against Southern corn rootworm larvae was determined to be similar to that observed with highly potent Bt toxins against lepidopteran pests. In addition, it was found that the two peptides present in toxin B could be processed in vitro from a 281-kDa protoxin by endogenous P. luminescens proteases. Proteolytic processing was shown to enhance insecticidal activity.

Phase Variation in Xenorhabdus nematophilus

Xenorhabdus nematophilus is a symbiotic bacterium that inhabits the intestine of entomopathogenic nematodes. The bacterium-nematode symbiotic pair is pathogenic for larval-stage insects. The phase I cell type is the form of the bacterium normally associated with the nematode. A variant cell type, referred to as phase II, can form spontaneously under stationary-phase conditions. Phase II cells do not elaborate products normally associated with the phase I cell type. To better define phase variation in X. nematophilus, several strains (19061, AN6, F1, N2-4) of this bacterium were analyzed for new phenotypic traits. An analysis of pathogenicity in Manduca sexta larvae revealed that the phase II form of AN6 (AN6/II) was significantly less virulent than the phase I form (AN6/I). The variant form of N2-4 was also avirulent. On the other hand, F1/II and 19061/II were as virulent as the respective phase I cells. Strain 19061/II was found to be motile, and AN6/II regained motility when the bacteria were grown in low-osmolarity medium. In contrast, F1/II remained nonmotile. The phase II cells did not produce the outer membrane protein, OpnB, that is normally induced during the stationary phase. Both phase I and phase II cells were able to support nematode growth and development. These findings indicate that while certain phenotypic traits are common to all phase II cells, other characteristics, such as virulence and motility, are variable and can be influenced by environmental conditions.

Insertional inactivation of genes encoding the crystalline inclusion proteins of Photorhabdus luminescens results in mutants with pleiotropic phenotypes

The entomopathogenic bacterium Photorhabdus luminescens exhibits phase variation when cultured in vitro. The variant forms of P. luminescens are pleiotropic and are designated phase I and phase II variants. One of the characteristic phenotypes of phase I cells is the production of two types of intracellular protein inclusions. The genes encoding the protein monomers that form these inclusions, designated cipA and cipB, were cloned and characterized. cipA and cipB encode hydrophobic proteins of 11,648 and 11,308 Da, respectively. The deduced amino acid sequences of CipA and CipB have no significant amino acid sequence similarity to any other known protein but have 25% identity and 49% similarity to each other. Insertional inactivation of cipA or cipB in phase I cells of P. luminescens produced mutants that differ from phase I cells in bioluminescence, the pattern and activities of extracellular products, biochemical traits, adsorption of dyes, and ability to support nematode growth and reproduction. In general, the cip mutants were phenotypically more similar to each other than to either phase I or phase II variants.

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.

Cloning and nucleotide sequence of a flagellin encoding genetic locus from Xenorhabdus nematophilus: phase variation leads to differential transcription of two flagellar genes (fliCD)

The insect-pathogenic bacterium Xenorhabdus undergoes spontaneous phase variation involving a large number of phenotypes. Our previous study indicated that phase I variants were motile, whereas phase II variants of X. nematophilus F1 were nonflagellated cells which did not synthesize flagellin [Givaudan A., Baghdiguian, S., Lanois, A. and Boemare, N. (1995) Appl. Environ. Microbiol. 61, 1408-1413]. In order to approach the study of the flagellar switching, a locus containing two ORFs from X. nematophilus F1 (phase I) was identified by using functional complementation of flagellin-negative E. coli. The sequence analysis revealed that the first ORF corresponds to the fliC gene coding for flagellin, and showed a high degree of homology between the N-terminal and C-terminal of Xenorhabdus FliC and flagellins from other bacteria. The second identified ORF in the opposite orientation encodes a homologue of the enterobacterial hook-associated protein 2, FliD. Both Xenorhabdus fliCD genes were required for the entire restoration of E. coli motility. A sequence highly homologous to the sigma 28 consensus promoter was identified upstream from the coding sequences from both genes. The structure of the fliC gene and its surrounding region was shown to be the same in both phase variants, but Northern blot analysis revealed that fliC and fliD were, respectively, not and weakly transcribed in phase II variants. In addition, complementation experiments showed that motility and flagellin synthesis of phase II cannot be recovered by placing in trans fliCD genes from phase I. These latter results suggest that a gene(s) higher in the transcriptional hierarchy of the flagellar regulon is switched off in Xenorhabdus phase II variants.

The Phylloplane as a Source of Bacillus thuringiensis Variants

Novel variants of Bacillus thuringiensis were isolated from the phylloplane of deciduous and conifer trees as well as of other plants. These isolates displayed a range of toxicity towards Trichoplusia ni. Immunoblot and toxin protein analysis indicate that these strains included representatives of the three principal B. thuringiensis pathotypes active against larvae of the orders Lepidoptera, Diptera, and Coleoptera. We propose that B. thuringiensis be considered part of the common leaf microflora of many plants.

Cloning, organization, and expression of the bioluminescence genes of Xenorhabdus luminescens

The lux genes of Xenorhabdus luminescens, a symbiont of the nematode Heterorhabditis bacteriophora, were cloned and expressed in Escherichia coli. The expression of these genes in E. coli was qualitatively similar to their expression in X. luminescens. The organization of the genes is similar to that found in the marine luminous bacteria. Hybridization studies with the DNA that codes for the two subunits of luciferase revealed considerable homology among all of the strains of X. luminescens and with the DNA of other species of luminous bacteria, but none with the nonluminous Xenorhabdus species. Gross DNA alterations such as insertions, deletions, or inversions do not appear to be involved in the generation of dim variants known as secondary forms.