Motility is required for the competitive fitness of entomopathogenic Photorhabdus luminescens during insect infection

Function and Global Regulation of Type III Secretion System and Flagella in Entomopathogenic Nematode Symbiotic Bacteria

Currently, it is widely accepted that the type III secretion system (T3SS) serves as the transport platform for bacterial virulence factors, while flagella act as propulsion motors. However, there remains a noticeable dearth of comparative studies elucidating the functional disparities between these two mechanisms. Entomopathogenic nematode symbiotic bacteria (ENS), including Xenorhabdus and Photorhabdus, are Gram-negative bacteria transported into insect hosts by Steinernema or Heterorhabdus. Flagella are conserved in ENS, but the T3SS is only encoded in Photorhabdus. There are few reports on the function of flagella and the T3SS in ENS, and it is not known what role they play in the infection of ENS. Here, we clarified the function of the T3SS and flagella in ENS infection based on flagellar inactivation in X. stockiae (flhDC deletion), T3SS inactivation in P. luminescens (sctV deletion), and the heterologous synthesis of the T3SS of P. luminescens in X. stockiae. Consistent with the previous results, the swarming movement of the ENS and the formation of biofilms are dominated by the flagella. Both the T3SS and flagella facilitate ENS invasion and colonization within host cells, with minimal impact on secondary metabolite formation and secretion. Unexpectedly, a proteomic analysis reveals a negative feedback loop between the flagella/T3SS assembly and the type VI secretion system (T6SS). RT-PCR testing demonstrates the T3SS's inhibition of flagellar assembly, while flagellin expression promotes T3SS assembly. Furthermore, T3SS expression stimulates ribosome-associated protein expression.

Enhanced production of trans-cinnamic acid in Photorhabdus luminescens with homolog expression and deletion strategies

AIM

This study aimed to overproduce industrially relevant and safe bio-compound trans-cinnamic acid (tCA) from Photorhabdus luminescens with deletion strategies and homologous expression strategies that had not been applied before for tCA production.

METHODS AND RESULTS

The overproduction of the industrially relevant compound tCA was successfully performed in P. luminescens by deleting stlB (TTO1ΔstlB) encoding a cinnamic acid CoA ligase in the isopropylstilbene pathway and the hcaE insertion (knockout) mutation (hcaE::cat) in the phenylpropionate catabolic pathway, responsible for tCA degradation. A double mutant of both stlB deletion and hcaE insertion mutation (TTO1DM ΔstlB-hcaE::cat) was also generated. These deletion strategies and the phenylalanine ammonium lyase-producing (PI-PAL from Photorhabdus luminescens) plasmid, pBAD30C, carrying stlA (homologous expression mutants) are utilized together in the same strain using different media, a variety of cultivation conditions, and efficient anion exchange resin (Amberlite IRA402) for enhanced tCA synthesis. At the end of the 120-h shake flask cultivation, the maximum tCA production was recorded as 1281 mg l-1 in the TTO1pBAD30C mutant cultivated in TB medium, with the IRA402 resin keeping 793 mg l-1 and the remaining 488 mg l-1 found in the supernatant.

CONCLUSION

TCA production was successfully achieved with homologous expression, coupled with deletion and insertion strategies. 1281 mg l-1is the highest tCA concentration that achieved by bacterial tCA production in flask cultivation, according to our knowledge.

The ecological roles of microbial lipopeptides: Where are we going?

Lipopeptides (LPs) are secondary metabolites produced by a diversity of bacteria and fungi. Their unique chemical structure comprises both a peptide and a lipid moiety. LPs are of major biotechnological interest owing to their emulsification, antitumor, immunomodulatory, and antimicrobial activities. To date, these versatile compounds have been applied across multiple industries, from pharmaceuticals through to food processing, cosmetics, agriculture, heavy metal, and hydrocarbon bioremediation. The variety of LP structures and the diversity of the environments from which LP-producing microorganisms have been isolated suggest important functions in their natural environment. However, our understanding of the ecological role of LPs is limited. In this review, the mode of action and the role of LPs in motility, antimicrobial activity, heavy metals removal and biofilm formation are addressed. We include discussion on the need to characterise LPs from a diversity of microorganisms, with a focus on taxa inhabiting 'extreme' environments. We introduce the use of computational target fishing and molecular dynamics simulations as powerful tools to investigate the process of interaction between LPs and cell membranes. Together, these advances will provide new understanding of the mechanism of action of novel LPs, providing greater insights into the roles of LPs in the natural environment.

Two novel XRE-like transcriptional regulators control phenotypic heterogeneity in Photorhabdus luminescens cell populations

BACKGROUND

The insect pathogenic bacterium Photorhabdus luminescens exists in two phenotypically different forms, designated as primary (1°) and secondary (2°) cells. Upon yet unknown environmental stimuli up to 50% of the 1° cells convert to 2° cells. Among others, one important difference between the phenotypic forms is that 2° cells are unable to live in symbiosis with their partner nematodes, and therefore are not able to re-associate with them. As 100% switching of 1° to 2° cells of the population would lead to a break-down of the bacteria's life cycle the switching process must be tightly controlled. However, the regulation mechanism of phenotypic switching is still puzzling.

RESULTS

Here we describe two novel XRE family transcriptional regulators, XreR1 and XreR2, that play a major role in the phenotypic switching process of P. luminescens. Deletion of xreR1 in 1° or xreR2 in 2° cells as well as insertion of extra copies of xreR1 into 2° or xreR2 into 1° cells, respectively, induced the opposite phenotype in either 1° or 2° cells. Furthermore, both regulators specifically bind to different promoter regions putatively fulfilling a positive autoregulation. We found initial evidence that XreR1 and XreR2 constitute an epigenetic switch, whereby XreR1 represses xreR2 expression and XreR2 self-reinforces its own gene by binding to XreR1.

CONCLUSION

Regulation of gene expression by the two novel XRE-type regulators XreR1 and XreR2 as well as their interplay represents a major regulatory process in phenotypic switching of P. luminescens. A fine-tuning balance between both regulators might therefore define the fate of single cells to convert from the 1° to the 2° phenotype.

In vivo transcriptome analysis provides insights into host-dependent expression of virulence factors by Yersinia entomophaga MH96, during infection of Galleria mellonella

The function of microbes can be inferred from knowledge of genes specifically expressed in natural environments. Here, we report the in vivo transcriptome of the entomopathogenic bacterium Yersinia entomophaga MH96, captured during initial, septicemic, and pre-cadaveric stages of intrahemocoelic infection in Galleria mellonella. A total of 1285 genes were significantly upregulated by MH96 during infection; 829 genes responded to in vivo conditions during at least one stage of infection, 289 responded during two stages of infection, and 167 transcripts responded throughout all three stages of infection compared to in vitro conditions at equivalent cell densities. Genes upregulated during the earliest infection stage included components of the insecticidal toxin complex Yen-TC (chi1, chi2, and yenC1), genes for rearrangement hotspot element containing protein yenC3, cytolethal distending toxin cdtAB, and vegetative insecticidal toxin vip2. Genes more highly expressed throughout the infection cycle included the putative heat-stable enterotoxin yenT and three adhesins (usher-chaperone fimbria, filamentous hemagglutinin, and an AidA-like secreted adhesin). Clustering and functional enrichment of gene expression data also revealed expression of genes encoding type III and VI secretion system-associated effectors. Together these data provide insight into the pathobiology of MH96 and serve as an important resource supporting efforts to identify novel insecticidal agents.

Phenotypic Heterogeneity of the Insect Pathogen Photorhabdus luminescens: Insights into the Fate of Secondary Cells

Photorhabdus luminescens is a Gram-negative bacterium that lives in symbiosis with soil nematodes and is simultaneously highly pathogenic toward insects. The bacteria exist in two phenotypically different forms, designated primary (1°) and secondary (2°) cells. Yet unknown environmental stimuli as well as global stress conditions induce phenotypic switching of up to 50% of 1° cells to 2° cells. An important difference between the two phenotypic forms is that 2° cells are unable to live in symbiosis with nematodes and are therefore believed to remain in the soil after a successful infection cycle. In this work, we performed a transcriptomic analysis to highlight and better understand the role of 2° cells and their putative ability to adapt to living in soil. We could confirm that the major phenotypic differences between the two cell forms are mediated at the transcriptional level as the corresponding genes were downregulated in 2° cells. Furthermore, 2° cells seem to be adapted to another environment as we found several differentially expressed genes involved in the cells' metabolism, motility, and chemotaxis as well as stress resistance, which are either up- or downregulated in 2° cells. As 2° cells, in contrast to 1° cells, chemotactically responded to different attractants, including plant root exudates, there is evidence for the rhizosphere being an alternative environment for the 2° cells. Since P. luminescens is biotechnologically used as a bio-insecticide, investigation of a putative interaction of 2° cells with plants is also of great interest for agriculture.IMPORTANCE The biological function and the fate of P. luminescens 2° cells were unclear. Here, we performed comparative transcriptomics of P. luminescens 1° and 2° cultures and found several genes, not only those coding for known phenotypic differences of the two cell forms, that are up- or downregulated in 2° cells compared to levels in 1° cells. Our results suggest that when 1° cells convert to 2° cells, they drastically change their way of life. Thus, 2° cells could easily adapt to an alternative environment such as the rhizosphere and live freely, independent of a host, putatively utilizing plant-derived compounds as nutrient sources. Since 2° cells are not able to reassociate with the nematodes, an alternative lifestyle in the rhizosphere would be conceivable.

Role of the Photorhabdus Dam methyltransferase during interactions with its invertebrate hosts

Photorhabdus luminescens is an entomopathogenic bacterium found in symbiosis with the nematode Heterorhabditis. Dam DNA methylation is involved in the pathogenicity of many bacteria, including P. luminescens, whereas studies about the role of bacterial DNA methylation during symbiosis are scarce. The aim of this study was to determine the role of Dam DNA methylation in P. luminescens during the whole bacterial life cycle including during symbiosis with H. bacteriophora. We constructed a strain overexpressing dam by inserting an additional copy of the dam gene under the control of a constitutive promoter in the chromosome of P. luminescens and then achieved association between this recombinant strain and nematodes. The dam overexpressing strain was able to feed the nematode in vitro and in vivo similarly as a control strain, and to re-associate with Infective Juvenile (IJ) stages in the insect. No difference in the amount of emerging IJs from the cadaver was observed between the two strains. Compared to the nematode in symbiosis with the control strain, a significant increase in LT50 was observed during insect infestation with the nematode associated with the dam overexpressing strain. These results suggest that during the life cycle of P. luminescens, Dam is not involved the bacterial symbiosis with the nematode H. bacteriophora, but it contributes to the pathogenicity of the nemato-bacterial complex.

Structural characterization of peptides fromLocusta migratoria manilensis(Meyen, 1835) and anti-aging effect inCaenorhabditis elegans

Locusts are a kind of agricultural pest rich in protein and widely eaten by people, yet, the nutritional and antioxidant activities of locust peptide have never been explored. In the current study, the locust peptides (LPs) were isolated from the Locusta migratoria manilensis (Meyen, 1835) and the anti-aging effects on Caenorhabditis elegans (C. elegans) were evaluated. The mean lifespan of C. elegans was significantly extended using LPs with a concentration of 1.0 mg mL⁻¹. Out of the 23 peptides, LP-1, a pentapeptide with The-Phe-Lys-His-Gly sequence, with a concentration of 2.5 mg mL⁻¹ significantly extended the lifespan of the worms by 23.5%. Additionally, LP-1 was observed to be a strong free radical-scavenger which can improve the survival of the C. elegans under oxidative stress, thermal stress and UV radiation. Furthermore, the LP-1 can up-regulate the expression of the transcription factor DAF-16 and jnk-1, suggesting that LP-1 may promote the C. elegans lifespan and stress resistance through a JNK-1-DAF-16 pathway. This study will be significant for the development of locusts and improvement of functional insect peptide production.

Locusts are a kind of agricultural pest rich in protein and widely eaten by people, yet, the nutritional and antioxidant activities of locust peptide have never been explored.

Secondary Metabolites Produced by Heterorhabditis Symbionts and Their Application in Agriculture: What We Know and What to Do Next

Gram-negative Photorhabdus bacteria have a dual lifestyle: they are mutualists of Heterorhabditis nematodes and are pathogens of insects. Together, this nematode-bacterium partnership has been used to successfully control a wide range of agricultural insect pests. Photorhabdus produce a diverse array of small molecules that play key biological roles in regulating their dual roles. In particular, several secondary metabolites (SM) produced by this bacterium are known to play a critical role in the maintenance of a monoxenic infection in the insect host and are also known to prevent contamination of the cadaver from soil microbes and/or predation by arthropods. A few of the SM this bacteria produce have been isolated and identified, and their biological activities have also been tested in laboratory assays. Over the past two decades, analyses of the genomes of several Photorhabdus spp. have revealed the presence of SM numerous gene clusters that comprise more than 6% of these bacteria genomes. Furthermore, genome mining and characterization of biosynthetic pathways, have uncovered the richness of these compounds, which are predicted to vary across different Photorhabdus spp. and strains. Although progress has been made in the identification and function of SM genes and gene clusters, the targeted testing for the bioactivity of molecules has been scarce or mostly focused on medical applications. In this review, we summarize the current knowledge of Photorhabdus SM, emphasizing on their activity against plant pathogens and parasites. We further discuss their potential in the management of agricultural pests and the steps that need to be taken for the implementation of Photorhabdus SM in pest management.

DNA Adenine Methyltransferase (Dam) Overexpression Impairs Photorhabdus luminescens Motility and Virulence

Dam, the most described bacterial DNA-methyltransferase, is widespread in gamma-proteobacteria. Dam DNA methylation can play a role in various genes expression and is involved in pathogenicity of several bacterial species. The purpose of this study was to determine the role played by the dam ortholog identified in the entomopathogenic bacterium Photorhabdus luminescens. Complementation assays of an Escherichia coli dam mutant showed the restoration of the DNA methylation state of the parental strain. Overexpression of dam in P. luminescens did not impair growth ability in vitro. In contrast, compared to a control strain harboring an empty plasmid, a significant decrease in motility was observed in the dam-overexpressing strain. A transcriptome analysis revealed the differential expression of 208 genes between the two strains. In particular, the downregulation of flagellar genes was observed in the dam-overexpressing strain. In the closely related bacterium Xenorhabdus nematophila, dam overexpression also impaired motility. In addition, the dam-overexpressing P. luminescens strain showed a delayed virulence compared to that of the control strain after injection in larvae of the lepidopteran Spodoptera littoralis. These results reveal that Dam plays a major role during P. luminescens insect infection.

HexA is a versatile regulator involved in the control of phenotypic heterogeneity of Photorhabdus luminescens

Phenotypic heterogeneity in microbial communities enables genetically identical organisms to behave differently even under the same environmental conditions. Photorhabdus luminescens, a bioluminescent Gram-negative bacterium, contains a complex life cycle, which involves a symbiotic interaction with nematodes as well as a pathogenic association with insect larvae. P. luminescens exists in two distinct phenotypic cell types, designated as the primary (1°) and secondary (2°) cells. The 1° cells are bioluminescent, pigmented and can support nematode growth and development. Individual 1° cells undergo phenotypic switching after prolonged cultivation and convert to 2° cells, which lack the 1° specific phenotypes. The LysR-type regulator HexA has been described as major regulator of this switching process. Here we show that HexA controls phenotypic heterogeneity in a versatile way, directly and indirectly. Expression of hexA is enhanced in 2° cells, and the corresponding regulator inhibits 1° specific traits in 2° cells. HexA does not directly affect bioluminescence, a predominant 1° specific phenotype. Since the respective luxCDABE operon is repressed at the post-transcriptional level and transcriptional levels of the RNA chaperone gene hfq are also enhanced in 2° cells, small regulatory RNAs are presumably involved that are under control of HexA. Another phenotypic trait that is specific for 1° cells is quorum sensing mediated cell clumping. The corresponding pcfABCDEF operon could be identified as the first direct target of HexA, since the regulator binds to the pcfA promoter region and thereby blocks expression of the target operon. In summary, our data show that HexA fulfills the task as repressor of 1° specific features in 2° cells in a versatile way and gives first insights into the complexity of regulating phenotypic heterogeneity in Photorhabdus bacteria.

Genes Required for the Anti-fungal Activity of a Bacterial Endophyte Isolated from a Corn Landrace Grown Continuously by Subsistence Farmers Since 1000 BC

Endophytes are microbes that inhabit internal plant tissues without causing disease. Some endophytes are known to combat pathogens. The corn (maize) landrace Chapalote has been grown continuously by subsistence farmers in the Americas since 1000 BC, without the use of fungicides, and the crop remains highly valued by farmers, in part for its natural tolerance to pests. We hypothesized that the pathogen tolerance of Chapalote may, in part, be due to assistance from its endophytes. We previously identified a bacterial endophyte from Chapalote seeds, Burkholderia gladioli strain 3A12, for its ability to combat a diversity of crop pathogens, including Sclerotinia homoeocarpa, the most important fungal disease of creeping bentgrass, a relative of maize used here as a model system. Strain 3A12 represents a unique opportunity to understand the anti-fungal activities of an endophyte associated with a crop variety grown by subsistence farmers since ancient times. Here, microscopy combined with Tn5-mutagenesis demonstrates that the anti-fungal mode of action of 3A12 involves flagella-dependent swarming toward its pathogen target, attachment and biofilm-mediated microcolony formation. The mutant screen revealed that YajQ, a receptor for the secondary messenger c-di-GMP, is a critical signaling system that mediates this endophytic mobility-based defense for its host. Microbes from the traditional seeds of farmers may represent a new frontier in elucidating host-microbe mutualistic interactions.

The Regulation of Secondary Metabolism and Mutualism in the Insect Pathogenic Bacterium Photorhabdus luminescens

Photorhabdus is a genus of insect-pathogenic Gram-negative bacteria that also maintain a mutualistic interaction with nematodes from the family Heterorhabditis. This complex life cycle, involving different interactions with different invertebrate hosts, coupled with the amenability of the system to laboratory culture has resulted in the development of Photorhabdus as a model system for studying bacterial-host interactions. Photorhabdus is predicted to have an extensive secondary metabolism with the genetic potential to produce >20 different small secondary metabolites. Therefore, this system also presents us with a unique opportunity to study the contribution of secondary metabolism to the environmental fitness of the producing organism in its natural habitat (i.e., the insect and/or the nematode). In vivo and in vitro studies have revealed that the vast majority of the genetic loci in Photorhabdus predicted to be involved in the production of secondary metabolites appear to be cryptic and, to date, although several have been characterized, only three compounds have been studied in any great detail: 3,5-dihydroxy-4-isopropylstilbene, the β-lactam antibiotic carbapenem, and an anthraquinone pigment. In this chapter, we describe how these compounds are made and the role (if any) that they have during the interactions between Photorhabdus and its invertebrate hosts. We will also outline recent work on the regulation of secondary metabolism in Photorhabdus and comment on how this has led to an increased understanding of mutualism in this bacterium.

Alarmone (p)ppGpp regulates the transition from pathogenicity to mutualism in Photorhabdus luminescens

The enteric gamma-proteobacterium Photorhabdus luminescens kills a wide range of insects, whilst also maintaining a mutualistic relationship with soil nematodes from the family Heterorhabditis. Pathogenicity is associated with bacterial exponential growth, whilst mutualism is associated with post-exponential (stationary) phase. During post-exponential growth, P. luminescens also elaborates an extensive secondary metabolism, including production of bioluminescence, antibiotics and pigment. However, the regulatory network that controls the expression of this secondary metabolism is not well understood. The stringent response is a well-described global regulatory system in bacteria and mediated by the alarmone (p)ppGpp. In this study, we disrupted the genes relA and spoT, encoding the two predicted (p)ppGpp synthases of P. luminescens TTO1, and we showed that (p)ppGpp is required for secondary metabolism. Moreover, we found the (p)ppGpp is not required for pathogenicity of P. luminescens, but is required for bacterial survival within the insect cadaver. Finally, we showed that (p)ppGpp is required for P. luminescens to support normal nematode growth and development. Therefore, the regulatory network that controls the transition from pathogenicity to mutualism in P. luminescens requires (p)ppGpp. This is the first report outlining a role for (p)ppGpp in controlling the outcome of an interaction between a bacteria and its host.

Flagellar Regulation and Virulence in the Entomopathogenic Bacteria-Xenorhabdus nematophila and Photorhabdus luminescens

There is a complex interplay between the regulation of flagellar motility and the expression of virulence factors in many bacterial pathogens. Here, we review the literature on the direct and indirect roles of flagellar motility in mediating the tripartite interaction between entomopathogenic bacteria (Photorhabdus and Xenorhabdus), their nematode hosts, and their insect targets. First, we describe the swimming and swarming motility of insect pathogenic bacteria and its impact on insect colonization. Then, we describe the coupling between the expression of flagellar and virulence genes and the dynamic of expression of the flagellar regulon during invertebrate infection. We show that the flagellar type 3 secretion system (T3SS) is also an export apparatus for virulence proteins in X. nematophila. Finally, we demonstrate that phenotypic variation, a common property of the bacterial symbionts of nematodes, also alters flagellar motility in Photorhabdus and Xenorhabdus. Finally, the so-called phenotypic heterogeneity phenomenon in the flagellar gene expression network will be also discussed. As the main molecular studies were performed in X. nematophila, future perspectives for the study of the interplay between flagellum and invertebrate interactions in Photorhabdus will be discussed.

Bacterial cell motility of Burkholderia gut symbiont is required to colonize the insect gut

We generated a Burkholderia mutant, which is deficient of an N-acetylmuramyl-l-alanine amidase, AmiC, involved in peptidoglycan degradation. When non-motile ΔamiC mutant Burkholderia cells harboring chain form were orally administered to Riptortus insects, ΔamiC mutant cells were unable to establish symbiotic association. But, ΔamiC mutant complemented with amiC gene restored in vivo symbiotic association. ΔamiC mutant cultured in minimal medium restored their motility with single-celled morphology. When ΔamiC mutant cells harboring single-celled morphology were administered to the host insect, this mutant established normal symbiotic association, suggesting that bacterial motility is essential for the successful symbiosis between host insect and Burkholderia symbiont.

Elucidation of the Photorhabdus temperata Genome and Generation of a Transposon Mutant Library To Identify Motility Mutants Altered in Pathogenesis

The entomopathogenic nematode Heterorhabditis bacteriophora forms a specific mutualistic association with its bacterial partner Photorhabdus temperata. The microbial symbiont is required for nematode growth and development, and symbiont recognition is strain specific. The aim of this study was to sequence the genome of P. temperata and identify genes that plays a role in the pathogenesis of the Photorhabdus-Heterorhabditis symbiosis. A draft genome sequence of P. temperata strain NC19 was generated. The 5.2-Mb genome was organized into 17 scaffolds and contained 4,808 coding sequences (CDS). A genetic approach was also pursued to identify mutants with altered motility. A bank of 10,000 P. temperata transposon mutants was generated and screened for altered motility patterns. Five classes of motility mutants were identified: (i) nonmotile mutants, (ii) mutants with defective or aberrant swimming motility, (iii) mutant swimmers that do not require NaCl or KCl, (iv) hyperswimmer mutants that swim at an accelerated rate, and (v) hyperswarmer mutants that are able to swarm on the surface of 1.25% agar. The transposon insertion sites for these mutants were identified and used to investigate other physiological properties, including insect pathogenesis. The motility-defective mutant P13-7 had an insertion in the RNase II gene and showed reduced virulence and production of extracellular factors. Genetic complementation of this mutant restored wild-type activity. These results demonstrate a role for RNA turnover in insect pathogenesis and other physiological functions.

IMPORTANCE

The relationship between Photorhabdus and entomopathogenic nematode Heterorhabditis represents a well-known mutualistic system that has potential as a biological control agent. The elucidation of the genome of the bacterial partner and role that RNase II plays in its life cycle has provided a greater understanding of Photorhabdus as both an insect pathogen and a nematode symbiont.

The genetic basis of the symbiosis between Photorhabdus and its invertebrate hosts

Photorhabdus is a pathogen of insects that also maintains a mutualistic association with nematodes from the family Heterorhabditis. Photorhabdus colonizes the gut of the infective juvenile (IJ) stage of the nematode. The IJ infects an insect and regurgitates the bacteria and the bacteria reproduce to kill the insect. The nematodes feed on the resulting bacterial biomass until a new generation of IJs emerges from the insect cadaver. Therefore, during its life cycle, Photorhabdus must (1) kill the insect host, (2) support nematode growth and development, and (3) be able to colonize the new generation of IJs. In this review, functional genomic studies that have been aimed at understanding the molecular mechanisms underpinning each of these roles will be discussed. These studies have begun to reveal that distinct gene sets may be required for each of these interactions, suggesting that there is only a minimal genetic overlap between pathogenicity and mutualism in Photorhabdus.

The expression of stlA in Photorhabdus luminescens is controlled by nutrient limitation

Photorhabdus is a genus of Gram-negative entomopathogenic bacteria that also maintain a mutualistic association with nematodes from the family Heterorhabditis. Photorhabdus has an extensive secondary metabolism that is required for the interaction between the bacteria and the nematode. A major component of this secondary metabolism is a stilbene molecule, called ST. The first step in ST biosynthesis is the non-oxidative deamination of phenylalanine resulting in the production of cinnamic acid. This reaction is catalyzed by phenylalanine-ammonium lyase, an enzyme encoded by the stlA gene. In this study we show, using a stlA-gfp transcriptional fusion, that the expression of stlA is regulated by nutrient limitation through a regulatory network that involves at least 3 regulators. We show that TyrR, a LysR-type transcriptional regulator that regulates gene expression in response to aromatic amino acids in E. coli, is absolutely required for stlA expression. We also show that stlA expression is modulated by σ^S and Lrp, regulators that are implicated in the regulation of the response to nutrient limitation in other bacteria. This work is the first that describes pathway-specific regulation of secondary metabolism in Photorhabdus and, therefore, our study provides an initial insight into the complex regulatory network that controls secondary metabolism, and therefore mutualism, in this model organism.

Serratia marcescens induces apoptotic cell death in host immune cells via a lipopolysaccharide- and flagella-dependent mechanism

Injection of Serratia marcescens into the blood (hemolymph) of the silkworm, Bombyx mori, induced the activation of c-Jun NH(2)-terminal kinase (JNK), followed by caspase activation and apoptosis of blood cells (hemocytes). This process impaired the innate immune response in which pathogen cell wall components, such as glucan, stimulate hemocytes, leading to the activation of insect cytokine paralytic peptide. S. marcescens induced apoptotic cell death of silkworm hemocytes and mouse peritoneal macrophages in vitro. We searched for S. marcescens transposon mutants with attenuated ability to induce apoptosis of silkworm hemocytes. Among the genes identified, disruption mutants of wecA (a gene involved in lipopolysaccharide O-antigen synthesis), and flhD and fliR (essential genes in flagella synthesis) showed reduced motility and impaired induction of mouse macrophage cell death. These findings suggest that S. marcescens induces apoptosis of host immune cells via lipopolysaccharide- and flagella-dependent motility, leading to the suppression of host innate immunity.

Swarming motility by Photorhabdus temperata is influenced by environmental conditions and uses the same flagella as that used in swimming motility

Photorhabdus temperata, an insect pathogen and nematode symbiont, is motile in liquid medium by swimming. We found that P. temperata was capable of surface movement, termed swarming behavior. Several lines of evidence indicate that P. temperata use the same flagella for both swimming and swarming motility. Both motility types required additional NaCl or KCl in the medium and had peritrichous flagella, which were composed of the same flagellin as detected by immunoblotting experiments. Mutants defective in flagellar structural proteins were nonmotile for both motility types. Unlike swimming, we observed swarming behavior to be a social form of movement in which the cells coordinately formed intricate channels covering a surface. The constituents of the swarm media affected motility. Swarming was optimal on low agar concentrations; as agar concentrations increased, swarm ring diameters decreased.

A MotN mutant of Ralstonia solanacearum is hypermotile and has reduced virulence

Ralstonia solanacearum is a soil-borne plant pathogen that causes bacterial wilt disease on many plant species. We previously showed that swimming motility contributes to virulence of this bacterium in the early stages of host invasion and colonization. In this study we identified a new negative regulator of motility, named motN, that is located in a cluster of motility-related genes. A motN mutant was hypermotile both on 0.3% agar motility plates and in rich and minimal medium broth. However, like its wild-type parent, it was largely nonmotile inside plants. The motN mutant cells appeared hyperflagellated, and sheared cell protein preparations from motN contained more flagellin than preparations from wild-type cells. The motN strain was significantly reduced in virulence in a naturalistic soil soak assay on tomato plants. However, the motN mutant had wild-type virulence when it was inoculated directly into the plant vascular system. This suggests that motN makes its contribution to virulence early in disease development. The motN mutant formed weaker biofilms than the wild type, but it attached normally to tomato roots and colonized tomato stems as well as its wild-type parent. Phenotypic analysis and gene expression studies indicated that MotN directly or indirectly represses transcription of the major motility regulator FlhDC. MotN was also connected with other known motility and virulence regulators, PehSR, VsrBC, and VsrAD, via uncertain mechanisms. Together, these results demonstrate the importance of precise regulation of flagellum-mediated motility in R. solanacearum.

Molecular mechanisms of persistence of mutualistic bacteria Photorhabdus in the entomopathogenic nematode host

Symbioses between microbes and animals are ubiquitous, yet little is known about the intricate mechanisms maintaining such associations. In an emerging mutualistic model system, insect-pathogenic bacteria Photorhabdus and their insect-parasitic nematode partner Heterorhabditis, we found that the bacteria undergo major transcriptional reshaping in the nematode intestine. Besides general starvation mechanisms, the bacteria induce cellular acidification to slow down growth, switch to pentose phosphate pathway to overcome oxidative stress and nutrition limitation, and shed motility but develop biofilm to persist in the nematode intestine until being released into the insect hemolymph. These findings demonstrate how the symbiotic bacteria reduce their nutritional dependence on the enduring nematode partner to ensure successful transmission of the couple to the next insect host.

Economical evolution: microbes reduce the synthetic cost of extracellular proteins

Protein evolution is not simply a race toward improved function. Because organisms compete for limited resources, fitness is also affected by the relative economy of an organism’s proteome. Indeed, many abundant proteins contain relatively high percentages of amino acids that are metabolically less taxing for the cell to make, thus reducing cellular cost. However, not all abundant proteins are economical, and many economical proteins are not particularly abundant. Here we examined protein composition and found that the relative synthetic cost of amino acids constrains the composition of microbial extracellular proteins. In Escherichia coli, extracellular proteins contain, on average, fewer energetically expensive amino acids independent of their abundance, length, function, or structure. Economic pressures have strategically shaped the amino acid composition of multicomponent surface appendages, such as flagella, curli, and type I pili, and extracellular enzymes, including type III effector proteins and secreted serine proteases. Furthermore, in silico analysis of Pseudomonas syringae, Mycobacterium tuberculosis, Saccharomyces cerevisiae, and over 25 other microbes spanning a wide range of GC content revealed a broad bias toward more economical amino acids in extracellular proteins. The synthesis of any protein, especially those rich in expensive aromatic amino acids, represents a significant investment. Because extracellular proteins are lost to the environment and not recycled like other cellular proteins, they present a greater burden on the cell, as their amino acids cannot be reutilized during translation. We hypothesize that evolution has optimized extracellular proteins to reduce their synthetic burden on the cell.

Microbes secrete proteins to perform essential interactions with their environment, such as motility, pathogenesis, biofilm formation, and resource acquisition. However, because microbes generally lack protein import systems, secretion is often a one-way street. Consequently, secreted proteins are less likely to be recycled by the cell due to environmental loss. We demonstrate that evolution has in turn selected these extracellular proteins for increased economy at the level of their amino acid composition. Compared to their cellular counterparts, extracellular proteins have fewer synthetically expensive amino acids and more inexpensive amino acids. The resulting bias lessens the loss of cellular resources due to secretion. Furthermore, this economical bias was observed regardless of the abundance, length, structure, or function of extracellular proteins. Thus, it appears that economy may address the compositional bias seen in many extracellular proteins and deliver further insight into the forces driving their evolution.

Experimental adaptation of Burkholderia cenocepacia to onion medium reduces host range

It is unclear whether adaptation to a new host typically broadens or compromises host range, yet the answer bears on the fate of emergent pathogens and symbionts. We investigated this dynamic using a soil isolate of Burkholderia cenocepacia, a species that normally inhabits the rhizosphere, is related to the onion pathogen B. cepacia, and can infect the lungs of cystic fibrosis patients. We hypothesized that adaptation of B. cenocepacia to a novel host would compromise fitness and virulence in alternative hosts. We modeled adaptation to a specific host by experimentally evolving 12 populations of B. cenocepacia in liquid medium composed of macerated onion tissue for 1,000 generations. The mean fitness of all populations increased by 78% relative to the ancestor, but significant variation among lines was observed. Populations also varied in several phenotypes related to host association, including motility, biofilm formation, and quorum-sensing function. Together, these results suggest that each population adapted by fixing different sets of adaptive mutations. However, this adaptation was consistently accompanied by a loss of pathogenicity to the nematode Caenorhabditis elegans; by 500 generations most populations became unable to kill nematodes. In conclusion, we observed a narrowing of host range as a consequence of prolonged adaptation to an environment simulating a specific host, and we suggest that emergent pathogens may face similar consequences if they become host-restricted.

Identification of genes involved in the mutualistic colonization of the nematode Heterorhabditis bacteriophora by the bacterium Photorhabdus luminescens

Background

Photorhabdus are Gram negative entomopathogenic bacteria that also have a mutualistic association with nematodes from the family Heterorhabditis. An essential part of this symbiosis is the ability of the bacterium to colonize the gut of the freeliving form of the nematode called the infective juvenile (IJ). Although the colonization process (also called transmission) has been described phenomonologically very little is known about the underlying molecular mechanisms. Therefore, in this study, we were interested in identifying genes in Photorhabdus that are important for IJ colonization.

Results

In this work we genetically tagged P. luminescens TT01 with gfp and constructed a library containing over 3200 mutants using the suicide vector, pUT-Km2. Using a combination of in vitro symbiosis assays and fluorescent microscopy we screened this library for mutants that were affected in their ability to colonize the IJ i.e. with decreased transmission frequencies. In total 8 mutants were identified with transmission frequencies of ≤ 30% compared to wild-type. These mutants were mapped to 6 different genetic loci; the pbgPE operon, galE, galU, proQ, asmA and hdfR. The pbgPE, galE and galU mutants were all predicted to be involved in LPS biosynthesis and, in support of this, we have shown that these mutants are avirulent and sensitive to the cationic antimicriobial peptide, polymyxin B. On the other hand the proQ, asmA and hdfR mutants were not affected in virulence and were either as resistant (proQ) or slightly more sensitive (asmA, hdfR) to polymyxin B than the wild-type (WT).

Conclusions

This is the first report describing the outcome of a comprehensive screen looking for transmission mutants in Photorhabdus. In total 6 genetic loci were identified and we present evidence that all of these loci are involved in the assembly and/or maintenance of LPS and other factors associated with the cell surface. Interestingly several, but not all, of the transmission mutants identified were also avirulent suggesting that there is a significant, but not complete, genetic overlap between pathogenicity and mutualism. Therefore, this study highlights the importance of the cell surface in mediating the symbiotic and pathogenic interactions of Photorhabdus.

Photorhabdus and a host of hosts

Photorhabdus is a member of the family Enterobacteriaceae that lives in a mutualistic association with a Heterorhabditis nematode worm. The nematode worm burrows into insect prey and regurgitates Photorhabdus, which goes on to kill the insect. The nematode feeds off the growing bacteria until the insect tissues are exhausted, whereupon they reassociate and leave the cadaver in search of new prey. This highly efficient partnership has been used for many years as a biological crop protection agent. The dual nature of Photorhabdus as a pathogen and mutualist makes it a superb model for understanding these apparently exclusive activities. Furthermore, recently identified clinical isolates of Photorhabdus are helping us to understand how human pathogens can emerge from the enormous reservoir of invertebrate pathogens in the environment. As Photorhabdus has never been found outside a host animal, its niche represents an entirely biotic landscape. In this review we discuss what molecular adaptations allow this bacterium to complete this fascinating and complex life cycle.