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Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

Photorhabdus are entomopathogenic members of the family Enterobacteriaceae. In addition to killing insects Photorhabdus also have a mutualistic association with nematodes from the family Heterorhabditidiae. Therefore, the bacteria have a complex life cycle that involves temporally separated pathogenic and mutualistic associations with two different invertebrate hosts. This tripartite Photorhabdus–insect–nematode association provides researchers with a unique opportunity to characterize the prokaryotic contribution to two different symbioses, i.e. pathogenicity and mutualism while also studying the role of the host in determining the outcome of association with the bacteria. In this review I will outline the life cycle of Photorhabdus and describe recent important advances in our understanding of the symbiology of Photorhabdus. Finally, the contribution made by this model to our understanding of the nature of symbiotic associations will be discussed.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

Studies using different model systems of bacteria–host interactions have described many of the mechanisms and pathways involved in the initiation, maintenance and outcome of these associations (Nyholm and McFall-Ngai, 2004; Moran, 2006). A significant outcome of these studies has been the realization that, in many aspects, pathogenicity and mutualism are quite similar and there is now an emerging acceptance that these bacteria–host interactions can be considered as bacterial infections with different outcomes (Hentschel et al., 2000).

In recent years two groups of entomopathogenic bacteria, Xenorhabdus and Photorhabdus, have emerged as useful models in the study of bacteria–host interactions as these bacteria have contrasting interactions with different invertebrate hosts during their normal life cycle (Ffrench-Constantet al., 2003; Goodrich-Blair and Clarke, 2007). Accordingly, the bacteria are highly virulent to a wide range of insect larvae while also maintaining a mutualistic interaction with nematodes from the family Steinernematidiae (for Xenorhabdus) and Heterorhabditiae (for Photorhabdus). The similarity in lifestyles appears to be a consequence of convergent evolution as there is substantial evidence that Xenorhabdus and Photorhabdus have evolved to this complex lifestyle independently (Goodrich-Blair and Clarke, 2007). In this review, I will discuss recent developments in our understanding of the interactions between Photorhabdus and its different invertebrate hosts [for a recent review on Xenorhabdus (Herbert and Goodrich-Blair, 2007]. In particular, I will focus on aspects of Photorhabdus biology that have been shown to be involved in the different symbioses and I will discuss the potential of post-genomic analyses in directing future research in this interesting model system.

Life cycle

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

Photorhabdus are a bioluminescent genus of Gram-negative bacteria that belong to γ-subdivision of the Proteobacteria, more specifically the family Enterobactericeae (Fischer-Le Saux et al., 1999). The enterics are arguably the best studied family of bacteria and the family includes important model organisms such as the mammalian pathogens Escherichia coli, Salmonella and Yersinia spp.; the phytopathogen Erwinia spp. and the reduced genome endosymbionts Buchnera, Wigglesworthia and Sodalis.

Photorhabdus are normally found in the gut of the infective juvenile (IJ) stage of the soil-dwelling heterorhabditid nematode. The IJ is, in many ways, analogous to the well-characterized dauer stage of the model nematode, Caenorhabditis elegans (Hu, 2007). Therefore, the IJ is a non-feeding, yet long-lived, stage of the nematode that is programmed to search for, and infect, susceptible insect hosts either through natural openings in the cuticle (e.g. the mouth, anus or spiracles) or by slicing the cuticle with the aid of a buccal tooth-like appendage. Once inside the insect the IJ migrates to the haemolymph (i.e. the insect blood) where partially characterized insect-derived signals induce the IJ to regurgitate the Photorhabdus from their gut (Ciche and Ensign, 2003). The bacteria grow exponentially in the insect reaching cell densities of up to 109 cfu/insect within 48 h (Watson et al., 2005). This rapid growth is facilitated by the secretion of toxins and other molecules that damage host tissues and reduce the effectiveness of the normally highly potent insect innate immune system (Goodrich-Blair and Clarke, 2007). Death of the insect is normally concomitant with the entry of the bacteria into the post-exponential phase of growth and, at this stage, all of the internal organs and tissues of the insect will have been converted into bacteria-associated biomass. Insect death also signals the end of the pathogenic period, and the start of the mutualistic period, of the Photorhabdus life cycle (Ffrench-Constant et al., 2003).

While Photorhabdus is growing exponentially, the nematode recovers from the IJ into an adult hermaphrodite. Recovery is defined as the exit from the diapause associated with the IJ and the resumption of nematode development. The completion of nematode recovery generally occurs after insect death and, at this point, the hermaphrodite will lay 200–300 eggs that will develop through four larval stages (L1–L4) into either male or hermaphrodite adult nematodes (Ciche et al., 2008). Remarkably, there is no subsequent external egg-laying observed and intrauterine nematode reproduction continues for a further 1–2 generations until, in response to an unknown environmental signal, the final generation of developing nematodes enter the alternative L3 juvenile stage, the IJ. During development Photorhabdus infect the gut of the IJ before it emerges from the insect cadaver into the soil (Ciche et al., 2008). Under optimal laboratory conditions, the life cycle (from IJ infection to emergence) takes 10–20 days to complete and > 100 000 IJs can be produced as a result of a single IJ infecting an insect. The bacteria–nematode complex is so effective at killing target insects that it is mass-produced and marketed as a biocontrol agent for the control of pests of high-value crops such as strawberries, mushrooms and turf grass (Ehlers, 2001).

Therefore, Photorhabdus has three essential roles to fulfil during its life cycle: (i) Photorhabdus must be a highly efficient pathogen of the insect; (ii) Photorhabdus must support nematode growth and development; and (iii) Photorhabdus must be able to colonize the IJ stage of the nematode (see Fig. 1) and the bacterial contribution to each of these roles will now be discussed.

image

Figure 1. The multiple personalities of Photorhabdus. The Photorhabdus life cycle can be defined in terms of the three distinct roles that the bacteria has during its interaction with the insect and the nematode. First, Photothabdus must be able to colonize the gut of the IJ in a highly specific process. A population of approximately 100 cfu is maintained in the IJ (for periods of several months to 1 year) and the top micrograph shows GFP-labelled Photorhabdus in the foregut of the IJ. Second, after regurgitation into the insect haemolymph, Photorhabdus must be able to kill the insect larvae. The cuticle of the insect cadaver remains intact after death and Photorhabdus-infected insects often change colour due to the production of an anthraquinone pigment by the bacteria. Finally, Photorhabdus must provide conditions within the cadaver that will support nematode growth and development. This includes the provision of nutrients to the increasing nematode population and protecting the cadavar from predation by saprophytic organisms.

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Photorhabdus and pathogenicity

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

Photorhabdus is a highly virulent pathogen of a wide range of insect larvae. In one model moth species, Galleria mellonella (the Greater Wax Moth), the LD50 of Photorhabdus injected directly into the insect haemocoel is < 5 cfu (our unpublished data). Once in the haemolymph Photorhabdus grow exponentially and there is a strong positive correlation between growth rate and the time taken to kill 50% of infected insects, i.e. LT50 (Clarke and Dowds, 1995; Watson et al., 2005). In response to the insects highly sophisticated innate immune response, Photorhabdus has evolved mechanisms that effectively nuetralize this response to facilitate rapid bacterial growth (see Fig. 2).

image

Figure 2. Photorhabdus interacts with both branches of insect innate immunity. During infection Photorhabdus produces factors that affect the activity of both the cellular and humoral innate immune reponse. Photorhabdus is recognized by the insect immune system and the bacteria adapt to the humoral response through the activity of a two-component pathway, PhoPQ. Photorhabdus produce a range of toxins that kill insect immune cells while also producing uncharacterized compounds (indicated by ?) that inhibit insect PLA2 activity and nodulation. In addition, a type III secretion system (TTSS) secretes effectors that inhibit phagocytosis but induce the formation of nodules, an important cellular response to infection. The small bioactive ST molecule inhibits prophenoloxidase (PO) activity and therefore prevents nodule maturation.

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The insect innate immune system has both cellular and humoral branches (Kanost et al., 2004). The cellular innate immune response is mediated by circulating haemocytes and includes both phagocytosis and nodulation. The latter is probably the most important cellular response to bacterial infection and involves the aggregation of haemocytes around the invading microorganism. Photorhabdus produces an array of toxins, including the Tc and the Mcf toxins, that have been shown to target, and induce apoptosis in, insect immune cells (Waterfield et al., 2001; Daborn et al., 2002; Au et al., 2004; Dowling et al., 2004; Waterfield et al., 2005). Photorhabdus also has a locus that encodes a type III secretion system (TTSS) with a single obvious effector protein, LopT, that has been shown to inhibit phagocytosis (Brugirard-Ricaud et al., 2004; 2005). Moreover, a mutation in sctC (encoding a protein required for effector secretion) resulted in decreased nodule formation suggesting that the TTSS also secretes effector(s) required for effective nodule formation (Brugirard-Ricaud et al., 2005). In addition, Photorhabdus produce compound(s) that inhibit the activity of host phospholipase A2 (PLA2), the enzyme involved in the activation of the insect eicosanoid signalling pathway that controls aspects of the cellular immune response (Kim et al., 2005; Stanley, 2006). Taken together, these results suggest that Photorhabdus manipulate nodule formation, perhaps as an adaptation to survive within this niche. In support of this it has been shown that Photorhabdus produces a small bioactive molecule, 3,5-dihydroxy-4-isopropylstilbene (ST), which inhibits the activity of phenoloxidase, an enzyme that is required for the production of melanin, a molecule that is involved in the stabilization of the nodule (Eleftherianos et al., 2007).

The humoral innate immune response primarily involves the production of a range of antimicrobial peptides (although insects can also produce lysozyme). Photorhabdus are recognized by pattern recognition receptors in the insect and recognition does lead to an increase in the production of an insect-specific repertoire of antimicrobial peptides (Eleftherianos et al., 2007; Hallem et al., 2007). Interestingly, Photorhabdus cannot infect insects that have been pre-immunized with E. coli and therefore already have relatively high levels of circulating antimicrobial peptides (Eleftherianos et al., 2006a). Therefore, Photorhabdus are not inherently resistant to the humoral response and the bacteria will have to adapt to the increase in the level of these antimicrobial peptides after infection. In other enteric pathogens adaptation is achieved by remodelling the outer membrane, primarily the LPS (Raetz et al., 2007). In Salmonella these modifications are mediated by the PhoPQ and PmrAB two-component pathways (Ernst et al., 2001). These regulatory pathways co-ordinate the expression of a number of genes that are involved in the modification of the lipid A moiety of the LPS, including the pmrHFIJKLM operon. This operon is required for the production and ligation of l-aminoarabinose onto the lipid A resulting in an alteration of the surface charge of the bacteria and therefore an increase in the resistance of the cell to the positively charged antimicrobial peptides (Gunn et al., 1998). Although Photorhabdus does not have pmrAB orthologues deletion of the phoP gene does result in a strain that is avirulent and sensitive to the antimicrobial peptide polymyxin B (Derzelle et al., 2004). In Photorhabdus the PhoPQ two-component pathway has been shown to control the expression of the seven gene pbgPE operon which is orthologous to the pmrHFIJKLM operon in Salmonella (Derzelle et al., 2004). Mutations in the pbgPE operon led to severe attenuation in virulence and sensitivity to polymyxin B (Bennett and Clarke, 2005). Therefore, it appears that Photorhabdus employ a similar strategy to mammalian pathogens in order to adapt to the humoral branch of the innate immune system.

Photorhabdus and nematode growth and development

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

The first step of nematode development is recovery from the IJ to the adult hermaphrodite. Within the insect, the recovery of the infecting IJ appears to be controlled by partially characterized haemolymph-derived signal(s) (Ciche and Ensign, 2003). However, IJs also recover when inoculated directly onto high densities of Photorhabdus grown on agar plates implying that the bacteria also produce a signal, termed ‘the food signal’, which can stimulate nematode development (Strauch and Ehlers, 1998). In 2001 Ciche and colleagues isolated an insertion mutation in a gene, ngrA, that did not support nematode growth and development (Ciche et al., 2001). This gene encodes a phosphopantetheinyl transferase that is involved in the production of small bioactive molecules such as siderophores and antibiotics (Ciche et al., 2001). Indeed, it has recently been established that a major component of this food signal is the ST molecule previously discussed as an inhibitor of phenoloxidase (Joyce et al., 2008). Under normal conditions approximately 50% of the IJs that are inoculated onto a lawn of wild-type Photorhabdus grown on an agar plate will recover to adult hermaphrodites. However, if the IJs are inoculated onto a mutant that is unable to produce ST this recovery rate decreases to approximately 5%. This decrease can be reversed by the addition of purified ST to the agar plate (Joyce et al., 2008).

The ASJ chemosensory neurons have been shown to be involved in IJ recovery in both C. elegans and the human parasite, Strongyloides stercoralis (Bargmann and Horvitz, 1991; Ashton et al. 2007). Laser ablation studies have confirmed the role of the ASJ neurons in IJ recovery in Heterorhabditis, suggesting that ST may be a ligand for these neurons (Hallem et al., 2007). However, IJ recovery also appears to involve both cGMP signalling and muscarinic acetylcholine receptors, and therefore the exact mode of action of ST remains to be determined (Hallem et al., 2007). Nonetheless, it is clear that ST is a multipotent molecule with important roles on both pathogenicity and mutualism.

The heterorhabditid nematode partner of Photorhabdus is bacteriophorous and feeds on the biomass (essentially a monoculture of Photorhabdus) present in the insect cadaver. Therefore, Photorhabdus must satisfy most, if not all, of the nutritional requirements of the nematode. Moreover, the nematodes are fastidious eaters and each nematode will only feed on particular strain(s) of Photorhabdus. Therefore, there is a degree of specificity in the nutritional interaction between the bacteria and the nematode although the basis of this specificity is not well understood. Photorhabdus produce two types of crystalline inclusion proteins (CIP) during the post-exponential phase of growth, CipA and CipB, and deletion of the cipA and cipB genes results in a mutant that is unable to support nematode growth and development (Bintrim and Ensign, 1998; Bowen and Ensign, 2001). Unfortunately, the cipA cipB double mutant is plieotropic, and therefore it is difficult to establish whether this lack of symbiosis is due exclusively to the absence of the CIPs. However, recent feeding studies do suggest that the CIPs do have a role in nematode nutrition (You et al., 2006).

Iron is an essential nutrient and iron uptake by Photorhabdus does play a role in nematode growth and development. It was shown that a mutation in exbD in P. temperata K122, encoding ExbD (a component of the TonB complex), was unable to support the growth and development of its nematode partner, H. downseii (Watson et al., 2005). The TonB complex is required for energizing the outer membrane receptors of siderophores and facilitating the uptake of the iron-siderophore complex into the periplasm (Postle and Larsen, 2007). Interestingly, the defect in nematode growth and development could be rescued by growing the exbD mutants on agar that was supplemented with high levels of iron, suggesting that the level of iron in the bacteria is important for symbiosis (Watson et al., 2005). However, the importance of iron in the nutritional interaction between Photorhabdus and the nematode appears to be species-specific as an exbD mutation in P. luminescens TT01 has no effect on the growth and development of its nematode partner, H. bacteriophora (our unpublished data). Interestingly, although H. downseii normally grows well on P. luminescens TT01 this nematode does not grow on the TT01 exbD mutant, suggesting that different nematode species have different requirements for iron (our unpublished data).

Photorhabdus and colonization of the IJ

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

It was generally accepted that transmission of Photorhabdus to the IJ was from the environment (i.e. the insect cadaver) and, as the IJ developed, it retained some Photorhabdus in its gut rather than digest it as food. However, a recent detailed microscopic analysis has revealed that transmission is, in fact, maternal (Ciche et al., 2008). As the adult hermaphrodite feeds some bacteria, which are not crushed by the pharynx, enter the gut where they bind specifically to the distal INT9 gut cells. The bacteria then migrate and infect the neighbouring rectal gland cells where they replicate inside vacuoles. All IJs appear to develop inside the adult hermaphrodite in a process called endotokia matricida whereby the eggs hatch internally and the mother is used as food by the developing IJs. During IJ development the infected rectal glands of the adult hermaphrodite rupture releasing the Photorhabdus into the body cavity of the mother. Each developing IJ is then colonized by a single bacterium that attaches to the pre-intestinal valve cell (PIVC) and replicates to give a final bacterial population of around 100 cfu in each mature IJ (Ciche et al., 2008). The Photorhabdus genome contains 11 loci predicted to encode different fimbria or pili and this would be appropriate for attachment to the different cells involved in transmission (Duchaud et al., 2003). Indeed, this transmission mechanism could also be used to explain why nematodes are generally only colonized by their cognate bacteria (or very closely related strains). Whole-genome comparisons between Photorhabdus isolates have shown that some fimbrial-encoding loci are part of the variable genome implying that they may have a role in the evolved specificity between the bacteria and the nematode (Gaudriault et al., 2006).

This remarkably intricate transmission mechanism is poorly understood at the molecular level and, to date, only one genetic locus has been described as being required for transmission, the pbgPE operon (Bennett and Clarke, 2005). Therefore, the pbgPE operon in Photorhabdus is required for both pathogenicity and mutualism, highlighting that mutualism can be genetically similar to pathogenicity. As outlined above, the pbgPE operon is involved in resistance to antimicrobial peptides and it is noteworthy that the ability to resist antimicrobial peptides facilitated the persistent infection of C. elegans by S. typhimurium (Alegado and Tan, 2008). Perhaps Photorhabdus must also overcome the humoral innate immune response of its nematode host?

The genomics of Photorhabdus

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

The publication of the genome sequence of P. luminescens TT01 in 2003 opened the door to the post-genomic analysis of this bacterium (Duchaud et al., 2003). In addition to TT01, the genome of P. asymbiotica (a species associated with human infections) has been sequenced by the Sanger Centre (although the sequence has not yet been completed) and two other sequencing projects (P. temperata K122 and P. asymbiotica Kingscliffe) are in progress or planned for the near future. Finally, a genome sample sequence of P. luminescens W14, comprising of 2000 random sequencing reads, is also available in GenBank (Ffrench-Constant et al., 2000). The availability of all of these sequences will be an important resource for the development of this bacterium as a model for studying bacteria–host interactions. P. luminescens TT01 has a single, circular genome of 5.6 Mb that is predicted to contain nearly 5000 ORFs. As expected, many of these ORFs have close orthologues in E. coli permitting the use of the available E. coli resources to facilitate gene annotation. However, as expected, the genome has also revealed interesting features that may be significant in terms of how Photorhabdus interacts with its different hosts. Here I will briefly describe some features of the genome that have helped to focus our current research.

Secondary metabolism

Photorhabdus has 22 genetic loci that contain genes encoding polyketide synthases (PKSs), non-ribosomal peptide synthases (NRPSs) and PKS-NRPS chimeras. In total, nearly 6% of the genome appears to be put aside for the production of small bioactive molecules (compared with 3% in Streptomyces). To date, only three of the 22 loci have been characterized and these have been shown to produce a carbapenam antibiotic, an anthraquinone pigment (through the activity of a Type II PKS) and the stilbene, ST (Derzelle et al., 2002; Brachmann et al., 2007; Joyce et al., 2008). Photorhabdus is the only bacterium that has been shown to produce a stilbene, an important family of bioactive molecules normally produced by plants. As already mentioned, in addition to having antimicrobial activity (particularly against Gram-positive bacteria), the ST molecule has been shown to have key roles in both pathogenicity and mutualism (Eleftherianos et al., 2007; Joyce et al., 2008). The biochemical pathway leading to the production of ST in P. luminescens TT01 has recently been elucidated and, although similar in some aspects to the plant pathway, it is essentially a novel pathway for stilbene biosynthesis (Joyce et al., 2008). This highlights the potential of Photorhabdus as a source of novel biochemistry. Production of the ST molecule is achieved by a branched biosynthetic pathway that involves both amino acid and fatty acid metabolism. The first step is the non-oxidative deamination of phenylalanine by the phenylalanine ammonium-lyase encoded by stlA (Williams et al., 2005). This results in the production of cinnamic acid which is then elongated by the addition of a single malonyl-CoA before being ligated with an intermediate in branched-chain fatty acid metabolism to form the mature molecule (Joyce et al., 2008).

Energy production

Photorhabdus is a facultative anaerobe and energy production in the absence of O2 can occur by either the use of alternative electron acceptors or, in their absence, fermentation. However, it is notable that, in comparison to E. coli, P. luminescens TT01 has a very restricted range of terminal electron acceptors. Indeed, fumarate and O2 are the only terminal electron acceptors predicted to be available to Photorhabdus. During the early stages of an infection, it is likely that O2 will be readily available to the growing bacterial population by simple diffusion through the insect spiracles. However, once the insect is dead and the bacteria have reached a high cell density, O2 has been shown to be essentially absent from the interior of the insect cadaver (Rosner et al., 1996). Therefore, during the post-pathogenicity stage, the availability of fumarate will determine whether the cells respire anaerobically or produce energy by fermentation. During anaerobic respiration fumarate is reduced to succinate by the enzyme fumarate reductase (encoded by frdABCD). The retention of fumarate as a terminal electron acceptor suggests an important role for this dicarboxylic acid in the interaction between the bacteria and the nematode.

Regulatory pathways

Photorhabdus has an obligate requirement to switch from a pathogen to a mutualist during its life cycle and there is some interest in analysing the regulatory networks that control this switch (Joyce et al., 2006). HexA, a LysR-type transcriptional repressor, has been shown to repress genes involved in mutualism in Photorhabdus (Joyce and Clarke, 2003). Therefore, a mutation in hexA resulted in a strain that constitutively expressed genes associated with mutualism. However, the hexA mutant strain was severly attenuated in virulence, suggesting that HexA plays an important role in the temporal separation of pathogenicity and mutualism (Joyce and Clarke, 2003).

The beginning of nematode reproduction temporally coincides with the transition of the bacteria from exponential growth to post-exponential growth (i.e. stationary phase). The BarA–UvrY two-component pathway (also known as the GacA–GacS pathway) has been implicated in the regulation of virulence and post-exponential metabolism in different bacteria (Lapouge et al., 2008). In P. luminescens TT01 the expression of uvrY has been reported as maximal at the end of exponential growth, suggesting a similar role for this pathway in this bacterium (Krin et al., 2008). Expression profiling of a uvrY deletion mutant revealed that this regulator represses motility but positively regulates genes involved in iron uptake, protease and peptidase production, bioluminescence and pigment production. Many of these genes are, in fact, indirectly regulated by UvrY through the small stable RNA, CsrB and its cognate RNA-binding protein CsrA (Krin et al., 2008). Finally, there is some evidence of a link between HexA and CsrB expression, suggesting a role for a HexA/UvrY/CsrB regulatory network in controlling the transition from pathogen to mutualist (our unpublished data).

Intercellular signalling

The transition from pathogen to mutualist occurs at high cell density and is associated with the production, by Photorhabdus, of a number of activities associated with mutualism, e.g. bioluminescence, antibiotic production. This co-ordinated change in gene expression would suggest that there is some degree of intercellular signalling in Photorhabdus. However, Photorhabdus does not produce any acyl-homoserine lactone (AHL)-type molecule and is therefore not considered to utilize this type of quorum sensing. Indeed, genes encoding LuxI-like or LuxM-like AHL synthases could not be identified in the genome. However, the family of LuxR-type trascriptional regulators is considerabley over-represented, with 39 copies in the genome (Duchaud et al., 2003; Heermann and Fuchs, 2008). Most of the luxR-like genes are clustered in two genetic loci that encode for 19 and 8 tandem repeats of a LuxR-type regulator. The function of these loci is not clear; however, similar clusters have been identified in the insect colonizing bacteriam, Sodalis glossinidius, suggesting a possible role in insect infection (Heermann and Fuchs, 2008). On the other hand, P. luminescens TT01 does have a luxS gene and deletion of this gene has been shown to affect virulence, suggesting a possible role for AI-2 in pathogenicity (Krin et al., 2006). Finally, cyclic-di-GMP has been shown to play a key role in contributing to lifestyle decisions in a variety of enteric pathogens but analysis of the Photorhabdus genome reveals a complete absence of all domains associated with cyclic-di-GMP turnover, i.e. GGDEF, EAL and HDGYP, suggesting that this signalling molecule is absent from Photorhabdus. Therefore, the molecular mechanism(s) utilized by Photorhabdus to co-ordinate the expression of genes involved in mutualism remains to be determined.

Conclusion

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

The genetics of pathogenicity in Photorhabdus appears to be very similar to what has been described in other enteric pathogens, e.g. E. coli, Salmonella and Yersinia highlighting the universal nature of pathogenicity (i.e. bacteria that infect insects use the same genes as bacteria that infect humans). Photorhabdus produce toxins and other molecules to inhibit the host cellular innate immune system while adapting to the humural response by remodelling the lipid A moiety of their LPS in a PhoPQ-dependent manner. However, Photorhabdus must also satisfy all of the nutritional requirements of its nematode partner while remaining competent to recolonize the IJ stage of the nematode at the appropriate time. Although genetic studies are still in their early stages, it is now apparent that there is some overlap between pathogenicity and mutualism. Genes involved in iron uptake in the bacteria and remodelling of the lipid A have been shown to be required for both pathogenicity and mutualism. Research has just started to elucidate the regulatory networks that temporally control the expression of these genes during the interaction between the bacteria, the nematode and the insect. These studies will be greatly facilitated by the imminent publication of a draft genomic sequence of the H. bacteriophora nematode and the genetic accessibility of both the insect and the nematode (Ciche, 2007). In addition, both the insect and the nematode have been shown to be amenable to knock-down experiments using RNAi (Eleftherianos et al., 2006b; Ciche and Sternberg, 2007). Moreover, the hosts are easy and relatively inexpensive to culture in the laboratory or purchase from suppliers and they represent natural environments for Photorhabdus enabling in vivo experiments to be done on an in vitro scale. Therefore, the tripartite Photorhabdus-nematode-insect system provides a unique opportunity to address important fundamental questions into the nature of pathogenity and mutualism, for example, how similar are these interactions at the genetic level, i.e. what is the degree of genetic overlap between pathogenicity and mutualism? Armed with the appropriate post-genomic tools this model system is now poised to take a major step forward in terms of understanding the molecular events that underpin the different associations between Photorhabdus and its hosts. Moreover, given that Photorhabdus is a member of the Enterobacteriaceae, it is likely that these studies will also contribute to similar studies with important mammalian pathogens such as E. coli and Salmonella spp.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References

The author would like to thank all members of the DJC lab, past and present, for their work on Photorhabdus. Work in the DJC lab has been supported by the BBSRC and the Leverhulme Trust and current research in UCC is supported by Science Foundation Ireland (SFI). D.J.C. would especially like to thank Susan Joyce, Max Dow and Rob Ryan for their critical reading of the manuscript.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Life cycle
  5. Photorhabdus and pathogenicity
  6. Photorhabdus and nematode growth and development
  7. Photorhabdus and colonization of the IJ
  8. The genomics of Photorhabdus
  9. Conclusion
  10. Acknowledgements
  11. References