Drosophila as a model host for Pseudomonas aeruginosa infection

Assessment of virulence diversity of methicillin-resistant Staphylococcus aureus strains with a Drosophila melanogaster infection model

Background

Staphylococcus aureus strains with distinct genetic backgrounds have shown different virulence in animal models as well as associations with different clinical outcomes, such as causing infection in the hospital or the community. With S. aureus strains carrying diverse genetic backgrounds that have been demonstrated by gene typing and genomic sequences, it is difficult to compare these strains using mammalian models. Invertebrate host models provide a useful alternative approach for studying bacterial pathogenesis in mammals since they have conserved innate immune systems of biological defense. Here, we employed Drosophila melanogaster as a host model for studying the virulence of S. aureus strains.

Results

Community-associated methicillin-resistant S. aureus (CA-MRSA) strains USA300, USA400 and CMRSA2 were more virulent than a hospital-associated (HA)-MRSA strain (CMRSA6) and a colonization strain (M92) in the D. melanogaster model. These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data. Moreover, MRSA killing activities in the D. melanogaster model are associated with bacterial replication within the flies. Different MRSA strains induced similar host responses in D. melanogaster, but demonstrated differential expression of common bacterial virulence factors, which may account for the different killing activities in the model. In addition, hemolysin α, an important virulence factor produced by S. aureus in human infections is postulated to play a role in the fly killing.

Conclusions

Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity. Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.

SseF, a type III effector protein from the mammalian pathogen Salmonella enterica, requires resistance-gene-mediated signalling to activate cell death in the model plant Nicotiana benthamiana

Type III effector proteins (T3Es) of many Gram-negative pathogenic bacteria manipulate highly conserved cellular processes, indicating conservation in virulence mechanisms during the infection of hosts of divergent evolutionary origin. In order to identify conserved effector functions, we used a cross-kingdom approach in which we expressed selected T3Es from the mammalian pathogen Salmonella enterica in leaves of Nicotiana benthamiana and searched for possible virulence or avirulence phenotypes. We show that the T3E SseF of S. enterica triggers hypersensitive response (HR)-like symptoms, a hallmark of effector-triggered immunity in plants, either when transiently expressed in leaves of N. benthamiana by Agrobacterium tumefaciens infiltration or when delivered by Xanthomonas campestris pv vesicatoria (Xcv) through the type III secretion system. The ability of SseF to elicit HR-like symptoms was lost upon silencing of suppressor of G2 allele of skp1 (SGT1), indicating that the S. enterica T3E is probably recognized by an R protein in N. benthamiana. Xcv translocating an AvrRpt2-SseF fusion protein was restricted in multiplication within leaves of N. benthamiana. Bacterial growth was not impaired but symptom development was rather accelerated in a compatible interaction with susceptible pepper (Capsicum annuum) plants. We conclude that the S. enterica T3E SseF is probably recognized by the plant immune system in N. benthamiana, resulting in effector-triggered immunity.

Lethality and cooperation of Pseudomonas aeruginosa quorum-sensing mutants in Drosophila melanogaster infection models

The virulence profiles of Pseudomonas aeruginosa quorum-sensing (QS) mutants were assessed in Drosophila melanogaster feeding and nicking infection models. Functional RhlIR and LasIR QS systems were required for killing in the fly feeding infection model but were not essential in the fly nicking infection model. Mixed infections between PAO1 and strains harbouring mutations in lasR, rhlI and lasI rhlI resulted in increased lethality in the fly feeding model compared with either isolate alone. These results suggested that the parental strain could cooperate with QS mutants in the Drosophila feeding infection model. Finally, the mixed infection between PAO1 and an rhlR mutant resulted in spiteful behaviour and reduced pathogenicity of the mixed culture.

Exploring the applications of invertebrate host-pathogen models for in vivo biofilm infections

In the natural environment, microorganisms exist together in self-produced polymeric matrix biofilms. Often, several species, which can belong to both bacterial and fungal kingdoms, coexist and interact in ways which are not completely understood. Biofilm infections have become prevalent largely in medical settings because of the increasing use of indwelling medical devices such as catheters or prosthetics. These infections are resistant to common antimicrobial therapies because of the inherent nature of their structure. In terms of infectious biofilms, it is important to understand the microbe-microbe interactions and how the host immune system reacts in order to discover therapeutic targets. Currently, single infection immune response studies are thriving with the use of invertebrate models. This review highlights the advances in single microbial-host immune response as well as the promising aspects of polymicrobial biofilm study in five invertebrate models: Lemna minor (duckweed), Arabidopsis thaliana (thale cress), Dictyostelium discoideum (slime mold), Drosophila melanogaster (common fruit fly), and Caenorhabditis elegans (roundworm).

The Drosophila melanogaster host model

The deleterious and sometimes fatal outcomes of bacterial infectious diseases are the net result of the interactions between the pathogen and the host, and the genetically tractable fruit fly, Drosophila melanogaster, has emerged as a valuable tool for modeling the pathogen-host interactions of a wide variety of bacteria. These studies have revealed that there is a remarkable conservation of bacterial pathogenesis and host defence mechanisms between higher host organisms and Drosophila. This review presents an in-depth discussion of the Drosophila immune response, the Drosophila killing model, and the use of the model to examine bacterial-host interactions. The recent introduction of the Drosophila model into the oral microbiology field is discussed, specifically the use of the model to examine Porphyromonas gingivalis-host interactions, and finally the potential uses of this powerful model system to further elucidate oral bacterial-host interactions are addressed.

Virulence of an exotoxin A-deficient strain of Pseudomonas aeruginosa toward the silkworm, Bombyx mori

We studied the contribution of exotoxin A to the virulence of Pseudomonas aeruginosa against the silkworm, Bombyx mori. First, an exotoxin A-deficient mutant strain (PAO1toxA) was created, and its virulence compared with that of the parental PAO1 strain. In a short-term mortality assay, the mutant harboring pBBR1MCS2 did not kill B. mori until 120 h after inoculation and complementation of the corresponding gene in trans restored the strain's virulence. Next, to ascertain whether or not it lost all virulence, PAO1toxA (pBBR1MCS2, pGFP) was used in a long-term mortality assay. B. mori inoculated with the mutant strain did not die until early in the 5th instar (240 h after inoculation). However, 50% of the inoculated B. mori died late in the 5th instar or in the early pupal stage (408 h after inoculation). All had died by the pupal stage (600 h after inoculation). The mutant strain was isolated from dead larvae and cocoons. The bacterial population of PAO1toxA in hemolymph reached 4.77 × 10(7) cfu/ml. These results indicated that exotoxin A acts as a virulence factor in B. mori and that other virulence factor(s) are involved during the late stages of infection.

Pseudomonas aeruginosa RhlR is required to neutralize the cellular immune response in a Drosophila melanogaster oral infection model

An in-depth mechanistic understanding of microbial infection necessitates a molecular dissection of host-pathogen relationships. Both Drosophila melanogaster and Pseudomonas aeruginosa have been intensively studied. Here, we analyze the infection of D. melanogaster by P. aeruginosa by using mutants in both host and pathogen. We show that orally ingested P. aeruginosa crosses the intestinal barrier and then proliferates in the hemolymph, thereby causing the infected flies to die of bacteremia. Host defenses against ingested P. aeruginosa included an immune deficiency (IMD) response in the intestinal epithelium, systemic Toll and IMD pathway responses, and a cellular immune response controlling bacteria in the hemocoel. Although the observed cellular and intestinal immune responses appeared to act throughout the course of the infection, there was a late onset of the systemic IMD and Toll responses. In this oral infection model, P. aeruginosa PA14 did not require its type III secretion system or other well-studied virulence factors such as the two-component response regulator GacA or the protease AprA for virulence. In contrast, the quorum-sensing transcription factor RhlR, but surprisingly not LasR, played a key role in counteracting the cellular immune response against PA14, possibly at an early stage when only a few bacteria are present in the hemocoel. These results illustrate the power of studying infection from the dual perspective of host and pathogen by revealing that RhlR plays a more complex role during pathogenesis than previously appreciated.

Drosophila melanogaster as an animal model for the study of Pseudomonas aeruginosa biofilm infections in vivo

Pseudomonas aeruginosa is an opportunistic pathogen capable of causing both acute and chronic infections in susceptible hosts. Chronic P. aeruginosa infections are thought to be caused by bacterial biofilms. Biofilms are highly structured, multicellular, microbial communities encased in an extracellular matrix that enable long-term survival in the host. The aim of this research was to develop an animal model that would allow an in vivo study of P. aeruginosa biofilm infections in a Drosophila melanogaster host. At 24 h post oral infection of Drosophila, P. aeruginosa biofilms localized to and were visualized in dissected Drosophila crops. These biofilms had a characteristic aggregate structure and an extracellular matrix composed of DNA and exopolysaccharide. P. aeruginosa cells recovered from in vivo grown biofilms had increased antibiotic resistance relative to planktonically grown cells. In vivo, biofilm formation was dependent on expression of the pel exopolysaccharide genes, as a pelB::lux mutant failed to form biofilms. The pelB::lux mutant was significantly more virulent than PAO1, while a hyperbiofilm strain (PAZHI3) demonstrated significantly less virulence than PAO1, as indicated by survival of infected flies at day 14 postinfection. Biofilm formation, by strains PAO1 and PAZHI3, in the crop was associated with induction of diptericin, cecropin A1 and drosomycin antimicrobial peptide gene expression 24 h postinfection. In contrast, infection with the non-biofilm forming strain pelB::lux resulted in decreased AMP gene expression in the fly. In summary, these results provide novel insights into host-pathogen interactions during P. aeruginosa oral infection of Drosophila and highlight the use of Drosophila as an infection model that permits the study of P. aeruginosa biofilms in vivo.

Insect natural products and processes: new treatments for human disease

In this overview, some of the more significant recent developments in bioengineering natural products from insects with use or potential use in modern medicine are described, as well as in utilisation of insects as models for studying essential mammalian processes such as immune responses to pathogens. To date, insects have been relatively neglected as sources of modern drugs although they have provided valuable natural products, including honey and silk, for at least 4-7000 years, and have featured in folklore medicine for thousands of years. Particular examples of Insect Folk Medicines will briefly be described which have subsequently led through the application of molecular and bioengineering techniques to the development of bioactive compounds with great potential as pharmaceuticals in modern medicine. Insect products reviewed have been derived from honey, venom, silk, cantharidin, whole insect extracts, maggots, and blood-sucking arthropods. Drug activities detected include powerful antimicrobials against antibiotic-resistant bacteria and HIV, as well as anti-cancer, anti-angiogenesis and anti-coagulant factors and wound healing agents. Finally, the many problems in developing these insect products as human therapeutic drugs are considered and the possible solutions emerging to these problems are described.

In-vivo expression profiling of Pseudomonas aeruginosa infections reveals niche-specific and strain-independent transcriptional programs

Pseudomonas aeruginosa is a threatening, opportunistic pathogen causing disease in immunocompromised individuals. The hallmark of P. aeruginosa virulence is its multi-factorial and combinatorial nature. It renders such bacteria infectious for many organisms and it is often resistant to antibiotics. To gain insights into the physiology of P. aeruginosa during infection, we assessed the transcriptional programs of three different P. aeruginosa strains directly after isolation from burn wounds of humans. We compared the programs to those of the same strains using two infection models: a plant model, which consisted of the infection of the midrib of lettuce leaves, and a murine tumor model, which was obtained by infection of mice with an induced tumor in the abdomen. All control conditions of P. aeruginosa cells growing in suspension and as a biofilm were added to the analysis. We found that these different P. aeruginosa strains express a pool of distinct genetic traits that are activated under particular infection conditions regardless of their genetic variability. The knowledge herein generated will advance our understanding of P. aeruginosa virulence and provide valuable cues for the definition of prospective targets to develop novel intervention strategies.

Paraoxonase 1, quorum sensing, and P. aeruginosa infection: a novel model

Pseudomonas aeruginosa is a Gram-negative bacterium which exacts a heavy burden on immunocompromised patients, but is non-pathogenic in a healthy host. Using small signaling molecules called acyl-homoserine lactones (AHLs), populations of P. aeruginosa can coordinate phenotypic changes, including biofilm formation and virulence factor secretion. This concentration-dependent process is called quorum sensing (QS). Interference with QS has been identified as a potential source of new treatments for P. aeruginosa infection. The human enzyme paraoxonase 1 (PON1) degrades AHL molecules, and is a promising candidate for QS interference therapy. Although paraoxonase orthologs exist in many species, genetic redundancy in humans and other mammals has made studying the specific effects of PON1 quite difficult. Arthropods, however, do not express any PON homologs. We generated a novel model to study the specific effects of PON1 by transgenically expressing human PON1 in Drosophila melanogaster. Using this model, we showed that P. aeruginosa infection lethality is QS-dependent, and that expression of PON1 has a protective effect. This work demonstrates the value of a D. melanogaster model for investigating the specific functions of members of the paraoxonase family in vivo, and suggests that PON1 plays a role in innate immunity.

Adult Drosophila melanogaster evolved for antibacterial defense invest in infection-induced expression of both humoral and cellular immunity genes

Background

While the transcription of innate immunity genes in response to bacterial infection has been well-characterised in the Drosophila model, we recently demonstrated the capacity for such transcription to evolve in flies selected for improved antibacterial defense. Here we use this experimental system to examine how insects invest in constitutive versus infection-induced transcription of immunity genes. These two strategies carry with them different consequences with respect to energetic and pleiotropic costs and may be more or less effective in improving defense depending on whether the genes contribute to humoral or cellular aspects of immunity.

Findings

Contrary to expectation we show that selection preferentially increased the infection-induced expression of both cellular and humoral immunity genes. Given their functional roles, infection induced increases in expression were expected for the humoral genes, while increases in constitutive expression were expected for the cellular genes. We also report a restricted ability to improve transcription of immunity genes that is on the order of 2-3 fold regardless of total transcription level of the gene.

Conclusions

The evolved increases in infection-induced expression of the cellular genes may result from specific cross talk with humoral pathways or from generalised strategies for enhancing immunity gene transcription. A failure to see improvements in constitutive expression of the cellular genes suggests either that increases might come at too great a cost or that patterns of expression in adults are decoupled from the larval phase where increases would be most effective. The similarity in fold change increase across all immunity genes may suggest a shared mechanism for the evolution of increased transcription in small, discrete units such as duplication of cis-regulatory elements.

The stringent response is essential for Pseudomonas aeruginosa virulence in the rat lung agar bead and Drosophila melanogaster feeding models of infection

The stringent response is a regulatory system that allows bacteria to sense and adapt to nutrient-poor environments. The central mediator of the stringent response is the molecule guanosine 3',5'-bispyrophosphate (ppGpp), which is synthesized by the enzymes RelA and SpoT and which is also degraded by SpoT. Our laboratory previously demonstrated that a relA mutant of Pseudomonas aeruginosa, the principal cause of lung infections in cystic fibrosis patients, was attenuated in virulence in a Drosophila melanogaster feeding model of infection. In this study, we examined the role of spoT in P. aeruginosa virulence. We generated an insertion mutation in spoT within the previously constructed relA mutant, thereby producing a ppGpp-devoid strain. The relA spoT double mutant was unable to establish a chronic infection in D. melanogaster and was also avirulent in the rat lung agar bead model of infection, a model in which the relA mutant is fully virulent. Synthesis of the virulence determinants pyocyanin, elastase, protease, and siderophores was impaired in the relA spoT double mutant. This mutant was also defective in swarming and twitching, but not in swimming motility. The relA spoT mutant and, to a lesser extent, the relA mutant were less able to withstand stresses such as heat shock and oxidative stress than the wild-type strain PAO1, which may partially account for the inability of the relA spoT mutant to successfully colonize the rat lung. Our results indicate that the stringent response, and SpoT in particular, is a crucial regulator of virulence processes in P. aeruginosa.

Shift in ribonucleotide reductase gene expression in Pseudomonas aeruginosa during infection

The roles of different ribonucleotide reductases (RNRs) in bacterial pathogenesis have not been studied systematically. In this work we analyzed the importance of the different Pseudomonas aeruginosa RNRs in pathogenesis using the Drosophila melanogaster host-pathogen interaction model. P. aeruginosa codes for three different RNRs with different environmental requirements. Class II and III RNR chromosomal mutants exhibited reduced virulence in this model. Translational reporter fusions of RNR gene nrdA, nrdJ, or nrdD to the green fluorescent protein were constructed to measure the expression of each class during the infection process. Analysis of the P. aeruginosa infection by flow cytometry revealed increased expression of nrdJ and nrdD and decreased nrdA expression during the infection process. Expression of each RNR class fits with the pathogenicities of the chromosomal deletion mutants. An extended understanding of the pathogenicity and physiology of P. aeruginosa will be important for the development of novel drugs against infections in cystic fibrosis patients.

Identification of putative innate immune related genes from a cell line of the mosquito Aedes albopictus following bacterial challenge

We report identification of putative innate immune related genes from a cell line of the mosquito Aedes albopictus challenged with heat-killed bacteria. Using a subtractive hybridization and sequencing approach, we analyzed a total 309 expressed sequence tags (ESTs) which clustered in 40 contigs. Thirty-five percent of genes yielded homology to known immune genes corresponding to antimicrobial peptides (AMPs), pathogen-associated molecular patterns, protease and immune signaling cascades. Interestingly, most of the genes have not been previously described from this mosquito and thus represent a class of novel immune genes. Further, 25% sequences did not match to any known species in the non-redundant databases, appear to be specific to the mosquito A. albopictus and merit further study.

Phenotypic diversification in vivo: Pseudomonas aeruginosa gacS− strains generate small colony variants in vivo that are distinct from in vitro variants

Pseudomonas aeruginosa has long been known to produce phenotypic variants during chronic mucosal surface infections. These variants are thought to be generated to ensure bacterial survival against the diverse challenges in the mucosal environment. Studies have begun to elucidate the mechanisms by which these variants emerge in vitro; however, too little information exists on phenotypic variation in vivo to draw any links between variants generated in vitro and in vivo. Consequently, in this study, the P. aeruginosa gacS gene, which has previously been linked to the generation of small colony variants (SCVs) in vitro, was studied in an in vivo mucosal surface infection model. More specifically, the rat prostate served as a model mucosal surface to test for the appearance of SCVs in vivo following infections with P. aeruginosa gacS− strains. As in in vitro studies, deletion of the gacS gene led to SCV production in vivo. The appearance of these in vivo SCVs was important for the sustainability of a chronic infection. In the subset of rats in which P. aeruginosa gacS− did not convert to SCVs, clearance of the bacteria took place and healing of the tissue ensued. When comparing the SCVs that arose at the mucosal surface (MS-SCVs) with in vitro SCVs (IV-SCVs) from the same gacS− parent, some differences between the phenotypic variants were observed. Whereas both MS-SCVs and IV-SCVs formed dense biofilms, MS-SCVs exhibited a less diverse resistance profile to antimicrobial agents than IV-SCVs. Additionally, MS-SCVs were better suited to initiate an infection in the rat model than IV-SCVs. Together, these observations suggest that phenotypic variation in vivo can be important for maintenance of infection, and that in vivo variants may differ from in vitro variants generated from the same genetic parent.

The peptidoglycan-binding protein FimV promotes assembly of the Pseudomonas aeruginosa type IV pilus secretin

The Pseudomonas aeruginosa inner membrane protein FimV is among several proteins of unknown function required for type IV pilus-mediated twitching motility, arising from extension and retraction of pili from their site of assembly in the inner membrane. The pili transit the periplasm and peptidoglycan (PG) layer, ultimately exiting the cell through the PilQ secretin. Although fimV mutants are nonmotile, they are susceptible to killing by pilus-specific bacteriophage, a hallmark of retractable surface pili. Here we show that levels of recoverable surface pili were markedly decreased in fimV pilT retraction-deficient mutants compared with levels in the pilT control, demonstrating that FimV acts at the level of pilus assembly. Levels of inner membrane assembly subcomplex proteins PilM/N/O/P were decreased in fimV mutants, but supplementation of these components in trans did not restore pilus assembly or motility. Loss of FimV dramatically reduced the levels of the PilQ secretin multimer through which pili exit the cell, in part due to decreased levels of PilQ monomers, while PilF pilotin levels were unchanged. Expression of pilQ in trans in the wild type or fimV mutants increased total PilQ monomer levels but did not alter secretin multimer levels or motility. PG pulldown assays showed that the N terminus of FimV bound PG in a LysM motif-dependent manner, and a mutant with an in-frame chromosomal deletion of the LysM motif had reduced motility, secretin levels, and surface piliation. Together, our data show that FimV's role in pilus assembly is to promote secretin formation and that this function depends upon its PG-binding domain.

Porphyromonas gingivalis-host interactions in a Drosophila melanogaster model

Porphyromonas gingivalis is a Gram-negative obligate anaerobe that has been implicated in the etiology of adult periodontitis. We recently introduced a Drosophila melanogaster killing model for examination of P. gingivalis-host interactions. In the current study, the Drosophila killing model was used to characterize the host response to P. gingivalis infection by identifying host components that play a role during infection. Drosophila immune response gene mutants were screened for altered susceptibility to killing by P. gingivalis. The Imd signaling pathway was shown to be important for the survival of Drosophila infected by nonencapsulated P. gingivalis strains but was dispensable for the survival of Drosophila infected by encapsulated P. gingivalis strains. The P. gingivalis capsule was shown to mediate resistance to killing by Drosophila antimicrobial peptides (Imd pathway-regulated cecropinA and drosocin) and human beta-defensin 3. Drosophila thiol-ester protein II (Tep II) and Tep IV and the tumor necrosis factor (TNF) homolog Eiger were also involved in the immune response against P. gingivalis infection, while the scavenger receptors Eater and Croquemort played no roles in the response to P. gingivalis infection. This study demonstrates that the Drosophila killing model is a useful high-throughput model for characterizing the host response to P. gingivalis infection and uncovering novel interactions between the bacterium and the host.

Porphyromonas gingivalis virulence in a Drosophila melanogaster model

Porphyromonas gingivalis has been implicated in the etiology of adult periodontitis. In this study, we examined the viability of Drosophila melanogaster as a new model for examining P. gingivalis-host interactions. P. gingivalis (W83) infection of Drosophila resulted in a systemic infection that killed in a dose-dependent manner. Differences in the virulence of several clinically prevalent P. gingivalis strains were observed in the Drosophila killing model, and the results correlated well with studies in mammalian infection models and human epidemiologic studies. P. gingivalis pathobiology in Drosophila did not result from uncontrolled growth of the bacterium in the Drosophila hemolymph (blood) or overt damage to Drosophila tissues. P. gingivalis killing of Drosophila was multifactorial, involving several bacterial factors that are also involved in virulence in mammals. The results from this study suggest that many aspects of P. gingivalis pathogenesis in mammals are conserved in Drosophila, and thus the Drosophila killing model should be useful for characterizing P. gingivalis-host interactions and, potentially, polymicrobe-host interactions.

Drosophila melanogaster as a model host for the Burkholderia cepacia complex

Background

Colonization with bacterial species from the Burkholderia cepacia complex (Bcc) is associated with fast health decline among individuals with cystic fibrosis. In order to investigate the virulence of the Bcc, several alternative infection models have been developed. To this end, the fruit fly is increasingly used as surrogate host, and its validity to enhance our understanding of host-pathogen relationships has been demonstrated with a variety of microorganisms. Moreover, its relevance as a suitable alternative to mammalian hosts has been confirmed with vertebrate organisms.

Methodology/Principal Findings

The aim of this study was to establish Drosophila melanogaster as a surrogate host for species from the Bcc. While the feeding method proved unsuccessful at killing the flies, the pricking technique did generate mortality within the populations. Results obtained with the fruit fly model are comparable with results obtained using mammalian infection models. Furthermore, validity of the Drosophila infection model was confirmed with B. cenocepacia K56-2 mutants known to be less virulent in murine hosts or in other alternative models. Competitive index (CI) analyses were also performed using the fruit fly as host. Results of CI experiments agree with those obtained with mammalian models.

Conclusions/Significance

We conclude that Drosophila is a useful alternative infection model for Bcc and that fly pricking assays and competition indices are two complementary methods for virulence testing. Moreover, CI results indicate that this method is more sensitive than mortality tests.

Contribution of oxygen-limiting conditions to persistent infection of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a versatile opportunistic human pathogen that is able to colonize a broad spectrum of different aquatic and soil habitats. In the environment and during pathogenesis, P. aeruginosa encounters oxygen-limited and anaerobic environments. Particularly during chronic infection of the cystic fibrosis lung, oxygen-limiting conditions seem to contribute to persistent infection. Oxygen limitation increases antibiotic tolerance, robust biofilms and alginate biosynthesis, which contribute to the persistence of this opportunistic pathogen. Despite the importance of anaerobic metabolism during persistent infection of P. aeruginosa, we are just beginning to understand the underlying regulatory network and the molecular basis of how anaerobic metabolism contributes to a persistent infection. A deeper understanding of the anaerobic physiology of P. aeruginosa will allow the identification of new antibiotic targets and new therapeutic strategies.

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

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

Molecular bases of proliferation of Francisella tularensis in arthropod vectors

Arthropod vectors are important vehicles for transmission of Francisella tularensis between mammals, but very little is known about the F. tularensis-arthropod vector interaction. Drosophila melanogaster has been recently developed as an arthropod vector model for F. tularensis. We have shown that intracellular trafficking of F. tularensis within human monocytes-derived macrophages and D. melanogaster-derived S2 cells is very similar. Within both evolutionarily distant host cells, the Francisella-containing phagosome matures to a late endosome-like phagosome with limited fusion to lysosomes followed by rapid bacterial escape into the cytosol where the bacterial proliferate. To decipher the molecular bases of intracellular proliferation of F. tularensis within arthropod-derived cells, we screened a comprehensive library of mutants of F. tularensis ssp. novicida for their defect in intracellular proliferation within D. melanogaster-derived S2 cells. Our data show that 394 genes, representing 22% of the genome, are required for intracellular proliferation within D. melanogaster-derived S2 cells, including many of the Francisella Pathogenicity Island (FPI) genes that are also required for proliferation within mammalian macrophages. Functional gene classes that exhibit growth defect include metabolic (25%), FPI (2%), type IV pili (1%), transport (16%) and DNA modification (5%). Among 168 most defective mutants in intracellular proliferation in S2 cells, 80 are defective in lethality and proliferation within adult D. melanogaster. The observation that only 135 of the 394 mutants that are defective in S2 cells are also defective in human macrophages indicates that F. tularensis utilize common as well as distinct mechanisms to proliferate within mammalian and arthropod cells. Our studies will facilitate deciphering the molecular aspects of F. tularensis-arthropod vector interaction and its patho-adaptation to infect mammals.

The Pseudomonas aeruginosa Chp chemosensory system regulates intracellular cAMP levels by modulating adenylate cyclase activity

Multiple virulence systems in the opportunistic pathogen Pseudomonas aeruginosa are regulated by the second messenger signalling molecule adenosine 3', 5'-cyclic monophosphate (cAMP). Production of cAMP by the putative adenylate cyclase enzyme CyaB represents a critical control point for virulence gene regulation. To identify regulators of CyaB, we screened a transposon insertion library for mutants with reduced intracellular cAMP. The majority of insertions resulting in reduced cAMP mapped to the Chp gene cluster encoding a putative chemotaxis-like chemosensory system. Further genetic analysis of the Chp system revealed that it has both positive and negative effects on intracellular cAMP and that it regulates cAMP levels by modulating CyaB activity. The Chp system was previously implicated in the production and function of type IV pili (TFP). Given that cAMP and the cAMP-dependent transcriptional regulator Vfr control TFP biogenesis gene expression, we explored the relationship between cAMP, the Chp system and TFP regulation. We discovered that the Chp system controls TFP production through modulation of cAMP while control of TFP-dependent twitching motility is cAMP-independent. Overall, our data define a novel function for a chemotaxis-like system in controlling cAMP production and establish a regulatory link between the Chp system, TFP and other cAMP-dependent virulence systems.

Genetic analysis of the regulation of type IV pilus function by the Chp chemosensory system of Pseudomonas aeruginosa

The virulence of the opportunistic pathogen Pseudomonas aeruginosa involves the coordinate expression of many virulence factors, including type IV pili, which are required for colonization of host tissues and for twitching motility. Type IV pilus function is controlled in part by the Chp chemosensory system, which includes a histidine kinase, ChpA, and two CheY-like response regulators, PilG and PilH. How the Chp components interface with the type IV pilus motor proteins PilB, PilT, and PilU is unknown. We present genetic evidence confirming the role of ChpA, PilG, and PilB in the regulation of pilus extension and the role of PilH and PilT in regulating pilus retraction. Using informative double and triple mutants, we show that (i) ChpA, PilG, and PilB function upstream of PilH, PilT, and PilU; (ii) that PilH enhances PilT function; and (iii) that PilT and PilB retain some activity in the absence of signaling input from components of the Chp system. By site-directed mutagenesis, we demonstrate that the histidine kinase domain of ChpA and the phosphoacceptor sites of both PilG and PilH are required for type IV pilus function, suggesting that they form a phosphorelay system important in the regulation of pilus extension and retraction. Finally, we present evidence suggesting that pilA transcription is regulated by intracellular PilA levels. We show that PilA is a negative regulator of pilA transcription in P. aeruginosa and that the Chp system functionally regulates pilA transcription by controlling PilA import and export.

The pathogenic properties of a novel and conserved gene product, KerV, in proteobacteria

Identification of novel virulence factors is essential for understanding bacterial pathogenesis and designing antibacterial strategies. In this study, we uncover such a factor, termed KerV, in Proteobacteria. Experiments carried out in a variety of eukaryotic host infection models revealed that the virulence of a Pseudomonas aeruginosa kerV null mutant was compromised when it interacted with amoebae, plants, flies, and mice. Bioinformatics analyses indicated that KerV is a hypothetical methyltransferase and is well-conserved across numerous Proteobacteria, including both well-known and emerging pathogens (e.g., virulent Burkholderia, Escherichia, Shigella, Vibrio, Salmonella, Yersinia and Brucella species). Furthermore, among the 197 kerV orthologs analyzed in this study, about 89% reside in a defined genomic neighborhood, which also possesses essential DNA replication and repair genes and detoxification gene. Finally, infection of Drosophila melanogaster with null mutants demonstrated that KerV orthologs are also crucial in Vibrio cholerae and Yersinia pseudotuberculosis pathogenesis. Our findings suggested that KerV has a novel and broad significance as a virulence factor in pathogenic Proteobacteria and it might serve as a new target for antibiotic drug design.

Drosophila melanogaster as a model host for studying Pseudomonas aeruginosa infection

Conservation of host signaling pathways and tissue physiology between Drosophila melanogaster and mammals allows for the modeling of human host-pathogen interactions in Drosophila. Here we present the use of genetically tractable Drosophila models of bacterial pathogenesis to study infection with the human opportunistic pathogen Pseudomonas aeruginosa. We describe and compare two protocols commonly used to infect Drosophila with P. aeruginosa: needle-pricking and injector-pumping. Each model has relevance for examining host components and bacterial factors in host defense and virulence. Fly survival and bacterial proliferation within host flies can be assessed as a measure of host susceptibility and pathogen virulence potential. The profiles of host responses toward P. aeruginosa virulent and non-virulent strains can be determined, enabling the identification of interaction-specific genes that could potentially favor or limit the initiation and progression of infection. Both of the protocols presented herein may be adapted for the inoculation and study of other microbial pathogens. P. aeruginosa cell preparation requires 24 h, fly inoculation 1 h, and fly survival and bacterial proliferation 1-4 d.

Genetic diversity and multihost pathogenicity of clinical and environmental strains of Burkholderia cenocepacia

A collection of 54 clinical and agricultural isolates of Burkholderia cenocepacia was analyzed for genetic relatedness by using multilocus sequence typing (MLST), pathogenicity by using onion and nematode infection models, antifungal activity, and the distribution of three marker genes associated with virulence. The majority of clinical isolates were obtained from cystic fibrosis (CF) patients in Michigan, and the agricultural isolates were predominantly from Michigan onion fields. MLST analysis resolved 23 distinct sequence types (STs), 11 of which were novel. Twenty-six of 27 clinical isolates from Michigan were genotyped as ST-40, previously identified as the Midwest B. cenocepacia lineage. In contrast, the 12 agricultural isolates represented eight STs, including ST-122, that were identical to clinical isolates of the PHDC lineage. In general, pathogenicity to onions and the presence of the pehA endopolygalacturonase gene were detected only in one cluster of related strains consisting of agricultural isolates and the PHDC lineage. Surprisingly, these strains were highly pathogenic in the nematode Caenorhabditis elegans infection model, killing nematodes faster than the CF pathogen Pseudomonas aeruginosa PA14 on slow-kill medium. The other strains displayed a wide range of pathogenicity to C. elegans, notably the Midwest clonal lineage which displayed high, moderate, and low virulence. Most strains displayed moderate antifungal activity, although strains with high and low activities were also detected. We conclude that pathogenicity to multiple hosts may be a key factor contributing to the potential of B. cenocepacia to opportunistically infect humans both by increasing the prevalence of the organism in the environment, thereby increasing exposure to vulnerable hosts, and by the selection of virulence factors that function in multiple hosts.

Effective but costly, evolved mechanisms of defense against a virulent opportunistic pathogen in Drosophila melanogaster

Drosophila harbor substantial genetic variation for antibacterial defense, and investment in immunity is thought to involve a costly trade-off with life history traits, including development, life span, and reproduction. To understand the way in which insects invest in fighting bacterial infection, we selected for survival following systemic infection with the opportunistic pathogen Pseudomonas aeruginosa in wild-caught Drosophila melanogaster over 10 generations. We then examined genome-wide changes in expression in the selected flies relative to unselected controls, both of which had been infected with the pathogen. This powerful combination of techniques allowed us to specifically identify the genetic basis of the evolved immune response. In response to selection, population-level survivorship to infection increased from 15% to 70%. The evolved capacity for defense was costly, however, as evidenced by reduced longevity and larval viability and a rapid loss of the trait once selection pressure was removed. Counter to expectation, we observed more rapid developmental rates in the selected flies. Selection-associated changes in expression of genes with dual involvement in developmental and immune pathways suggest pleiotropy as a possible mechanism for the positive correlation. We also found that both the Toll and the Imd pathways work synergistically to limit infectivity and that cellular immunity plays a more critical role in overcoming P. aeruginosa infection than previously reported. This work reveals novel pathways by which Drosophila can survive infection with a virulent pathogen that may be rare in wild populations, however, due to their cost.

The fruit fly is commonly used as a model organism to understand the mechanistic nature of the immune response to bacterial pathogens. The fly is also commonly used to understand what immunity costs hosts in terms of other traits such as life span and reproductive success. Here, we examine these two questions together in flies selected for improved defense against the bacterium Pseudomonas aeruginosa. We show that selected flies develop from egg to adult more rapidly than unselected flies. It appears that the selected flies invest more heavily in a wing of the immune system that involves engulfment and walling off of invading bacteria. This investment can also explain the shift in developmental rate, as these two biological pathways are controlled by shared sets of genes. These latter two findings are counter to the conventional wisdom and reveal a costly, but effective, means for the fly to circumvent the virulence of Pseudomonas aeruginosa. This bacterium is normally deadly, as it has specific mechanisms to evade the host immune response. Our work is significant for demonstrating a pathway for flies to survive bacterial infection with Pseudomonas aeruginosa and for offering a reason why such a defense is not normally present in wild populations.

Pseudomonas aeruginosa Type III secretion system interacts with phagocytes to modulate systemic infection of zebrafish embryos

Pseudomonas aeruginosa is an opportunistic human pathogen that can cause serious infection in those with deficient or impaired phagocytes. We have developed the optically transparent and genetically tractable zebrafish embryo as a model for systemic P. aeruginosa infection. Despite lacking adaptive immunity at this developmental stage, zebrafish embryos were highly resistant to P. aeruginosa infection, but as in humans, phagocyte depletion dramatically increased their susceptibility. The virulence of an attenuated P. aeruginosa strain lacking a functional Type III secretion system was restored upon phagocyte depletion, suggesting that this system influences virulence through its effects on phagocytes. Intravital imaging revealed bacterial interactions with multiple blood cell types. Neutrophils and macrophages rapidly phagocytosed and killed P. aeruginosa, suggesting that both cell types play a role in protection against infection. Intravascular aggregation of erythrocytes and other blood cells with resultant circulatory blockage was observed immediately upon infection, which may be relevant to the pathogenesis of thrombotic complications of human P. aeruginosa infections. The real-time visualization capabilities and genetic tractability of the zebrafish infection model should enable elucidation of molecular and cellular details of P. aeruginosa pathogenesis in conditions associated with neutropenia or impaired phagocyte function.

Drosophila are protected from Pseudomonas aeruginosa lethality by transgenic expression of paraoxonase-1

Pseudomonas aeruginosa uses quorum sensing, an interbacterial communication system, to regulate gene expression. The signaling molecule N-3-oxododecanoyl homoserine lactone (3OC12-HSL) is thought to play a central role in quorum sensing. Since 3OC12-HSL can be degraded by paraoxonase (PON) family members, we hypothesized that PONs regulate P. aeruginosa virulence in vivo. We chose Drosophila melanogaster as our model organism because it has been shown to be a tractable model for investigating host-pathogen interactions and lacks PONs. By using quorum-sensing-deficient P. aeruginosa, synthetic acyl-HSLs, and transgenic expression of human PON1, we investigated the role of 3OC12-HSL and PON1 on P. aeruginosa virulence. We found that P. aeruginosa virulence in flies was dependent upon 3OC12-HSL. PON1 transgenic flies expressed enzymatically active PON1 and thereby exhibited arylesterase activity and resistance to organophosphate toxicity. Moreover, PON1 flies were protected from P. aeruginosa lethality, and protection was dependent on the lactonase activity of PON1. Our findings show that PON1 can interfere with quorum sensing in vivo and provide insight into what we believe is a novel role for PON1 in the innate immune response to quorum-sensing-dependent pathogens. These results raise intriguing possibilities about human-pathogen interactions, including potential roles for PON1 as a modifier gene and for PON1 protein as a regulator of normal bacterial florae, a link between infection/inflammation and cardiovascular disease, and a potential therapeutic modality.

Drosophila melanogaster-based screening for multihost virulence factors of Pseudomonas aeruginosa PA14 and identification of a virulence-attenuating factor, HudA

Pseudomonas aeruginosa is an important opportunistic human pathogen that interacts with phylogenetically diverse nonmammalian hosts, including plants, nematodes, and insects. Here, we exploited the P. aeruginosa-induced killing of the fruit fly Drosophila melanogaster as an assay system to screen for virulence-attenuated mutants of P. aeruginosa PA14. Fifteen nonredundant mutants were isolated from 4,018 random transposon (TnphoA) insertion clones, and 13 out of them (86.7%) displayed significantly reduced virulence in a murine peritonitis model as well. The TnphoA insertion sites of the 15 mutants were determined; already known virulence genes (dsbA, pvdI, fhlB, pilF, and wspF) and new virulence genes such as PA0253 (hudR), PA0369, PA2077, PA0272, PA2113, PA2965 (fabF1), and PA2002 were identified; one insertion was located at the intergenic region between PA1928 and PA1929; and the other two insertions were located in the genes (PA14_35740 and PA14_36000) within a putative genomic island, indicating a potential pathogenicity island of PA14. Further characterization of hudR, a virulence gene which encodes a MarR/SlyA family transcription factor, revealed that elevated expression of PA0254 (hudA [homologous to UbiD]) was necessary and sufficient for the virulence attenuation of the hudR mutant. The HudR protein repressed the hudAR operon by directly binding to its upstream promoter region. Collectively, these results validate the relevance of the D. melanogaster model for the high-throughput identification of new virulence factors involved in the multihost pathogenesis of P. aeruginosa.

Potassium and sodium transporters of Pseudomonas aeruginosa regulate virulence to barley

We investigated the role of three uncharacterized cation transporters of Pseudomonas aeruginosa PAO1 as virulence factors for barley: PA1207, PA5021, and PA2647. PAO1 displayed reduced barley virulence with inactivated PA1207, PA5021, and PA2647 as well as with one known Na(+)/H(+) antiporter, PA1820. Using the Escherichia coli LB2003 mutant lacking three K(+) uptake systems, the expression of the PA5021 gene repressed LB2003 growth with low K(+), but the strain acquired tolerance to high K(+). In contrast, the expression of the PA1207 gene enhanced growth of LB2003 with low K(+) but repressed its growth with high K(+); therefore, the PA5021 protein exports K(+), while the PA1207 protein imports K(+). The PA5021 mutant of P. aeruginosa also showed impaired growth at 400 mM KCl and at 400 mM NaCl; therefore, the PA5021 protein may also export Na(+). The loss of the PA5021 protein also decreased production of the virulence factor pyocyanin; corroborating this result, pyocyanin production decreased in wild-type PAO1 under high salinity. Whole-genome transcriptome analysis showed that PAO1 induced more genes in barley upon infection compared to the PA5021 mutant. Additionally, PAO1 infection induced water stress-related genes in barley, which suggests that barley may undergo water deficit upon infection by this pathogen.

Molecular analysis of a novel gene cluster encoding an insect toxin in plant-associated strains of Pseudomonas fluorescens

Pseudomonas fluorescens CHA0 and the related strain Pf-5 are well-characterized representatives of rhizosphere bacteria that have the capacity to protect crop plants from fungal root diseases, mainly by releasing a variety of exoproducts that are toxic to plant pathogenic fungi. Here, we report that the two plant-beneficial pseudomonads also exhibit potent insecticidal activity. Anti-insect activity is linked to a novel genomic locus encoding a large protein toxin termed Fit (for P. fluorescensinsecticidal toxin) that is related to the insect toxin Mcf (Makes caterpillars floppy) of the entomopathogen Photorhabdus luminescens, a mutualist of insect-invading nematodes. When injected into the haemocoel, even low doses of P. fluorescens CHA0 or Pf-5 killed larvae of the tobacco hornworm Manduca sexta and the greater wax moth Galleria mellonella. In contrast, mutants of CHA0 or Pf-5 with deletions in the Fit toxin gene were significantly less virulent to the larvae. When expressed from an inducible promoter in a non-toxic Escherichia coli host, the Fit toxin gene was sufficient to render the bacterium toxic to both insect hosts. Our findings establish the Fit gene products of P. fluorescens CHA0 and Pf-5 as potent insect toxins that define previously unappreciated anti-insect properties of these plant-colonizing bacteria.

Cyanide produced by human isolates of Pseudomonas aeruginosa contributes to lethality in Drosophila melanogaster

Some Pseudomonas aeruginosa strains are cyanogenic, and cyanide may contribute to the bacterium's virulence. Using human isolates of P. aeruginosa, we have shown that Drosophila melanogaster suspended above cyanogenic strains become motionless and develop bradycardia and that flies injected with cyanogenic bacterial strains die more rapidly than those injected with noncyanogenic strains. Flies exposed to cyanogenic strains had high cyanide and low adenosine triphosphate (ATP) concentrations in body extracts, and treatment with a cyanide antidote equalized survival of flies injected with cyanogenic and noncyanogenic strains. P. aeruginosa PAO1 strain with a mutation in the hydrogen cyanide synthase gene cluster was much less toxic to flies than the parental cyanogenic strain or 2 knock-in strains. Transgenic flies overexpressing rhodanese, which detoxifies cyanide by converting it to thiocyanate, were resistant to cyanide and the increased virulence of cyanogenic strains. We conclude that D. melanogaster is a good model for studying cyanide produced by P. aeruginosa.

Pseudomonas aeruginosa cystic fibrosis isolates from individual patients demonstrate a range of levels of lethality in two Drosophila melanogaster infection models

Recently, two Drosophila melanogaster models of infection, fly feeding and fly nicking, have been developed that allow a determination of pathogenic potential of Pseudomonas aeruginosa isolates. In this study, control strains, isolates from burn wounds, and isolates from the sputa of cystic fibrosis (CF) patients were used to compare the two infection models to determine whether any of the isolates might be better adapted to either of the models. In addition, our goal was to determine the variability of isolates from individual CF patients. Three of four control strains (PAO1, PAK, and PA14) caused significant mortality in the flies in both models of infection. The remaining control strain, PA103, was lethal to flies in the nicking model but lacked significant lethality in the feeding model. The burn wound isolates had a high level of lethality in both models. Interestingly, the CF isolates had the largest diversity of lethality in both models of infection. The range of pathogenic potentials of the CF isolates occurred across a cohort of patients, both at the patient level and down to the level of individual sputum samples. The majority of all isolates had similar levels of lethality in both fly infection models. However, two CF isolates were significantly more lethal in the nicking model, and three CF isolates were significantly more lethal in the feeding model. In conclusion, the two Drosophila infection models were useful for the analysis of the diversity of pathogenic potentials of P. aeruginosa isolates.

Drosophila melanogaster as a model for elucidating the pathogenicity of Francisella tularensis

Drosophila melanogaster is a widely used model organism for research on innate immunity and serves as an experimental model for infectious diseases. The aetiological agent of the zoonotic disease tularaemia, Francisella tularensis, can be transmitted by ticks and mosquitoes and Drosophila might be a useful, genetically amenable model host to elucidate the interactions between the bacterium and its arthropod vectors. We found that the live vaccine strain of F. tularensis was phagocytosed by Drosophila and multiplied in fly haemocytes in vitro and in vivo. Bacteria injected into flies resided both inside haemocytes and extracellularly in the open circulatory system. A continuous activation of the humoral immune response, i.e. production of antimicrobial peptides under control of the imd/Relish signalling pathway, was observed and it may have contributed to the relative resistance to F. tularensis as flies defective in the imd/Relish pathway died rapidly. Importantly, bacterial strains deficient for genes of the F. tularensis intracellular growth locus or the macrophage growth locus were attenuated in D. melanogaster. Our results demonstrate that D. melanogaster is a suitable model for the analysis of interactions between F. tularensis and its arthropod hosts and that it can also be used to identify F. tularensis virulence factors relevant for mammalian hosts.

RNAi screen in Drosophila cells reveals the involvement of the Tom complex in Chlamydia infection

Chlamydia spp. are intracellular obligate bacterial pathogens that infect a wide range of host cells. Here, we show that C. caviae enters, replicates, and performs a complete developmental cycle in Drosophila SL2 cells. Using this model system, we have performed a genome-wide RNA interference screen and identified 54 factors that, when depleted, inhibit C. caviae infection. By testing the effect of each candidate's knock down on L. monocytogenes infection, we have identified 31 candidates presumably specific of C. caviae infection. We found factors expected to have an effect on Chlamydia infection, such as heparansulfate glycosaminoglycans and actin and microtubule remodeling factors. We also identified factors that were not previously described as involved in Chlamydia infection. For instance, we identified members of the Tim-Tom complex, a multiprotein complex involved in the recognition and import of nuclear-encoded proteins to the mitochondria, as required for C. caviae infection of Drosophila cells. Finally, we confirmed that depletion of either Tom40 or Tom22 also reduced C. caviae infection in mammalian cells. However, C. trachomatis infection was not affected, suggesting that the mechanism involved is C. caviae specific.

Chlamydia spp. are intracellular bacterial pathogens that infect a wide range of hosts and cause various diseases, including preventable blindness in developing countries, sexually transmitted disease, and pneumonia. Chlamydia spp. are able to establish their replication niche inside the host cell, residing in a membrane-bound compartment that serves as a protector shield against immune surveillance and antimicrobial agents but also acts as a “filter” to exchange factors with the host cell. Despite the primary importance of Chlamydia for human health, little is known about the mechanisms underlying the infection process. The study of Chlamydia pathogenesis is challenging because Chlamydia spp. are not amenable to genetic manipulation and it is difficult to conduct extensive genetic approaches in the mammalian host. To circumvent these difficulties, we have used Drosophila cells to model Chlamydia infection. We conducted a genome-wide RNA interference screen and identified host factors that, when depleted, reduce Chlamydia infection. Validating our approach, we further showed that the identified factors were also required for infection in mammalian cells. This work will help us better understand the complex interaction between Chlamydia and its host and potentially identify novel targets for therapeutic treatment.

Involvement of skeletal muscle gene regulatory network in susceptibility to wound infection following trauma

Despite recent advances in our understanding the pathophysiology of trauma, the basis of the predisposition of trauma patients to infection remains unclear. A Drosophila melanogaster/Pseudomonas aeruginosa injury and infection model was used to identify host genetic components that contribute to the hyper-susceptibility to infection that follows severe trauma. We show that P. aeruginosa compromises skeletal muscle gene (SMG) expression at the injury site to promote infection. We demonstrate that activation of SMG structural components is under the control of cJun-N-terminal Kinase (JNK) Kinase, Hemipterous (Hep), and activation of this pathway promotes local resistance to P. aeruginosa in flies and mice. Our study links SMG expression and function to increased susceptibility to infection, and suggests that P. aeruginosa affects SMG homeostasis locally by restricting SMG expression in injured skeletal muscle tissue. Local potentiation of these host responses, and/or inhibition of their suppression by virulent P. aeruginosa cells, could lead to novel therapies that prevent or treat deleterious and potentially fatal infections in severely injured individuals.

Pseudomonas aeruginosa virulence genes identified in a Dictyostelium host model

The human pathogen Pseudomonas aeruginosa has been shown previously to use similar virulence factors when infecting mammalian hosts or Dictyostelium amoebae. Here we randomly mutagenized a clinical isolate of P. aeruginosa, and identified mutants with attenuated virulence towards Dictyostelium. These mutant strains also exhibited a strong decrease in virulence when infecting Drosophila and mice, confirming that P. aeruginosa makes use of similar virulence traits to confront these very different hosts. Further characterization of these bacterial mutants showed that TrpD is important for the induction of the quorum-sensing circuit, while PchH and PchI are involved in the induction of the type III secretion system. These results demonstrate the usefulness and the relevance of the Dictyostelium host model to identify and analyse new virulence genes in P. aeruginosa.

Rac2 is a major actor of Drosophila resistance to Pseudomonas aeruginosa acting in phagocytic cells

Pathogen recognition and engulfment by phagocytic cells of the blood cell lineage constitute the first line of defense against invading pathogens. This cellular immune response is conserved throughout evolution and depends strictly on cytoskeletal changes regulated by the RhoGTPases family. Many pathogens have developed toxins modifying RhoGTPases activity to their own benefit. In particular, the Exoenzyme S (ExoS) toxin of the Gram-negative bacteria Pseudomonas aeruginosa is directly injected into the host cell cytoplasm and contains a GAP domain (ExoSGAP) targeting RhoGTPases. Searching for the contribution of each RhoGTPases, Rho1, Rac1, Rac2, Mtl (Mig2-like) and Cdc42 to fly resistance to P. aeruginosa infections, we found that Rac2 is required to resist to P. aeruginosa and to other Gram-negative or Gram-positive bacteria. The Rac2 immune-deficient phenotype is attributable to defective engulfment of pathogens since Rac2-mutant macrophages exhibited strong reduction in the phagocytosis level of both Gram-negative and Gram-positive bacterial particles whereas systemic immune signaling pathways, including Toll, Immune deficiency and Jun kinases, were not affected. Co-expression of Rac2 and ExoSGAP rescued the increased sensitivity to P. aeruginosa observed in ExoSGAP-expressing flies suggesting that Rac2 is the main host factor whose function is inhibited by the GAP domain of the ExoS toxin.

The host defense of Drosophila melanogaster

To combat infection, the fruit fly Drosophila melanogaster relies on multiple innate defense reactions, many of which are shared with higher organisms. These reactions include the use of physical barriers together with local and systemic immune responses. First, epithelia, such as those beneath the cuticle, in the alimentary tract, and in tracheae, act both as a physical barrier and local defense against pathogens by producing antimicrobial peptides and reactive oxygen species. Second, specialized hemocytes participate in phagocytosis and encapsulation of foreign intruders in the hemolymph. Finally, the fat body, a functional equivalent of the mammalian liver, produces humoral response molecules including antimicrobial peptides. Here we review our current knowledge of the molecular mechanisms underlying Drosophila defense reactions together with strategies evolved by pathogens to evade them.

The Neff strain of Acanthamoeba castellanii, a tool for testing the virulence of Mycobacterium kansasii

Virulent Mycobacterium kansasii (mainly subtype 1) may cause lung infections, whereas certain other strains (essentially subtype 3) are commonly non-pathogenic mycobacteria colonizing the human lower respiratory tract of patients. Determining the clinical significance of a strain isolated from a respiratory sample represents a major challenge for clinicians. Since some mycobacteria may use free-living amoebae as a training ground to select virulence traits, we wondered whether the Acanthamoeba castellanii amoeba could be used to determine the virulence of these intracellular bacteria. We investigated whether the growth and cytopathic effect of M. kansasii in A. castellanii correlate with the virulence of M. kansasii determined clinically and by subtyping. Pathogenic subtype 1 M. kansasii strains grew better in A. castellanii than non-pathogenic subtype 3 strains when considering both the number of bacteria per amoeba and the percentage of infected amoebae. Moreover, a subtype 3 M. kansasii strain isolated from blood culture, and thus considered pathogenic, was revealed to grow in A. castellanii similarly to pathogenic subtype 1 strains. These results suggest that amoebae may represent useful tools for testing the virulence of intracellular mycobacteria and other amoeba-resisting bacteria. This is important, since identification of novel bacterial virulence factors relies largely on in vitro assessment of virulence.