Identification of putative effectors of the Type IV secretion system from the Wolbachia endosymbiont of Brugia malayi

Three feminizing Wolbachia strains in a single host species: comparative genomics paves the way for identifying sex reversal factors

Introduction

Endosymbiotic bacteria in the genus Wolbachia have evolved numerous strategies for manipulating host reproduction in order to promote their own transmission. This includes the feminization of males into functional females, a well-studied phenotype in the isopod Armadillidium vulgare. Despite an early description of this phenotype in isopods and the development of an evolutionary model of host sex determination in the presence of Wolbachia, the underlying genetic mechanisms remain elusive.

Methods

Here we present the first complete genomes of the three feminizing Wolbachia (wVulC, wVulP, and wVulM) known to date in A. vulgare. These genomes, belonging to Wolbachia B supergroup, contain a large number of mobile elements such as WO prophages with eukaryotic association modules. Taking advantage of these data and those of another Wolbachia-derived feminizing factor integrated into the host genome (f element), we used a comparative genomics approach to identify putative feminizing factors.

Results

This strategy has enabled us to identify three prophage-associated genes secreted by the Type IV Secretion System: one ankyrin repeat domain-containing protein, one helix-turn-helix transcriptional regulator and one hypothetical protein. In addition, a latrotoxin-related protein, associated with phage relic genes, was shared by all three genomes and the f element.

Conclusion

These putative feminization-inducing proteins shared canonical interaction features with eukaryotic proteins. These results pave the way for further research into the underlying functional interactions.

Bactericidal effectors of the Stenotrophomonas maltophilia type IV secretion system: functional definition of the nuclease TfdA and structural determination of TfcB

Stenotrophomonas maltophilia expresses a type IV protein secretion system (T4SS) that promotes contact-dependent killing of other bacteria and does so partly by secreting the effector TfcB. Here, we report the structure of TfcB, comprising an N-terminal domain similar to the catalytic domain of glycosyl hydrolase (GH-19) chitinases and a C-terminal domain for recognition and translocation by the T4SS. Utilizing a two-hybrid assay to measure effector interactions with the T4SS coupling protein VirD4, we documented the existence of five more T4SS substrates. One of these was protein 20845, an annotated nuclease. A S. maltophilia mutant lacking the gene for 20845 was impaired for killing Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. Moreover, the cloned 20845 gene conferred robust toxicity, with the recombinant E. coli being rescued when 20845 was co-expressed with its cognate immunity protein. The 20845 effector was an 899 amino-acid protein, comprised of a GHH-nuclease domain in its N-terminus, a large central region of indeterminant function, and a C-terminus for secretion. Engineered variants of the 20845 gene that had mutations in the predicted catalytic site did not impede E. coli, indicating that the antibacterial effect of 20845 involves its nuclease activity. Using flow cytometry with DNA staining, we determined that 20845, but not its mutant variants, confers a loss in DNA content of target bacteria. Database searches revealed that uncharacterized homologs of 20845 occur within a range of bacteria. These data indicate that the S. maltophilia T4SS promotes interbacterial competition through the action of multiple toxic effectors, including a potent, novel DNase.IMPORTANCEStenotrophomonas maltophilia is a multi-drug-resistant, Gram-negative bacterium that is an emerging pathogen of humans. Patients with cystic fibrosis are particularly susceptible to S. maltophilia infection. In hospital water systems and various types of infections, S. maltophilia co-exists with other bacteria, including other pathogens such as Pseudomonas aeruginosa. We previously demonstrated that S. maltophilia has a functional VirB/D4 type VI protein secretion system (T4SS) that promotes contact-dependent killing of other bacteria. Since most work on antibacterial systems involves the type VI secretion system, this observation remains noteworthy. Moreover, S. maltophilia currently stands alone as a model for a human pathogen expressing an antibacterial T4SS. Using biochemical, genetic, and cell biological approaches, we now report both the discovery of a novel antibacterial nuclease (TfdA) and the first structural determination of a bactericidal T4SS effector (TfcB).

Unravelling bacterial virulence factors in yeast: From identification to the elucidation of their mechanisms of action

Pathogenic bacteria employ virulence factors (VF) to establish infection and cause disease in their host. Yeasts, Saccharomyces cerevisiae and Saccharomyces pombe, are useful model organisms to study the functions of bacterial VFs and their interaction with targeted cellular processes because yeast processes and organelle structures are highly conserved and similar to higher eukaryotes. In this review, we describe the principles and applications of the yeast model for the identification and functional characterisation of bacterial VFs to investigate bacterial pathogenesis. The growth inhibition phenotype caused by the heterologous expression of bacterial VFs in yeast is commonly used to identify candidate VFs. Then, subcellular localisation patterns of bacterial VFs can provide further clues about their target molecules and functions during infection. Yeast knockout and overexpression libraries are also used to investigate VF interactions with conserved eukaryotic cell structures (e.g., cytoskeleton and plasma membrane), and cellular processes (e.g., vesicle trafficking, signalling pathways, and programmed cell death). In addition, the yeast growth inhibition phenotype is also useful for screening new drug leads that target and inhibit bacterial VFs. This review provides an updated overview of new tools, principles and applications to study bacterial VFs in yeast.

The cellular lives of Wolbachia

Wolbachia are successful Gram-negative bacterial endosymbionts, globally infecting a large fraction of arthropod species and filarial nematodes. Efficient vertical transmission, the capacity for horizontal transmission, manipulation of host reproduction and enhancement of host fitness can promote the spread both within and between species. Wolbachia are abundant and can occupy extraordinary diverse and evolutionary distant host species, suggesting that they have evolved to engage and manipulate highly conserved core cellular processes. Here, we review recent studies identifying Wolbachia-host interactions at the molecular and cellular levels. We explore how Wolbachia interact with a wide array of host cytoplasmic and nuclear components in order to thrive in a diversity of cell types and cellular environments. This endosymbiont has also evolved the ability to precisely target and manipulate specific phases of the host cell cycle. The remarkable diversity of cellular interactions distinguishes Wolbachia from other endosymbionts and is largely responsible for facilitating its global propagation through host populations. Finally, we describe how insights into Wolbachia-host cellular interactions have led to promising applications in controlling insect-borne and filarial nematode-based diseases.

Toward novel treatment against filariasis: Insight into genome-wide co-evolutionary analysis of filarial nematodes and Wolbachia

Infectious diseases caused by filarial nematodes are major health problems for humans and animals globally. Current treatment using anti-helminthic drugs requires a long treatment period and is only effective against the microfilarial stage. Most species of filarial nematodes harbor a specific strain of Wolbachia bacteria, which are essential for the survival, development, and reproduction of the nematodes. This parasite-bacteria obligate symbiosis offers a new angle for the cure of filariasis. In this study, we utilized publicly available genome data and putative protein sequences from seven filarial nematode species and their symbiotic Wolbachia to screen for protein-protein interactions that could be a novel target against multiple filarial nematode species. Genome-wide in silico screening was performed to predict molecular interactions based on co-evolutionary signals. We identified over 8,000 pairs of gene families that show evidence of co-evolution based on high correlation score and low false discovery rate (FDR) between gene families and obtained a candidate list that may be keys in filarial nematode-Wolbachia interactions. Functional analysis was conducted on these top-scoring pairs, revealing biological processes related to various signaling processes, adult lifespan, developmental control, lipid and nucleotide metabolism, and RNA modification. Furthermore, network analysis of the top-scoring genes with multiple co-evolving pairs suggests candidate genes in both Wolbachia and the nematode that may play crucial roles at the center of multi-gene networks. A number of the top-scoring genes matched well to known drug targets, suggesting a promising drug-repurposing strategy that could be applicable against multiple filarial nematode species.

Wbm0076, a candidate effector protein of the Wolbachia endosymbiont of Brugia malayi, disrupts eukaryotic actin dynamics

Brugia malayi, a parasitic roundworm of humans, is colonized by the obligate intracellular bacterium, Wolbachia pipientis. The symbiosis between this nematode and bacterium is essential for nematode reproduction and long-term survival in a human host. Therefore, identifying molecular mechanisms required by Wolbachia to persist in and colonize B. malayi tissues will provide new essential information regarding the basic biology of this endosymbiosis. Wolbachia utilize a Type IV secretion system to translocate so-called "effector" proteins into the cytosol of B. malayi cells to promote colonization of the eukaryotic host. However, the characterization of these Wolbachia secreted proteins has remained elusive due to the genetic intractability of both organisms. Strikingly, expression of the candidate Wolbachia Type IV-secreted effector protein, Wbm0076, in the surrogate eukaryotic cell model, Saccharomyces cerevisiae, resulted in the disruption of the yeast actin cytoskeleton and inhibition of endocytosis. Genetic analyses show that Wbm0076 is a member of the family of Wiskott-Aldrich syndrome proteins (WAS [p]), a well-conserved eukaryotic protein family required for the organization of actin skeletal structures. Thus, Wbm0076 likely plays a central role in the active cell-to-cell movement of Wolbachia throughout B. malayi tissues during nematode development. As most Wolbachia isolates sequenced to date encode at least partial orthologs of wBm0076, we find it likely that the ability of Wolbachia to directly manipulate host actin dynamics is an essential requirement of all Wolbachia endosymbioses, independent of host cell species.

Restriction of Wolbachia Bacteria in Early Embryogenesis of Neotropical Drosophila Species via Endoplasmic Reticulum-Mediated Autophagy

Wolbachia are maternally transmitted intracellular bacteria that are not only restricted to the reproductive organs but also found in various somatic tissues of their native hosts. The abundance of the endosymbiont in the soma, usually a dead end for vertically transmitted bacteria, causes a multitude of effects on life history traits of their hosts, which are still not well understood. Thus, deciphering the host-symbiont interactions on a cellular level throughout a host's life cycle is of great importance to understand their homeostatic nature, persistence, and spreading success. Using fluorescent and transmission electron microscopy, we conducted a comprehensive analysis of Wolbachia tropism in soma and germ line of six Drosophila species at the intracellular level during host development. Our data uncovered diagnostic patterns of infections to embryonic primordial germ cells and to particular cells of the soma in three different neotropical Drosophila species that have apparently evolved independently. We further found that restricted patterns of Wolbachia tropism are determined in early embryogenesis via selective autophagy, and their spatially restricted infection patterns are preserved in adult flies. We observed tight interactions of Wolbachia with membranes of the endoplasmic reticulum, which might play a scaffolding role for autophagosome formation and subsequent elimination of the endosymbiont. Finally, by analyzing D. simulans lines transinfected with nonnative Wolbachia, we uncovered that the host genetic background regulates tissue tropism of infection. Our data demonstrate a novel and peculiar mechanism to limit and spatially restrict bacterial infection in the soma during a very early stage of host development. IMPORTANCE All organisms are living in close and intimate interactions with microbes that cause conflicts but also cooperation between both unequal genetic partners due to their different innate interests of primarily enhancing their own fitness. However, stable symbioses often result in homeostatic interaction, named mutualism, by balancing costs and benefits, where both partners profit. Mechanisms that have evolved to balance and stably maintain homeostasis in mutualistic relationships are still quite understudied; one strategy is to "domesticate" potentially beneficial symbionts by actively controlling their replication rate below a critical and, hence, costly threshold, and/or to spatially and temporally restrict their localization in the host organism, which, in the latter case, in its most extreme form, is the formation of a specialized housing organ for the microbe (bacteriome). However, questions remain: how do these mutualistic associations become established in their first place, and what are the mechanisms for symbiont control and restriction in their early stages? Here, we have uncovered an unprecedented symbiont control mechanism in neotropical Drosophila species during early embryogenesis. The fruit fly evolved selective autophagy to restrict and control the proliferation of its intracellular endosymbiont Wolbachia in a defined subset of the stem cells as soon as the host's zygotic genome is activated.

Wolbachia endosymbionts in two Anopheles species indicates independent acquisitions and lack of prophage elements

Wolbachia is a genus of obligate bacterial endosymbionts that infect a diverse range of arthropod species as well as filarial nematodes, with its single described species, Wolbachia pipientis, divided into several ‘supergroups’ based on multilocus sequence typing. Wolbachia strains in mosquitoes have been shown to inhibit the transmission of human pathogens, including Plasmodium malaria parasites and arboviruses. Despite their large host range, Wolbachia strains within the major malaria vectors of the Anopheles gambiae and Anopheles funestus complexes appear at low density, established solely on PCR-based methods. Questions have been raised as to whether this represents a true endosymbiotic relationship. However, recent definitive evidence for two distinct, high-density strains of supergroup B Wolbachia within Anopheles demeilloni and Anopheles moucheti has opened exciting possibilities to explore naturally occurring Wolbachia endosymbionts in Anopheles for biocontrol strategies to block Plasmodium transmission. Here, we utilize genomic analyses to demonstrate that both Wolbachia strains have retained all key metabolic and transport pathways despite their smaller genome size, with this reduction potentially attributable to degenerated prophage regions. Even with this reduction, we confirmed the presence of cytoplasmic incompatibility (CI) factor genes within both strains, with wAnD maintaining intact copies of these genes while the cifB gene was interrupted in wAnM, so functional analysis is required to determine whether wAnM can induce CI. Additionally, phylogenetic analysis indicates that these Wolbachia strains may have been introduced into these two Anopheles species via horizontal transmission events, rather than by ancestral acquisition and subsequent loss events in the Anopheles gambiae species complex. These are the first Wolbachia genomes, to our knowledge, that enable us to study the relationship between natural strain Plasmodium malaria parasites and their anopheline hosts.

Endosymbionts Reduce Microbiome Diversity and Modify Host Metabolism and Fecundity in the Planthopper Sogatella furcifera

Endosymbionts can strongly affect bacterial microbiota in pests. The white-backed planthopper Sogatella furcifera, a notorious pest in rice, is usually co-infected with Cardinium and Wolbachia, but the effects of these endosymbionts together or individually on the host microbiome and fecundity are unclear. Here, we established three S. furcifera lines (Cardinium and Wolbachia double-infected, Cardinium single-infected, and both-uninfected lines) backcrossed to a common nuclear background and found that single and double infections reduced bacterial diversity and changed bacterial community structure across nymph and adult stages and across adult tissues. The endosymbionts differed in densities between adults and nymphs as well as across adult tissues, with the distribution of Cardinium affected by Wolbachia. Both the single infection and particularly the double infection reduced host fecundity. Lines also differed in levels of metabolites, some of which may influence fecundity (e.g., arginine biosynthesis and nicotinamide metabolism). Cardinium in the single-infected line upregulated metabolic levels, while Wolbachia in the double-infected line appeared to mainly downregulate them. Association analysis pointed to possible connections between various bacteria and differential metabolites. These results reveal that Cardinium by itself and in combination with Wolbachia affect bacterial microbiota and levels of metabolites, with likely effects on host fecundity. Many of the effects of these metabolically limited endosymbionts that are dependent on the hosts may be exerted through manipulation of the microbiome.

IMPORTANCE Endosymbionts can profoundly affect the nutrition, immunity, development, and reproduction of insect hosts, but the effects of multiple endosymbiont infections on microbiota and the interaction of these effects with insect host fitness are not well known. By establishing S. furcifera lines with different endosymbiont infection status, we found that Cardinium and the combined Cardinium + Wolbachia infections differentially reduced bacterial diversity as well as changing bacterial community structure and affecting metabolism, which may connect to negative fitness effects of the endosymbionts on their host. These results established the connections between reduced bacterial diversity, decreased fecundity and metabolic responses in S. furcifera.

Effectors of the Stenotrophomonas maltophilia Type IV Secretion System Mediate Killing of Clinical Isolates of Pseudomonas aeruginosa

Previously, we documented that Stenotrophomonas maltophilia encodes a type IV secretion system (T4SS) that allows the organism to kill, in contact-dependent fashion, heterologous bacteria, including wild-type Pseudomonas aeruginosa. Bioinformatic screens based largely on the presence of both a C-terminal consensus sequence and an adjacent gene encoding a cognate immunity protein identified 13 potential antibacterial effectors, most of which were highly conserved among sequenced strains of S. maltophilia. The immunity proteins of two of these proved especially capable of protecting P. aeruginosa and Escherichia coli against attack from the Stenotrophomonas T4SS. In turn, S. maltophilia mutants lacking the putative effectors RS14245 and RS14255 were impaired for killing not only laboratory E. coli but clinical isolates of P. aeruginosa, including ones isolated from the lungs of cystic fibrosis patients. That complemented mutants behaved as wild type did confirmed that RS14245 and RS14255 are required for the bactericidal activity of the S. maltophilia T4SS. Moreover, a mutant lacking both of these proteins was as impaired as a mutant lacking the T4SS apparatus, indicating that RS14245 and RS14255 account for (nearly) all of the bactericidal effects seen. Utilizing an interbacterial protein translocation assay, we determined that RS14245 and RS14255 are bona fide substrates of the T4SS, a result confirmed by examination of mutants lacking both the T4SS and the individual effectors. Delivery of the cloned 14245 protein (alone) into the periplasm resulted in the killing of target bacteria, indicating that this effector, a putative lipase, is both necessary and sufficient for bactericidal activity.

Cells within cells: Rickettsiales and the obligate intracellular bacterial lifestyle

The Rickettsiales are a group of obligate intracellular vector-borne Gram-negative bacteria that include many organisms of clinical and agricultural importance, including Anaplasma spp., Ehrlichia chaffeensis, Wolbachia, Rickettsia spp. and Orientia tsutsugamushi. This Review provides an overview of the current state of knowledge of the biology of these bacteria and their interactions with host cells, with a focus on pathogenic species or those that are otherwise important for human health. This includes a description of rickettsial genomics, bacterial cell biology, the intracellular lifestyles of Rickettsiales and the mechanisms by which they induce and evade the innate immune response.

The Wolbachia Symbiont: Here, There and Everywhere

Wolbachia symbionts, first observed in the 1920s, are now known to be present in about 30-70% of tested arthropod species, in about half of tested filarial nematodes (including the majority of human filarial nematodes), and some plant-parasitic nematodes. In arthropods, they are generally viewed as parasites while in nematodes they appear to be mutualists although this demarcation is not absolute. Their presence in arthropods generally leads to reproductive anomalies, while in nematodes, they are generally required for worm development and reproduction. In mosquitos, Wolbachia inhibit RNA viral infections, leading to populational reductions in human RNA virus pathogens, whereas in filarial nematodes, their requirement for worm fertility and survival has been channeled into their use as drug targets for filariasis control. While much more research on these ubiquitous symbionts is needed, they are viewed as playing significant roles in biological processes, ranging from arthropod speciation to human health.

Dual RNAseq analyses at soma and germline levels reveal evolutionary innovations in the elephantiasis-agent Brugia malayi, and adaptation of its Wolbachia endosymbionts

Brugia malayi is a human filarial nematode responsible for elephantiasis, a debilitating condition that is part of a broader spectrum of diseases called filariasis, including lymphatic filariasis and river blindness. Almost all filarial nematode species infecting humans live in mutualism with Wolbachia endosymbionts, present in somatic hypodermal tissues but also in the female germline which ensures their vertical transmission to the nematode progeny. These α-proteobacteria potentially provision their host with essential metabolites and protect the parasite against the vertebrate immune response. In the absence of Wolbachia wBm, B. malayi females become sterile, and the filarial nematode lifespan is greatly reduced. In order to better comprehend this symbiosis, we investigated the adaptation of wBm to the host nematode soma and germline, and we characterized these cellular environments to highlight their specificities. Dual RNAseq experiments were performed at the tissue-specific and ovarian developmental stage levels, reaching the resolution of the germline mitotic proliferation and meiotic differentiation stages. We found that most wBm genes, including putative effectors, are not differentially regulated between infected tissues. However, two wBm genes involved in stress responses are upregulated in the hypodermal chords compared to the germline, indicating that this somatic tissue represents a harsh environment to which wBm have adapted. A comparison of the B. malayi and C. elegans germline transcriptomes reveals a poor conservation of genes involved in the production of oocytes, with the filarial germline proliferative zone relying on a majority of genes absent from C. elegans. The first orthology map of the B. malayi genome presented here, together with tissue-specific expression enrichment analyses, indicate that the early steps of oogenesis are a developmental process involving genes specific to filarial nematodes, that likely result from evolutionary innovations supporting the filarial parasitic lifestyle.

Sensing, Signaling, and Secretion: A Review and Analysis of Systems for Regulating Host Interaction in Wolbachia

Wolbachia (Anaplasmataceae) is an endosymbiont of arthropods and nematodes that resides within host cells and is well known for manipulating host biology to facilitate transmission via the female germline. The effects Wolbachia has on host physiology, combined with reproductive manipulations, make this bacterium a promising candidate for use in biological- and vector-control. While it is becoming increasingly clear that Wolbachia's effects on host biology are numerous and vary according to the host and the environment, we know very little about the molecular mechanisms behind Wolbachia's interactions with its host. Here, I analyze 29 Wolbachia genomes for the presence of systems that are likely central to the ability of Wolbachia to respond to and interface with its host, including proteins for sensing, signaling, gene regulation, and secretion. Second, I review conditions under which Wolbachia alters gene expression in response to changes in its environment and discuss other instances where we might hypothesize Wolbachia to regulate gene expression. Findings will direct mechanistic investigations into gene regulation and host-interaction that will deepen our understanding of intracellular infections and enhance applied management efforts that leverage Wolbachia.

Quantitative methods for assessing local and bodywide contributions to Wolbachia titer in maternal germline cells of Drosophila

BACKGROUND

Little is known about how bacterial endosymbionts colonize host tissues. Because many insect endosymbionts are maternally transmitted, egg colonization is critical for endosymbiont success. Wolbachia bacteria, carried by approximately half of all insect species, provide an excellent model for characterizing endosymbiont infection dynamics. To date, technical limitations have precluded stepwise analysis of germline colonization by Wolbachia. It is not clear to what extent titer-altering effects are primarily mediated by growth rates of Wolbachia within cell lineages or migration of Wolbachia between cells.

RESULTS

The objective of this work is to inform mechanisms of germline colonization through use of optimized methodology. The approaches are framed in terms of nutritional impacts on Wolbachia. Yeast-rich diets in particular have been shown to suppress Wolbachia titer in the Drosophila melanogaster germline. To determine the extent of Wolbachia sensitivity to diet, we optimized 3-dimensional, multi-stage quantification of Wolbachia titer in maternal germline cells. Technical and statistical validation confirmed the identity of Wolbachia in vivo, the reproducibility of Wolbachia quantification and the statistical power to detect these effects. The data from adult feeding experiments demonstrated that germline Wolbachia titer is distinctly sensitive to yeast-rich host diets in late oogenesis. To investigate the physiological basis for these nutritional impacts, we optimized methodology for absolute Wolbachia quantification by real-time qPCR. We found that yeast-rich diets exerted no significant effect on bodywide Wolbachia titer, although ovarian titers were significantly reduced. This suggests that host diets affects Wolbachia distribution between the soma and late stage germline cells. Notably, relative qPCR methods distorted apparent wsp abundance, due to altered host DNA copy number in yeast-rich conditions. This highlights the importance of absolute quantification data for testing mechanistic hypotheses.

CONCLUSIONS

We demonstrate that absolute quantification of Wolbachia, using well-controlled cytological and qPCR-based methods, creates new opportunities to determine how bacterial abundance within the germline relates to bacterial distribution within the body. This methodology can be applied to further test germline infection dynamics in response to chemical treatments, genetic conditions, new host/endosymbiont combinations, or potentially adapted to analyze other cell and tissue types.

Wolbachia endosymbionts subvert the endoplasmic reticulum to acquire host membranes without triggering ER stress

The reproductive parasites Wolbachia are the most common endosymbionts on earth, present in a plethora of arthropod species. They have been introduced into mosquitos to successfully prevent the spread of vector-borne diseases, yet the strategies of host cell subversion underlying their obligate intracellular lifestyle remain to be explored in depth in order to gain insights into the mechanisms of pathogen-blocking. Like some other intracellular bacteria, Wolbachia reside in a host-derived vacuole in order to replicate and escape the immune surveillance. Using here the pathogen-blocking Wolbachia strain from Drosophila melanogaster, introduced into two different Drosophila cell lines, we show that Wolbachia subvert the endoplasmic reticulum to acquire their vacuolar membrane and colonize the host cell at high density. Wolbachia redistribute the endoplasmic reticulum, and time lapse experiments reveal tight coupled dynamics suggesting important signalling events or nutrient uptake. Wolbachia infection however does not affect the tubular or cisternal morphologies. A fraction of endoplasmic reticulum becomes clustered, allowing the endosymbionts to reside in between the endoplasmic reticulum and the Golgi apparatus, possibly modulating the traffic between these two organelles. Gene expression analyses and immunostaining studies suggest that Wolbachia achieve persistent infections at very high titers without triggering endoplasmic reticulum stress or enhanced ERAD-driven proteolysis, suggesting that amino acid salvage is achieved through modulation of other signalling pathways.

Wolbachia are a genus of intracellular bacteria living in symbiosis with millions of arthropod species. They have the ability to block the transmission of arboviruses when introduced into mosquito vectors, by interfering with the cellular resources exploited by these viruses. Despite the biomedical interest of this symbiosis, little is known about the mechanisms by which Wolbachia survive and replicate in the host cell. We show here that the membrane composing the Wolbachia vacuole is acquired from the endoplasmic reticulum, a central organelle required for protein and lipid synthesis, and from which originates a vesicular trafficking toward the Golgi apparatus and the secretory pathway. Wolbachia modify the distribution of this organelle which is a potential source of membrane and likely of nutrients as well. In contrast to some intracellular pathogenic bacteria, the effect of Wolbachia on the cell homeostasis does not induce a stress on the endoplasmic reticulum. One of the consequences of such a stress would be an increased proteolysis used to relieve the cell from an excess of misfolded proteins. Incidentally, this suggests that Wolbachia do not acquire amino acids from the host cell through this strategy.

The Wolbachia Endosymbionts

The Wolbachia endosymbionts encompass a large group of intracellular bacteria of biomedical and veterinary relevance, closely related to Anaplasma, Ehrlichia, and Rickettsia. This genus of Gram-negative members of the Alphaproteobacteria does not infect vertebrates but is instead restricted to ecdysozoan species, including terrestrial arthropods and a family of parasitic filarial nematodes, the Onchocercidae. The Wolbachia profoundly impact not only the ecology and evolution but also the reproductive biology of their hosts, through a wide range of symbiotic interactions. Because they are essential to the survival and reproduction of their filarial nematode hosts, they represent an attractive target to fight filariasis. Their abilities to spread through insect populations and to affect vector competence through pathogen protection have made Wolbachia a staple for controlling vector-borne diseases. Estimated to be present in up to 66% of insect species, the Wolbachia are probably the most abundant endosymbionts on earth. Their success resides in their unique capacity to infect and manipulate the host germ line to favor their vertical transmission through the maternal lineage. Because the Wolbachia resist genetic manipulation and growth in axenic culture, our understanding of their biology is still in its infancy. Despite these limitations, the "-omics" revolution combined with the use of well-established and emerging experimental host models is accelerating our comprehension of the host phenotypes caused by Wolbachia, and the identification of Wolbachia effectors is ongoing.