Waddlia chondrophila enters and multiplies within human macrophages

Host cell death during infection with Chlamydia: a double-edged sword

The phylum Chlamydiae constitutes a group of obligate intracellular bacteria that infect a remarkably diverse range of host species. Some representatives are significant pathogens of clinical or veterinary importance. For instance, Chlamydia trachomatis is the leading infectious cause of blindness and the most common bacterial agent of sexually transmitted diseases. Chlamydiae are exceptionally dependent on their eukaryotic host cells as a consequence of their developmental biology. At the same time, host cell death is an integral part of the chlamydial infection cycle. It is therefore not surprising that the bacteria have evolved exquisite and versatile strategies to modulate host cell survival and death programs to their advantage. The recent introduction of tools for genetic modification of Chlamydia spp., in combination with our increasing awareness of the complexity of regulated cell death in eukaryotic cells, and in particular of its connections to cell-intrinsic immunity, has revived the interest in this virulence trait. However, recent advances also challenged long-standing assumptions and highlighted major knowledge gaps. This review summarizes current knowledge in the field and discusses possible directions for future research, which could lead us to a deeper understanding of Chlamydia's virulence strategies and may even inspire novel therapeutic approaches.

Waddlia chondrophila, a Chlamydia-related bacterium, has a negative impact on human spermatozoa

STUDY QUESTION

What is the impact of Waddlia chondrophila, an emerging Chlamydia-related bacterium associated with miscarriage, on human spermatozoa?

SUMMARY ANSWER

W. chondrophila had a negative impact on human spermatozoa (decrease in viability and mitochondrial membrane potential) and was not entirely removed from infected samples by density gradient centrifugation.

WHAT IS KNOWN ALREADY

Bacterial infection or colonization might have a deleterious effect on male fertility. Waddlia chondrophila was previously associated with miscarriage, but its impact on male reproductive function has never been studied.

STUDY DESIGN SIZE, DURATION

An in vitro model of human spermatozoa infection was used to assess the effects of W. chondrophila infection. Controls included Chlamydia trachomatis serovar D and latex beads with similar size to bacteria.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Purified motile spermatozoa were infected with W. chondrophila (multiplicity of infection of 1). Immunohistochemistry combined with confocal microscopy was used to evaluate how bacteria interact with spermatozoa. The impact on physiology was assessed by monitoring cell viability, mitochondrial membrane potential and DNA fragmentation.

MAIN RESULTS AND THE ROLE OF CHANCE

Using super-resolution confocal microscopy, bacteria were localized on spermatozoa surface, as well as inside the cytoplasm. Compared to controls, W. chondrophila caused a 20% increase in mortality over 72 h of incubation (P < 0.05). Moreover, higher bacterial loads significantly reduced mitochondrial membrane potential. Bacteria present on spermatozoa surface were able to further infect a cell-monolayer, indicating that sperm might vector bacteria during sexual intercourse.

LIMITATIONS REASONS FOR CAUTION

The main limitation of the study is the use of an in vitro model of infection, which might be too simplistic compared to an actual infection. An animal model of infection should be developed to better evaluate the in vivo impact of W. chondrophila.

WIDER IMPLICATIONS OF THE FINDINGS

Intracellular bacteria, including C. trachomatis, Mycoplasma spp. and Ureaplasma spp., are associated with male infertility. Waddlia chondrophila might represent yet another member of this group, highlighting the need for more rigorous microbiological analysis during investigations for male infertility.

STUDY FUNDING/COMPETING INTEREST(S)

This work has been funded by the Department of Obstetrics and Gynecology, Lausanne University Hospital, Switzerland, and by the Swiss National Science Foundation (Grant nos. 310030-156169/1, 320030-169853/1 and 320030-169853/2 attributed to D.B.). D.B. is also supported by the 'Fondation Leenaards' through the 'Bourse pour la relève académique', by the 'Fondation Divesa' and by the 'Loterie Romande'. No conflicts of interest to declare.

A Zebrafish Model for Chlamydia Infection with the Obligate Intracellular Pathogen Waddlia chondrophila

Obligate intracellular chlamydial bacteria of the Planctomycetes-Verrucomicrobia-Chlamydiae (PVC) superphylum are important pathogens of terrestrial and marine vertebrates, yet many features of their pathogenesis and host specificity are still unknown. This is particularly true for families such as the Waddliacea which, in addition to epithelia, cellular targets for nearly all Chlamydia, can infect and replicate in macrophages, an important arm of the innate immune system or in their free-living amoebal counterparts. An ideal pathogen model system should include both host and pathogen, which led us to develop the first larval zebrafish model for chlamydial infections with Waddlia chondrophila. By varying the means and sites of application, epithelial cells of the swim bladder, endothelial cells of the vasculature and phagocytosing cells of the innate immune system became preferred targets for infection in zebrafish larvae. Through the use of transgenic zebrafish, we could observe recruitment of neutrophils to the infection site and demonstrate for the first time that W. chondrophila is taken up and replicates in these phagocytic cells and not only in macrophages. Furthermore, we present evidence that myeloid differentiation factor 88 (MyD88) mediated signaling plays a role in the innate immune reaction to W. chondrophila, eventually by Toll-like receptor (TLRs) recognition. Infected larvae with depleted levels of MyD88 showed a higher infection load and a lower survival rate compared to control fish. This work presents a new and potentially powerful non-mammalian experimental model to study the pathology of chlamydial virulence in vivo and opens up new possibilities for investigation of other members of the PVC superphylum.

Prevalence and diversity of Chlamydiales and other amoeba-resisting bacteria in domestic drinking water systems

A growing number of human infections incriminate environmental bacteria that have evolved virulent mechanisms to resist amoebae and use them as a replicative niche. These bacteria are designated amoeba-resisting bacteria (ARB). Despite the isolation of these ARB in various human clinical samples, the possible source of infection remains undetermined in most cases. However, it is known that the ARB Legionella pneumophila, for instance, causes a respiratory infection in susceptible hosts after inhalation of contaminated water aerosols from various sources. The Chlamydiales order contains many ARB, such as Parachlamydia acanthamoebae or Simkania negevensis, previously implicated in human respiratory infections with no identified contamination sources. We thus investigated whether domestic water systems are a potential source of transmission of these Chlamydiales to humans by using amoebal culture and molecular methods. Other important ARB such as mycobacteria and Legionella were also investigated, as were their possible amoebal hosts. This work reports for the first time a very high prevalence and diversity of Chlamydiales in drinking water, being detected in 35 (72.9%) of 48 investigated domestic water systems, with members of the Parachlamydiaceae family being dominantly detected. Furthermore, various Legionella and mycobacteria species were also recovered, some species of which are known to be causal agents of human infections.

Simkania negevensis, an insight into the biology and clinical importance of a novel member of the Chlamydiales order

Simkania negevensis is a Chlamydia-related bacterium discovered in 1993 and represents the founding member of the Simkaniaceae family within the Chlamydiales order. As other Chlamydiales, it is an obligate intracellular bacterium characterized by a biphasic developmental cycle. Its similarities with the pathogenic Chlamydia trachomatis and Chlamydia pneumoniae make it an interesting bacterium. So far, little is known about its biology, but S. negevensis harbors various microbiological characteristics of interest, including a strong association of the Simkania-containing vacuole with the ER and the presence of an intron in the 23S rRNA encoding gene. Evidence of human exposition has been reported worldwide. However, there is a lack of robust clinical studies evaluating its implication in human diseases; current data suggest an association with pneumonia and bronchiolitis making S. negevensis a potential emerging pathogen. Owing to its fastidious growth requirements, the clinical relevance of S. negevensis is probably underestimated. In this review, we summarize the current knowledge on S. negevensis and explore future research challenges.

Mouse Model of Respiratory Tract Infection Induced by Waddlia chondrophila

Waddlia chondrophila, an obligate intracellular bacterium belonging to the Chlamydiales order, is considered as an emerging pathogen. Some clinical studies highlighted a possible role of W. chondrophila in bronchiolitis, pneumonia and miscarriage. This pathogenic potential is further supported by the ability of W. chondrophila to infect and replicate within human pneumocytes, macrophages and endometrial cells. Considering that W. chondrophila might be a causative agent of respiratory tract infection, we developed a mouse model of respiratory tract infection to get insight into the pathogenesis of W. chondrophila. Following intranasal inoculation of 2 x 10⁸W. chondrophila, mice lost up to 40% of their body weight, and succumbed rapidly from infection with a death rate reaching 50% at day 4 post-inoculation. Bacterial loads, estimated by qPCR, increased from day 0 to day 3 post-infection and decreased thereafter in surviving mice. Bacterial growth was confirmed by detecting dividing bacteria using electron microscopy, and living bacteria were isolated from lungs 14 days post-infection. Immunohistochemistry and histopathology of infected lungs revealed the presence of bacteria associated with pneumonia characterized by an important multifocal inflammation. The high inflammatory score in the lungs was associated with the presence of pro-inflammatory cytokines in both serum and lungs at day 3 post-infection. This animal model supports the role of W. chondrophila as an agent of respiratory tract infection, and will help understanding the pathogenesis of this strict intracellular bacterium.

Waddlia chondrophila induces systemic infection, organ pathology, and elicits Th1-associated humoral immunity in a murine model of genital infection

Waddlia chondrophila is a known bovine abortigenic Chlamydia-related bacterium that has been associated with adverse pregnancy outcomes in human. However, there is a lack of knowledge regarding how W. chondrophila infection spreads, its ability to elicit an immune response and induce pathology. A murine model of genital infection was developed to investigate the pathogenicity and immune response associated with a W. chondrophila infection. Genital inoculation of the bacterial agent resulted in a dose-dependent infection that spread to lumbar lymph nodes and successively to spleen and liver. Bacterial-induced pathology peaked on day 14, characterized by leukocyte infiltration (uterine horn, liver, and spleen), necrosis (liver) and extramedullary hematopoiesis (spleen). Immunohistochemistry demonstrated the presence of a large number of W. chondrophila in the spleen on day 14. Robust IgG titers were detected by day 14 and remained high until day 52. IgG isotypes consisted of high IgG2a, moderate IgG3 and no detectable IgG1, indicating a Th1-associated immune response. This study provides the first evidence that W. chondrophila genital infection is capable of inducing a systemic infection that spreads to major organs, induces uterus, spleen, and liver pathology and elicits a Th1-skewed humoral response. This new animal model will help our understanding of the mechanisms related to intracellular bacteria-induced miscarriages, the most frequent complication of pregnancy that affects one in four women.

Waddlia: An emerging pathogen and a model organism to study the biology of chlamydiae

Waddlia chondrophila is an emerging pathogen associated with abortion in cattle. In humans, a growing body of evidence supports its pathogenic role in miscarriage and in respiratory tract infection. The human pathogenicity of W. chondrophila is further supported by the presence of several virulence factors including a catalase, a functional T3SS and several adhesins. Despite this medical importance, no commercial tests are available and diagnostic of this strict intracellular bacterium mainly relies on serology, PCR and immunohistochemistry. So far, the epidemiology of W. chondrophila remains largely unexplored and zoonotic, waterborne or interhuman transmission has been considered. Apart from its pathogenic role, chlamydiologists are also interested in W. chondrophila in order to better understand biological mechanisms conserved and shared with Chlamydia spp. Indeed, W. chondrophila proved to be a useful model organism to study the pathobiology of chlamydiae thanks to its rapid replication, its large size allowing precise subcellular protein localization, as well as its growth in Dictyostelium amoebae.

Current and past strategies for bacterial culture in clinical microbiology

A pure bacterial culture remains essential for the study of its virulence, its antibiotic susceptibility, and its genome sequence in order to facilitate the understanding and treatment of caused diseases. The first culture conditions empirically varied incubation time, nutrients, atmosphere, and temperature; culture was then gradually abandoned in favor of molecular methods. The rebirth of culture in clinical microbiology was prompted by microbiologists specializing in intracellular bacteria. The shell vial procedure allowed the culture of new species of Rickettsia. The design of axenic media for growing fastidious bacteria such as Tropheryma whipplei and Coxiella burnetii and the ability of amoebal coculture to discover new bacteria constituted major advances. Strong efforts associating optimized culture media, detection methods, and a microaerophilic atmosphere allowed a dramatic decrease of the time of Mycobacterium tuberculosis culture. The use of a new versatile medium allowed an extension of the repertoire of archaea. Finally, to optimize the culture of anaerobes in routine bacteriology laboratories, the addition of antioxidants in culture media under an aerobic atmosphere allowed the growth of strictly anaerobic species. Nevertheless, among usual bacterial pathogens, the development of axenic media for the culture of Treponema pallidum or Mycobacterium leprae remains an important challenge that the patience and innovations of cultivators will enable them to overcome.

In contrast to Chlamydia trachomatis, Waddlia chondrophila grows in human cells without inhibiting apoptosis, fragmenting the Golgi apparatus, or diverting post-Golgi sphingomyelin transport

The Chlamydiales are an order of obligate intracellular bacteria sharing a developmental cycle inside a cytosolic vacuole, with very diverse natural hosts, from amoebae to mammals. The clinically most important species is Chlamydia trachomatis. Many uncertainties remain as to how Chlamydia organizes its intracellular development and replication. The discovery of new Chlamydiales species from other families permits the comparative analysis of cell-biological events and may indicate events that are common to all or peculiar to some species and more or less tightly linked to "chlamydial" development. We used this approach in the infection of human cells with Waddlia chondrophila, a species from the family Waddliaceae whose natural host is uncertain. Compared to C. trachomatis, W. chondrophila had slightly different growth characteristics, including faster cytotoxicity. The embedding in cytoskeletal structures was not as pronounced as for the C. trachomatis inclusion. C. trachomatis infection generates proteolytic activity by the protease Chlamydia protease-like activity factor (CPAF), which degrades host substrates upon extraction; these substrates were not cleaved in the case of W. chondrophila. Unlike Chlamydia, W. chondrophila did not protect against staurosporine-induced apoptosis. C. trachomatis infection causes Golgi apparatus fragmentation and redirects post-Golgi sphingomyelin transport to the inclusion; both were absent from W. chondrophila-infected cells. When host cells were infected with both species, growth of both species was reduced. This study highlights differences between bacterial species that both depend on obligate intracellular replication inside an inclusion. Some features seem principally dispensable for intracellular development of Chlamydiales in vitro but may be linked to host adaptation of Chlamydia and the higher virulence of C. trachomatis.

Trafficking of Estrella lausannensis in human macrophages

Estrella lausannensis is a new member of the Chlamydiales order. Like other Chlamydia-related bacteria, it is able to replicate in amoebae and in fish cell lines. A preliminary study investigating the pathogenic potential of Chlamydia-related bacteria found a correlation between antibody response to E. lausannensis and pneumonia in children. To further investigate the pathogenic potential of E. lausannensis, we determined its ability to grow in human macrophages and its intracellular trafficking. The replication in macrophages resulted in viable E. lausannensis; however, it caused a significant cytopathic effect. The intracellular trafficking of E. lausannensis was analyzed by determining the interaction of the Estrella-containing inclusions with various endocytic markers as well as host organelles. The E. lausannensis inclusion escaped the endocytic pathway rapidly avoiding maturation into phagolysosomes by preventing both EEA-1 and LAMP-1 accumulation. Compared to Waddlia chondrophila, another Chlamydia-related bacteria, the recruitment of mitochondria and endoplasmic reticulum was minimal for E. lausannensis inclusions. Estrella lausannensis appears to use a distinct source of nutrients and energy compared to other members of the Chlamydiales order. In conclusion, we hypothesize that E. lausannensis has a restricted growth in human macrophages, due to its reduced capacity to control programmed cell death.

Babela massiliensis, a representative of a widespread bacterial phylum with unusual adaptations to parasitism in amoebae

Background

Only a small fraction of bacteria and archaea that are identifiable by metagenomics can be grown on standard media. Recent efforts on deep metagenomics sequencing, single-cell genomics and the use of specialized culture conditions (culturomics) increasingly yield novel microbes some of which represent previously uncharacterized phyla and possess unusual biological traits.

Results

We report isolation and genome analysis of Babela massiliensis, an obligate intracellular parasite of Acanthamoeba castellanii. B. massiliensis shows an unusual, fission mode of cell multiplication whereby large, polymorphic bodies accumulate in the cytoplasm of infected amoeba and then split into mature bacterial cells. This unique mechanism of cell division is associated with a deep degradation of the cell division machinery and delayed expression of the ftsZ gene. The genome of B. massiliensis consists of a circular chromosome approximately 1.12 megabase in size that encodes, 981 predicted proteins, 38 tRNAs and one typical rRNA operon. Phylogenetic analysis shows that B. massiliensis belongs to the putative bacterial phylum TM6 that so far was represented by the draft genome of the JCVI TM6SC1 bacterium obtained by single cell genomics and numerous environmental sequences.

Conclusions

Currently, B. massiliensis is the only cultivated member of the putative TM6 phylum. Phylogenomic analysis shows diverse taxonomic affinities for B. massiliensis genes, suggestive of multiple gene acquisitions via horizontal transfer from other bacteria and eukaryotes. Horizontal gene transfer is likely to be facilitated by the cohabitation of diverse parasites and symbionts inside amoeba. B. massiliensis encompasses many genes encoding proteins implicated in parasite-host interaction including the greatest number of ankyrin repeats among sequenced bacteria and diverse proteins related to the ubiquitin system. Characterization of B. massiliensis, a representative of a distinct bacterial phylum, thanks to its ability to grow in amoeba, reaffirms the critical role of diverse culture approaches in microbiology.

Reviewers

This article was reviewed by Dr. Igor Zhulin, Dr. Jeremy Selengut, and Pr Martijn Huynen.

Electronic supplementary material

The online version of this article (doi:10.1186/s13062-015-0043-z) contains supplementary material, which is available to authorized users.

Importance of amoebae as a tool to isolate amoeba-resisting microorganisms and for their ecology and evolution: the Chlamydia paradigm

Free-living amoebae are distributed worldwide and are frequently in contact with humans and animals. As cysts, they can survive in very harsh conditions and resist biocides and most disinfection procedures. Several microorganisms, called amoeba-resisting microorganisms (ARMs), have evolved to survive and multiply within these protozoa. Among them are many important pathogens, such as Legionella and Mycobacteria, and also several newly discovered Chlamydia-related bacteria, such as Parachlamydia acanthamoebae, Estrella lausannensis, Simkania negevensis or Waddlia chondrophila whose pathogenic role towards human or animal is strongly suspected. Amoebae represent an evolutionary crib for their resistant microorganisms since they can exchange genetic material with other ARMs and develop virulence traits that will be further used to infect other professional phagocytes. Moreover, amoebae constitute an ideal tool to isolate strict intracellular microorganisms from complex microbiota, since they will feed on other fast-growing bacteria, such as coliforms potentially present in the investigated samples. The paradigm that ARMs are likely resistant to macrophages, another phagocytic cell, and that they are likely virulent towards humans and animals is only partially true. Indeed, we provide examples of the Chlamydiales order that challenge this assumption and suggest that the ability to multiply in protozoa does not strictly correlate with pathogenicity and that we should rather use the ability to replicate in multiple and diverse eukaryotic cells as an indirect marker of virulence towards mammals. Thus, cell-culture-based microbial culturomics should be used in the future to try to discover new pathogenic bacterial species.

Amoebae as a tool to isolate new bacterial species, to discover new virulence factors and to study the host-pathogen interactions

Amoebae are unicellular protozoan present worldwide in several environments mainly feeding on bacteria. Some of them, the amoebae-resistant bacteria (ARBs), have evolved mechanisms to survive and replicate inside amoebal species. These mainly include legionella, mycobacteria and Chlamydia-related bacteria. Amoebae can provide a replicative niche, can act as reservoir for bacteria whereas the cystic form can protect the internalized bacteria. Moreover, the amoebae represent a Trojan horse for ARBs to infect animals. The long interaction between amoebae and bacteria has likely selected for bacterial virulence traits leading to the adaptation towards an intracellular lifestyle, and some ARBs have acquired the ability to infect mammals. This review intends to highlight the important uses of amoebae in several fields in microbiology by describing the main tools developed using amoebal cells. First, amoebae such as Acanthamoeba are used to isolate and discover new intracellular bacterial species by two main techniques: the amoebal co-culture and the amoebal enrichment. In the second part, taking Waddlia chondrophila as example, we summarize some important recent applications of amoebae to discover new bacterial virulence factors, in particular thanks to the amoebal plaque assay. Finally, the genetically tractable Dictyostelium discoideum is used as a model organism to study host-pathogen interactions, in particular with the development of several approaches to manipulate its genome that allowed the creation of a wide range of mutated strains largely shared within the Dictyostelium community.

Waddlia chondrophila infects and multiplies in ovine trophoblast cells stimulating an inflammatory immune response

Background

Waddlia chondrophila (W. chondrophila) is an emerging abortifacient organism which has been identified in the placentae of humans and cattle. The organism is a member of the order Chlamydiales, and shares many similarities at the genome level and in growth studies with other well-characterised zoonotic chlamydial abortifacients, such as Chlamydia abortus (C. abortus). This study investigates the growth of the organism and its effects upon pro-inflammatory cytokine expression in a ruminant placental cell line which we have previously utilised in a model of C. abortus pathogenicity.

Methodology/Principal Findings

Using qPCR, fluorescent immunocytochemistry and electron microscopy, we characterised the infection and growth of W. chondrophila within the ovine trophoblast AH-1 cell line. Inclusions were visible from 6 h post-infection (p.i.) and exponential growth of the organism could be observed over a 60 h time-course, with significant levels of host cell lysis being observed only after 36 h p.i. Expression of CXCL8, TNF-α, IL-1α and IL-1β were determined 24 h p.i. A statistically significant response in the expression of CXCL8, TNF-α and IL-1β could be observed following active infection with W. chondrophila. However a significant increase in IL-1β expression was also observed following the exposure of cells to UV-killed organisms, indicating the stimulation of multiple innate recognition pathways.

Conclusions/Significance

W. chondrophila infects and grows in the ruminant trophoblast AH-1 cell line exhibiting a complete chlamydial replicative cycle. Infection of the trophoblasts resulted in the expression of pro-inflammatory cytokines in a dose-dependent manner similar to that observed with C. abortus in previous studies, suggesting similarities in the pathogenesis of infection between the two organisms.

Cell wall precursors are required to organize the chlamydial division septum

Members of the Chlamydiales order are major bacterial pathogens that divide at mid-cell, without a sequence homologue of the FtsZ cytokinetic tubulin and without a classical peptidoglycan cell wall. Moreover, the spatiotemporal mechanisms directing constriction in Chlamydia are not known. Here we show that the MreB actin homologue and its conserved regulator RodZ localize to the division furrow in Waddlia chondrophila, a member of the Chlamydiales order implicated in human miscarriage. RodZ is recruited to the septal site earlier than MreB and in a manner that depends on biosynthesis of the peptidoglycan precursor lipid II by the MurA enzyme. By contrast, crosslinking of lipid II peptides by the Pbp3 transpeptidase disperses RodZ from the septum. Altogether, these findings provide a cytological framework for understanding chlamydial cytokinesis driven by septal cell wall synthesis.

Chlamydial metabolism revisited: interspecies metabolic variability and developmental stage-specific physiologic activities

Chlamydiae are a group of obligate intracellular bacteria comprising important human and animal pathogens as well as symbionts of ubiquitous protists. They are characterized by a developmental cycle including two main morphologically and physiologically distinct stages, the replicating reticulate body and the infectious nondividing elementary body. In this review, we reconstruct the history of studies that have led to our current perception of chlamydial physiology, focusing on their energy and central carbon metabolism. We then compare the metabolic capabilities of pathogenic and environmental chlamydiae highlighting interspecies variability among the metabolically more flexible environmental strains. We discuss recent findings suggesting that chlamydiae may not live as energy parasites throughout the developmental cycle and that elementary bodies are not metabolically inert but exhibit metabolic activity under appropriate axenic conditions. The observed host-free metabolic activity of elementary bodies may reflect adequate recapitulation of the intracellular environment, but there is evidence that this activity is biologically relevant and required for extracellular survival and maintenance of infectivity. The recent discoveries call for a reconsideration of chlamydial metabolism and future in-depth analyses to better understand how species- and stage-specific differences in chlamydial physiology may affect virulence, tissue tropism, and host adaptation.

Discovery of new intracellular pathogens by amoebal coculture and amoebal enrichment approaches

Intracellular pathogens such as legionella, mycobacteria and Chlamydia-like organisms are difficult to isolate because they often grow poorly or not at all on selective media that are usually used to cultivate bacteria. For this reason, many of these pathogens were discovered only recently or following important outbreaks. These pathogens are often associated with amoebae, which serve as host-cell and allow the survival and growth of the bacteria. We intend here to provide a demonstration of two techniques that allow isolation and characterization of intracellular pathogens present in clinical or environmental samples: the amoebal coculture and the amoebal enrichment. Amoebal coculture allows recovery of intracellular bacteria by inoculating the investigated sample onto an amoebal lawn that can be infected and lysed by the intracellular bacteria present in the sample. Amoebal enrichment allows recovery of amoebae present in a clinical or environmental sample. This can lead to discovery of new amoebal species but also of new intracellular bacteria growing specifically in these amoebae. Together, these two techniques help to discover new intracellular bacteria able to grow in amoebae. Because of their ability to infect amoebae and resist phagocytosis, these intracellular bacteria might also escape phagocytosis by macrophages and thus, be pathogenic for higher eukaryotes.

Developmental cycle and host interaction of Rhabdochlamydia porcellionis, an intracellular parasite of terrestrial isopods

Environmental chlamydiae are a diverse group of obligate intracellular bacteria related to well-known pathogens of humans. To date, only very little is known about chlamydial species infecting arthropods. In this study, we used cocultivation with insect cells for recovery and maintenance of Rhabdochlamydia porcellionis, a parasite of the crustacean host Porcellio scaber. In vitro, the infection cycle of R. porcellionis was completed within 7 days, resulting in the release of infectious particles by host cell lysis. Lack of apoptosis induction during the entire course of infection, combined with a reduced sensitivity of infected cultures to experimentally induced programmed cell death, indicates that R. porcellionis like its human pathogenic relatives counteracts this host defence mechanism. Interestingly, the rod-shaped variant of R. porcellionis, proposed to represent their mature infective stage, was not detected in cell culture, suggesting that its development may require prolonged maturation or may be triggered by specific conditions encountered only in the animal host. This first cell culture-based system for the cultivation and investigation of an arthropod-associated chlamydial species will help to better understand the biology of a so far neglected group of chlamydiae and its recently suggested potential to cause disease in humans.

Early expression of the type III secretion system of Parachlamydia acanthamoebae during a replicative cycle within its natural host cell Acanthamoeba castellanii

The type three secretion system (T3SS) operons of Chlamydiales bacteria are distributed in different clusters along their chromosomes and are conserved at both the level of sequence and genetic organization. A complete characterization of the temporal expression of multiple T3SS components at the transcriptional and protein levels has been performed in Parachlamydia acanthamoebae, replicating in its natural host cell Acanthamoeba castellanii. The T3SS components were classified in four different temporal clusters depending on their pattern of expression during the early, mid- and late phases of the infectious cycle. The putative T3SS transcription units predicted in Parachlamydia are similar to those described in Chlamydia trachomatis, suggesting that T3SS units of transcriptional expression are highly conserved among Chlamydiales bacteria. The maximal expression and activation of the T3SS of Parachlamydia occurred during the early to mid-phase of the infectious cycle corresponding to a critical phase during which the intracellular bacterium has (1) to evade and/or block the lytic pathway of the amoeba, (2) to differentiate from elementary bodies (EBs) to reticulate bodies (RBs), and (3) to modulate the maturation of its vacuole to create a replicative niche able to sustain efficient bacterial growth.

Discovery of catalases in members of the Chlamydiales order

Catalase is an important virulence factor for survival in macrophages and other phagocytic cells. In Chlamydiaceae, no catalase had been described so far. With the sequencing and annotation of the full genomes of Chlamydia-related bacteria, the presence of different catalase-encoding genes has been documented. However, their distribution in the Chlamydiales order and the functionality of these catalases remain unknown. Phylogeny of chlamydial catalases was inferred using MrBayes, maximum likelihood, and maximum parsimony algorithms, allowing the description of three clade 3 and two clade 2 catalases. Only monofunctional catalases were found (no catalase-peroxidase or Mn-catalase). All presented a conserved catalytic domain and tertiary structure. Enzymatic activity of cloned chlamydial catalases was assessed by measuring hydrogen peroxide degradation. The catalases are enzymatically active with different efficiencies. The catalase of Parachlamydia acanthamoebae is the least efficient of all (its catalytic activity was 2 logs lower than that of Pseudomonas aeruginosa). Based on the phylogenetic analysis, we hypothesize that an ancestral class 2 catalase probably was present in the common ancestor of all current Chlamydiales but was retained only in Criblamydia sequanensis and Neochlamydia hartmannellae. The catalases of class 3, present in Estrella lausannensis and Parachlamydia acanthamoebae, probably were acquired by lateral gene transfer from Rhizobiales, whereas for Waddlia chondrophila they likely originated from Legionellales or Actinomycetales. The acquisition of catalases on several occasions in the Chlamydiales suggests the importance of this enzyme for the bacteria in their host environment.

Well water as a possible source of Waddlia chondrophila infections

Waddlia chondrophila is an emerging pathogen considered as a potential agent of abortion in humans and bovines, and is related with human respiratory disease. Despite these findings, the infection source and transmission pathways have not been identified. The evidence of growth into amoeba suggests water as a possible environmental source. The presence of Waddlia chondrophila was determined in drinking and well water samples (n=70) by quantitative PCR (Q-PCR). Positive results were observed in 10 (25%) of the 40 well samples analyzed; therefore, well water could be a potential reservoir and possible infection source of Waddlia chondrophila in animals and humans.

Intracellular bacteria and adverse pregnancy outcomes

This review considers the role of intracellular bacteria in adverse pregnancy outcomes, such as miscarriage, stillbirths, and preterm labour. The cause of miscarriage, stillbirth and preterm labour often remains unexplained. Intracellular bacteria that grow either poorly or not at all on media used routinely to detect human pathogens could be the aetiological agents of these obstetric conditions. For example, Listeria monocytogenes and Coxiella burnetti are intracellular bacteria that have a predilection for the fetomaternal unit and may induce fatal disease in the mother and/or fetus. Both are important foodborne or zoonotic pathogens in pregnancy. Preventive measures, diagnostic tools and treatment will be reviewed. Moreover, we will also address the importance in adverse pregnancy outcomes of other intracellular bacteria, including Brucella abortus and various members of the order Chlamydiales. Indeed, there is growing evidence that Chlamydia trachomatis, Chlamydia abortus and Chlamydia pneumoniae infections may also result in adverse pregnancy outcomes in humans and/or animals. Moreover, newly discovered Chlamydia-like organisms have recently emerged as new pathogens of both animals and humans. For example, Waddlia chondrophila, a Chlamydia-related bacterium isolated from aborted bovine fetuses, has also been implicated in human miscarriages. Future research should help us to better understand the pathophysiology of adverse pregnancy outcomes caused by intracellular bacteria and to determine the precise mode of transmission of newly identified bacteria, such as Waddlia and Parachlamydia. These emerging pathogens may represent the tip of the iceberg of a large number of as yet unknown intracellular pathogenic agents.

Identification of immunogenic proteins of Waddlia chondrophila

Evidence is growing for a role of Waddlia chondrophila as an agent of adverse pregnancy outcomes in both humans and ruminants. This emerging pathogen, member of the order Chlamydiales, is also implicated in bronchiolitis and lower respiratory tract infections. Until now, the serological diagnosis of W. chondrophila infection has mainly relied on manually intensive tests including micro-immunofluorescence and Western blotting. Thus, there is an urgent need to establish reliable high throughput serological assays. Using a combined genomic and proteomic approach, we detected 57 immunogenic proteins of W. chondrophila, of which 17 were analysed by mass spectrometry. Two novel hypothetical proteins, Wim3 and Wim4, were expressed as recombinant proteins in Escherichia coli, purified and used as antigens in an ELISA test. Both proteins were recognized by sera of rabbits immunized with W. chondrophila as well as by human W. chondrophila positive sera but not by rabbit pre-immune sera nor human W. chondrophila negative sera. These results demonstrated that the approach chosen is suitable to identify immunogenic proteins that can be used to develop a serological test. This latter will be a valuable tool to further clarify the pathogenic potential of W. chondrophila.

Endemic and emerging chlamydial infections of animals and their zoonotic implications

The Chlamydiae are a diverse group of obligate intracellular Gram-negative bacteria that are known to infect a wide variety of host species and are responsible for a wide range of diseases in animals and man. Many of these organisms have been extensively characterized and their zoonotic implications recognized. Studies of human disease first provided evidence for the disease-causing potential of Chlamydia-related bacteria; however, there is now increasing evidence that a number of these organisms may also be the causative agents for a number of pathogenic conditions of livestock that had previously remained undiagnosed. The aim of this review is to draw together the evidence for the role of the newly emerging chlamydial infections in livestock disease, the current understanding of their roles in human disease and highlight the potential for zoonotic transmission.

Permissivity of fish cell lines to three Chlamydia-related bacteria: Waddlia chondrophila, Estrella lausannensis and Parachlamydia acanthamoebae

Epitheliocystis is an infectious disease affecting gills and skin of various freshwater and marine fishes, associated with high mortality and reduced growth of survivors. Candidatus Piscichlamydia salmonis and Clavochlamydia salmonicola have recently been identified as aetiological agents of epitheliocystis in Atlantic Salmon. In addition, several other members of the Chlamydiales order have been identified in other fish species. To clarify the pathogenicity of Chlamydia-like organisms towards fishes, we investigated the permissivity of two fish cell lines, EPC-175 (Fathead Minnow) and RTG-2 (rainbow trout) to three Chlamydia-related bacteria: Waddlia chondrophila, Parachlamydia acanthamoebae and Estrella lausannensis. Quantitative PCR and immunofluorescence demonstrated that W. chondrophila and, to a lesser extent, E. lausannensis were able to replicate in the two cell lines tested. Waddlia chondrophila multiplied rapidly in its host cell and a strong cytopathic effect was observed. During E. lausannensis infection, we observed a limited replication of the bacteria not followed by host cell lysis. Very limited replication of P. acanthamoebae was observed in both cell lines tested. Given its high infectivity and cytopathic effect towards fish cell lines, W. chondrophila represents the most interesting Chlamydia-related bacteria to be used to develop an in vivo model of epitheliocystis disease in fishes.

Permissivity of Vero cells, human pneumocytes and human endometrial cells to Waddlia chondrophila

Growing evidence suggests that the bacterium Waddlia chondrophila, a novel member of the Chlamydiales order, is an agent of miscarriage in humans and abortion in ruminants. We thus investigated the permissivity of three epithelial cell lines, primate Vero kidney cells, human A549 pneumocytes and human Ishikawa endometrial cells to this strict intracellular bacteria. Bacterial growth kinetics in these cell lines was assessed by quantitative PCR and immunofluorescence and our results demonstrated that W. chondrophila enters and efficiently multiplies in these epithelial cell lines. Additionally, confocal and electron microscopy indicated that the bacteria co-localize with host cell mitochondria. Within Vero and A549 cells, intracellular growth of W. chondrophila was associated with a significant decrease in host cell viability while no such cytophatic effect was detected in Ishikawa cells. Bacterial cell growth in this endometrial cell line stopped 48 h after infection. This stop in the replication of W. chondrophila coincided with the appearance of large aberrant bodies, a form of the bacteria also observed in Chlamydiaceae and associated with persistence. This persistent state of W. chondrophila may explain recurrent episodes of miscarriage in vivo, since the bacteria might reactivate within endometrial cells following hormonal changes that occur during pregnancy.

Chlamydiales and the innate immune response: friend or foe?

Pathogenicity of Chlamydia and Chlamydia-related bacteria could be partially mediated by an enhanced activation of the innate immune response. The study of this host pathogen interaction has proved challenging due to the restricted in vitro growth of these strict intracellular bacteria and the lack of genetic tools to manipulate their genomes. Despite these difficulties, the interactions of Chlamydiales with the innate immune cells and their effectors have been studied thoroughly. This review aims to point out the role of pattern recognition receptors and signal molecules (cytokines, reactive oxygen species) of the innate immune response in the pathogenesis of chlamydial infection. Besides inducing clearance of the bacteria, some of these effectors may be used by the Chlamydia to establish chronic infections or to spread. Thus, the induced innate immune response seems to be variable depending on the species and/or the serovar, making the pattern more complex. It remains crucial to determine the common players of the innate immune response in order to help define new treatment strategies and to develop effective vaccines. The excellent growth in phagocytic cells of some Chlamydia-related organisms such as Waddlia chondrophila supports their use as model organisms to study conserved features important for interactions between the innate immunity and Chlamydia.

Role of MyD88 and Toll-like receptors 2 and 4 in the sensing of Parachlamydia acanthamoebae

Parachlamydia acanthamoebae is a Chlamydia-related organism whose pathogenic role in pneumonia is supported by serological and molecular clinical studies and an experimental mouse model of lung infection. Toll-like receptors (TLRs) play a seminal role in sensing microbial products and initiating innate immune responses. The aim of this study was to investigate the roles of MyD88, TLR2, and TLR4 in the interaction of Parachlamydia with macrophages. Here, we showed that Parachlamydia entered bone-marrow derived macrophages (BMDMs) in a TLR-independent manner but did not multiply intracellularly. Interestingly, compared to live bacteria, heat-inactivated Parachlamydia induced the production of substantial amounts of tumor necrosis factor alpha (TNF), interleukin-6 (IL-6), and IL-12p40 by BMDMs and of TNF and IL-6 by peritoneal macrophages as well as RAW 264.7 and J774 macrophage cell lines. Cytokine production by BMDMs, which was partially inhibited upon trypsin treatment of Parachlamydia, was dependent on MyD88, TLR4, and, to a lesser extent, TLR2. Finally, MyD88(-/-), TLR4(-/-), and TLR2(-/-) mice were as resistant as wild-type mice to lung infection following the intratracheal instillation of Parachlamydia. Thus, in contrast to Chlamydia pneumoniae, Parachlamydia acanthamoebae weakly stimulates macrophages, potentially compensating for its low replication capacity in macrophages by escaping the innate immune surveillance.

Exploitation of Host Polyubiquitination Machinery through Molecular Mimicry by Eukaryotic-Like Bacterial F-Box Effectors

Microbial pathogens have evolved exquisite mechanisms to interfere and intercept host biological processes, often through molecular mimicry of specific host proteins. Ubiquitination is a highly conserved eukaryotic post-translational modification essential in determining protein fate, and is often hijacked by pathogenic bacteria. The conserved SKP1/CUL1/F-box (SCF) E3 ubiquitin ligase complex plays a key role in ubiquitination of proteins in eukaryotic cells. The F-box protein component of the SCF complex provides specificity to ubiquitination by binding to specific cellular proteins, targeting them to be ubiquitinated by the SCF complex. The bacterial pathogens. Legionella pneumophila, Agrobacterium tumefaciens, and Ralstonia solanacearum utilize type III or IV translocation systems to inject into the host cell eukaryotic-like F-box effectors that interact with the host SKP1 component of the SCF complex to trigger ubiquitination of specific host cells targets, which is essential to promote proliferation of these pathogens. Our bioinformatic analyses have identified at least 74 genes encoding putative F-box proteins belonging to 22 other bacterial species, including human pathogens, plant pathogens, and amebal endosymbionts. Therefore, subversion of the host ubiquitination machinery by bacterial F-box proteins may be a widespread strategy amongst pathogenic bacteria. The findings that bacterial F-box proteins harbor Ankyrin repeats as protein–protein interaction domains, which are present in F-box proteins of primitive but not higher eukaryotes, suggest acquisition of many bacterial F-box proteins from primitive eukaryotic hosts rather than the mammalian host.

The Waddlia genome: a window into chlamydial biology

Growing evidence suggests that a novel member of the Chlamydiales order, Waddlia chondrophila, is a potential agent of miscarriage in humans and abortion in ruminants. Due to the lack of genetic tools to manipulate chlamydia, genomic analysis is proving to be the most incisive tool in stimulating investigations into the biology of these obligate intracellular bacteria. 454/Roche and Solexa/Illumina technologies were thus used to sequence and assemble de novo the full genome of the first representative of the Waddliaceae family, W. chondrophila. The bacteria possesses a 2′116′312bp chromosome and a 15′593 bp low-copy number plasmid that might integrate into the bacterial chromosome. The Waddlia genome displays numerous repeated sequences indicating different genome dynamics from classical chlamydia which almost completely lack repetitive elements. Moreover, W. chondrophila exhibits many virulence factors also present in classical chlamydia, including a functional type III secretion system, but also a large complement of specific factors for resistance to host or environmental stresses. Large families of outer membrane proteins were identified indicating that these highly immunogenic proteins are not Chlamydiaceae specific and might have been present in their last common ancestor. Enhanced metabolic capability for the synthesis of nucleotides, amino acids, lipids and other co-factors suggests that the common ancestor of the modern Chlamydiales may have been less dependent on their eukaryotic host. The fine-detailed analysis of biosynthetic pathways brings us closer to possibly developing a synthetic medium to grow W. chondrophila, a critical step in the development of genetic tools. As a whole, the availability of the W. chondrophila genome opens new possibilities in Chlamydiales research, providing new insights into the evolution of members of the order Chlamydiales and the biology of the Waddliaceae.

Early intracellular trafficking of Waddlia chondrophila in human macrophages

Waddlia chondrophila is an obligate intracellular bacterium considered as a potential agent of abortion in both humans and bovines. This member of the order Chlamydiales multiplies rapidly within human macrophages and induces lysis of the infected cells. To understand how this Chlamydia-like micro-organism invades and proliferates within host cells, we investigated its trafficking within monocyte-derived human macrophages. Vacuoles containing W. chondrophila acquired the early endosomal marker EEA1 during the first 30 min following uptake. However, the live W. chondrophila-containing vacuoles never co-localized with late endosome and lysosome markers. Instead of interacting with the endosomal pathway, W. chondrophila immediately co-localized with mitochondria and, shortly after, with endoplasmic reticulum- (ER-) resident proteins such as calnexin and protein disulfide isomerase. The acquisition of mitochondria and ER markers corresponds to the beginning of bacterial replication. It is noteworthy that mitochondrion recruitment to W. chondrophila inclusions is prevented only by simultaneous treatment with the microtubule and actin cytoskeleton-disrupting agents nocodazole and cytochalasin D. In addition, brefeldin A inhibits the replication of W. chondrophila, supporting a role for COPI-dependent trafficking in the biogenesis of the bacterial replicating vacuole. W. chondrophila probably survives within human macrophages by evading the endocytic pathway and by associating with mitochondria and the ER. The intracellular trafficking of W. chondrophila in human macrophages represents a novel route that differs strongly from that used by other members of the order Chlamydiales.

Development of a real-time PCR for the specific detection of Waddlia chondrophila in clinical samples

Waddlia chondrophila is considered as an emerging human pathogen likely involved in miscarriage and lower respiratory tract infections. Given the low sensitivity of cell culture to recover such an obligate intracellular bacteria, molecular-based diagnostic approaches are warranted. We thus developed a real-time PCR that amplifies Waddlia chondrophila DNA. Specific primers and probe were selected to target the 16S rRNA gene. The PCR specifically amplified W. chondrophila but did not amplify other related-bacteria such as Parachlamydia acanthamoebae, Simkania negevensis and Chlamydia pneumoniae. The PCR exhibited a good intra-run and inter-run reproducibility and a sensitivity of less than ten copies of the positive control. This real-time PCR was then applied to 32 nasopharyngeal aspirates taken from children with bronchiolitis not due to respiratory syncytial virus (RSV). Three samples revealed to be Waddlia positive, suggesting a possible role of this Chlamydia-related bacteria in this setting.

Parachlamydia acanthamoebae, an emerging agent of pneumonia

Parachlamydia acanthamoebae is a Chlamydia-like organism that easily grows within Acanthamoeba spp. Thus, it probably uses these widespread free-living amoebae as a replicative niche, a cosmopolite aquatic reservoir and a vector. A potential role of P. acanthamoebae as an agent of lower respiratory tract infection was initially suggested by its isolation within an Acanthamoeba sp. recovered from the water of a humidifier during the investigation of an outbreak of fever. Additional serological and molecular-based investigations further supported its pathogenic role, mainly in bronchiolitis, bronchitis, aspiration pneumonia and community-acquired pneumonia. P. acanthamoebae was shown to survive and replicate within human macrophages, lung fibroblasts and pneumocytes. Moreover, this strict intracellular bacterium also causes severe pneumonia in experimentally infected mice, thus fulfilling the third and fourth Koch criteria for a pathogenic role. Consequently, new tools have been developed for the diagnosis of parachlamydial infections. It will be important to routinely search for this emerging agent of pneumonia, as P. acanthamoebae is apparently resistant to quinolones, which are antibiotics often used for the empirical treatment of atypical pneumonia. Other Chlamydia-related bacteria, including Protochlamydia naegleriophila, Simkania negevensis and Waddlia chondrophila, might also cause lung infections. Moreover, several additional novel chlamydiae, e.g. Criblamydia sequanensis and Rhabdochlamydia crassificans, have been discovered and are now being investigated for their human pathogenicity.

Antibiotic susceptibility of Waddlia chondrophila in Acanthamoeba castellanii amoebae

Waddlia chondrophila is an emerging cause of miscarriage in bovines and humans. Given the strict intracellular growth of this Chlamydia-like organism, its antibiotic susceptibility was tested by amoebal coculture, cell culture, and real-time PCR. W. chondrophila was susceptible to doxycycline and azithromycin but resistant to beta-lactams and fluoroquinolones.