Type III secretion in Chlamydia: a case of déjà vu?

Type III Secretion in Chlamydia

Type III secretion systems (T3SSs) are utilized by Gram-negative pathogens to enhance their pathogenesis. This secretion system is associated with the delivery of effectors through a needle-like structure from the bacterial cytosol directly into a target eukaryotic cell. These effector proteins then manipulate specific eukaryotic cell functions to benefit pathogen survival within the host. The obligate intracellular pathogens of the family Chlamydiaceae have a highly evolutionarily conserved nonflagellar T3SS that is an absolute requirement for their survival and propagation within the host with about one-seventh of the genome dedicated to genes associated with the T3SS apparatus, chaperones, and effectors. Chlamydiae also have a unique biphasic developmental cycle where the organism alternates between an infectious elementary body (EB) and replicative reticulate body (RB). T3SS structures have been visualized on both EBs and RBs. And there are effector proteins that function at each stage of the chlamydial developmental cycle, including entry and egress. This review will discuss the history of the discovery of chlamydial T3SS and the biochemical characterization of components of the T3SS apparatus and associated chaperones in the absence of chlamydial genetic tools. These data will be contextualized into how the T3SS apparatus functions throughout the chlamydial developmental cycle and the utility of heterologous/surrogate models to study chlamydial T3SS. Finally, there will be a targeted discussion on the history of chlamydial effectors and recent advances in the field.

Chlamydia muridarum Induces Pathology in the Female Upper Genital Tract via Distinct Mechanisms

Sexually transmitted infection with Chlamydia trachomatis may lead to fibrotic blockage in women's upper genital tracts, resulting in tubal infertility. Intravaginal inoculation with C. muridarum readily induces fibrotic blockage or hydrosalpinx in mice and is used for investigating C. trachomatis pathogenicity. Using this model in combination with an antibody depletion approach, we confirmed CD4⁺ T cell-mediated protective immunity and a CD8⁺ T cell-dependent pathogenic mechanism during chlamydial infection in C57BL/6J mice. However, when mice genetically deficient in CD8⁺ T cells were evaluated, we found, surprisingly, that these mice were still able to develop robust hydrosalpinx following C. muridarum infection, both contradicting the observation made in C57BL/6J mice and suggesting a pathogenic mechanism that is independent of CD8⁺ T cells. We further found that depletion of CD4⁺ T cells from CD8⁺ T cell-deficient mice significantly reduced chlamydial induction of hydrosalpinx, indicating that CD4⁺ T cells became pathogenic in mice genetically deficient in CD8⁺ T cells. Since depletion of CD4⁺ T cells both promoted chlamydial infection and reduced chlamydial pathogenicity in CD8⁺ T cell-deficient mice, we propose that in the absence of CD8⁺ T cells, some CD4⁺ T cells may remain protective (as in C57BL/6J mice), while others may directly contribute to chlamydial pathogenicity. Thus, chlamydial pathogenicity can be mediated by distinct host mechanisms, depending upon host genetics and infection conditions. The CD8⁺ T cell-deficient mouse model may be useful for further investigating the mechanisms by which CD4⁺ T cells promote chlamydial pathogenicity.

Chlamydia Spreading from the Genital Tract to the Gastrointestinal Tract - A Two-Hit Hypothesis

Chlamydia trachomatis, a leading bacterial cause of sexually transmitted infection-induced infertility, is frequently detected in the gastrointestinal tract. Chlamydia muridarum, a model pathogen for investigating C. trachomatis pathogenesis, readily spreads from the mouse genital tract to the gastrointestinal tract, establishing long-lasting colonization. C. muridarum mutants, despite their ability to activate acute oviduct inflammation, are attenuated in inducing tubal fibrosis and are no longer able to colonize the gastrointestinal tract, suggesting that the spread of C. muridarum to the gastrointestinal tract may contribute to its pathogenicity in the upper genital tract. However, gastrointestinal C. muridarum cannot directly autoinoculate the genital tract. Both antigen-specific CD8⁺ T cells and profibrotic cytokines, such as TNFα and IL-13, are essential for C. muridarum to induce tubal fibrosis; this may be induced by the gastrointestinal C. muridarum, as a second hit, to transmucosally convert tubal repairing - initiated by C. muridarum infection of tubal epithelial cells (serving as the first hit) - into pathogenic fibrosis. Testing the two-hit mouse model should both add new knowledge to the growing list of mechanisms by which gastrointestinal microbes contribute to pathologies in extragastrointestinal tissues and provide information for investigating the potential role of gastrointestinal C. trachomatis in human chlamydial pathogenesis.

A small-molecule compound belonging to a class of 2,4-disubstituted 1,3,4-thiadiazine-5-ones inhibits intracellular growth and persistence of Chlamydia trachomatis

Chlamydia trachomatis is one of the most common sexually transmitted pathogens in the world and often causes chronic inflammatory diseases that are insensitive to antibiotics. The type 3 secretion system (T3SS) of pathogenic bacteria is a promising target for therapeutic intervention aimed at bacterial virulence and can be an attractive alternative for the treatment of chronic infections. Recently, we have shown that a small-molecule compound belonging to a class of 2,4-disubstituted 1,3,4-thiadiazine-5-ones produced through the chemical modification of the thiohydrazides of oxamic acids, designated CL-55, inhibited the intracellular growth of C. trachomatis in a T3SS-dependent manner. To assess the feasibility of CL-55 as a therapeutic agent, our aim was to determine which point(s) in the developmental cycle CL-55 affects. We found that CL-55 had no effect on the adhesion of elementary bodies (EBs) to host cells but significantly suppressed EB internalization. We further found that CL-55 inhibited the intracellular division of reticulate bodies (RBs). An ultrastructural analysis revealed loss of contact between the RBs and the inclusion membrane in the presence of CL-55. Finally, we found that our T3SS inhibitor prevented the persistence of Chlamydia in cell culture and its reversion to the infectious state. Our findings indicate that our T3SS inhibitor may be effective in the treatment of both productive and persistent infections.

Chlamydial Lytic Exit from Host Cells Is Plasmid Regulated

Chlamydia trachomatis is an obligate intracellular bacterium that is a globally important human pathogen. The chlamydial plasmid is an attenuating virulence factor, but the molecular basis for attenuation is not understood. Chlamydiae replicate within a membrane-bound vacuole termed an inclusion, where they undergo a biphasic developmental growth cycle and differentiate from noninfectious into infectious organisms. Late in the developmental cycle, the fragile chlamydia-laden inclusion retains its integrity by surrounding itself with scaffolds of host cytoskeletal proteins. The ability of chlamydiae to developmentally free themselves from this cytoskeleton network is a fundamental virulence trait of the pathogen. Here, we show that plasmidless chlamydiae are incapable of disrupting their cytoskeletal entrapment and remain intracellular as stable mature inclusions that support high numbers of infectious organisms. By using deletion mutants of the eight plasmid-carried genes (Δpgp1 to Δpgp8), we show that Pgp4, a transcriptional regulator of multiple chromosomal genes, is required for exit. Exit of chlamydiae is dependent on protein synthesis and is inhibited by the compound C1, an inhibitor of the type III secretion system (T3S). Exit of plasmid-free and Δpgp4 organisms, which failed to lyse infected cells, was rescued by latrunculin B, an inhibitor of actin polymerization. Our findings describe a genetic mechanism of chlamydial exit from host cells that is dependent on an unknown pgp4-regulated chromosomal T3S effector gene.

IMPORTANCE

Chlamydia's obligate intracellular life style requires both entry into and exit from host cells. Virulence factors that function in exiting are unknown. The chlamydial inclusion is stabilized late in the infection cycle by F-actin. A prerequisite of chlamydial exit is its ability to disassemble actin from the inclusion. We show that chlamydial plasmid-free organisms, and also a plasmid gene protein 4 (pgp4) null mutant, do not disassociate actin from the inclusion and fail to exit cells. We further provide evidence that Pgp4-regulated exit is dependent on the chlamydial type III secretion system. This study is the first to define a genetic mechanism that functions in chlamydial lytic exit from host cells. The findings also have practical implications for understanding why plasmid-free chlamydiae are highly attenuated and have the ability to elicit robust protective immune responses.

Electron tomography and cryo-SEM characterization reveals novel ultrastructural features of host-parasite interaction during Chlamydia abortus infection

Chlamydia (C.) abortus is a widely spread pathogen among ruminants that can be transmitted to women during pregnancy leading to severe systemic infection with consecutive abortion. As a member of the Chlamydiaceae, C. abortus shares the characteristic feature of an obligate intracellular biphasic developmental cycle with two morphological forms including elementary bodies (EBs) and reticulate bodies (RBs). In contrast to other chlamydial species, C. abortus ultrastructure has not been investigated yet. To do so, samples were fixed by high-pressure freezing and processed by different electron microscopic methods. Freeze-substituted samples were analysed by transmission electron microscopy, scanning transmission electron microscopical tomography and immuno-electron microscopy, and freeze-fractured samples were analysed by cryo-scanning electron microscopy. Here, we present three ultrastructural features of C. abortus that have not been reported up to now. Firstly, the morphological evidence that C. abortus is equipped with the type three secretion system. Secondly, the accumulation and even coating of whole inclusion bodies by membrane complexes consisting of multiple closely adjacent membranes which seems to be a C. abortus specific feature. Thirdly, the formation of small vesicles in the periplasmic space of RBs in the second half of the developmental cycle. Concerning the time point of their formation and the fact that they harbour chlamydial components, these vesicles might be morphological correlates of an intermediate step during the process of redifferentiation of RBs into EBs. As this feature has also been shown for C. trachomatis and C. pneumoniae, it might be a common characteristic of the family of Chlamydiaceae.

Chlamydial biology and its associated virulence blockers

Chlamydiales are obligate intracellular parasites of eukaryotic cells. They can be distinguished from other Gram-negative bacteria through their characteristic developmental cycle, in addition to special biochemical and physical adaptations to subvert the eukaryotic host cell. The host spectrum includes humans and other mammals, fish, birds, reptiles, insects and even amoeba, causing a plethora of diseases. The first part of this review focuses on the specific chlamydial infection biology and metabolism. As resistance to classical antibiotics is emerging among Chlamydiae as well, the second part elaborates on specific compounds and tools to block chlamydial virulence traits, such as adhesion and internalization, Type III secretion and modulation of gene expression.

Inclusion biogenesis and reactivation of persistent Chlamydia trachomatis requires host cell sphingolipid biosynthesis

Chlamydiae are obligate intracellular pathogens that must coordinate the acquisition of host cell-derived biosynthetic constituents essential for bacterial survival. Purified chlamydiae contain several lipids that are typically found in eukaryotes, implying the translocation of host cell lipids to the chlamydial vacuole. Acquisition and incorporation of sphingomyelin occurs subsequent to transport from Golgi-derived exocytic vesicles, with possible intermediate transport through endosomal multivesicular bodies. Eukaryotic host cell-derived sphingomyelin is essential for intracellular growth of Chlamydia trachomatis, but the precise role of this lipid in development has not been delineated. The present study identifies specific phenotypic effects on inclusion membrane biogenesis and stability consequent to conditions of sphingomyelin deficiency. Culturing infected cells in the presence of inhibitors of serine palmitoyltransferase, the first enzyme in the biosynthetic pathway of host cell sphingomyelin, resulted in loss of inclusion membrane integrity with subsequent disruption in normal chlamydial inclusion development. Surprisingly, this was accompanied by premature redifferentiation to and release of infectious elementary bodies. Homotypic fusion of inclusions was also disrupted under conditions of sphingolipid deficiency. In addition, host cell sphingomyelin synthesis was essential for inclusion membrane stability and expansion that is vital to reactivation of persistent chlamydial infection. The present study implicates both the Golgi apparatus and multivesicular bodies as key sources of host-derived lipids, with multivesicular bodies being essential for normal inclusion development and reactivation of persistent C. trachomatis infection.

The genus Chlamydia is composed of a group of obligate intracellular bacterial pathogens that cause several human diseases of medical significance. C. trachomatis is the most commonly encountered sexually transmitted pathogen, as well as the leading cause of preventable blindness worldwide. The prevalence of chlamydial infections, and the extraordinary morbidity and health care costs associated with chronic persisting disease, justifies the research efforts in this area of microbial pathogenesis. Despite their clinical importance, the mechanisms by which these intracellular bacteria obtain nutrients essential to their growth remain enigmatic. Acquisition of sphingolipids, from the cells that chlamydiae infect, is essential for bacterial propagation. This study identifies a requirement for the lipid sphingomyelin from the infected host cell for bacterial replication during infection, and for long-term subsistence in persistent chlamydial infection. Blockage of sphingomyelin acquisition results in premature release of bacteria, a reduced bacterial number, and failure of the bacteria to cause a persisting infection. In this study, we have identified and subsequently disrupted specific sphingomyelin transport pathways, providing important implications on therapeutic intervention targeting this successful microbial pathogen.

Identification of Chlamydia trachomatis CT621, a protein delivered through the type III secretion system to the host cell cytoplasm and nucleus

Chlamydiae are obligate intracellular bacteria, developing inside host cells within chlamydial inclusions. From these inclusions, the chlamydiae secrete proteins into the host cell cytoplasm. A pathway through which secreted proteins can be delivered is the type III secretion system (T3SS). The T3SS is common to several gram-negative bacteria and the secreted proteins serve a variety of functions often related to the modulation of host signalling. To identify new potentially secreted proteins, the cytoplasm was extracted from Chlamydia trachomatis L2-infected HeLa cells, and two-dimensional polyacrylamide gel electrophoresis profiles of [³⁵S]-labelled chlamydial proteins from this extract were compared with profiles of chlamydial proteins from the lysate of infected cells. In this way, CT621 was identified. CT621 is a member of a family of proteins containing a domain of unknown function DUF582 that is only found within the genus Chlamydia. Immunofluorescence microscopy and immunoblotting demonstrated that CT621 is secreted late in the chlamydial developmental cycle and that it is the first chlamydial protein found to be localized within both the host cell cytoplasm and the nucleus. To determine whether CT621 is secreted through the T3SS, an inhibitor of this apparatus was added to the infection medium, resulting in retention of the protein inside the chlamydiae. Hence, the so far uncharacterized CT621 is a new type III secretion effector protein.

Small molecule inhibitors of the Yersinia type III secretion system impair the development of Chlamydia after entry into host cells

Background

Chlamydiae are obligate intracellular pathogens that possess a type III secretion system to deliver proteins into the host cell during infection. Small molecule inhibitors of type III secretion in Yersinia, termed INPs (Innate Pharmaceuticals AB) were reported to strongly inhibit Chlamydia growth in epithelial cells. In this study we have analyzed the effect of these drugs on bacterial invasiveness.

Results

We demonstrate that INPs affect Chlamydia growth in a dose dependent manner after bacterial invasion. The efficiency of C. trachomatis L2 and C. caviae GPIC entry into host cells was not altered in the presence of INPs. In C. caviae, entry appears to proceed normally with recruitment of actin and the small GTPases Rac, Cdc42 and Arf6 to the site of bacterial entry.

Conclusion

INPs have a strong inhibitory effect on Chlamydia growth. However, bacterial invasion is not altered in the presence of these drugs. In the light of these results, we discuss several hypotheses regarding the mode of action of INPs on type III secretion during the Chlamydia infectious cycle.

Host complement regulatory protein CD59 is transported to the chlamydial inclusion by a Golgi apparatus-independent pathway

Chlamydia is an obligate intracellular bacterium that grows and replicates inside a cytoplasmic inclusion. We report that a host protein, CD59, which regulates complement function at the surfaces of uninfected cells, can be detected at the membrane of the chlamydial inclusion. This localization to the inclusion membrane was specific for CD59 and not a general feature of other glycosylphosphatidylinositol (GPI)-anchored proteins or representative cell surface proteins. Using differential permeabilization studies, we showed that CD59 is localized to the luminal but not the cytoplasmic face of the inclusion membrane, consistent with membrane association via its GPI anchor. Furthermore, CD59 was present at the inclusion even when we prevented it from associating with membrane microdomains via the GPI anchor or when we inhibited general protein transport to the cell surface, indicating that a conventional Golgi apparatus-dependent trafficking mechanism was not involved. Based on these findings, we propose that selected host proteins are trafficked to the inclusion by a Golgi apparatus-independent pathway during a Chlamydia infection.

Chlamydia trachomatis polymorphic membrane protein D is an oligomeric autotransporter with a higher-order structure

Chlamydia trachomatis is a globally important obligate intracellular bacterial pathogen that is a leading cause of sexually transmitted disease and blinding trachoma. Effective control of these diseases will likely require a preventative vaccine. C. trachomatis polymorphic membrane protein D (PmpD) is an attractive vaccine candidate as it is conserved among C. trachomatis strains and is a target of broadly cross-reactive neutralizing antibodies. We show here that immunoaffinity-purified native PmpD exists as an oligomer with a distinct 23-nm flower-like structure. Two-dimensional blue native-sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses showed that the oligomers were composed of full-length PmpD (p155) and two proteolytically processed fragments, the p73 passenger domain (PD) and the p82 translocator domain. We also show that PmpD undergoes an infection-dependent proteolytic processing step late in the growth cycle that yields a soluble extended PD (p111) that was processed into a p73 PD and a novel p30 fragment. Interestingly, soluble PmpD peptides possess putative eukaryote-interacting functional motifs, implying potential secondary functions within or distal to infected cells. Collectively, our findings show that PmpD exists as two distinct forms, a surface-associated oligomer exhibiting a higher-order flower-like structure and a soluble form restricted to infected cells. We hypothesize that PmpD is a multifunctional virulence factor important in chlamydial pathogenesis and could represent novel vaccine or drug targets for the control of human chlamydial infections.

Induction of type III secretion by cell-free Chlamydia trachomatis elementary bodies

Chlamydiae secrete type III effector proteins at two distinct stages of their developmental cycle. Elementary bodies (EBs) secrete at least one pre-formed effector protein, Tarp, across the host plasma membrane from an extracellular location. Once internalized, a set of newly transcribed proteins are secreted to modify the inclusion membrane. In an effort to better understand the triggers for chlamydial type III secretion and develop means to identify new effectors, we investigated various inducers of T3SS in other Gram-negative bacterial systems to determine if they were able to activate chlamydial type III secretion from EBs using Tarp secretion as an indicator of activation. Chlamydial EBs are induced to secrete Tarp by exposure to FBS, BSA, or sphingolipid and cholesterol-rich liposomes (SCRLs). The induction by FBS and BSA, but not SCRL, is enhanced in the presence of the calcium-chelator, EGTA. This secretion was temperature dependent and inhibited by paraformaldehyde fixation of the EBs.

Chlamydophila psittaci infections in birds: a review with emphasis on zoonotic consequences

The first part of the present review gives an overview on the history of infectious agents of the order Chlamydiales and the general infection biology of Chlamydophila (C.) psittaci, the causative agent of psittacosis. In the second part, the classification of C. psittaci strains, as well as issues of epidemiology of avian chlamydiosis., disease transmission routes, clinical disease, public health significance, present legislation and recommendations for prevention and control are reviewed.

Bioinformatic and biochemical evidence for the identification of the type III secretion system needle protein of Chlamydia trachomatis

Chlamydia spp. express a functional type III secretion system (T3SS) necessary for pathogenesis and intracellular growth. However, certain essential components of the secretion apparatus have diverged to such a degree as to preclude their identification by standard homology searches of primary protein sequences. One example is the needle subunit protein. Electron micrographs indicate that chlamydiae possess needle filaments, and yet database searches fail to identify a SctF homologue. We used a bioinformatics approach to identify a likely needle subunit protein for Chlamydia. Experimental evidence indicates that this protein, designated CdsF, has properties consistent with it being the major needle subunit protein. CdsF is concentrated in the outer membrane of elementary bodies and is surface exposed as a component of an extracellular needle-like projection. During infection CdsF is detectable by indirect immunofluorescence in the inclusion membrane with a punctuate distribution adjacent to membrane-associated reticulate bodies. Biochemical cross-linking studies revealed that, like other SctF proteins, CdsF is able to polymerize into multisubunit complexes. Furthermore, we identified two chaperones for CdsF, termed CdsE and CdsG, which have many characteristics of the Pseudomonas spp. needle chaperones PscE and PscG, respectively. In aggregate, our data are consistent with CdsF representing at least one component of the extended Chlamydia T3SS injectisome. The identification of this secretion system component is essential for studies involving ectopic reconstitution of the Chlamydia T3SS. Moreover, we anticipate that CdsF could serve as an efficacious target for anti-Chlamydia neutralizing antibodies.

Type III secretion à la Chlamydia

Type III secretion (T3S) is a mechanism that is central to the biology of the Chlamydiaceae and many other pathogens whose virulence depends on the translocation of toxic effector proteins to cytosolic targets within infected eukaryotic cells. Biomathematical simulations, using a previously described model of contact-dependent, T3S-mediated chlamydial growth and late differentiation, suggest that chlamydiae contained in small non-fusogenic inclusions will persist. Here, we further discuss the model in the context of in vitro-persistent, stress-induced aberrantly enlarged forms and of recent studies using small molecule inhibitors of T3S. A general mechanism is emerging whereby both early- and mid-cycle T3S-mediated activities and late T3S inactivation upon detachment of chlamydiae from the inclusion membrane are crucial for chlamydial intracellular development.

Ultrastructural analysis of chlamydial antigen-containing vesicles everting from the Chlamydia trachomatis inclusion

Several chlamydial antigens have been detected in the infected epithelial cell cytosol and on the host cell surface prior to their presumed natural release at the end of the 72-96 h developmental cycle. These extra-inclusion antigens are proposed to influence vital host cell functions, antigen trafficking and presentation and, ultimately, contribute to a prolonged inflammatory response. To begin to dissect the mechanisms for escape of these antigens from the chlamydial inclusion, which are enhanced on exposure to antibiotics, polarized endometrial epithelial cells (HEC-1B) were infected with Chlamydia trachomatis serovar E for 36 h or 48 h. Infected cells were then exposed to chemotactic human polymorphonuclear neutrophils not loaded or pre-loaded in vitro with the antibiotic azithromycin. Viewed by electron microscopy, the azithromycin-mediated killing of chlamydiae involved an increase in chlamydial outer membrane blebbing followed by the appearance of the blebs in larger vesicles (i) everting from but still associated with the inclusion as well as (ii) external to the inclusion. Evidence that the vesicles originated from the chlamydial inclusion membrane was shown by immuno-localization of inclusion membrane proteins A, F, and G on the vesicular membranes. Chlamydial heat shock protein 60 (chsp60) copies 2 and 3, but not copy 1, were released from RB and incorporated into the everted inclusion membrane vesicles and delivered to the infected cell surface. These data represent direct evidence for one mechanism of early antigen delivery, albeit membrane-bound, beyond the confines of the chlamydial inclusion.

Type III secretion, contact-dependent model for the intracellular development of chlamydia

The medically significant genus Chlamydia is a class of obligate intracellular bacterial pathogens that replicate within vacuoles in host eukaryotic cells termed inclusions. Chlamydia's developmental cycle involves two forms; an infectious extracellular form, known as an elementary body (EB), and a non-infectious form, known as the reticulate body (RB), that replicates inside the vacuoles of the host cells. The RB surface is covered in projections that are in intimate contact with the inclusion membrane. Late in the developmental cycle, these reticulate bodies differentiate into the elementary body form. In this paper, we present a hypothesis for the modulation of these developmental events involving the contact-dependent type III secretion (TTS) system. TTS surface projections mediate intimate contact between the RB and the inclusion membrane. Below a certain number of projections, detachment of the RB provides a signal for late differentiation of RB into EB. We use data and develop a mathematical model investigating this hypothesis. If the hypothesis proves to be accurate, then we have shown that increasing the number of inclusions per host cell will increase the number of infectious progeny EB until some optimal number of inclusions. For more inclusions than this optimum, the infectious yield is reduced because of spatial restrictions. We also predict that a reduction in the number of projections on the surface of the RB (and as early as possible during development) will significantly reduce the burst size of infectious EB particles. Many of the results predicted by the model can be tested experimentally and may lead to the identification of potential targets for drug design.

Population-based genetic epidemiologic analysis of Chlamydia trachomatis serotypes and lack of association between ompA polymorphisms and clinical phenotypes

Chlamydia trachomatis is the leading cause of bacterial sexually transmitted diseases worldwide. Urogenital strains are classified into serotypes and genotypes based on the major outer membrane protein and its gene, ompA, respectively. Studies of the association of serotypes with clinical signs and symptoms have produced conflicting results while no studies have evaluated associations with ompA polymorphisms. We designed a population-based cross-sectional study of 344 men and women with urogenital chlamydial infections (excluding co-pathogen infections) presenting to clinics serving five U.S. cities from 1995 to 1997. Signs, symptoms and sequelae of chlamydial infection (mucopurulent cervicitis, vaginal or urethral discharge; dysuria; lower abdominal pain; abnormal vaginal bleeding; and pelvic inflammatory disease) were analyzed for associations with serotype and ompA polymorphisms. One hundred and fifty-three (44.5%) of 344 patients had symptoms consistent with urogenital chlamydial infection. Gender, reason for visit and city were significant independent predictors of symptom status. Men were 2.2 times more likely than women to report any symptoms (P=0.03) and 2.8 times more likely to report a urethral discharge than women were to report a vaginal discharge in adjusted analyses (P=0.007). Differences in serotype or ompA were not predictive except for an association between serotype F and pelvic inflammatory disease (P=0.046); however, the number of these cases was small. While there was no clinically prognostic value associated with serotype or ompA polymorphism for urogenital chlamydial infections except for serotype F, future studies might utilize multilocus genomic typing to identify chlamydial strains associated with clinical phenotypes.

TheYersinia pestistype III secretion needle plays a role in the regulation of Yop secretion

Activation of bacterial virulence-associated type III secretion systems (T3SSs) requires direct contact between a bacterium and a eukaryotic cell. In Yersinia pestis, the cytosolic LcrG protein and a cytosolic YopN-TyeA complex function to block T3S in the presence of extracellular calcium and prior to contact with a eukaryotic cell. The mechanism by which the bacterium senses extracellular calcium and/or cell contact and transmits these signals to the cytosolic compartment is unknown. We report here that YscF, a small protein that polymerizes to form the external needle of the T3SS, is essential for the calcium-dependent regulation of T3S. Alanine-scanning mutagenesis was used to identify YscF mutants that secrete virulence proteins in the presence and absence of calcium and prior to contact with a eukaryotic cell. Interestingly, one of the YscF mutants that exhibited constitutive T3S was unable to translocate secreted proteins across the eukaryotic plasma membrane. These data indicate that the YscF needle is a multifunctional structure that participates in virulence protein secretion, in translocation of virulence proteins across eukaryotic membranes and in the cell contact- and calcium-dependent regulation of T3S.

The chlamydial inclusion: escape from the endocytic pathway

Chlamydiae, bacterial obligate intracellular pathogens, are the etiologic agents of several human diseases. A large part of the chlamydial intracellular survival strategy involves the formation of a unique organelle called the inclusion that provides a protected site within which they replicate. The chlamydial inclusion is effectively isolated from endocytic pathways but is fusogenic with a subset of exocytic vesicles that deliver sphingomyelin from the Golgi apparatus to the plasma membrane. A combination of host and parasite functions contribute to the biogenesis of this compartment. Establishment of the mature inclusion is accompanied by the insertion of multiple chlamydial proteins, suggesting that chlamydiae actively modify the inclusion to define its interactions with the eukaryotic host cell. Despite being sequestered within a membrane-bound vacuole, chlamydiae clearly communicate with and manipulate the host cell from within this privileged intracellular niche.

Temporal expression of type III secretion genes of Chlamydia pneumoniae

Chlamydia pneumoniae has been shown to possess at least 13 genes that are homologous with other known type III secretion (TTS) systems. Upon infection of HEp-2 cells with C. pneumoniae, the expression of these genes was followed by reverse transcriptase PCR throughout the developmental cycle of this obligate intracellular pathogen. In addition, expression was analyzed when C. pneumoniae was grown in the presence of human gamma interferon (IFN-gamma). The groEL-1, ompA, and omcB genes were used as markers for the early, middle, and late stages of the developmental cycle, respectively, and the inhibition of expression of the fstK gene was used as a marker for the effect of IFN-gamma on the maturation of C. pneumoniae. In the absence of IFN-gamma, the TTS genes were expressed as follows: early stage (1.5 to 8 h), yscC, yscS, yscL, yscJ and lcrH-2; middle stage (by 12 to 18 h), lcrD, yscN, and yscR; and late stage (by 24 h), lcrE, sycE, lcrH-1, and yscT. Of the genes expressed early, the lcrH-2 gene was detected the earliest, at 1.5 h. Expression of the yscU gene was not detected at any of the time points examined. Under the influence of IFN-gamma, the cluster of TTS genes that were normally not expressed until the middle to late stages of the developmental cycle, namely, lcrD, lcrE, and sycE, as well as lcrH-1, were down-regulated, and expression could not be detected up to 48 h. In contrast, the expression of the other TTS genes appeared to be unchanged in the presence of IFN-gamma. The lcrH-1 and lcrH-2 genes differed from one another in both their temporal expression and response to IFN-gamma. In other TTS systems, these genes code for proteins that function in regulation of effector protein synthesis as well as serve as chaperones for proteins that provide for the translocation of the effector proteins into the host cell. In summary, the expression pattern of the TTS genes of C. pneumoniae examined suggests that they are temporally regulated throughout the developmental cycle. Furthermore, paralleling the inhibition of the maturation of the reticulate body to the elementary body, TTS genes expressed in the later stages of the cycle appear to be down-regulated when the organism is grown in the presence of IFN-gamma.

Genome sequence of Chlamydophila caviae (Chlamydia psittaci GPIC): examining the role of niche-specific genes in the evolution of the Chlamydiaceae

The genome of Chlamydophila caviae (formerly Chlamydia psittaci, GPIC isolate) (1 173 390 nt with a plasmid of 7966 nt) was determined, representing the fourth species with a complete genome sequence from the Chlamydiaceae family of obligate intracellular bacterial pathogens. Of 1009 annotated genes, 798 were conserved in all three other completed Chlamydiaceae genomes. The C.caviae genome contains 68 genes that lack orthologs in any other completed chlamydial genomes, including tryptophan and thiamine biosynthesis determinants and a ribose-phosphate pyrophosphokinase, the product of the prsA gene. Notable amongst these was a novel member of the virulence-associated invasin/intimin family (IIF) of Gram-negative bacteria. Intriguingly, two authentic frameshift mutations in the ORF indicate that this gene is not functional. Many of the unique genes are found in the replication termination region (RTR or plasticity zone), an area of frequent symmetrical inversion events around the replication terminus shown to be a hotspot for genome variation in previous genome sequencing studies. In C.caviae, the RTR includes several loci of particular interest including a large toxin gene and evidence of ancestral insertion(s) of a bacteriophage. This toxin gene, not present in Chlamydia pneumoniae, is a member of the YopT effector family of type III-secreted cysteine proteases. One gene cluster (guaBA-add) in the RTR is much more similar to orthologs in Chlamydia muridarum than those in the phylogenetically closest species C.pneumoniae, suggesting the possibility of horizontal transfer of genes between the rodent-associated Chlamydiae. With most genes observed in the other chlamydial genomes represented, C.caviae provides a good model for the Chlamydiaceae and a point of comparison against the human atherosclerosis-associated C.pneumoniae. This crucial addition to the set of completed Chlamydiaceae genome sequences is enabling dissection of the roles played by niche-specific genes in these important bacterial pathogens.

Chlamydia trachomatis induces remodeling of the actin cytoskeleton during attachment and entry into HeLa cells

To elucidate the host cell machinery utilized by Chlamydia trachomatis to invade epithelial cells, we examined the role of the actin cytoskeleton in the internalization of chlamydial elementary bodies (EBs). Treatment of HeLa cells with cytochalasin D markedly inhibited the internalization of C. trachomatis serovar L2 and D EBs. Association of EBs with HeLa cells induced localized actin polymerization at the site of attachment, as visualized by either phalloidin staining of fixed cells or the active recruitment of GFP-actin in viable infected cells. The recruitment of actin to the specific site of attachment was accompanied by dramatic changes in the morphology of cell surface microvilli. Ultrastructural studies revealed a transient microvillar hypertrophy that was dependent upon C. trachomatis attachment, mediated by structural components on the EBs, and cytochalasin D sensitive. In addition, a mutant CHO cell line that does not support entry of C. trachomatis serovar L2 did not display such microvillar hypertrophy following exposure to L2 EBs, which is in contrast to infection with serovar D, to which it is susceptible. We propose that C. trachomatis entry is facilitated by an active actin remodeling process that is induced by the attachment of this pathogen, resulting in distinct microvillar reorganization throughout the cell surface and the formation of a pedestal-like structure at the immediate site of attachment and entry.

Proteins in the chlamydial inclusion membrane

The chlamydiae are obligate intracellular pathogens that occupy a nonacidified vacuole (the inclusion) during their entire developmental cycle. Several proteins have recently been identified that are localized to the inclusion membrane. The following is a discussion of how inclusion membrane proteins might participate in the chlamydial developmental process.

Comparative proteome analysis of Chlamydia trachomatis serovar A, D and L2

Chlamydia trachomatis represents a group of human pathogenic obligate intracellular and gram-negative bacteria. The genome of C. trachomatis D comprises 894 open reading frames (ORFs). In this study the global expression of genes in C. trachomatis A, D and L2, which are responsible for different chlamydial diseases, was investigated using a proteomics approach. Based on silver stained two-dimensional polyacrylamide gel electrophoresis (2-D PAGE), gels with purified elementary bodies (EB) and auto-radiography of gels with 35S-labeled C. trachomatis proteins up to 700 protein spots were detectable within the range of the immobilized pH gradient (IPG) system used. Using mass spectrometry and N-terminal sequencing followed by database searching we identified 250 C. trachomatis proteins from purified EB of which 144 were derived from different genes representing 16% of the ORFs predicted from the C. trachomatis D genome and the 7.5 kb C. trachomatis plasmid. Important findings include identification of proteins from the type III secretion apparatus, enzymes from the central metabolism and confirmation of expression of 25 hypothetical ORFs and five polymorphic membrane proteins. Comparison of serovars generated novel data on genetic variability as indicated by electrophoretic variation and potentially important examples of serovar specific differences in protein abundance. The availability of the complete genome made it feasible to map and to identify proteins of C. trachomatis on a large scale and the integration of our data in a 2-D PAGE database will create a basis for post genomic research, important for the understanding of chlamydial development and pathogenesis.

Genomic approach for analysis of surface proteins in Chlamydia pneumoniae

Chlamydia pneumoniae, a human pathogen causing respiratory infections and probably contributing to the development of atherosclerosis and heart disease, is an obligate intracellular parasite which for replication needs to productively interact with and enter human cells. Because of the intrinsic difficulty in working with C. pneumoniae and in the absence of reliable tools for its genetic manipulation, the molecular definition of the chlamydial cell surface is still limited, thus leaving the mechanisms of chlamydial entry largely unknown. In an effort to define the surface protein organization of C. pneumoniae, we have adopted a combined genomic-proteomic approach based on (i) in silico prediction from the available genome sequences of peripherally located proteins, (ii) heterologous expression and purification of selected proteins, (iii) production of mouse immune sera against the recombinant proteins to be used in Western blotting and fluorescence-activated cell sorter (FACS) analyses for the identification of surface antigens, and (iv) mass spectrometry analysis of two-dimensional electrophoresis (2DE) maps of chlamydial protein extracts to confirm the presence of the FACS-positive antigens in the chlamydial cell. Of the 53 FACS-positive sera, 41 recognized a protein species with the expected size on Western blots, and 28 of the 53 antigens shown to be surface-exposed by FACS were identified on 2DE maps of elementary-body extracts. This work represents the first systematic attempt to define surface protein organization in C. pneumoniae.

Chlamydia pneumoniae and chronic skin wounds: a focused review

The genus, Chlamydophilia, as obligate intracellular pathogens, induce chronic scarring in humans. Chlamydia pneumoniae, a common cause of pneumonia, infects endothelial cells and circulating macrophages. Evidence that C. pneumoniae is an opportunistic pathogen in chronic skin ulcers and other inflammatory skin conditions analogous to its role in atherosclerosis is reviewed.

Proteome analysis of theChlamydia pneumoniaeelementary body

Chlamydia pneumoniae is an obligate intracellular human pathogen that causes acute and chronic respiratory tract diseases and that has been implicated as a possible risk factor in the development of atherosclerotic heart disease. C. pneumoniae cultivated in Hep-2 cells were 35S-labeled and infectious elementary bodies (EB) were purified. The EB proteins were separated by two-dimensional gel electrophoresis. Excised protein spots were in-gel digested with trypsin and peptides were concentrated on reverse-phase chromatographic beads for identification analysis by matrix-assisted laser desorption/ionization-mass spectrometry. In the pH range from 3-11, 263 C. pneumoniae protein spots encoded from 167 genes were identified. These genes constitute 15% of the genome. The identified proteins include 31 hypothetical proteins. It has recently been suggested that EB should be able to synthesize ATP. This view may be strengthened by the identification of several proteins involved in energy metabolism. Furthermore, proteins have been found which are involved in the type III secretion apparatus important for pathogenesis of intracellular bacteria. Proteome maps and a table of all identified proteins have been made available on the world wide web at www.gram.au.dk.

Revisiting the chlamydial type III protein secretion system: clues to the origin of type III protein secretion

The bacterial type III secretion pathway delivers effector proteins into eukaryotic cells. Analysis of the type III system and flagellar export genes in the obligate parasites of the family Chlamydiales suggests that the type III system arose from the flagellar export system in chlamydiae or related bacteria.

Evidence for the secretion of Chlamydia trachomatis CopN by a type III secretion mechanism

The medically significant, obligate intracellular pathogen Chlamydia trachomatis replicates within vacuoles termed inclusions. A developmental cycle is initiated after entry into a host cell and is manifested by the transformation of infectious elementary bodies (EBs) to larger, non-infectious reticulate bodies (RBs). Analysis of the C. trachomatis genome has revealed that chlamydiae possess genes that may encode a type III secretion apparatus. In other Gram-negative pathogens, the type III secretion mechanism is used to target virulence factors directly to the host cell cytoplasm and is essential for full virulence. To evaluate the possibility of a functional type III secretion mechanism in C. trachomatis, we initially focused on a locus containing genes encoding products with similarity to chaperones (Scc1), secretion pore components (Cds1 and Cds2) and secreted proteins (CopN) from other type III systems. Gene expression was tested by reverse transcriptase-polymerase chain reaction (RT-PCR) of total RNA extracted from infected HeLa cell monolayers at 2, 6, 12 and 20 h after infection and normalized for the number of C. trachomatis genomes present. Message was detected for Scc1 at all times, whereas message for all other tested genes was detected in significant amounts at 12 h and 20 h. Immunoblot analysis with Scc1- and CopN-specific antibodies revealed that CopN and Scc1 were present in EBs, RBs and whole-culture extracts harvested 20 h after infection. CopN is homologous to the secreted protein YopN of Yersinia sp., and analysis of monolayers 20 h after infection via indirect immunofluorescence showed specific labelling of inclusion membranes when probed with CopN-specific antibodies but not with Scc1-specific antibodies. His-tagged CopN and a chlamydial cytoplasmic control protein (NrdB) were expressed in Yersinia enterocolitica containing or lacking the virulence plasmid pYV. CopN, but not NrdB, was secreted by Y. enterocolitica in a Ca2+- and pYV-dependent fashion. These data indicate that components of the putative type III apparatus of C. trachomatis are expressed and that at least one of these products is secreted by chlamydiae to the inclusion membrane. The observation that CopN is also secreted by the Yersinia type III apparatus provides support for the notion that chlamydiae secrete proteins via a type III mechanism.

Intracellular survival by Chlamydia

Chlamydiae are obligate intracellular bacterial pathogens whose entry into mucosal epithelial cells is required for intracellular survival and subsequent growth. After a seemingly stealthy entry, chlamydiae quickly modify their vacuole (i) for exit from the endosomal pathway to the exocytic pathway and (ii) to permit fusion with intercepted endoplasmic reticulum- and Golgi-derived vesicles carrying glycerophospholipids and sphingolipids for chlamydiae-containing vacuole membrane expansion. Chlamydiae possess novel hollow proteinaceous structures, termed projections, which they use to pierce the inclusion membrane, possibly to acquire from the epithelial cytoplasm nutrients they cannot synthesize; whether or not these truncated flagellar-like structures serve a dual exchange function for secretion of molecules to programme host cell signalling is unknown. Despite the accumulation of some 500-1000 progeny in the enormously enlarged inclusion, host cell function is surprisingly little disrupted, and progeny escape can be unobtrusive. This elegant adaptive pathogen strategy, which leads to silent, chronic human infection, is fascinating from a cellular microbiology perspective.

Type III secretion system in Chlamydia species: identified members and candidates

Chlamydia trachomatis and Chlamydia pneumoniae genomes contain genes coding for type III secretion apparatuses. Like other pathogens, Chlamydia probably uses this system to secrete proteins in the host cell. With the aim of identifying such proteins, we analyzed the organization of Chlamydia type III secretion genes.

A system for identifying post-invasion functions of invasion genes: requirements for the Mxi-Spa type III secretion pathway of Shigella flexneri in intercellular dissemination

Invasion and intercellular spread are hallmarks of Shigella pathogenicity. Invasion of the eukaryotic cell cytosol requires a type III secretion system (Mxi-Spa) and its cognate set of secreted Ipa invasins. Once intracellular, the IcsA protein directs a form of actin-based motility that helps to drive intracellular bacterial movement, formation of cellular protrusions and cell-to-cell spread. Work in our laboratory has focused on identifying additional factors required for this intercellular form of dissemination. In this study, we sought to identify novel contributions of the type III secretion pathway to post-invasion-specific processes, distinct from its previously characterized roles in invasion. Studies of post-invasion Ipa and Mxi-Spa functions are complicated by an absolute requirement for these virulence proteins in invasion. To circumvent this problem, we developed a system called TIER (for test of intracellular expression requirements), whereby specific ipa, mxi or spa loci are transiently expressed before infection of tissue culture cell monolayers (thus supporting invasion), but then repressed after invasion in the intracellular environment. Such invasive type III secretion mutants (called TIER mutants) were severely restricted in their ability to spread intercellularly and form plaques in confluent tissue culture cell monolayers. Intercellular spread defects were associated with the repression of most type III pathway components examined, including structural (MxiM and Spa33), secreted effector (IpaB, IpaC and IpaD) and regulatory elements (VirF and VirB). A kinetic analysis of bacterial growth in L2 cell monolayers showed that each of the TIER mutants was defective with respect to long-term intracellular proliferation and viability. Examination of TIER mutant-infected monolayers by electron microscopy revealed that the type III pathway was required for a late step in intercellular spread - bacterial escape from protrusion-derived, double-membrane-bound vacuoles. The TIER mutants were eventually degraded in a process involving vacuolar acidification. Based on these findings, we propose that Ipa secretion via Mxi-Spa is required in the protrusion vacuole for double-membrane lysis.

Identification and characterization of a Chlamydia trachomatis early operon encoding four novel inclusion membrane proteins

Chlamydia trachomatis is a bacterial obligate intracellular parasite that replicates within a vacuole, termed an inclusion, that does not fuse with lysosomes. Within 2 h after internalization, the C. trachomatis inclusion ceases to interact with the endocytic pathway and, instead, becomes fusogenic with exocytic vesicles containing exogenously synthesized NBD-sphingomyelin. Both fusion of exocytic vesicles and long-term avoidance of lysosomal fusion require early chlamydial gene expression. Modification of the chlamydial inclusion probably occurs through the expression and insertion of chlamydial protein(s) into the inclusion membrane. To identify candidate inclusion membrane proteins, antisera were raised against a total membrane fraction purified from C. trachomatis-infected HeLa cells. By indirect immunofluorescence, this antisera recognized the inclusion membrane and, by immunoblot analysis, recognized three chlamydial-specific antigens of approximate molecular weights 15, 18 and 21 kDa. IncG, encoding an 18 kDa and 21 kDa doublet chlamydial antigen, was identified by screening a C. trachomatis, serovar L2, genomic expression library. Three additional genes, incD, incE and incF, were co-transcribed with incG. Monospecific antisera against each of the four genes of this operon demonstrated that the gene products were localized to the chlamydial inclusion membrane. Immediately downstream from the operon containing incD-G was the C. trachomatis homologue of incA. Like IncD, E, F and G, C. trachomatis IncA is also localized to the inclusion membrane. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis demonstrated that IncD-G, but not incA, are transcribed within the first 2 h after internalization, making them candidates for chlamydial factors required for the modification of the nascent chlamydial inclusion.

Protein translocation: delivering virulence into the host cell

A wide variety of plant and human bacterial pathogens use a specialized 'type III' protein secretion system to deliver virulence factors into host cells. Appendage-like surface structures have recently been identified on several bacterial pathogens and there are indications that these may be conduits for virulence factor delivery.