The intracellular life of Chlamydia psittaci: how do the bacteria interact with the host cell?

Psittacosis: An Underappreciated and Often Undiagnosed Disease

The bacterial agent Chlamydia psittaci, and the resulting disease of psittacosis, is a little-known and underappreciated infectious disease by healthcare practitioners and in public health in general. C. psittaci infections can cause significant psittacosis outbreaks, with person-to-person transmission documented in the last decade. In this publication, we review the pathogen and its disease, as well as examine the potential for genetic manipulation in this organism to create a more deadly pathogen. Recent disease surveys indicate that currently, the highest incidences of human disease exist in Australia, Germany and the UK. We recommend the universal public health reporting of C. psittaci and psittacosis disease and increasing the promotion of public health awareness.

Chlamydia psittaci: update on an underestimated zoonotic agent

Chlamydia (C.) psittaci is an economically relevant pathogen in poultry and pet birds, where it causes psittacosis/ornithosis, and also a human pathogen causing atypical pneumonia after zoonotic transmission. Despite its well-documented prevalence, the agent has received less attention by researchers than other Chlamydia spp. in the last decades. In the present paper, we review recently published data on C. psittaci infection and attempt to single out characteristic features distinguishing it from related chlamydial agents. It is remarkable that C. psittaci is particularly efficient in disseminating in the host organism causing systemic disease, which occasionally can take a fulminant course. At the cellular level, the pathogen's broad host cell spectrum (from epithelial cells to macrophages), its rapid entry and fast replication, proficient use of intracellular transport routes to mitochondria and the Golgi apparatus, the pronounced physical association of chlamydial inclusions with energy-providing cell compartments, as well as the subversive regulation of host cell survival during productive and persistent states facilitate the characteristic efficient growth and successful host-to-host spread of C. psittaci. At the molecular level, the pathogen was shown to upregulate essential chlamydial genes when facing the host immune response. We hypothesize that this capacity, in concert with expression of specific effectors of the type III secretion system and efficient suppression of selected host defense signals, contributes to successful establishment of the infection in the host. Concerning the immunology of host-pathogen interactions, C. psittaci has been shown to distinguish itself by coping more efficiently than other chlamydiae with pro-inflammatory mediators during early host response, which can, to some extent, explain the effective evasion and adaptation strategies of this bacterium. We conclude that thorough analysis of the large number of whole-genome sequences already available will be essential to identify genetic markers of the species-specific features and trigger more in-depth studies in cellular and animal models to address such vital topics as treatment and vaccination.

Chlamydia psittaci inclusion membrane protein IncB associates with host protein Snapin

Chlamydia (C.) psittaci, the causative agent of psittacosis in birds and humans, is the most important zoonotic pathogen of the family Chlamydiaceae. During a unique developmental cycle of this obligate intracellular pathogen, the infectious elementary body gains access to the susceptible host cell, where it transforms into the replicative reticulate body. C. psittaci uses dynein motor proteins for optimal early development. Chlamydial proteins that mediate this process are unknown. Two-hybrid screening with the C. psittaci inclusion protein IncB as bait against a HeLa Yeast Two-hybrid (YTH) library revealed that the host protein Snapin interacts with IncB. Snapin is a cytoplasmic protein that plays a multivalent role in intracellular trafficking. Confocal fluorescence microscopy using an IncB-specific antibody demonstrated that IncB, Snapin, and dynein were co-localized near the inclusion of C. psittaci-infected HEp-2 cells. This co-localization was lost when Snapin was depleted by RNAi. The interaction of Snapin with both IncB and dynein has been shown in vitro and in vivo. We propose that Snapin connects chlamydial inclusions with the microtubule network by interacting with both IncB and dynein.

A novel co-infection model with Toxoplasma and Chlamydia trachomatis highlights the importance of host cell manipulation for nutrient scavenging

Toxoplasma and Chlamydia trachomatis are obligate intracellular pathogens that have evolved analogous strategies to replicate within mammalian cells. Both pathogens are known to extensively remodel the cytoskeleton, and to recruit endocytic and exocytic organelles to their respective vacuoles. However, how important these activities are for infectivity by either pathogen remains elusive. Here, we have developed a novel co-infection system to gain insights into the developmental cycles of Toxoplasma and C. trachomatis by infecting human cells with both pathogens, and examining their respective ability to replicate and scavenge nutrients. We hypothesize that the common strategies used by Toxoplasma and Chlamydia to achieve development results in direct competition of the two pathogens for the same pool of nutrients. We show that a single human cell can harbour Chlamydia and Toxoplasma. In co-infected cells, Toxoplasma is able to divert the content of host organelles, such as cholesterol. Consequently, the infectious cycle of Toxoplasma progresses unimpeded. In contrast, Chlamydia's ability to scavenge selected nutrients is diminished, and the bacterium shifts to a stress-induced persistent growth. Parasite killing engenders an ordered return to normal chlamydial development. We demonstrate that C. trachomatis enters a stress-induced persistence phenotype as a direct result from being barred from its normal nutrient supplies as addition of excess nutrients, e.g. amino acids, leads to substantial recovery of Chlamydia growth and infectivity. Co-infection of C. trachomatis with slow growing strains of Toxoplasma or a mutant impaired in nutrient acquisition does not restrict chlamydial development. Conversely, Toxoplasma growth is halted in cells infected with the highly virulent Chlamydia psittaci. This study illustrates the key role that cellular remodelling plays in the exploitation of host intracellular resources by Toxoplasma and Chlamydia. It further highlights the delicate balance between success and failure of infection by intracellular pathogens in a co-infection system at the cellular level.

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.

Chlamydia trachomatis vaccine research through the years

Chlamydia trachomatis is a Gram-negative obligate intracellular bacterium. It is the leading cause of bacterial sexual transmitted infections (STIs). World Health Organization figures estimated that over 90 million new cases of genital C. trachomatis infections occur worldwide each year. A vaccination program is considered to be the best approach to reduce the prevalence of C. trachomatis infections, as it would be much cheaper and have a greater impact on controlling C. trachomatis infections worldwide rather than a screening program or treating infections with antibiotics. Currently, there are no vaccines available which effectively protect against a C. trachomatis genital infection despite the many efforts that have been made throughout the years. In this paper, the many attempts to develop a protective vaccine against a genital C. trachomatis infection will be reviewed.

Chlamydia pneumoniae survival in macrophages is regulated by free Ca2+ dependent reactive nitrogen and oxygen species

OBJECTIVES

Despite an efficient macrophage immune capability, Chlamydia pneumoniae infects host cells and causes chronic diseases. To gain better insights into C. pneumoniae survival mechanisms in macrophages, its growth in regular RAW-264.7 cells (nitric oxide sufficient NO (+)) and RAW-264.7 cells (nitric oxide insufficient NO (-)) were studied.

METHODS

Role of Ca(2+), NO and reactive oxygen species (ROS) during C. pneumoniae infection in macrophages were determined.

RESULTS

RAW-264.7 NO (-) cells supported significantly Chlamydia growth, showing an upregulation of ROS, superoxide dismutase (SOD) and catalase activities as compared with RAW-264.7 NO (+) cell. Ascorbic acid, inducible nitric oxide synthase inhibitor and glutathione significantly prompted Chlamydia inclusion formation. Cytosolic Ca(2+) had regulatory effect on organism growth, NO generation, SOD and catalase activities in both cell types.

CONCLUSIONS

These findings suggest that minimal Ca(2+) signaling in macrophages at early stages of infection, NO and ROS release have modulatory effects onC. pneumoniae survival, onset of persistence and chronicity, processes which are needed for the initiation of diseases in which C. pneumoniae has been implicated as a possible etiologic agent.

Optimal development of Chlamydophila psittaci in L929 fibroblast and BGM epithelial cells requires the participation of microfilaments and microtubule-motor proteins

The cytoskeleton is involved in several cellular activities, including internalization and transport of foreign particles. Although particular functions to each cytoskeleton component have been described, interactions between those components seem to occur. The involvement of the different host cell cytoskeletal components in uptake and development of Chlamydophila psittaci is incompletely understood. In this study, the participation of the microfilament network along with the kinesin and dynein microtubule motor proteins in the internalization and further development of Chlamydophila psittaci were investigated in L929 fibroblast and BGM epithelial cells. Cytochalasin D disruption of actin filaments, and blockage of the motor proteins through the introduction of monoclonal antibodies into the host cells were carried out, either single or combined, at different moments around bacterial inoculation, and Chlamydophila infectivity determined 24 h post- inoculation by direct immunofluorescence. Our results show that, although Chlamydophila Ipsittaci can make use of both microfilament-dependent and independent entry pathways in both cell types, Chlamydophila internalization and development in the fibroblast cells mainly concerned processes mediated by microfilaments while in the epithelial cells mechanisms that require microtubule motor proteins were the ones predominantly involved. Evidence that mutual participation of the actin and tubulin networks in both host cells are required for optimal growth of Chlamydophila psittaci is also presented.

Significance of host cell kinesin in the development of Chlamydia psittaci

The influence of the microtubule-associated motor protein kinesin on Chlamydia psittaci inclusion development in epithelial and fibroblast cell lines was addressed. Kinesin was blocked early after chlamydial internalization (4 h postinfection [p.i.]) and before the initiation of active chlamydial multiplication (8 h p.i.). Chlamydia development was monitored by fluorescence and transmission electron microscopy at different times during the cycle. In both host cell lines, kinesin blockage restricted mitochondria from the chlamydial vacuole. The effects of kinesin blockage on the C. psittaci replication cycle included the presence of multiple inclusions up to late in the cycle, the presence of enlarged pleomorphic reticulate bodies, and a delayed reappearance of elementary bodies. The last effect seems to be greater when kinesin is blocked early after infection. Our results show that kinesin activity is required for optimal development of these microorganisms, most probably acting through the apposition of mitochondria to the C. psittaci inclusions.

Cytokine release by ovine macrophages following infection with Chlamydia psittaci

Chlamydia psittaci is an obligate intracellular pathogen that causes abortion in both sheep and humans. The disease in sheep (but not humans) is characterized by a long-term persistent phase that appears to be under the control of interferon-gamma. However, nothing is known about cytokine induction that precedes the persistent phase in sheep. Primary alveolar lavage cells recovered from normal adult sheep were used to study cytokine production in the first 72 h of infection with C. psittaci. These cells were phenotypically characteristic of macrophages, being adherent, phagocytic, CD14+ and staining positive for non-specific esterase. In vitro infection of the macrophages with C. psittaci resulted in the release of IL-1beta, IL-8 and granulocyte-macrophage colony-stimulating factor (GM-CSF) as measured by ovine-specific ELISAs. Heat-treated chlamydiae (1 h at 65 degrees C) did not induce the release of IL-1beta, but the release of IL-8 was similar to that induced by untreated organisms. The cells from different sheep varied most notably in their patterns of GM-CSF release in response to heat-treated and untreated organisms.

Obligate intracellular parasites: Rickettsia prowazekii and Chlamydia trachomatis

Transitions to obligate intracellular parasitism have occurred at numerous times in the evolutionary past. The genome sequences of two obligate intracellular parasites, Rickettsia prowazekii and Chlamydia trachomatis, were published last year. A comparative analysis of these two genomes has revealed examples of reductive convergent evolution, such as a massive loss of genes involved in biosynthetic functions. In addition, both genomes were found to encode transport systems for ATP and ADP, not otherwise found in bacteria. Here, we discuss adaptations to intracellular habitats by comparing the information obtained from the recently published genome sequences of R. prowazekii and C. trachomatis.