In vitro studies of Rickettsia-host cell interactions: Confocal laser scanning microscopy of Rickettsia helvetica-infected eukaryotic cell lines

Rickettsia helvetica in C3H/HeN mice: A model for studying pathogen-host interactions

An infection with the tick-borne Rickettsia helvetica has been associated with a broad spectrum of clinical manifestations in humans, but patients are only seldomly reported. Understanding its disease etiology necessitates well-stablished infection models, improving to recognize and diagnose patients with R. helvetica infection and facilitating the development of effective control strategies. In this study, we used C3H/HeN mice as a model to establish R. helvetica infection, achieving a high infection prevalence (89-100 %). While the liver and the spleen DNA consistently tested positive for infection in all challenged mice, additional infected organs included the kidneys, heart, and the lungs. Notably, a low prevalence of infection was observed in I. ricinus nymphs fed on R. helvetica-challenged mice. In addition, larvae were refractory to infection, suggesting that ticks exhibit low susceptibility to the pathogen. To the best of our knowledge, this is the first study of an animal model for R. helvetica infection. It serves as a valuable tool for advancing research on the interactions among the bacterium and its vertebrate host.

Spotted Fever Group Rickettsiae in Ticks and Small Mammals from Grassland and Forest Habitats in Central Germany

Rickettsiae of the spotted fever group (SFG) are zoonotic tick-borne pathogens. Small mammals are important hosts for the immature life stages of two of the most common tick species in Europe, Ixodes ricinus and Dermacentor reticulatus. These hosts and vectors can be found in diverse habitats with different vegetation types like grasslands and forests. To investigate the influence of environmental and individual factors on Rickettsia prevalence, this study aimed to analyse the prevalence of SFG rickettsiae in ticks and small mammals in different small-scale habitats in central Germany for the first time. Small mammals of ten species and ticks of two species were collected from grasslands and forests in the Hainich-Dün region, central Germany. After species identification, DNA samples from 1098 ticks and ear snips of 1167 small mammals were screened for Rickettsia DNA by qPCR targeting the gltA gene. Positive samples were retested by conventional PCR targeting the ompB gene and sequencing. Rickettsia DNA was detected in eight out of ten small mammal species. Small mammal hosts from forests (14.0%) were significantly more often infected than those from grasslands (4.4%) (p < 0.001). The highest prevalence was found in the mostly forest-inhabiting genus Apodemus (14.8%) and the lowest in Microtus (6.6%), which inhabits grasslands. The prevalence was higher in D. reticulatus (46.3%) than in the I. ricinus complex (8.6%). Adult ticks were more often infected than nymphs (p = 0.0199). All sequenced rickettsiae in I. ricinus complex ticks were R. helvetica, and the ones in D. reticulatus were R. raoultii. Unlike adults, questing nymphs have had only one blood meal, which explains the higher prevalence in I. ricinus adults. Interestingly, habitat type did influence infection probability in small mammals, but did not in ticks. A possible explanation may be the high prevalence in Apodemus flavicollis and A. sylvaticus which were more abundant in the forest.

High Prevalence of Rickettsia raoultii Found in Dermacentor Ticks Collected in Barnaul, Altai Krai, Western Siberia

The prevalence of the tick-borne spotted fever group rickettsioses pathogens in ticks collected in Barnaul, the administrative center of Altai Krai, Western Siberia, was studied. The causative agent of tick-borne lymphadenopathy (TIBOLA) Rickettsia raoultii was revealed to be present in 61.9% of the samples from Dermacentor ticks. Moreover, Rickettsia helvetica has been identified in 5.1% of Ixodes ticks.

Signatures in in vitro infection of NSC-34 mouse neurons and their cell nucleus with Rickettsia helvetica

BACKGROUND

Rickettsia helvetica, a spotted fever rickettsia, is transmitted to humans via ticks in Europe, North Africa, and Asia. The central nervous system is a crucial target for rickettsial diseases, which has been reported for 12 of the 31 species, of which R. helvetica is one. This study aimed, in an experimental model, to identify characteristics of R. helvetica infection in a mouse neuronal cell line, NSC-34.

RESULTS

NSC-34, a fusion cell line of mouse motor spinal cord neurons and neuroblastoma cells, was used as a model. Propagation of R. helvetica in neurons was confirmed. Short actin tails were shown at the polar end of the bacteria, which makes it likely that they can move intracellularly, and even spread between cells. Another protein, Sca4, which with the cell adhesion protein vinculin enables the passage of the cell membrane, was expressed during infection. No significant increase in TNFα levels was seen in the infected neurons, which is of interest because TNFα protects the host cell from infection-induced apoptotic death which is crucial for host cell survival. The bacteria were also shown to invade and grow in the cell nucleus of the neuron.

CONCLUSIONS

The findings suggest that a R. helvetica infection may be harmful to NSC-34 neurons under these in vitro conditions, but the full effects of the infection on the cell need to be studied further, also on human neurons, to also understand the possible significance of this infection in relation to pathogenetic mechanisms.

Pathogenicity and virulence of Rickettsia

Rickettsiae include diverse Gram-negative microbial species that exhibit obligatory intracellular lifecycles between mammalian hosts and arthropod vectors. Human infections with arthropod-borne Rickettsia continue to cause significant morbidity and mortality as recent environmental changes foster the proliferation of arthropod vectors and increased exposure to humans. However, the technical difficulties in working with Rickettsia have delayed our progress in understanding the molecular mechanisms involved in rickettsial pathogenesis and disease transmission. Recent advances in developing genetic tools for Rickettsia have enabled investigators to identify virulence genes, uncover molecular functions, and characterize host responses to rickettsial determinants. Therefore, continued efforts to determine virulence genes and their biological functions will help us understand the underlying mechanisms associated with arthropod-borne rickettsioses.

Coinfection of Two Rickettsia Species in a Single Tick Species Provides New Insight into Rickettsia-Rickettsia and Rickettsia-Vector Interactions

Rickettsiae are obligate intracellular bacteria that can cause life-threatening illnesses. There is an ongoing debate as to whether established infections by one Rickettsia species preclude the maintenance of the second species in ticks. Here, we identified two Rickettsia species in inoculum from Haemaphysalis montgomeryi ticks and subsequently obtained pure isolates of each species by plaque selection. The two isolates were classified as a transitional group and spotted fever group rickettsiae and named Rickettsia hoogstraalii str CS and Rickettsia rhipicephalii str EH, respectively. The coinfection of these two Rickettsia species was detected in 25.6% of individual field-collected H. montgomeryi. In cell culture infection models, R. hoogstraalii str CS overwhelmed R. rhipicephalii str EH with more obvious cytopathic effects, faster plaque formation, and increased cellular growth when cocultured, and R. hoogstraalii str CS seemed to polymerize actin tails differently from R. rhipicephalii str EH in vitro. This work provides a model to investigate the mechanisms of both Rickettsia-Rickettsia and Rickettsia-vector interactions. IMPORTANCE The rickettsiae are a group of obligate intracellular Gram-negative bacteria that include human pathogens causing an array of clinical symptoms and even death. There is an important question in the field, that is whether one infection can block the superinfection of other rickettsiae. This work demonstrated the coinfection of two Rickettsia species in individual ticks and further highlighted that testing the rickettsial competitive exclusion hypothesis will undoubtedly be a promising area as methods for bioengineering and pathogen biocontrol become amenable for rickettsiae.

Procedure for spotted fever group Rickettsia isolation from limited clinical blood specimens

BACKGROUND

Current isolation techniques for spotted fever group Rickettsia from clinical samples are laborious and are limited to tissue, blood and blood derivatives with volumes ideally greater than 1 mL. We validated the use of simplified methodologies for spotted fever group Rickettsia culture isolation that overcome sample volume limitations and provide utility in clinical diagnostics and research studies.

METHODOLOGY/PRINCIPAL FINDINGS

A modified cell culture method is evaluated for the isolation of Rickettsia ssp. from human diagnostic samples. Culture sampling method, culture platform, and growth phase analysis were evaluated to determine best practices for optimal culture isolation conditions. Rickettsial isolates (R. conorii, R. rickettsii, and R. parkeri) were grown in Vero E6 cells over a course of 5 to 7 days at low inoculum treatments (~40 bacterial copies) to standardize the sampling strategy at a copy number reflective of the bacteremia in acute diagnostic samples. This methodology was verified using small volumes (50 μL) of 25 unprocessed clinical whole blood, plasma, and serum samples from acute samples of patients suspected of having Rocky Mountain Spotted Fever, of which 10 were previously confirmed positive via the PanR8 qPCR assay, 13 had no detectable Rickettsia DNA by the PanR8 qPCR assay, and 2 were not previously tested; these samples resulted in the cultivation of 7 new R. rickettsii isolates.

CONCLUSIONS/SIGNIFICANCE

We observed that rickettsial isolate growth in culture is reproducibly identified by real-time PCR testing of culture media within 72 hours after inoculation. Additionally, specimen sedimentation prior to isolation to remove red blood cells was found to decrease the amount of total organism available in the inoculum. A small volume culture method was established focusing on comparative qPCR detection rather than bacterial visualization, taking significantly shorter time to detect, and requiring less manipulation compared to traditional clinical isolate culture methods.

Detection of potentially pathogenic bacteria from Ixodes ricinus carried by pets in Tuscany, Italy

Background

Ticks are vectors of disease-causing pathogens that pose a serious threat to animals and people. Dogs and cats are exposed to tick infestation in multiple ways and can easily transport infected ticks into domestic environments and potentially transfer them to people. Pet owners are at increased risk of picking up ticks from their pets and developing tickborne diseases. This study aims to detect the presence of pathogens of potential public health interest in ticks removed from cats and dogs in Tuscany, Italy.

Methods

The collected ticks were screened for the presence of protozoan (Theileria species and Babesia species) and bacterial (Rickettsia species, Anaplasma species, Ehrlichia species, Chlamydia species, Bartonella species and Coxiella burnetii) pathogens using PCR.

Results

PCR and sequencing analysis revealed that 3 per cent of the ticks were PCR-positive for the presence of Rickettsia helvetica DNA, 5 per cent of ticks were PCR-positive for Bartonella henselae DNA, and 46 per cent of ticks were PCR-positive for Chlamydia psittaci and Chlamydia abortus DNA. None of the examined ticks was PCR-positive for Theileria species, Babesia species, Anaplasma species, Ehrlichia canis or Coxiella burnetii DNA.

Conclusion

The results of this preliminary study highlight the importance of monitoring companion animals as indicators to evaluate the health status of their owners. Preventive measures are necessary to limit the spread of zoonotic pathogens from companion animals to people within the home environment.

Growth Characteristics of Rickettsia Species LON Strains Closely Related to Rickettsia japonica Isolated from Haemaphysalis longicornis Ticks in Mouse Derived L929 and Human-Derived THP-1 Host Cell Lines

Non-pathogenic Rickettsia species LON strains closely related to an agent of Japanese spotted fever (JSF), R. japonica, were isolated in Japan from Haemaphysalis longicornis ticks in 2001. However, the biological properties of LONs in mammalian host cells are poorly understood. In this study, microscopic analysis showed that LONs in a mouse-derived L929 host cell line were rod shaped with sizes of 0.3-0.5 × 0.5-2.0 μm. Molecular analysis revealed the existence of a LON-specific disrupted open reading frame in R. japonica-related group-specific DNA regions. Growth kinetics of LON-2 and LON-13 strains analyzed by a quantitative real-time PCR showed 100-fold or more increment of LONs cultured in L929 host cells at 30°C and slightly less increment at 33°C, and 25-fold increment in human-derived THP-1 host cells at 35°C on day 7 (168 h) post infection. The generation times of the two LON strains cultured in L929 and THP-1 were estimated to be 9.4-12.9 h and 9.6-10.9 h, respectively. To our knowledge, this is the first report on the biological characteristics of Rickettsia sp. LON strains in mammalian cells, which may provide significant information for the experimental approaches for other rickettsiae.

Host Epigenetics in Intracellular Pathogen Infections

Some intracellular pathogens are able to avoid the defense mechanisms contributing to host epigenetic modifications. These changes trigger alterations tothe chromatin structure and on the transcriptional level of genes involved in the pathogenesis of many bacterial diseases. In this way, pathogens manipulate the host cell for their own survival. The better understanding of epigenetic consequences in bacterial infection may open the door for designing new vaccine approaches and therapeutic implications. This article characterizes selected intracellular bacterial pathogens, including Mycobacterium spp., Listeria spp., Chlamydia spp., Mycoplasma spp., Rickettsia spp., Legionella spp. and Yersinia spp., which can modulate and reprogram of defense genes in host innate immune cells.

Counterattacking the tick bite: towards a rational design of anti-tick vaccines targeting pathogen transmission

Hematophagous arthropods are responsible for the transmission of a variety of pathogens that cause disease in humans and animals. Ticks of the Ixodes ricinus complex are vectors for some of the most frequently occurring human tick-borne diseases, particularly Lyme borreliosis and tick-borne encephalitis virus (TBEV). The search for vaccines against these diseases is ongoing. Efforts during the last few decades have primarily focused on understanding the biology of the transmitted viruses, bacteria and protozoans, with the goal of identifying targets for intervention. Successful vaccines have been developed against TBEV and Lyme borreliosis, although the latter is no longer available for humans. More recently, the focus of intervention has shifted back to where it was initially being studied which is the vector. State of the art technologies are being used for the identification of potential vaccine candidates for anti-tick vaccines that could be used either in humans or animals. The study of the interrelationship between ticks and the pathogens they transmit, including mechanisms of acquisition, persistence and transmission have come to the fore, as this knowledge may lead to the identification of critical elements of the pathogens' life-cycle that could be targeted by vaccines. Here, we review the status of our current knowledge on the triangular relationships between ticks, the pathogens they carry and the mammalian hosts, as well as methods that are being used to identify anti-tick vaccine candidates that can prevent the transmission of tick-borne pathogens.