Propagation of arthropod-borne Rickettsia spp. in two mosquito cell lines

Water and mosquitoes as key components of the infective cycle of Francisella tularensis in Europe: a review

Francisella tularensis is the pathogen of tularemia, a zoonotic disease that have a broad range of hosts. Its epidemiology is related to aquatic environments, particularly in the subspecies holarctica. In this review, we explore the role of water and mosquitoes in the epidemiology of Francisella in Europe. F. tularensis epidemiology has been linked to natural waters, where its persistence has been associated with biofilm and amebas. In Sweden and Finland, the European countries where most human cases have been reported, mosquito bites are a main route of transmission. F. tularensis is present in other European countries, but to date positive mosquitoes have not been found. Biofilm and amebas are potential sources of Francisella for mosquito larvae, however, mosquito vector capacity has not been demonstrated experimentally, with the need to be studied using local species to uncover a potential transmission adaptation. Transstadial, for persistence through life stages, and mechanical transmission, suggesting contaminated media as a source for infection, have been studied experimentally for mosquitoes, but their natural occurrence needs to be evaluated. It is important to clear up the role of different local mosquito species in the epidemiology of F. tularensis and their importance in all areas where tularemia is present.

Booklice Liposcelis bostrychophila naturally infected by Rickettsia felis cause fever and experimental pneumonia in mammals

BACKGROUND

Rickettsia felis is emergent in tropical areas. Despite its high morbidity, its natural history has not yet been fully determined. We investigated the role of the common household booklouse, Liposcelis bostrychophila, recently found to harbour R. felis.

METHODS

Blood samples from 372 febrile patients from Senegalese villages, as well as nasal and skin samples from 264 asymptomatic individuals, were tested for cat flea-associated and booklice-associated strains of R. felis. Dust samples from beds were collected to isolate booklice and R. felis. Mice were infected with aerosol of R. felis strain from naturally infected booklice.

RESULTS

Forty febrile patients (11%) were infected by R. felis, including 26 (7%) by the booklice-associated strain. Nine nasal samples (3.4%) and 28 skin samples (10.6%) contained R. felis, including seven and 24, respectively, with the booklice-associated strain. The presence of live L. bostrychophila was observed in 32 dust samples (16.8%); R. felis was identified in 62 dust samples (32.5%). Several mice samples were positive for R. felis; interstitial lymphohistiocytic infiltrates were identified in lungs.

CONCLUSIONS

L. bostrychophila may be a reservoir of R. felis. The booklice-associated strain is pathogenic in mammals causing pneumonia. Human infection may be acquired via inhalation of infected booklice particles.

Culture Isolate of Rickettsia felis from a Tick

Although the cat flea, Ctenocephalides felis, has been identified as the primary vector of Rickettsia felis, additional flea, tick, mite, and louse species have also been associated with this bacterium by molecular means; however, the role of these arthropods in the transmission of R. felis has not been clarified. Here, we succeeded in culture isolation of R. felis from a host-seeking castor bean tick, Ixodes ricinus, the most common tick in Slovakia. The bacterial isolation was performed on XTC-2 cells at 28 °C using the shell-vial technique. An evaluation of the growth properties was performed for both the XTC-2 and Vero cell lines. We observed R. felis in the infected host cells microscopically by Gimenez staining and immunofluorescence assay. The R. felis isolate was purified by gradient ultracentrifugation and visualized by electron microscopy. Fragments of the genes gltA, ompA, ompB, htrA, rpoB, sca4, rffE, and rrs were amplified and compared with the corresponding sequences of the type strain URRWXCal2 and other R. felis culture -isolated strains. We did not detect any nucleotide polymorphisms; however, plasmid pRFδ, characteristic of the standard strain, was absent in our isolate. Herein, we describe the first successful isolation and characterization of a tick-derived R. felis strain “Danube”, obtained from an I. ricinus nymph.

Regulator of Actin-Based Motility (RoaM) Downregulates Actin Tail Formation by Rickettsia rickettsii and Is Negatively Selected in Mammalian Cell Culture

The etiological agent of Rocky Mountain spotted fever, Rickettsia rickettsii, is an obligately intracellular pathogen that induces the polymerization of actin filaments to propel the bacterium through the cytoplasm and spread to new host cells. Cell-to-cell spread via actin-based motility is considered a key virulence determinant for spotted fever group rickettsiae, as interruption of sca2, the gene directly responsible for actin polymerization, has been shown to reduce fever in guinea pigs. However, little is known about how, or if, motility is regulated by the bacterium itself. We isolated a hyperspreading variant of R. rickettsii Sheila Smith that produces actin tails at an increased rate. A1G_06520 (roaM [regulator of actin-based motility]) was identified as a negative regulator of actin tail formation. Disruption of RoaM significantly increased the number of actin tails compared to the wild-type strain but did not increase virulence in guinea pigs; however, overexpression of RoaM dramatically decreased the presence of actin tails and moderated fever response. Localization experiments suggest that RoaM is not secreted, while reverse transcription-quantitative PCR (RT-qPCR) data show that various levels of RoaM do not significantly affect the expression of the known rickettsial actin-regulating proteins sca2, sca4, and rickA. Taken together, the data suggest a previously unrecognized level of regulation of actin-based motility in spotted fever group rickettsiae. Although this gene is intact in many isolates of spotted fever, transitional, and ancestral group Rickettsia spp., it is often ablated in highly passaged laboratory strains. Serial passage experiments revealed strong negative selection of roaM in Vero 76 cells.

Association between Growth Rate and Pathogenicity of Spotted Fever Group Rickettsia

Rickettsia parkeri and Rickettsia amblyommatis are spotted fever group Rickettsia (SFGR) associated with Amblyomma ticks. R. parkeri is a recognized human pathogen that causes an eschar-associated febrile illness, while R. amblyommatis has not been confirmed as a causative agent of human disease. We hypothesized that the rate of replication is one of the factors contributing to rickettsial pathogenicity. In this study, growth and infectivity of R. parkeri and R. amblyommatis in mammalian (Vero E6) and tick-derived (ISE6) cell lines were assessed and compared over a 96-hour time course of infection using quantitative real-time polymerase chain reaction and microscopy. The pathogenic R. parkeri displayed a significantly higher level of infection in both Vero E6 and ISE6 cells than R. amblyommatis at 72 hours post-inoculation (hpi). Distinct growth profiles between rickettsial species with known and uncertain pathogenicity were identified. R. parkeri burdens were significantly greater than those of R. amblyommatis from 24 to 96 hpi. The relative fold changes of load were significantly higher in the pathogenic agent than in R. amblyommatis from 48 hpi onward and reached the maximum fold increase of ~2002- and ~296-fold in Vero E6 cells and ~1363- and ~161-fold in ISE6 cells, respectively, at 96 hpi. The results from the present study demonstrate that growth rate is associated with the pathogenicity of rickettsiae. Understanding SFGR growth characteristics in mammalian and tick cells will provide insight into rickettsial biology and pathogenesis.

Bioinformatic and cell-based tools for pooled CRISPR knockout screening in mosquitos

Mosquito-borne diseases present a worldwide public health burden. Current efforts to understand and counteract them have been aided by the use of cultured mosquito cells. Moreover, application in mammalian cells of forward genetic approaches such as CRISPR screens have identified essential genes and genes required for host-pathogen interactions, and in general, aided in functional annotation of genes. An equivalent approach for genetic screening of mosquito cell lines has been lacking. To develop such an approach, we design a new bioinformatic portal for sgRNA library design in several mosquito genomes, engineer mosquito cell lines to express Cas9 and accept sgRNA at scale, and identify optimal promoters for sgRNA expression in several mosquito species. We then optimize a recombination-mediated cassette exchange system to deliver CRISPR sgRNA and perform pooled CRISPR screens in an Anopheles cell line. Altogether, we provide a platform for high-throughput genome-scale screening in cell lines from disease vector species.

Cells within cells: Rickettsiales and the obligate intracellular bacterial lifestyle

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

Molecular-based Survey of Rickettsia spp. and Coxiella burnetii in Mosquitoes (Diptera: Culicidae) from Fars Province, Southern Iran, during 2017-18

Objectives:

Since there have not been any studies on the roles of the Iranian mosquitoes in the transmission of Rickettsia spp. and Coxiella burneti, the present study investigates the roles of mosquitoes in the transmission of the pathogens using the PCR techniques for the first time in Iran.

Methods:

The present study was conducted in Fars province during the activity seasons of mosquitoes in 2017-18. The primer design was done to investigate the probability of mosquito’s contamination with Rickettsia spp. and Coxiella burnetii. The conventional PCR was used after the extraction of DNA from mosquitoes to study the contamination.

Results:

A total of 1103 adult mosquitoes were collected and identified. Among them, 3 genera and 11 species were identified, including Anopheles (25.74%), Culex (51.84%) and Culiseta (22.39%) genera. All tested mosquitoes were negative in terms of contamination to Rickettsia spp. and Coxiella burnetii.

Conclusion:

Based on the results, mosquitoes are not considered as vectors of Rickettsia spp. and Coxiella burnetii in this part of the country currently. Further studies on a larger scale are needed to examine the exact role of mosquitoes (as a possible vector with high abundance and mobility) in the transmission of these pathogens in tropical areas of Iran.

Wolbachia successfully replicate in a newly established horn fly, Haematobia irritans irritans (L.) (Diptera: Muscidae) cell line

BACKGROUND

Haematobia spp., horn flies (HF) and buffalo flies (BF), are economically important ectoparasites of dairy and beef cattle. Control of these flies relies mainly on treating cattle with chemical insecticides. However, the development of resistance to commonly used compounds is compromising the effectiveness of these treatments and alternative methods of control are required. Wolbachia are maternally transmitted endosymbiotic bacteria of arthropods that cause various reproductive distortions and fitness effects, making them a potential candidate for use in the biological control of pests. The first step towards this is the establishment and adaptation of xenobiotic infections of Wolbachia in target host cell lines.

RESULTS

Here, we report the successful establishment of a continuous HF cell line (HIE-18) from embryonic cells and its stable transinfection with Wolbachia strains wAlbB native to mosquitoes, and wMel and wMelPop native to Drosophila melanogaster. HIE-18 cells were typically round and diploid with ten chromosomes (2n = 10) or tetraploid with 20 chromosomes (4n = 20), with a doubling time of 67.2 h. Wolbachia density decreased significantly in HIE-18 cells in the first 48 h of infection, possibly due to overexpression of antimicrobial peptides through the Imd immune signalling pathway. However, density recovered after this time and HIE-18 cell lines stably infected with the three strains of Wolbachia have now each been subcultured more than 50 times as persistently infected lines.

CONCLUSION

The amenability of HF cells to infection with different strains of Wolbachia and the establishment of stable sustaining infections suggest the potential for use of Wolbachia in novel approaches for the control of Haematobia spp. Further, the availability of the HIE-18 cell line will provide an important resource for the study of genetics, host-parasite interactions and chemical resistance in Haematobia populations. © 2020 Society of Chemical Industry.

John H, Jarman RG, Hang J, Richards AL. Molecular characterization of novel mosquito-borne Rickettsia spp. from mosquitoes collected at the Demilitarized Zone of the Republic of Korea

Rickettsiae are associated with a diverse range of invertebrate hosts. Of these, mosquitoes could emerge as one of the most important vectors because of their ability to transmit significant numbers of pathogens and parasites throughout the world. Recent studies have implicated Anopheles gambiae as a potential vector of Rickettsia felis. Herein we report that a metagenome sequencing study identified rickettsial sequence reads in culicine mosquitoes from the Republic of Korea. The detected rickettsiae were characterized by a genus-specific quantitative real-time PCR assay and sequencing of rrs, gltA, 17kDa, ompB, and sca4 genes. Three novel rickettsial genotypes were detected (Rickettsia sp. A12.2646, Rickettsia sp. A12.2638 and Rickettsia sp. A12.3271), from Mansonia uniformis, Culex pipiens, and Aedes esoensis, respectively. The results underscore the need to determine the Rickettsia species diversity associated with mosquitoes, their evolution, distribution and pathogenic potential.

Extensive genetic diversity of Rickettsiales bacteria in multiple mosquito species

Rickettsiales are important zoonotic pathogens, causing severe disease in humans globally. Although mosquitoes are an important vector for diverse pathogens, with the exception of members of the genus Wolbachia little is known about their role in the transmission of Rickettsiales. Herein, Rickettsiales were identified by PCR in five species of mosquitoes (Anopheles sinensis, Armigeres subalbatus, Aedes albopictus, Culex quinquefasciatus and Cu. tritaeniorhynchus) collected from three Chinese provinces during 2014–2015. Subsequent phylogenetic analyses of the rrs, groEL and gltA genes revealed the presence of Anaplasma, Ehrlichia, Candidatus Neoehrlichia, and Rickettsia bacteria in mosquitoes, comprising nine documented and five tentative species bacteria, as well as three symbionts/endosybionts. In addition, bacteria were identified in mosquito eggs, larvae, and pupae sampled from aquatic environments. Hence, these data suggest that Rickettsiales circulate widely in mosquitoes in nature. Also of note was that Ehrlichia and Rickettsia bacteria were detected in each life stage of laboratory cultured mosquitoes, suggesting that Rickettsiales may be maintained in mosquitoes through both transstadial and transovarial transmission. In sum, these data indicate that mosquitoes may have played an important role in the transmission and evolution of Rickettsiales in nature.

Strategies for detecting rickettsiae and diagnosing rickettsial diseases

Rickettsial diseases and scrub typhus constitute a group of the oldest known vector-borne diseases. The cosmopolitan distribution of the vectors that transmit rickettsiae and orientiae leads to a worldwide prevalence of these diseases. Despite their significant historical status, detection and diagnosis of these diseases are still evolving today. Serological methods remain among the most prevalent techniques used for the detection/diagnosis of rickettsial diseases and scrub typhus. Molecular techniques have been instrumental in increasing the sensitivity/specificity of diagnosis, identifying new Rickettsia and Orientia species and have enhanced epidemiological capabilities when used in combination with serological methods. In this review, we discuss these techniques and their associated pros and cons.

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.

Cultivation of Rickettsia amblyommii in tick cells, prevalence in Florida lone star ticks (Amblyomma americanum)

Background

Rickettsia amblyommii is a bacterium in the spotted fever group of organisms associated with the lone star tick (LST), Amblyomma americanum. The LST is the most commonly reported tick to parasitize humans in the southeastern US. Within this geographic region, there have been suspected cases of Rocky Mountain spotted fever (RMSF) where the causative agent, R. rickettsii, was not identified in the local tick population. In these areas, patients with clinical signs of RMSF had low or no detectable antibodies to R. rickettsii, resulting in an inability to confirm a diagnosis.

Methods

R. amblyommii was cultivated from host-seeking LSTs trapped in Central Florida and propagated in ISE6 (Ixodes scapularis) and AAE2 (A. americanum) cells. Quantitative PCR targeting the 17-kD gene of Rickettsia spp. identified the genus of the organism in culture. Variable regions of groEL, gtlA and rompA genes were amplified and sequenced to confirm the species. The prevalence of R. amblyommii in LSTs within the geographic region was determined by qPCR followed by conventional PCR and direct sequencing.

Results

Analyses of amplified sequences from the cultured organism were 100% homologous to R. amblyommii. The overall prevalence of Rickettsia spp. in the local population of LSTs was 57.1% and rompA sequence analysis identified only R. amblyommii in LSTs.

Conclusions

A Florida strain of R. amblyommii was successfully cultivated in two tick cell lines. Further evaluation of the new strain and comparisons to the other geographic strains is needed. The prevalence of this SFG organism in the tick population warrants further investigation into the organism’s ability to cause clinical disease in mammalian species.

Tropism and pathogenicity of rickettsiae

Rickettsiae are obligate intracellular parasitic bacteria that cause febrile exanthematous illnesses such as Rocky Mountain spotted fever, Mediterranean spotted fever, epidemic, and murine typhus, etc. Although the vector ranges of each Rickettsia species are rather restricted; i.e., ticks belonging to Arachnida and lice and fleas belonging to Insecta usually act as vectors for spotted fever group (SFG) and typhus group (TG) rickettsiae, respectively, it would be interesting to elucidate the mechanisms controlling the vector tropism of rickettsiae. This review discusses the factors determining the vector tropism of rickettsiae. In brief, the vector tropism of rickettsiae species is basically consistent with their tropism toward cultured tick and insect cells. The mechanisms responsible for rickettsiae pathogenicity are also described. Recently, genomic analyses of rickettsiae have revealed that they possess several genes that are homologous to those affecting the pathogenicity of other bacteria. Analyses comparing the genomes of pathogenic and non-pathogenic strains of rickettsiae have detected many factors that are related to rickettsial pathogenicity. It is also known that a reduction in the rickettsial genome has occurred during the course of its evolution. Interestingly, Rickettsia species with small genomes, such as Rickettsia prowazekii, are more pathogenic to humans than those with larger genomes. This review also examines the growth kinetics of pathogenic and non-pathogenic species of SFG rickettsiae (SFGR) in mammalian cells. The growth of non-pathogenic species is restricted in these cells, which is mediated, at least in part, by autophagy. The superinfection of non-pathogenic rickettsiae-infected cells with pathogenic rickettsiae results in an elevated yield of the non-pathogenic rickettsiae and the growth of the pathogenic rickettsiae. Autophagy is restricted in these cells. These results are discussed in this review.

An update on the detection and treatment of Rickettsia felis

Rickettsia felis was described as a human pathogen almost two decades ago, and human infection is currently reported in 18 countries in all continents. The distribution of this species is worldwide, determined by the presence of the main arthropod vector, Ctenocephalides felis (Bouché). The list of symptoms, which includes fever, headache, myalgia, and rash, keeps increasing as new cases with unexpected symptoms are described. Moreover, the clinical presentation of R. felis infection can be easily confused with many tropical and nontropical diseases, as well as other rickettsial infections. Although specific laboratory diagnosis and treatment for this flea-borne rickettsiosis are detailed in the scientific literature, it is possible that most human cases are not being diagnosed properly. Furthermore, since the cat flea infests different common domestic animals, contact with humans may be more frequent than reported. In this review, we provide an update on methods for specific detection of human infection by R. felis described in the literature, as well as the treatment prescribed to the patients. Considering advances in molecular detection tools, as well as options for as-yet-unreported isolation of R. felis from patients in cell culture, increased diagnosis and characterization of this emerging pathogen is warranted.

Tryptose phosphate broth improves Rickettsia felis replication in mammalian cells

In cell culture, Rickettsia felis grows only at low temperatures (< 31 °C). Therefore, its ability to enter, survive and grow in cell lines has primarily been tested in cells derived from amphibians and arthropods, which naturally grow at low temperatures, and only infrequently in mammalian cells. We subcultured R. felis in mammalian cells for more than 10 passages using media supplemented with tryptose phosphate broth (TPB) and found that TPB is critical for optimal growth of R. felis in mammalian cells.

In vitro propagation of Candidatus Rickettsia andeanae isolated from Amblyomma maculatum

Candidatus Rickettsia andeanae was identified during an investigation of a febrile outbreak in northwestern Peru (2002). DNA sequencing from two ticks (Amblyomma maculatum, Ixodes boliviensis) collected during the investigation revealed a novel Rickettsia agent with similarity to the spotted fever group rickettsiae. Since then, Candidatus R. andeanae has been detected in A. maculatum ticks collected in the southeastern and southcentral United States, Argentina, and Peru. To date, Candidatus R. andeanae has not been successfully cultivated in the laboratory. We present evidence for the continuous cultivation in three cell lines of Candidatus R. andeanae isolated from an A. maculatum tick (Portsmouth, Virginia).

Distribution and infection frequency of 'Candidatus Rickettsia amblyommii' in Maryland populations of the lone star tick (Amblyomma americanum) and culture in an Anopheles gambiae mosquito cell line

Amblyomma americanum (the lone star tick) is a broadly distributed tick that transmits multiple pathogens of humans and domestic animals. 'Candidatus Rickettsia amblyommii' is a spotted-fever group rickettsial species that is potentially associated with human disease. In 2008 and 2009, we assayed over 500 unfed adult ticks from 19 Maryland populations for the presence of 'Candidatus R. amblyommii'. Infection frequencies ranged from 33% to 100%, with an average infection rate of 60% in 2008 and 69% in 2009. Infection frequencies did not differ statistically between sexes. To develop a system in which to study 'Candidatus R. amblyommii' in the laboratory, we used a cell line developed from Anopheles gambiae mosquitoes (Sua5B) to isolate and culture 'Candidatus R. amblyommii' from field-collected A. americanum ticks from 2 localities in Maryland. After infection, Sua5B cells were infected for more than 40 passages. Infection was confirmed by Rickettsia-specific PCR, gene sequencing, and Rickettsia-specific fluorescence in situ hybridization (FISH). These data show that 'Candidatus R. amblyommii' is widespread in Maryland A. americanum populations and that Sua5B cells are a useful tool for culturing Rickettsia infections from wild ticks.

Wolbachia infections in Anopheles gambiae cells: transcriptomic characterization of a novel host-symbiont interaction

The endosymbiotic bacterium Wolbachia is being investigated as a potential control agent in several important vector insect species. Recent studies have shown that Wolbachia can protect the insect host against a wide variety of pathogens, resulting in reduced transmission of parasites and viruses. It has been proposed that compromised vector competence of Wolbachia-infected insects is due to up-regulation of the host innate immune system or metabolic competition. Anopheles mosquitoes, which transmit human malaria parasites, have never been found to harbor Wolbachia in nature. While transient somatic infections can be established in Anopheles, no stable artificially-transinfected Anopheles line has been developed despite numerous attempts. However, cultured Anopheles cells can be stably infected with multiple Wolbachia strains such as wAlbB from Aedes albopictus, wRi from Drosophila simulans and wMelPop from Drosophila melanogaster. Infected cell lines provide an amenable system to investigate Wolbachia-Anopheles interactions in the absence of an infected mosquito strain. We used Affymetrix GeneChip microarrays to investigate the effect of wAlbB and wRi infection on the transcriptome of cultured Anopheles Sua5B cells, and for a subset of genes used quantitative PCR to validate results in somatically-infected Anopheles mosquitoes. Wolbachia infection had a dramatic strain-specific effect on gene expression in this cell line, with almost 700 genes in total regulated representing a diverse array of functional classes. Very strikingly, infection resulted in a significant down-regulation of many immune, stress and detoxification-related transcripts. This is in stark contrast to the induction of immune genes observed in other insect hosts. We also identified genes that may be potentially involved in Wolbachia-induced reproductive and pathogenic phenotypes. Somatically-infected mosquitoes had similar responses to cultured cells. The data show that Wolbachia has a profound and unique effect on Anopheles gene expression in cultured cells, and has important implications for mechanistic understanding of Wolbachia-induced phenotypes and potential novel strategies to control malaria.

Transinfection and growth discrepancy of Drosophila Wolbachia strain wMel in cell lines of the mosquito Aedes albopictus

AIM

The Wolbachia strain wMel can protect Drosophila melanogaster against pathogenic RNA viruses. To analyse the potential of this inhibitory effect against arboviruses vectorized by these mosquitoes, we here first transinfected the Aedes albopictus Aa23 and C6/36 cell lines with the Wolbachia strain wMel and then monitored their infection dynamics.

METHODS AND RESULTS

Wolbachia strain wMel was transferred into A. albopictus Aa23 and C6/36 cell lines using the shell vial technique. The presence of the bacterium in the transinfected cells was monitored by quantitative PCR and fluorescence in situ hybridization. Bacteria could be detected in the cytoplasm of both the Aa23 and C6/36 cell lines. However, the dynamics and stability of the bacterial infection differed depending on the initial cell background. The Aa23 cell line, which had been treated with a tetracycline antibiotic 2 years previously to eliminate its natural Wolbachia wAlbB-infecting strain, lost the introduced Wolbachia wMel strain after 12 passages postinfection. In contrast, the C6/36 cell line, which had originally been aposymbiotic, displayed a stable infection with Wolbachia wMel. The bacterial density in C6/36 was greater than that of the A. albopictus RML12 cell line from which the wMel strain had originated.

CONCLUSIONS

Transient or persistent transinfection of A. albopictus Aa23 and C6/36 cell lines with Wolbachia wMel strain was achieved. The results indicate the influence of the genetic background of mosquito cells in maintaining Wolbachia originating from a distant dipteral host.

SIGNIFICANCE AND IMPACT OF THE STUDY

The cell model built here can now be used to investigate the viral inhibitory effect of the Wolbachia wMel strain against arboviruses such as dengue and chikungunya, which are transmitted by the mosquito A. albopictus.

Ecology of Rickettsia felis: a review

It has been two decades since the first description of Rickettsia felis, and although a nearly cosmopolitan distribution is now apparent, much of the ecology of this unique microorganism remains unresolved. The cat flea, Ctenocephalides felis, is currently the only known biological vector of R. felis; however, molecular evidence of R. felis in other species of fleas as well as in ticks and mites suggests a variety of arthropod hosts. Studies examining the transmission of R. felis using colonized cat fleas have shown stable vertical transmission but not horizontal transmission. Likewise, serological and molecular tools have been used to detect R. felis in a number of vertebrate hosts, including humans, in the absence of a clear mechanism of horizontal transmission. Considered an emerging flea-borne rickettsiosis, clinical manifestation of R. felis infection in humans, including, fever, rash, and headache is similar to other rickettsial diseases. Recent advances toward further understanding the ecology of R. felis have been facilitated by stable R. felis-infected cat flea colonies, several primary flea isolates and sustained maintenance of R. felis in cell culture systems, and highly sensitive quantitative molecular assays. Here, we provide a synopsis of R. felis including the known distribution and arthropods infected; transmission mechanisms; current understanding of vertebrate infection and human disease; and the tools available to further examine R. felis.

Characterization and growth of polymorphic Rickettsia felis in a tick cell line

Morphological differentiation in some arthropod-borne bacteria is correlated with increased bacterial virulence, transmission potential, and/or as a response to environmental stress. In the current study, we utilized an in vitro model to examine Rickettsia felis morphology and growth under various culture conditions and bacterial densities to identify potential factors that contribute to polymorphism in rickettsiae. We utilized microscopy (electron microscopy and immunofluorescence), genomic (PCR amplification and DNA sequencing of rickettsial genes), and proteomic (Western blotting and liquid chromatography-tandem mass spectrometry) techniques to identify and characterize morphologically distinct, long-form R. felis. Without exchange of host cell growth medium, polymorphic R. felis was detected at 12 days postinoculation when rickettsiae were seeded at a multiplicity of infection (MOI) of 5 and 50. Compared to short-form R. felis organisms, no change in membrane ultrastructure in long-form polymorphic rickettsiae was observed, and rickettsiae were up to six times the length of typical short-form rickettsiae. In vitro assays demonstrated that short-form R. felis entered into and replicated in host cells faster than long-form R. felis. However, when both short- and long-form R. felis organisms were maintained in cell-free medium for 12 days, the infectivity of short-form R. felis was decreased compared to long-form R. felis organisms, which were capable of entering host cells, suggesting that long-form R. felis is more stable outside the host cell. The relationship between rickettsial polymorphism and rickettsial survivorship should be examined further as the yet undetermined route of horizontal transmission of R. felis may utilize metabolically and morphologically distinct forms for successful transmission.