Nonselective Persistence of a Rickettsia conorii Extrachromosomal Plasmid during Mammalian Infection.

Scientific analysis of the genus Rickettsia is undergoing a rapid period of change with the emergence of viable genetic tools. The development of these tools for the mutagenesis of pathogenic bacteria will permit forward genetic analysis of Rickettsia pathogenesis. Despite these advances, uncertainty still remains regarding the use of plasmids to study these bacteria in in vivo mammalian models of infection, namely, the potential for virulence changes associated with the presence of extrachromosomal DNA and nonselective persistence of plasmids in mammalian models of infection. Here, we describe the transformation of Rickettsia conorii Malish 7 with the plasmid pRam18dRGA[AmTrCh]. Transformed R. conorii stably maintains this plasmid in infected cell cultures, expresses the encoded fluorescent proteins, and exhibits growth kinetics in cell culture similar to those of nontransformed R. conorii. Using a well-established murine model of fatal Mediterranean spotted fever, we demonstrate that R. conorii(pRam18dRGA[AmTrCh]) elicits the same fatal outcomes in animals as its untransformed counterpart and, importantly, maintains the plasmid throughout infection in the absence of selective antibiotic pressure. Interestingly, plasmid-transformed R. conorii was readily observed both in endothelial cells and within circulating leukocytes. Together, our data demonstrate that the presence of an extrachromosomal DNA element in a pathogenic rickettsial species does not affect either in vitro proliferation or in vivo infectivity in models of disease and that plasmids such as pRam18dRGA[AmTrCh] are valuable tools for the further genetic manipulation of pathogenic rickettsiae.

Identification of host proteins involved in rickettsial invasion of tick cells.

Tick-borne spotted fever group (SFG) Rickettsia species are obligate intracellular bacteria capable of infecting both vertebrate and invertebrate host cells, an essential process for subsequent bacterial survival in distinct hosts. The host cell signaling molecules involved in the uptake of Rickettsia into mammalian and Drosophila cells have been identified; however, invasion into tick cells is understudied. Considering the movement of SFG Rickettsia between vertebrate and invertebrate hosts, the hypothesis is that conserved mechanisms are utilized for host cell invasion. The current study employed biochemical inhibition assays to determine the tick proteins involved in Rickettsia montanensis infection of tick-derived cells from a natural host, Dermacentor variabilis. The results revealed several tick proteins important for rickettsial invasion, including actin filaments, actin-related protein 2/3 complex, phosphatidylinositol-3'-kinase, protein tyrosine kinases (PTKs), Src family PTK, focal adhesion kinase, Rho GTPase Rac1, and neural Wiskott-Aldrich syndrome protein. Delineating the molecular mechanisms of rickettsial infection is critical to a thorough understanding of rickettsial transmission in tick populations and the ecology of tick-borne rickettsial diseases.

Cofeeding intra- and interspecific transmission of an emerging insect-borne rickettsial pathogen.

Cat fleas (Ctenocephalides felis) are known as the primary vector and reservoir of Rickettsia felis, the causative agent of flea-borne spotted fever; however, field surveys regularly report molecular detection of this infectious agent from other blood-feeding arthropods. The presence of R. felis in additional arthropods may be the result of chance consumption of an infectious bloodmeal, but isolation of viable rickettsiae circulating in the blood of suspected vertebrate reservoirs has not been demonstrated. Successful transmission of pathogens between actively blood-feeding arthropods in the absence of a disseminated vertebrate infection has been verified, referred to as cofeeding transmission. Therefore, the principal route from systemically infected vertebrates to uninfected arthropods may not be applicable to the R. felis transmission cycle. Here, we show both intra- and interspecific transmission of R. felis between cofeeding arthropods on a vertebrate host. Analyses revealed that infected cat fleas transmitted R. felis to naïve cat fleas and rat fleas (Xenopsylla cheopis) via fleabite on a nonrickettsemic vertebrate host. Also, cat fleas infected by cofeeding were infectious to newly emerged uninfected cat fleas in an artificial system. Furthermore, we utilized a stochastic model to demonstrate that cofeeding is sufficient to explain the enzootic spread of R. felis amongst populations of the biological vector. Our results implicate cat fleas in the spread of R. felis amongst different vectors, and the demonstration of cofeeding transmission of R. felis through a vertebrate host represents a novel transmission paradigm for insect-borne Rickettsia and furthers our understanding of this emerging rickettsiosis.

Comparative vertical transmission of Rickettsia by Dermacentor variabilis and Amblyomma maculatum.

The geographical overlap of multiple Rickettsia and tick species coincides with the molecular detection of a variety of rickettsial agents in what may be novel tick hosts. However, little is known concerning transmissibility of rickettsial species by various tick hosts. To examine the vertical transmission potential between select tick and rickettsial species, two sympatric species of ticks, Dermacentor variabilis and Amblyomma maculatum, were exposed to five different rickettsial species, including Rickettsia rickettsii, Rickettsia parkeri, Rickettsia montanensis, Rickettsia amblyommatis, or flea-borne Rickettsia felis. Fitness-related metrics including engorgement weight, egg production index, nutrient index, and egg hatch percentage were then assessed. Subsamples of egg clutches and unfed larvae, nymphs, and adults for each cohort were assessed for transovarial and transstadial transmission of rickettsiae by qPCR. Rickettsial exposure had a minimal fitness effect in D. variabilis and transovarial transmission was observed for all groups except R. rickettsii. In contrast, rickettsial exposure negatively influenced A. maculatum fitness and transovarial transmission of rickettsiae was demonstrated only for R. amblyommatis- and R. parkeri-exposed ticks. Sustained maintenance of rickettsiae via transstadial transmission was diminished from F1 larvae to F1 adults in both tick species. The findings of this study suggest transovarial transmission specificity may not be tick species dependent, and sustained vertical transmission is not common.

Transmission mechanisms of an emerging insect-borne rickettsial pathogen.

BACKGROUND: Vector-borne pathogens must overcome arthropod infection and escape barriers (e.g. midgut and salivary glands) during the extrinsic incubation period (EIP) before subsequent transmission to another host. This particular timespan is undetermined for the etiological agent of flea-borne spotted fever (Rickettsia felis). Artificial acquisition of R. felis by blood-feeding cat fleas revealed dissemination to the salivary glands after seven days; however, this length of time is inconsistent with co-feeding studies that produced infectious cat fleas within 24 h of infection. In the current study, we demonstrated that an alternative mechanism is responsible for the early-phase transmission that typifies flea-borne R. felis spread. METHODS: Co-feeding transmission bioassays were constructed to assess temporal dynamics of R. felis amongst cat fleas, including exposure time to produce infectious fleas and association time to transmit infection to naïve fleas. Additional experiments examined the proportion of R. felis-exposed cat fleas with contaminated mouthparts, as well as the likelihood for cat fleas to release R. felis from their mouthparts following exposure to an infectious bloodmeal. The potential for mechanical transmission of R. felis by co-feeding cat fleas was further examined using fluorescent latex beads, as opposed to a live pathogen, which would not require a biological mechanism to achieve transmission. RESULTS: Analyses revealed that R. felis-infected cat fleas were infectious to naïve fleas less than 24 h after exposure to the pathogen, but showed no rickettsial dissemination to the salivary glands during this early-phase transmission. Additionally, the current study revealed that R. felis-infected cat fleas must co-feed with naïve fleas for more than 12 h in order for early-phase transmission to occur. Further evidence supported that contaminated flea mouthparts may be the source of the bacteria transmitted early, and demonstrated that R. felis is released from the mouthparts during brief probing events. Moreover, the use of fluorescent latex beads supports the notion that early-phase transmission of R. felis is a mechanical mechanism. CONCLUSIONS: Determination of the transmission mechanisms utilized by R. felis is essential to fully understand the vulnerability of susceptible vertebrate hosts, including humans, to this pathogen.

Amblyomma maculatum Feeding Augments Rickettsia parkeri Infection in a Rhesus Macaque Model: A Pilot Study.

Rickettsia parkeri is an emerging eschar-causing human pathogen in the spotted fever group of Rickettsia and is transmitted by the Gulf coast tick, Amblyomma maculatum. Tick saliva has been shown to alter both the cellular and humoral components of the innate and adaptive immune systems. However, the effect of this immunomodulation on Rickettsia transmission and pathology in an immunocompetent vertebrate host has not been fully examined. We hypothesize that, by modifying the host immune response, tick feeding enhances infection and pathology of pathogenic spotted fever group Rickettsia sp. In order to assess this interaction in vivo, a pilot study was conducted using five rhesus macaques that were divided into three groups. One group was intradermally inoculated with low passage R. parkeri (Portsmouth strain) alone (n = 2) and another group was inoculated during infestation by adult, R. parkeri-free A. maculatum (n = 2). The final macaque was infested with ticks alone (tick feeding control group). Blood, lymph node and skin biopsies were collected at several time points post-inoculation/infestation to assess pathology and quantify rickettsial DNA. As opposed to the tick-only animal, all Rickettsia-inoculated macaques developed inflammatory leukograms, elevated C-reactive protein concentrations, and elevated TH1 (interferon-γ, interleukin-15) and acute phase inflammatory cytokines (interleukin-6) post-inoculation, with greater neutrophilia and interleukin-6 concentrations in the tick plus R. parkeri group. While eschars formed at all R. parkeri inoculation sites, larger and slower healing eschars were observed in the tick feeding plus R. parkeri group. Furthermore, dissemination of R. parkeri to draining lymph nodes early in infection and increased persistence at the inoculation site were observed in the tick plus R. parkeri group. This study indicates that rhesus macaques can be used to model R. parkeri rickettsiosis, and suggests that immunomodulatory factors introduced during tick feeding may enhance the pathogenicity of spotted fever group Rickettsia.

Novel identification of Dermacentor variabilis Arp2/3 complex and its role in rickettsial infection of the arthropod vector.

Tick-borne spotted fever group (SFG) Rickettsia species must be able to infect both vertebrate and arthropod host cells. The host actin-related protein 2/3 (Arp2/3) complex is important in the invasion process and actin-based motility for several intracellular bacteria, including SFG Rickettsia in Drosophila and mammalian cells. To investigate the role of the tick Arp2/3 complex in tick-Rickettsia interactions, open reading frames of all subunits of the protein including Arp2, Arp3, ARPC1, ARPC2, ARPC3, ARPC4, and ARPC5 were identified from Dermacentor variabilis. Amino acid sequence analysis showed variation (ranging from 25-88%) in percent identity compared to the corresponding subunits of the complex from Drosophila melanogaster, Mus musculus, Homo sapiens, and Saccharomyces cerevisiae. Potential ATP binding sites were identified in D. variabilis (Dv) Arp2 and Arp3 subunits as well as five putative WD (Trp-Asp) motifs which were observed in DvARPC1. Transcriptional profiles of all subunits of the DvArp2/3 complex revealed greater mRNA expression in both Rickettsia-infected and -uninfected ovary compared to midgut and salivary glands. In response to R. montanensis infection of the tick ovary, the mRNA level of only DvARPC4 was significantly upregulated compared to uninfected tissues. Arp2/3 complex inhibition bioassays resulted in a decrease in the ability of R. montanensis to invade tick tissues with a significant difference in the tick ovary, indicating a role for the Arp2/3 complex in rickettsial invasion of tick cells. Characterization of tick-derived molecules associated with rickettsial infection is imperative in order to better comprehend the ecology of tick-borne rickettsial diseases.

Idiopathic solitary cutaneous xanthoma in a dog.

A 6-year-old female spayed Boxer mix dog was presented with multiple cutaneous masses, one of which was determined to be a xanthoma. Fine-needle aspirates of this mass revealed large round cells that were consistent with macrophages. These macrophages had lightly basophilic cytoplasm that was filled with many clear circular spaces that varied in size. The nuclei of these cells displayed mild anisokaryosis with condensed chromatin and lacked prominent nucleoli. The cytologic interpretation was lipid-laden histiocytic inflammation most consistent with a cutaneous xanthoma, which was confirmed histologically. Mild hypertriglyceridemia and persistent moderate hypercholesterolemia were present. After ruling out other causes of hyperlipidemia, we concluded that the dog likely had idiopathic hyperlipidemia with secondary xanthoma formation.