Abstract
Rickettsial diversity is intriguing in that some species are transmissible to vertebrates, while others appear exclusive to invertebrate hosts. Of particular interest is Rickettsia felis, identifiable in both stored product insect pests and hematophagous disease vectors. To understand rickettsial survival tactics in, and probable movement between, both insect systems will explicate the determinants of rickettsial pathogenicity. Towards this objective, a population of Liposcelis bostrychophila, common booklice, was successfully used for rickettsial isolation in ISE6 (tick-derived cells). Rickettsiae were also observed in L. bostrychophila by electron microscopy and in paraffin sections of booklice by immunofluorescence assay using anti-R. felis polyclonal antibody. The isolate, designated R. felis strain LSU-Lb, resembles typical rickettsiae when examined by microscopy. Sequence analysis of portions of the Rickettsia specific 17-kDa antigen gene, citrate synthase (gltA) gene, rickettsial outer membrane protein A (ompA) gene, and the presence of the R. felis plasmid in the cell culture isolate confirmed the isolate as R. felis. Variable nucleotide sequences from the isolate were obtained for R. felis-specific pRF-associated putative tldD/pmbA. Expression of rickettsial outer membrane protein B (OmpB) was verified in R. felis (LSU-Lb) using a monoclonal antibody. Additionally, a quantitative real-time PCR assay was used to identify a significantly greater median rickettsial load in the booklice, compared to cat flea hosts. With the potential to manipulate arthropod host biology and infect vertebrate hosts, the dual nature of R. felis provides an excellent model for the study of rickettsial pathogenesis and transmission. In addition, this study is the first isolation of a rickettsial pathogen from a non-hematophagous arthropod.
Highlights
The complexity of intracellular bacteria belonging to the genus Rickettsia is becoming more apparent as many species are being recognized as pathogens or endosymbionts of arthropods of medical or veterinary importance, seemly ubiquitous in many nonhematophagous insect pests, or both
Known to infect hematophagous arthropods, R. felis is primarily associated with cat fleas, Ctenocephalides felis
Detection of Rickettsia in booklice In order to investigate the presence of Rickettsia in booklice associated with the regular maintenance of the LSU colony of cat fleas, formalin-fixed paraffin-embedded booklice sections were stained with polyclonal antibody against R. felis followed by a secondary FITC-conjugated antibody
Summary
The complexity of intracellular bacteria belonging to the genus Rickettsia is becoming more apparent as many species are being recognized as pathogens or endosymbionts of arthropods of medical or veterinary importance, seemly ubiquitous in many nonhematophagous insect pests, or both. The mycetome consists of clustering mycetocytes containing bacteria and may become an integral part of the host, sometimes essential for host fitness and fecundity [6,7,10]. For many of these Rickettsia/host relationships currently not associated with human disease the Rickettsia sp. As an obligate endosymbiont of Liposcelis bostrychophila, rickettsiae target reproductive cells and have a dynamic cell association during insect development demonstrating a relatively synchronized relationship in this arthropod host [6]
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