Abstract
BackgroundSince its first detection, characterization of R. felis has been a matter of debate, mostly due to the contamination of an initial R. felis culture by R. typhi. However, the first stable culture of R. felis allowed its precise phenotypic and genotypic characterization, and demonstrated that this species belonged to the spotted fever group rickettsiae. Later, its genome sequence revealed the presence of two forms of the same plasmid, physically confirmed by biological data. In a recent article, Gillespie et al. (PLoS One. 2007;2(3):e266.) used a bioinformatic approach to refute the presence of the second plasmid form, and proposed the creation of a specific phylogenetic group for R. felis.Methodology/Principal FindingsIn the present report, we, and five independent international laboratories confirmed unambiguously by PCR the presence of two plasmid forms in R. felis strain URRWXCal2 T, but observed that the plasmid content of this species, from none to 2 plasmid forms, may depend on the culture passage history of the studied strain. We also demonstrated that R. felis does not cultivate in Vero cells at 37°C but generates plaques at 30°C. Finally, using a phylogenetic study based on 667 concatenated core genes, we demonstrated the position of R. felis within the spotted fever group.SignificanceWe demonstrated that R. felis, which unambiguously belongs to the spotted fever group rickettsiae, may contain up to two plasmid forms but this plasmid content is unstable.
Highlights
Rickettsia felis (R. felis) was first detected in 1990 in American Ctenocephalides felis fleas using electron microscopy, and named the ELB agent after the Elward laboratory (Soquel, CA) where the flea colony was raised [1]
Despite a first phylogenetic study clearly showing its classification within the spotted fever group (SFG) [7], confusion was brought by further reports, which attributed to the ELB agent, renamed R. felis, several characteristics of R. typhi [8,9,10,11]
We were surprised that the only hypothesis produced by Gillespie et al [19] to explain the discordance between our results, showing the presence of two plasmids in R. felis [18], and those of Pornwiroon et al who could detect only one plasmid [14], was that the small plasmid form of R. felis was an artefact of our genome assembly
Summary
Rickettsia felis (R. felis) was first detected in 1990 in American Ctenocephalides felis fleas using electron microscopy, and named the ELB agent after the Elward laboratory (Soquel, CA) where the flea colony was raised [1]. It was later detected by PCR in humans with a murine typhus-like illness [2,3,4,5,6]. In 2001, R. felis was cultivated from cat fleas at low temperature (28uC), and was established for the first time [2] It was deposited as strain URRWXCal2T in two official collections: the American Type Culture Collection Gillespie et al (PLoS One. 2007;2(3):e266.) used a bioinformatic approach to refute the presence of the second plasmid form, and proposed the creation of a specific phylogenetic group for R. felis
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