Genomic Diversification in Strains of Rickettsia felis Isolated from Different Arthropods

Abstract

Rickettsia felis (Alphaproteobacteria: Rickettsiales) is the causative agent of an emerging flea-borne rickettsiosis with worldwide occurrence. Originally described from the cat flea, Ctenocephalides felis, recent reports have identified R. felis from other flea species, as well as other insects and ticks. This diverse host range for R. felis may indicate an underlying genetic variability associated with host-specific strains. Accordingly, to determine a potential genetic basis for host specialization, we sequenced the genome of R. felis str. LSU-Lb, which is an obligate mutualist of the parthenogenic booklouse Liposcelis bostrychophila (Insecta: Psocoptera). We also sequenced the genome of R. felis str. LSU, the second genome sequence for cat flea-associated strains (cf. R. felis str. URRWXCal2), which are presumably facultative parasites of fleas. Phylogenomics analysis revealed R. felis str. LSU-Lb diverged from the flea-associated strains. Unexpectedly, R. felis str. LSU was found to be divergent from R. felis str. URRWXCal2, despite sharing similar hosts. Although all three R. felis genomes contain the pRF plasmid, R. felis str. LSU-Lb carries an additional unique plasmid, pLbaR (plasmid of L. bostrychophila associated Rickettsia), nearly half of which encodes a unique 23-gene integrative conjugative element. Remarkably, pLbaR also encodes a repeats-in-toxin-like type I secretion system and associated toxin, heretofore unknown from other Rickettsiales genomes, which likely originated from lateral gene transfer with another obligate intracellular parasite of arthropods, Cardinium (Bacteroidetes). Collectively, our study reveals unexpected genomic diversity across three R. felis strains and identifies several diversifying factors that differentiate facultative parasites of fleas from obligate mutualists of booklice.

Small Regulatory RNAs of Rickettsia conorii

“…Even greater genetic divergence is evident for R. felis , the etiological agent of a typhus-like flea borne rickettsiosis, whose genome is overrun by mobile genetic elements. Despite coding for ~1600–1800 ORFs, only 300 of these belong to the core set of genes shared by other rickettsial species14. It has also been suggested that typhus group genomes have a faster divergence rate in comparison to spotted fever group (SFG) species.…”

“…R. conorii genome harbors a single chromosome of 1268755 bp encoding for 1578 protein coding genes, 9 pseudogenes, 33 tRNAs, 2 rRNAs, and contains ~32% intergenic region910. This relatively high percentage of non-coding region in R. conorii and other rickettsial genomes has traditionally been considered to be the ‘junk DNA’ or defunct genes resulting from reductive evolution and pseudogenization711121314. However, recent advances in computational bioinformatics and bacterial molecular genetics have led to the appreciation that the intergenic regions, in addition to harboring transcription factor binding sites and mobile genetic elements, encode for small non-coding RNAs involved in the regulation of target genes.…”

Flea-Borne Rickettsioses and Rickettsiae

“…gambiae , the role that infected mosquitos play in the epidemiology of R. felis infections in humans is unknown. In addition to these aforementioned blood-feeding arthropods, R. felis has also been cultured from the nonhematophagous book louse ( Liposcelis bostrychophila ),41 where its genetic diversity, compared with isolates obtained from other arthropods, is surprisingly great 42. Finally, R. felis DNA has even been detected in ape feces43—the implications of this finding are yet to be elucidated.…”

Proteomic analysis of Rickettsia akari proposes a 44 kDa-OMP as a novel biomarker for Rickettsialpox diagnosis

“…Rickettsia akari, the causative agent of Rickettsialpox, was formerly placed into the SFG. However, current genomic studies reclassified this organism together with R. australis and R. felis into the TRG [4,5]. This mite-borne pathogen is usually transmitted to humans by a rodent mite Liponyssoides sanguineus [6,7].…”