Abstract
Rickettsia raoultii is one of the causative agents of tick-borne lymphadenopathy in humans. This bacterium was previously isolated and propagated in tick cell lines; however, the growth characteristics have not been investigated. Here, we present the replication kinetics of R. raoultii in cell lines derived from different tick genera (BME/CTVM23, RSE/PILS35, and IDE8). Tick cell cultures were infected in duplicate with cryopreserved R. raoultii prepared from homologous cell lines. By 12–14 days post infection, 100% of the cells were infected, as visualized in Giemsa-stained cytocentrifuge smears. R. raoultii growth curves, determined by rickettsiae-specific gltA qPCR, exhibited lag, exponential, stationary and death phases. Exponential phases of 4–12 days and generation times of 0.9–2.6 days were observed. R. raoultii in BME/CTVM23 and RSE/PILS35 cultures showed, respectively, 39.5- and 37.1-fold increases compared to the inoculum. In contrast, multiplication of R. raoultii in the IDE8 cultures was 110.1-fold greater than the inoculum with a 7-day stationary phase. These findings suggest variation in the growth kinetics of R. raoultii in the different tick cell lines tested, amongst which IDE8 cells could tolerate the highest levels of R. raoultii replication. Further studies of R. raoultii are needed for a better understanding of its persistence within tick populations.
Highlights
Rickettsia raoultii is a spotted fever group rickettsial species, which was first described in Dermacentor nuttalli and Rhipicephalus pumilio ticks from Siberia [1]
Sequencing of the PCR amplicon from the inoculated IDE8 and RSE/PILS35 cells confirmed the absence of the R. microplus 16S rRNA sequence in either of the recipient cultures at 7 and 13 dpi
Duplicate cultures of each of BME/CTVM23, RSE/PILS35 and IDE8 cell lines were infected with R. raoultii and monitored at the selected time points by Giemsa-stained cytocentrifuge smears for the presence of bacteria
Summary
Rickettsia raoultii is a spotted fever group rickettsial species, which was first described in Dermacentor nuttalli (reported as genotypes DnS14 and DnS28) and Rhipicephalus pumilio (genotype RpA4) ticks from Siberia [1]. It was later described from Dermacentor spp. ticks, mainly D. nuttalli, Dermacentor reticulatus, Dermacentor marginatus and Dermacentor silvarum, from Russia [2], countries in Europe [2,3,4,5], and Asia [6,7,8,9].
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