Abstract
Rickettsia parkeri and Rickettsia amblyommatis are spotted fever group Rickettsia (SFGR) associated with Amblyomma ticks. R. parkeri is a recognized human pathogen that causes an eschar-associated febrile illness, while R. amblyommatis has not been confirmed as a causative agent of human disease. We hypothesized that the rate of replication is one of the factors contributing to rickettsial pathogenicity. In this study, growth and infectivity of R. parkeri and R. amblyommatis in mammalian (Vero E6) and tick-derived (ISE6) cell lines were assessed and compared over a 96-hour time course of infection using quantitative real-time polymerase chain reaction and microscopy. The pathogenic R. parkeri displayed a significantly higher level of infection in both Vero E6 and ISE6 cells than R. amblyommatis at 72 hours post-inoculation (hpi). Distinct growth profiles between rickettsial species with known and uncertain pathogenicity were identified. R. parkeri burdens were significantly greater than those of R. amblyommatis from 24 to 96 hpi. The relative fold changes of load were significantly higher in the pathogenic agent than in R. amblyommatis from 48 hpi onward and reached the maximum fold increase of ~2002- and ~296-fold in Vero E6 cells and ~1363- and ~161-fold in ISE6 cells, respectively, at 96 hpi. The results from the present study demonstrate that growth rate is associated with the pathogenicity of rickettsiae. Understanding SFGR growth characteristics in mammalian and tick cells will provide insight into rickettsial biology and pathogenesis.
Highlights
A significant difference in infectivity was not identified at 96 hpi, Vero E6 cells exposed to R. parkeri showed a greater density of rickettsiae within the cells than those of R. amblyommatis
Quantitative analyses of R. parkeri and R. amblyommatis in both cell types showed that rickettsiae levels remained relatively constant for the first 12-24 hpi suggesting the length of the lag phase required for the bacteria to encounter the initial stages of their life cycles before beginning cell division
The lag phase duration of spotted fever group Rickettsia (SFGR) in cell culture system varied among species, e.g., 2-3 days for Rickettsia helvetica cultured in Vero cells,[37 6,7,8] days for Rickettsia felis grown in S2 and C6/36 cells,[38] except for Rickettsia slovaca that showed no lag phase when cultivated in Vero and L929 cells.[39]
Summary
This agent is detected in other species of ticks, comprising A. maculatum,[4,14] Amblyomma cajennense complex,[22] Amblyomma nr maculatum,[22] and I. scapularis.[14,23] The relatively high prevalence of R. amblyommatis detected in A. americanum[24,25] and its existence in other tick species have been suggested as a potential risk of rickettsial infection to humans; no confirmed cases of human infection by R. amblyommatis have been reported to date
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