ISSN: 0973-7510

E-ISSN: 2581-690X

Research Article | Open Access
Apichai Bourchookarn1,2, Christopher D. Paddock3, Kevin R. Macaluso2,4 and Walairat Bourchookarn1
1Faculty of Science and Technology, Prince of Songkla University, Pattani 94000, Thailand.
2Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Skip Bertman Drive, SVM-3213, Baton Rouge, LA 70803, USA.
3Rickettsial Zoonoses Branch, Mailstop G-13, Centers for Disease Control and Prevention, 1600 Clifton Rd., Atlanta, GA 30333, USA.
4Department of Microbiology and Immunology, College of Medicine, University of South Alabama, Mobile, AL 36688, USA.
J Pure Appl Microbiol. 2022;16(1):374-383 | Article Number: 7409 | © The Author(s). 2022
Received: 04/11/2021 | Accepted: 04/01/2022 | Published online: 15/02/2022
Issue online: March 2022

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.


Rickettsia parkeri, Rickettsia amblyommatis, Amblyomma spp., growth rate, qPCR

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© The Author(s) 2022. Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License which permits unrestricted use, sharing, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.