Growth characteristics of Rickettsia species LON strains closely related to Rickettsia japonica isolated from Haemaphysalis longicornis ticks in mouse-derived L929 and human-derived THP-1 host cell lines.

Non-pathogenic Rickettsia species LON strains closely related to an agent of Japanese spotted fever (JSF), R. japonica, were isolated from Haemaphysalis longicornis ticks in 2001, Japan. However, the biological properties of LONs in mammalian host cells are less understood. In this study, microscopic analysis showed that LONs in a mouse-derived L929 host cell line are rod shaped with sizes of 0.3-0.5 x 0.5- 2.0 μm. Molecular-based analysis revealed the existence of LON-specific disrupted open reading frame in R. japonica-related group specific DNA region. Growth kinetics of LON-2 and LON-13 strains analyzed by a quantitative real-time PCR showed 100-fold increment or more of LONs cultured in L929 host cells at 30oC and slightly less increment at 33oC, and 25-fold increment in human-derived THP-1 host cells at 35oC on day 7 (168 h) post infection. The generation times of 2 LON strains cultured in L929 and THP-1 were estimated to be 9.4-12.9 h and 9.6-10.9 h, respectively. As far as we know, this is for the first time a report regarding the biological characteristics of Rickettsia sp. LONs in mammalian cells and will provide the significant information for the experimental approaches of the other rickettsiae.