Tick Cell Culture Analysis of Growth Dynamics and Cellular Tropism of , an Endosymbiont of the Blacklegged Tick, .

The blacklegged tick, , a species of significant importance to human and animal health, harbors an endosymbiont sensu stricto. The symbiont is largely restricted to the ovaries, but all life stages can harbor various quantities or lack entirely. The endosymbiont is cultivable in cell lines isolated from embryos of ticks. most readily grows and is maintained in the cell line IRE11 from the European tick, . The line was characterized by light and electron microscopy and used to analyze the growth dynamics of wildtype and GFPuv-expressing . qPCR indicated that the genome copy doubling time in IRE11 was >7 days. Measurements of fluorescence using a plate reader indicated that the amount of green fluorescent protein doubled every 11 days. Two 23S rRNA probes were tested via RNA FISH on rickettsiae grown in vitro and adapted to evaluate the tissue tropism of in field-collected female . We observed strong positive signals of in the ovaries and surrounding the nucleus of the developing oocytes. Tissue tropism in and in vitro growth dynamics strengthen the contemporary understanding of as a transovarially transmitted, non-pathogenic endosymbiont.