Abstract
Simple SummaryThe blacklegged tick, Ixodes scapularis, a species of significant medical and veterinary importance, harbors an endosymbiont, Rickettsia buchneri. This bacterium is largely restricted to the ovaries, but all life stages can harbor different numbers or lack R. buchneri entirely. The endosymbiont is cultivable in cell lines isolated from embryos of Ixodes ticks. We characterized the cells using microscopy. The doubling time of wildtype R. buchneri and a transformant expressing green fluorescent protein was determined to be >7 days when measured by quantitative PCR. Quantification based on fluorescence indicated that 11 days were needed to double the amount of green fluorescent protein. Two rRNA probes were tested using rickettsiae grown in vitro and adapted to localize R. buchneri in different organs of field-collected female I. scapularis ticks. We observed strong positive signals of R. buchneri in the ovaries and surrounding the nucleus of the developing oocytes. The sequestration of rickettsia in ticks and the slow growth dynamics strengthen the contemporary understanding of R. buchneri as a transovarially transmitted, non-pathogenic endosymbiont.The blacklegged tick, Ixodes scapularis, a species of significant importance to human and animal health, harbors an endosymbiont Rickettsia buchneri sensu stricto. The symbiont is largely restricted to the ovaries, but all life stages can harbor various quantities or lack R. buchneri entirely. The endosymbiont is cultivable in cell lines isolated from embryos of Ixodes ticks. Rickettsia buchneri most readily grows and is maintained in the cell line IRE11 from the European tick, Ixodes ricinus. The line was characterized by light and electron microscopy and used to analyze the growth dynamics of wildtype and GFPuv-expressing R. buchneri. 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 R. buchneri in field-collected female I. scapularis. We observed strong positive signals of R. buchneri in the ovaries and surrounding the nucleus of the developing oocytes. Tissue tropism in I. scapularis and in vitro growth dynamics strengthen the contemporary understanding of R. buchneri as a transovarially transmitted, non-pathogenic endosymbiont.
Highlights
Introduction and HistoryOne of the most notorious North American ticks from the perspective of human and animal health is the black-legged tick, Ixodes scapularis
Given the ability of I. scapularis to transmit a wide variety of pathogens, it is puzzling that this tick has not been linked to the transmission of Rickettsia species, even though its European counterpart, the sheep tick, Ixodes ricinus, is a competent vector of several rickettsial agents, including Rickettsia monacensis, Rickettsia slovaca, and Rickettsia helvetica [15,16]
We describe the application of an I. ricinus cell line to develop a fluorescent RNA probe to map R. buchneri in different organs and cells of female blacklegged ticks
Summary
Introduction and HistoryOne of the most notorious North American ticks from the perspective of human and animal health is the black-legged tick, Ixodes scapularis. It is responsible for spreading bacteria that cause Lyme disease (Borrelia burgdorferi sensu stricto) and human anaplasmosis (Anaplasma phagocytophilum), as well as protozoan blood parasites (Babesia microti) and the deer tick lineage of Powassan virus [1,2,3,4], among others The role of this tick as a major vector of multiple human pathogens spurred the development of I. scapularis cell lines for the propagation and study of these microbes [5]. The range of blacklegged ticks overlaps that of known vectors of Rickettsia rickettsii, i.e., the American dog tick, Dermacentor variabilis, in areas where Rocky Mountain spotted fever is endemic [17] This suggests that I. scapularis has the opportunity to feed on rickettsiaemic hosts and ingest rickettsiae in the process, which highlights the question of why I. scapularis does not seem to transmit pathogens in the genus Rickettsia
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