Life cycle, growth characteristics and host cell response of Rickettsia helvetica in a Vero cell line

Rickettsia helvetica, a spotted fever rickettsia and emerging pathogen with Ixodes ricinus ticks as the main vector, is an agent of human disease and may cause febrile illness as well as meningitis. In three parallel series the isolated standard type of R. helvetica, obtained from a PCR-positive I. ricinus tick, was high-passaged and propagated in a Vero cell line. By using quantitative real-time PCR, the generation time from inoculation to stationary phase of growth was calculated to 20-22 h. In the static cultivation system the stationary phase was observed from the seventh day after inoculation, and there was no observed degradation of R. helvetica DNA during the 14 days studied. Microscopy showed that the organisms invaded the host cells rapidly and were primarily found free in the cytoplasm and only occasionally located in the nucleus. Four days after inoculation some of the host cells were broken and many indifferent stages of cytoplasmic organic decomposition were seen. However the R. helvetica organism did not show any morphologic alterations and the number of organisms was stable after the replication peak which may indicate that R. helvetica is adapted to growth in a Vero cell line and/or that the phase of degradation occurs later than the 14 days studied. The findings differ from what has been reported for other rickettsiae of the spotted fever group and may be of importance for invasiveness and virulence of R. helvetica.