Plaque assay and cloning of scrub typhus rickettsiae in irradiated L-929 cells

It was demonstrated that gamma-irradiated L-929 cells support plaque formation by three strains of Rickettsia tsutsugamushi and representative species of the spotted fever and typhus group rickettsiae. Sensitivity of the plaque assay for detection of viable scrub typhus rickettsiae was similar to that achieved with intraperitoneal inoculation of random-bred mice. The concentration of irradiated cells and the temperature and length of incubation were all found to affect plaque size. A technique combining terminal dilution and plaque purification was used to obtain clones of three strains of scrub typhus rickettsiae.