Toxoplasma gondii: in vivo and in vitro cystogenesis of the virulent RH strain.

Models of cystogenesis were developed in vivo and in vitro with the virulent RH Toxoplasma gondii strain. In vivo in a first assay, 7.5-wk-old Fischer rats were inoculated intraperitoneally with 10(7) tachyzoites from mouse peritoneal fluid. Between 9 and 17 wk after inoculation, 1-5 cysts of 40-60 microns, containing numerous bradyzoites, were isolated from the brain of 5 of the rats. In a second assay, 24 Wistar rats were inoculated intraperitoneally with 10(2), 10(4), and 10(8) tachyzoites at 7, 11, 24, and 46 days. An inoculum of 10(8) was lethal for the 6 rats inoculated before 46 days and there was no correlation between the inoculum and the number of cysts except in the rats inoculated at 46 days. Fibroblast cell cultures (MRC5), at pH 7.5, were destroyed in 72 hr by 10(5)/ml RH-strain tachyzoites. However, when pH culture was increased to pH 8, up to 140 cysts were observed with indirect fluorescent antibody staining. Electron microscopy showed a 15 microns cyst, containing bradyzoites and surrounded by a 0.5-1-micron irregular wall.