Toxoplasma gondii: Isolation and preliminary characterization of temperature-sensitive mutants

Toxoplasma gondii, strain RH, produced plaques in human fibroblast tissue cultures over the temperatures 30–41 C. Muta⋗enesis with N-methyl- N′-nitro- N-nitrosoguanidine yielded seven temperature-sensitive mutants that had lost the ability to form plaques at 40 C but still grew well at 33 C. No spontaneous mutants were detected. The temperature-sensitive mutants were not markedly thermolabile and adsorbed normally to tissue culture cells at 40 C. Three mutants differed from one another in their temperatures for optimal growth, and in their ability to remain infectious within cells incubated at 40 C. Both mutants that were tested were found to be markedly less virulent for mice than was the wild type RH strain.