Plaque Assay of Toxoplasma on Monolayers of Chick Embryo Fibroblasts.

The RH strain of Toxoplasma gondii induces formation of necrotic plaques on monolayers of chick embryo fibroblasts overlaid with nutrient agar. Correlation between number of organisms plated and number of plaques produced indicates that each plaque is induced by a single toxoplasma. Survival of toxoplasmata in various media has been tested, and it has been found that Hanks BSS containing chick embryo extract in concentrations of 5 to 50% preserves 80-100% of plaque forming units (pfu) for 3 to 4 hours, 50-60% for 6 hours. Adsorption of pfu on monolayers is maximal after 70 to 90 minutes (Kc = 0.024), can be retarded by increased viscosity of the medium, speeded by centrifugation. Artificial lysis of chick embryo fibroblasts infected at low multiplicity yields populations with a predictable efficiency of plating of about 50% of the counted organisms.