Antiproliferative Plaque Assay for Screening in vitro Bioactive Molecules against Toxoplasma gondii Tachyzoites.

Toxoplasmosis is a parasitic infection commonly related to ocular lesions and neonatal malformations. Since the early 1950s, the first-line treatment of this disease has relied on the combination of sulfadiazine, pyrimethamine, and folinic acid. Over those years, only a few alternative regimens have been introduced. This highlights the need for discovering new bioactive molecules against Toxoplasma gondii. Given that this pathogen is an obligate intracellular parasite, traditional drug screening in the laboratory typically requires expensive materials and equipment, such as assays using fluorescent proteins or β-galactosidase-expressing parasites. Additionally, methods like optical microscopy quantification can be time-consuming. The plaque assay is a method that evaluates the intensity of intracellular pathogen proliferation by measuring the number of regions and area of destruction in a cell monolayer damaged by the parasite's lytic cycle. This work describes an optimized plaque assay protocol designed for screening active molecules against intracellular tachyzoites of T. gondii in vitro. This protocol utilizes inexpensive materials and a straightforward laboratory setup, yielding rapid and reproducible results that facilitate the identification of new active molecules against this parasite by various research groups.