Ultrastructural determination of cystogenesis by various Toxoplasma gondii isolates in cell culture.

The tissue cyst stage of Toxoplasma gondii is important in relapsing disease seen in toxoplasmic encephalitis and retinochoroiditis. An in vitro culture system to examine the developmental biology of the tissue cyst stage would greatly aid in our understanding of this stage of the parasite's life cycle. We used transmission electron microscopy (TEM) and acid-pepsin digestion of infected cell cultures to determine the capability of 21 isolates of T. gondii to produce tissue cysts in cell cultures. All 21 of the isolates had acid-pepsin-resistant stages present, and tissue cysts could be demonstrated in 19 using TEM. The present study demonstrates that tissue cyst formation in vitro is a common phenomenon for T. gondii isolates.