Cyst Formation from Virulent RH Strain of Toxoplasma gondii Tachyzoite: In Vitro Cultivation

This study was performed to induce conversion of RH strain tachyzoites of Toxoplasma gondii to bradyzoites by pH changing of the cul­ture medium.
HeLa cell monolayers were infected at a 1:1 tachyzoite to cell ratio. Four hours after infection, the culture medium was removed and re­placed with culture medium and 5% FCS, adjusted to pH 8 with NaOH. The culture was maintained at 37 °C without CO2 until the end of the ex­periment. Cyst-like structures were collected and stained with periodic acid schiff (PAS) staining method. The soluble antigens of cyst-like structures of RH strain, in addition to RH tachyzoite, bradyzoites of avirulent Tehran strain and uninfected HeLa cells were electrophoresed on 12.5% poly­acrylamide gel. The gel was stained by coomassie brilliant blue R-250.
Four days after infection of HeLa cells with tachyzoites of T. gondii, RH strain, cyst- like structures were noticed and stained with PAS. In the SDS-PAGE, protein bands of these structures had some differences with tachyzoites of RH strain, but there was quite similarity between pro­tein bands of these structures and tissue cysts (bradyzoites) of Tehran strains. P34 and P36 (bradyzoite-specific proteins) were observed only in T. gondii bradyzoites of RH (cyst like structures) and bradyzoites of Tehran strains.
Alkalization of culture medium to pH 8 induced expression of bradyzoite- specific proteins and production of RH cysts in cell culture.