Short-term culture adaptation of Toxoplasma gondii archetypal II and III field isolates affects cystogenic capabilities and modifies virulence in mice.

[Display omitted] • The influence of in vitro maintenance on phenotypic traits of Toxoplasma gondii was studied. • Cell culture adaptation in the short term influences cyst formation and mouse virulence. • Toxoplasma gondii isolates experience a loss of cyst formation after 40 culture passages. • Changes in mouse virulence correlated with in vitro parasite growth and cyst formation. Most Toxoplasma gondii research has been carried out using strains maintained in the laboratory for long periods of time. Long-term passage in mice or cell culture influences T. gondii phenotypic traits such as the capability to produce oocysts in cats and virulence in mice. In this work, we investigated the effect of cell culture adaptation in the short term for recently obtained type II (Tg ShSp1 (Genotype ToxoDB#3), Tg ShSp2 (#1), Tg ShSp3 (#3) and Tg ShSp16 (#3)) and type III (#2) isolates (Tg ShSp24 and Tg PigSp1). With this purpose, spontaneous and alkaline stress-induced cyst formation in Vero cells during 40 passages, from passage 10 (p 10) to 50 (p 50), and isolate virulence at p 10 versus p 50 were studied using a harmonized bioassay method in Swiss/CD1 mice. T. gondii cell culture maintenance showed a drastic loss of spontaneous and induced production of mature cysts after ≈25–30 passages. The Tg ShSp1, Tg ShSp16 and Tg ShSp24 isolates failed to generate spontaneously formed mature cysts at p 50. Limited cyst formation was associated with an increase in parasite growth and a shorter lytic cycle. In vitro maintenance also modified T. gondii virulence in mice at p 50 with events of exacerbation, increasing cumulative morbidity for Tg ShSp2 and Tg ShSp3 isolates and mortality for Tg ShSp24 and Tg PigSp1 isolates, or attenuation, with absence of mortality and severe clinical signs for Tg ShSp16, and better control of the infection with the lowest parasite and cyst burdens in lungs and brain for the Tg ShSp1 isolate. The present findings show deep changes in relevant phenotypic traits in laboratory-adapted T. gondii isolates and open new discussion about their use for inferring keys to parasite biology and virulence. [ABSTRACT FROM AUTHOR]