Genome changes in the Toxoplasma gondii strains during laboratory passages in mice

Three strains of Toxoplasma gondii isolated from rabbits (K6 and K9) and a cat (K24) in the Czech Republic in 1994-95 were maintained in a tissue cyst form in laboratory mice by injections of brain homogenate every 4 - 6 months. These strains were passaged 12 to 13 times before tachyzoites, from which DNA was isolated, were obtained. PCR/RFLP (restriction fragment length polymorphism) (ROP1 gene/DdeI restriction endonuclease) and RFLP/DNA (RFLP with chromosomal DNA) (TGR1E probe/PstI and San, restriction endonucleases) were used to characterize their genotypes. In the course of subsequent tissue cyst-tissue cyst passages, the pathogenicity of the strains increased and intraperitoneal passaging of tachyzoites from the peritoneal exudate two times weekly had to be used to facilitate the continuous maintenance. After the pathogenicity increase, the DNA was isolated from the tachyzoites after 6 to 48 tachyzoite-tachyzoite passages. Genotype differences were observed in the strain before and after increasing their pathogenicity. Both PCR/RFLP and RRLP/DNA patterns of the strain before and after increasing their pathogenicity were similar, and even identical, with previously described genotype characteristics of avirulent and virulent strains of T. gondii.