Genome changes in the Toxoplasma gondii strains during laboratory passagesin 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 mont hs. These strains were passaged 12 to 13 times before tachyzoites, from whi ch DNA was isolated, were obtained. PCR/RFLP (restriction fragment length p olymorphism) (ROP1 gene/DdeI restriction endonuclease) and RFLP/DNA (RFLP w ith chromosomal DNA) (TGR1E probe/PstI and San, restriction endonucleases) were used to characterize their genotypes. In the course of subsequent tiss ue cyst-tissue cyst passages, the pathogenicity of the strains increased an d 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 afte r 6 to 48 tachyzoite-tachyzoite passages. Genotype differences were observe d in the strain before and after increasing their pathogenicity. Both PCR/R FLP and RRLP/DNA patterns of the strain before and after increasing their p athogenicity were similar, and even identical, with previously described ge notype characteristics of avirulent and virulent strains of T. gondii.