Abstract: Objective To study the biological characteristics of newly isolated Toxoplasma gondii Korean isolate-1 （KI-1）. Methods The morphology and infectivity of KI-1 were observed by transmission electron microscopy and animal inoculation. Both RH and KI-1 antigens were detected by an anti-T. gondii monoclonal antibody （mAb）, Tg563, and an anti-Neospora caninum mAb, 12B4. The genotype was determined by PCR-RFLP analysis of SAG2 locus. The sequence heterogeneity of the SAG1, ROP1 gene coding regions of the RH and KI-1 was detected by an automated sequencer. Results The morphology and infectivity of KI-1 were similar to those of RH strain. Both RH and KI-1 antigens reacted with Tg563, not with 12B4. In comparison to RH strain, the KI-1 showed a difference of seven positions of nucleotide substitutions and six positions of amino acid substitutions in both SAG1 and ROP1 gene coding regions. Conclusion KI-1 is an isolate of T. gondii with strong virulence. Compared with RH strain, certain genetic differences exist.

Key words: Toxoplasma gondii, Toxoplasma gondiiWestern blotting, PCR, RFLP, Gene sequencing