Genome Changes in the Toxoplasma Gondii Strains during Laboratory Passages in Mice

Literák I . , I . Rychl ík: Genome Changes in the Toxoplasma gondii Strains During Laboratory Passages in Mice. Acta Vet. Brno 1999, 68: 203–208. 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 SalI 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 RFLP/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. Toxoplasma gondii, PCR/RFLP, RFLP/DNA, DNA fingerprinting, strains, pathogenicity, virulence Toxoplasma gondii is a protozoan parasite of mammals and birds with worldwide occurrence (Dubey and Beat t ie 1988). In man and in sensitive animal species, it may be the cause of asymptomatic as well as lethal infections. In view of different manifestations of toxoplasmosis in the same host species, pathogenicity of individual strains of T. gondii has been studied to explain different manifestations of the disease by different pathogenicity of strains. Sibley and Boothroyd (1992) stated that “virulent” relates to strains with LD100 (100 % lethal dose) < 10 tachyzoites for outbred Swiss mice, and “non-virulent” (avirulent) to strains with LD100 > 10 3 tachyzoites. Genotype studies of individual T. gondii strains based on restriction fragment length polymorphism of the chromosomal DNA (RFLP/DNA) and RFLP of a specific DNA sequence amplified by the PCR (PCR/RFLP) fitted closely with the T. gondii pathogenicity. Those studies documented the characteristic genotypes of virulent and avirulent strains (Cris t ina et al. 1991ab; Sibley and Boothroyd 1992; Cris t ina et al. 1995; Howe and Sibley 1994, 1995; Li terák et al. 1998). Most T. gondii strains have been isolated by means of passages in laboratory mice. Laboratory mice have also been used to maintain a number of strains on a long-term basis, and many different types of strain analyses, including genotype analysis, have been performed after the strains have been multiplied in laboratory mice. An interesting phenomenon has been observed in some strains for several decades: their pathogenicity ACTA VET. BRNO 1999, 68: 203–208 Address for correspondence: Doc. MVDr. Ivan Literák, CSc. Department of Infectious Diseases and Veterinary Epidemiology, Faculty of Veterinary Medicine, University of Veterinary and Pharmaceutical Sciences Palackého 1-3, 612 42 Brno, Czech Republic Phone: ++420-5-41 56 23 07 Fax: ++420-5-74 88 41 E-mail: [email protected] http://www.vfu.cz/acta-vet/actavet.htm