Toxoplasma gondii: the growth characteristics of three virulent strains

We have studied the phenotype of three mouse virulent strains of Toxoplasma gondii (RH, Martin and ENT), monitoring cellular factors which may relate to virulence. There was variation between these three strains in three separate criteria: invasion, growth and tachyzoite–bradyzoite interconversion. The ENT strain exhibited consistently higher invasion rates, a shorter doubling time and a lower frequency of bradyzoite production than Martin or RH strains. In addition to variation in growth rate, there were also differences in the morphology of the parasites, with the ENT strain exhibiting highly synchronous division giving rise to characteristic rosettes. The Martin strain produced bradyzoites at a higher frequency and, in culture, parasites were often seen in tight clusters, which were reminiscent of early tissue cysts. These phenotypic variations amongst mouse-virulent strains of the parasite may imply underlying genetic differences within the group.