As is well known, Paramecium bursaria has many endosymbiotic algae in its cytoplasm. We already reported that these endosymbiotic algae can be removed from the cytoplasm by using the herbicide paraquat and symbiotic algae-free paramecia were prepared (Hosoya et al. 1995). In this study, we examined the characteristics of symbiotic algae-free paramecia obtained by paraquat treatment. Symbiotic algae-free paramecia had a shorter log phase than those with symbiotic algae, and showed lower cell density at the stationary phase. Additionally, they had the ability to form stable symbiotic relationships with symbiotic algae. Symbiotic algae obtained from P. bursaria in log phase showed a higher infection rate than those from P. bursaria in stationary phase. These results suggest that growth phase of the host influences the reinfection ability of their interior symbiotic algae.

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Culture medium in the stationary phase of Tetrahymena pyriformis (strain W) had activities to reduce the growth rate of the ciliate markedly, while the culture medium in the logarithmic phase had no growth-inhibitory activities. These activities were heat stable and reduced by dialysis against a fresh culture medium. When the pH of the culture medium in the stationary phase (pH 7.4) was changed to that of fresh culture medium (pH 6.5), the growth rate of the ciliate was recovered. Further, when the pH of the fresh culture medium (pH 6.5) was changed to that of the culture medium in the stationary phase (pH 7.4), the growth rate of the ciliate was reduced. These results suggest that the pH in the culture medium has very important roles for cell growth.

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We have coupled ¹⁴C-Samorin to bovine transferrin (Tf) using 2- azidophenyl glyoxal (APG), Samorin-APG-Tf was incubated with either the Trypanosoma congolense Samorin-sensitive (IL 1180) or Samorin-resistant (IL 3338.2) The APG-Tf conjugate without bound drug was used as the control. Trypanosoma congolense IL 1180 grew poorly even at Samorin/Tf concentrations as low as 0.12 ng drug/0.14 μg Tf per ml. Free Samorin kills Trypanosoma IL 1180 at a dose of 1ng/ml. The efficacy of the drug/Tf complex was as least as effective as free drug alone even in the presence of high amounts of serum Tf present in medium. The situation with the Samorin resistant T. congolense clone IL 3338.2 was quite different. This clone is sensitive to the effects of Samorin-APG-Tf at drug concentrations of 10ng/ml or higher. Surprisingly, the control APG-Tf conjugate was able to inhibit the growth of both Trypanosoma lL 1180 and IL 3338.2, albeit at similarly high Tf concentrations: 24.2-48.5 μg APG-Tf/ml. The data demonstrate the potential use of Tf as a novel drug delivery system in T. congolense. This finding has far reaching implications in the treatment of human sleeping sickness as Tf is one of the few serum proteins that is able to cross the blood-brain barrier.

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We have sequenced the cDNA encoding the major surface antigens P30 and P22, and the excreted-secreted major antigen P23 of the virulent RH strain and avirulent Beverley and S-273 strains of Toxoplasma gondii by direct PCR. The amplified sequences of antigen cDNA were compared. Complete homology was found in the cDNAs encoding the P22 antigen in the RH and S-273 strains of T. gondii, and high homology but not complete was found between the cDNA encoding P23 and P30 in the Beverley and S-273. An interesting finding was the amino acid sequence differences relating to surface charge of P30 of virulent and avirulent strains of T. gondii. These phenomena might be associated with infectious difference in cells and/or the differences in pathogenicity.

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