Experiments on cryopreservation of Plasmodium falciparum

A comparative study was carried out in order to assess the effectivity of the deep-freezing conditions for P. falciparum cultures: 1. by plunging them into liquid nitrogen, 2. by use of an established freezing program for lymphocytes. After thawing in a waterbath at 37 degrees C, samples were diluted with equal volumes of 3.5% sodium chloride solution or 15% phosphate-buffered saline supplemented glucose solution. Other samples were not diluted after thawing, but the medium was replaced twice during the first day in culture. Survival was evaluated by counting the parasites in Giemsa-stained smears taken on day 3 after thawing. We obtained the best survival rate using NaCl solution, whereas replacing the medium twice resulted in the lowest level of survival. In synchronized cultures we studied survival rates of the different developmental stages of the intraerythrocytic parasites: young trophozoites (ring stages) and schizonts. The results indicate that irrespective of the freezing technique used, the survival level of young trophozoites was high (40 and 60%). The more sensitive stages (late trophozoites and schizonts) were better protected by using the freezing program designed for lymphocytes.