Clones of the malaria parasite Plasmodium falciparum obtained by microscopic selection: their characterization with regard to knobs, chloroquine sensitivity, and formation of gametocytes.

A culture line of Plasmodium falciparum (FCR-3/Gambia) was used to select and place in culture cells containing a single parasite. The method depends on examination of minute droplets of dilute cell suspensions with oil immersion phase-contrast microscopy. Droplets found to contain a single parasite were maintained under appropriate culture conditions until detectable numbers of parasites were present (generally by day 21). Of nine clones that grew up, seven were knobless and two were knobby. The clones differed somewhat in chloroquine sensitivity. Their 50% inhibition point under one set of conditions in vitro ranged from 0.02 to 0.06 micrograms of base per ml, compared with only 0.003 micrograms for the highly sensitive line FCR-8/West Africa. All three tested clones formed gametocytes under appropriate in vitro conditions.