Concentration and separation of erythrocytes infected with Plasmodium falciparum by gradient centrifugation.

Concentration of infected erythrocytes was achieved in cell suspensions derived from long-term culture of Plasmodium falciparum growing asynchronously in human erythrocytes. This new procedure involves the slow centrifugation (at 33 g) of erythrocyte suspensions through 5% Ficoll solutions. Mature asexual erythrocytic forms are preferentially retained in the gradient solution (top fraction). After further gradient centrifugation of these parasitized cells, the concentration of mature forms is increased 15- to 31-fold and a mature form parasitaemia of 71-81% is obtained in the final erythrocyte suspension. Furthermore, at least 75% of the total number of the mature forms can be retrieved by this method. Parasitized cells that are not retained in the gradient are sedimented to the bottom of the tube (bottom fraction) and consist predominantly of ring forms. Parasites from both the top and bottom fractions are viable and have been used to initiate short-term synchronous cultures. By providing purified parasite preparations, this simple procedure will facilitate immunological, chemotherapeutic, and biochemical studies with P. falciparum.