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Malaria is caused by unicellular parasites of the genus Plasmodium, which reside in erythrocytes during the clinically relevant stage of infection. To separate parasite from host cell material, haemolytic agents such as saponin are widely used. Previous electron microscopy studies on saponin-treated parasites reported both, parasites enclosed by the erythrocyte membrane and liberated from the host cell. These ambiguous reports prompted us to investigate haemolysis by live-cell time-lapse microscopy. Using either saponin or streptolysin O to lyse Plasmodium falciparum-infected erythrocytes, we found that ring-stage parasites efficiently exit the erythrocyte upon haemolysis. For late-stage parasites, we found that only approximately half were freed, supporting the previous electron microscopy studies. Immunofluorescence imaging indicated that freed parasites were surrounded by the parasitophorous vacuolar membrane. These results may be of interest for future work using haemolytic agents to enrich for parasite material.

Fluorescence correlation spectroscopy (FCS) is a versatile tool for investigating the mobilities of fluorescent molecules in cells. In this article, we show that it is possible to distinguish between freely diffusing and membrane-bound forms of biomolecules involved in signal transduction in living cells. Fluorescence correlation spectroscopy was used to measure the mobility of phytochrome, which plays a role in phototropism and polarotropism in protonemal tip cells of the moss Ceratodon purpureus. The phytochrome was loaded with phycoerythrobilin, which is fluorescent only in the phytochrome-bound state. Confocal laser scanning microscopy was used for imaging and selecting the xy measuring position in the apical zone of the tip cell. Fluorescence correlation was measured at ancient z-positions in the cell. Analysis of the diffusion coefficients by nonlinear least-square fits showed a subcellular fraction of phytochrome at the cell periphery with a sixfold higher diffusion coefficient than in the core fraction. This phytochrome is apparently bound to the membrane and probably controls the phototropic and polarotropic response.