blood film, to keep it within the wanted limits, the culture can be thinned out with healthy red blood cells.[7]
Concentration of P. falciparum-infected erythrocytes by discontinuous density gradient centrifugation in Percoll.[8]
The original method for the successful ex vivo propagation of P. falciparum described culture of the parasite under static conditions (Trager-Jensen method).
RPMI 1640 (which turned out to be the best of the commercial media) in Petri dishes, placed in a candlejar and incubated. The line grew very well and became FCR-3/Gambia, one of the most widely used strains. Later, other lines would be established using similar methods and the impact of continuous cultivation of P. falciparum was phenomenal especially for the testing of putative antimalarials and for deciphering its genes. A number of subsequent reports (from as far back as the early 1980s), showed that cell suspension (using a shaking-incubator) significantly increased culture growth. Continuous agitation has also been shown to improve other parameters of culture growth relevant to researchers, such as the prolongation of culture synchrony after synchronization procedures, and a reduction of the rate of multiple infections.[9] Despite this, the practice of culturing the parasite under static conditions remains widespread. The greatest value of the candlejar method is that it can be used in laboratories almost anywhere in the world where there is an incubator, a candle and a desiccator.[10] Around 60% parasitized cells can be obtained using optimized culturing conditions.[1] Recent studies of P. falciparum isolated directly from infected patients indicate that alternative parasite biological states occur in the natural host that are not observed with ex vivo cultivated parasites.[11]
Concentration of infected cells
Blood stages of P.falciparum[1]Magnetic collection of P.falciparum infected blood[12][13]
To achieve synchronization and/or concentration of the parasites in culture several methods have been developed. A discontinuous Percollgradient procedure can be used to isolate
infected red blood cells because red cells containing plasmodia are less dense than normal ones. Young
trophozoites coincided with erythrocytes in a broad band corresponding to densities from 1.075 to 1.100 g/ml, whereas schizonts were concentrated at a density approximating 1.062 g/ml.[14]
There are studies, however, that suggest that some strains of P.falciparum are affected in their capacity of invasion after being exposed to this chemical.
The difference between
paramagnetic hemozoin in infected red blood cells can also be used for isolation. Magnetic columns have shown to be less harmful for the parasite and are simple and adjustable to the needs of the researcher.[15][16] The column is mounted in a potent magnet holder and the culture flowed through it. The column traps the erythrocytes infected with the latest stages of the parasites, which can then be eluted when the column is removed from the magnet. It is a simple method that does not need expensive equipment and it does not seem to affect the parasites as to their invasion capabilities afterwards.[12]
