Restoration of virulence of axenically cultivated Entamoeba histolytica by cholesterol.

The lost pathogenicity of two strains of Entamoeba histolytica, one isolated in 1924 and the other in 1967, grown in axenic culture for the past 5 and 6 years respectively, was restored by supplementing the culture medium with cholesterol through a number of transfers. The number of passages in the cholesterol-supplemented medium, necessary to restore a certain degree of pathogenicity of the two strains in hamsters, was proportional to the total time of in vitro cultivation of the strain, and not just the time of cultivation under axenic conditions. Pathogenicity, once restored, persisted for a long time after cholesterol treatment was stopped.