Cysts of Entamoeba protozoans, E. coli and E. histolityca, computer illustration. E. coli is a non-pathogenic amoeba, its cyst is 15-25 micrometres, has eight nuclei and chromatoidal bar elongated with splintered ends (dark red). E. histolytica is a parasitic protozoan that causes amoebic dysentery in humans after ingestion of infective cysts in contaminated food or water. The mature cyst of E. histolytica is 10-20 micrometres, has four nuclei and rounded elongated chromatoidal bar (dark red).